Final Report Summary - NWS4LIGHT (Nanowires for solid state lighting)
Executive Summary:
Final report
Executive summary:
This collaborative project concerns materials research for future solid state lighting solutions, i. e. illumination with white light indoors as well as outdoors. The project is coordinated by Lund University, Sweden, (ULUND), Prof. Lars Samuelson. Other academic partners are: The Technical University of Denmark, (DTU); Copenhagen University, (UCPH); Stiftung Deutsches Electronen Synchrotron, DESY, Hamburg, (DESY); and Forschungszentrum Juelich GMBH, Juelich,(JUELICH). Two industrial partners are involved: Glo AB, Lund, Sweden (GLO), and Glo-USA Inc. Sunnyvale, USA, (GLOINC). The partners have specific roles in the project: ULUND, JUELICH, GLO and GLOINC are producing the materials for the LEDs, and also perform characterization of these materials; DESY and UCPH provide high spatial resolution synchrotron based structural studies, DTU assists with high resolution electron microscopy and related material analysis. GLO and GLOINC grow, produce and characterize LED devices based on the grown materials.
White LEDs are just entering the market as the future solution for lighting. Today’s LED lamps are based on violet GaN-InGaN LEDs with a cover of a phosphor powder emitting in yellow (and red). This concept was awarded the Nobel Prize in Physics 2014. These planar LEDs are typically grown on sapphire, leading to a high defect density. The nano-wire based LEDs (NW-LEDs) can be grown defect free (even on foreign substrates), and contain a quantum well (QW) based active region just like the planar LEDs. The NW-LEDs are typically rods a couple of µm long and about 400 nm diameter, with a hexagonal cross section. The different layers in a NW-LED are stacked radially, with a thin core in the middle, buffer layers and active QW layers further out, blocking layer and p-layer outmost, in a so called core-shell configuration. As substrate sapphire or silicon can be used, with a thin GaN nucleation layer and an insulating layer (e.g SiNx) on top. A nano-imprint lithography (NIL) process is preferably used to expose the GaN nucleation layer in a regular pattern, leading to growth of NWs in the same regular way. The growth is done with the Metal Organic Chemical Vapor Deposition (MOCVD) method, well established for planar LEDs.
This project aims at developing efficient LEDs in four colours, blue, green, yellow and red. By combining and driving four such LEDs in the same package white light emission of higher efficiency, and with higher color rendering. than in the planar case is foreseen. We have produced NW-LEDs in both InGaN-GaN and AlGaInP materials systems. Blue and green InGaN –based NW-LEDs give comparable output to the planar versions. For the red-emitting AlGaInP based NW-LEDs the output is still not optimized.
We have also developed InGaN-based LEDs based on nano-templates with a c-plane InGaN surface. Growing a planar LED structure on top of such templates allows NW-LEDs which are defect free, have low built in strain and can cover the entire visible spectrum with InGaN QWs.
Project Context and Objectives:
Summary description of project context and objectives.
The invention of new efficient white light sources based on light emitting diodes (LEDs) was awarded the 2014 Nobel Prize in Physics. This can be seen as a recognition of an achievement in new material development involving the III-nitride materials (like GaN and InGaN) and the industrial process to bring the white LEDs to the market. The present white LED lamps are based on violet-emitting InGaN LEDs assembled together with phosphor materials emitting at longer wavelengths. The mix of these emissions is perceived as white light by the human eye, although with rather poor color rendering properties. These lamps are available on the market with efficiencies exceeding these of the fluorescent lamps, like the popular compact versions (CFLs), they typically produce of the order 130 – 150 lm per W of incoming electric effect. These nitride structures are usually grown on foreign substrate materials (like Sapphire, SiC or silicon) to reduce production cost. Still the cost of these white lamps must come down nearly an order of magnitude to dominate the market for lighting.
The drawback of these white LEDs is a loss of about 25% of the energy of the primary violet emission in the down conversion from violet to longer wavelengths, this energy is lost to heating in the phosphor material. In addition there is a rather strong droop (decrease of efficiency at high currents), which can be as high as 40% at high currents. This droop is entirely a property of the primary violet LEDs, and is in production handled by using a much larger emitting chip area that otherwise necessary. This is obviously a factor that contributes to the high cost of the present white lamps. Further, so called warm white light similar to the sunlight is difficult to obtain with high efficiency using the presently available phosphors. Future research will therefore need to address these major topics. There is some evidence that the droop has to do with the growth on foreign substrates, which creates a high dislocation density throughout the LED structure (up to 109 cm-2). Growth on low defect density GaN substrates (or preferably InGaN substrates) is presently unrealistic in LED production due to limited availability of such material, in combination with a high cost.
Avoiding structural defects is therefore one base line in the materials research related to future LEDs. It is well established that in nanostructures like nanowires the materials can be grown free of structural defects, mainly due to the limited dimensions. Defect free nanostructures may be obtained even for growth on a foreign substrate, provided a suitable buffer layer is used. These facts make the nanowire concept very interesting for development of a new generation of LEDs for solid state lighting (SSL).
In the present development work we want to explore whether the use of phosphors can also be avoided for white LEDs. The general goal of this project has been to establish new material technologies for efficient LEDs for four key light wavelengths in the visible region, i.e. blue, green, yellow and red. White light can then be efficiently produced by mixing of the output of all four LED colours produced by combining four different colour LEDs in the same LED package (the so called polychromatic solution). This solution would then potentially offer a much higher efficiency for conversion of electricity into visible light (up to 260 lm/W is predicted in LED production for this concept). The significant novel approach in this project is that this will be done based on nanowires as the active elements for light emission. An essential development is then that the nanowire based LEDs can be produced without any structural defects. The emphasis is on materials development and related characterization of relevant properties, but realistic device prototypes are also demonstrated via an industrial partner during the project duration. The bold long term ambition in the project is that NW-based LEDs will be demonstrated that are equally efficient than the corresponding commercially available LEDs from the present planar technology, for all four colours blue, green, yellow and red. The objectives expressed in the EC call for project applications in 2011 had a 10 years perspective for the fulfilment of somewhat more ambitious goals.
The growth development at ULUND and GLO is focused on two different material systems, InGaN and AlInGaP. The InGaN alloy system is the one employed for the planar violet-blue- green LEDs today available commercially. While such planar blue InGaN LEDs are very good, there are serious problems with the radiative efficiency in the planar green LEDs. In the planar InGaN LED technology there are severe limitations towards extension of the emitting wavelengths towards the red, mainly due to strain-induced defect creation in the In-rich active quantum well (QW) regions during growth. Such defects are known to cause non-radiative recombination in LEDs, reducing the light output, while also expected to reduce the life-time and reliability of the devices. It is expected that such strain related problems are somewhat less severe in our NW-based LED structures, where the active regions are positioned closer to an outer surface. In addition to regular nano-wire LED structures with concentric device layers around a vertical core (the so called core-shell configuration) we are presently investigating different solutions for the active region, including InGaN c-plane nano-templates, to explore the limits for obtaining efficient InGaN-based NW-LEDs towards the red spectral range. Obviously it would be a great advantage for LED processing to obtain all LEDs for different emission colors in one and the same material system.
The new development of c-plane InGaN nanotemplates may potentially resolve many of the presently discovered problems with InGaN-based LEDs. There are many reports on excessive strain in the present planar longer wavelength (green and longer) InGaN LED structures. Structural defects like misfit dislocations and stacking faults are observed as a result of the strain induced by the strong difference in lattice parameters between GaN barriers and the InGaN QWs. There is also a suspicion that this built-in strain also promotes a high concentration of point defects in the same regions of the LED structure. These problems may be eliminated by using InGaN templates as a base for growth of the LED structure. Defect free c-plane InGaN nano-templates with In composition potentially up to 20% can be produced in a single growth step, these platelets are typically strain-free and also free of structural defects. For green LEDs a template In composition about 10% is suitable, for longer wavelength the In content in the template has to be increased accordingly. Using this technology InGaN LEDs with emission from blue to red has been demonstrated in the project, although not yet optimised.
The III-V material system AlInGaP has been developed extensively for planar red-emitting LEDs during the last decade, and there is good hope that efficient yellow and red emission can be achieved using NW-based structures for these same materials. Our ongoing work explores the feasibility of growing these core-shell structures based on InGaP NW cores. The first red-emitting such NW-LEDs were recently demonstrated in the project.
In addition to these efforts there is a parallel activity at JUELICH to grow core-shell nano- crystals (NCs, about 5 nm size) in the InP-ZnS materials system. The idea is to test the suitability of such dots as light emitting agents in combination with the InGaN based NW-LED emitters, i e the dots will be optically excited by violet, blue, or green NW-LEDs and emit in the yellow-to-red wavelengths, when suitably applied in the NW-LED structure. So far the emission in the green range from the fabricated core-shell NCs is quite good (above 50 % efficiency), and recent work on the red-emitting NCs indicate an efficiency about 40%.
The materials characterization efforts have been conducted as planned, in close connection with the growth efforts. The beneficiaries working at the large scale synchrotron facilities have developed new world class techniques for specific studies at the nano scale, not possible in ordinary materials research laboratories. The much shorter wavelengths produced in the synchrotron facilities allow focusing of a nano-beam of such radiation to a specific part of a single NW-LED object, and then also produce a high resolution image of the result, such as composition and strain. This way strain inhomogeneities have been discovered in NW-LED samples which by ordinary inspection in a SEM picture appear perfectly symmetric. Low temperature cathodo-luminescence (CL) measurements have proven to be an invaluable tool to evaluate the emission properties in cross section for an individual NW-LED. This way important gradients in composition in the active QW layers vs wire length have been discovered, such properties do affect the radiative emission properties in a negative way. Upgraded equipment for the optical studies at ULUND (including a streak camera setup) has been installed at ULUND, and the first results on NW luminescence dynamics were presented. The excellent facilities for electron microscopy at DTU have produced important data (via electron holography) on profiling of the potential across the produced doped NWs intended for NW-LED structures.
A properly equipped pilot production line for NW-LEDs has been established at the GLOINC partner. This allows evaluation of fully processed NW-LEDs according to the work plan. Recently quite promising blue as well as green LEDs were demonstrated, with an external quantum efficiency (EQE) of close to 30% for blue NW-LEDs and more than 40% for green NW-LEDs. In parallel, work at ULUND develops fully contacted single NW LEDs, and characterization techniques for those (electrical and optical). More details from this device-related work is presented in the section about S&T results below.
Project Results:
The main S&T results in the project.
The overall goal with this project has been to develop nanowire structures for LED applications, notably for visible light emission, i.e. so called Solid State Lighting (SSL). This involves research on suitable growth procedures for the complex LED configurations involved. The nanowire topology means that the characterization of the relevant physical properties partly requires novel experimental techniques, adapted to the nanowire geometry. A complete characterization would ideally involve sub-nm resolution of the measured properties, not yet realistic. To verify that the results in growth and characterization lead to the projected properties of the nano-wire based LEDs (NW-LEDs), a pilot production facility has been established at one of the industry partners (GLOINC). The results obtained from the evaluation of the produced NW-LEDs form a relevant basis for the evaluation of the progress as well as remaining problems in the application of NW-LEDs for SSL purposes.
The most important details of the results obtained will be described in what follows, following the structure of the different work packages. In the growth area it is clear that the MOCVD technique has proven to be a useful vehicle to produce complex core-shell NW-LED structures with nm precision. This does not mean that problems are not present. It is clear that the obtained irregular results of the efficiency of blue NW-LED emitters involve a defect problem that is not directly revealed by the experimental investigations. Likewise the rather strong droop observed for green emitters is not expected from the presently accepted theory, it may be viewed as another evidence for nm size structural defects related to the complex geometry of these NWs. More research is obviously needed before such problems are under control.
In the characterization work package important developments have taken place in optical methods, electron microscopy and XRD techniques. The electron holography techniques can now be used with confidence since the influence of electron irradiation on defect creation during the experiments has been understood. The potential profile across a single NW-LED structure can now be mapped out, which is very valuable to understand other data, such as electrical measurements. Photoluminescence (PL) and cathodoluminescence (CL, both at temperatures down to 5 K) are valuable to understand the nature of the radiative transitions in the LEDs, the latter technique also has the spatial resolution to study variations of emission across a single NW-LED. A suitable platform for electrical measurements on single NW-LEDs has been developed and used in the project.
The third work package in focused on demonstrating a device processing technology for the studied NW-LEDs, by using a wafer process flow running on a commercial wafer fabrication line. One important task is to compare the predicted performance of the NW-LEDs with reality, in terms of lumen and color maintenance, wall-plug efficiency, reliability, and predicted production cost. GLOINC has demonstrated that it is possible to produce nanowire LEDs using a flip-chip process technology, thereby enabling the integration of these LEDs into systems with large numbers of devices. The reliability tests also show that these devices behave very similarly to the commercial planar LEDs. As mentioned above there are still some areas where more work is needed to understand the behavior, e.g the lower average emission intensity from the blue NW-LEDs (as compared to the planar violet-blue LEDs) and the excessive droop of the green NW-LEDs.
Below some important results in the project will be described in more detail.
WP1. MOCVD growth technology for III-nitride and III-V NW-LED structures.
The III-N NW LED structure consists typically of a Si-doped GaN core in the shape of a pillar grown perpendicular to the wafer surface. The GaN core is of wurtzite crystal structure and acts as a template for a radially grown InGaN quantum well acting as an active region for the LED device, an electron blocking AlGaN shell and a Mg-doped GaN: p-GaN shell as a source of holes injected to the active layer, where they can recombine with electrons and generate light. The vertical form of nanowire is beneficial for enhancing light extraction. A schematic representation of the NW LED cross-section along a cut perpendicular to the m-plane can be seen in Fig. 1a. A complementary cross-section view from SEM on a typical NW LED is shown in Fig. 1b.
The growth template consists of a silicon substrate, of (111) orientation, or a c-plane sapphire wafer. An AlGaN/AlN wetting layer is grown before the main buffer layer, which is a 2-4 µm Si-doped thick GaN layer, with Si doping in the range 1-5 E18 cm-3. A silicon nitride mask was deposited on the GaN layer, with a thickness ranging from 30 to 50nm. Electron beam lithography (EBL) or nanoimprint lithography (NIL) have been used to make openings in the silicon nitride mask down to the GaN layer. The typical opening aperture ranges between 70 to 140 nm, however in this project we have also tested openings up to 300 nm. The openings form a honeycomb pattern, with a pitch distance between the nearest neighbors varying between 1 and 1.5 µm. These openings define the positions and dimensions of the grown nanowires. To determine the surface morphology scanning electron microscopy was performed on nanowire samples. A Thermal Field Emission Scanning Electron Microscope (SEM) LEO 1560 was used, with 10 kV electron acceleration voltage. Cathodoluminescence (CL) was performed at Lund University (ULUND), using a Cambridge Instruments S250 M3 SEM, with a GaAs photomultiplier tube (PMT) recording the CL spectra, and with a cooling stage capable of cooling the sample to below 5 K. Photoluminescence (PL) data was recorded using a micro-PL setup at Lund University, with an excitation laser at 375nm, 9mW.
In order to ensure controlled growth of NW LEDs, nanowire cores must be grown uniformly from the mask openings. Since different pitch and different hole size was used, it is important to use proper nucleation conditions. In the initial nucleation step the flows of TEGa as Ga precursor and NH3 as N precursor were optimized for various pitch/hole sizes. Effects of thermal pre-annealing as well as nucleation and nanowire core growth temperature were taken into account. After nucleation, TEGa and NH3 were continuously flown to ensure the targeted nanowire length. The results of nanowire diameter in respect to growth time in the case of EBL 1 µm /70nm opening are shown in Figure 2a, and the dependence of m-plane length on growth time are shown in Fig. 2b.
As seen in Fig 2a. the diameter is enlarged in the first 30 seconds of growth, after which it has stabilized at about 160 nm. The m-plane length increases linearly with the growth time. Based on the initial growth stage findings and by applying volume growth rate calculations to different pitch/opening size we have obtained uniform growth of NW LEDs on available substrates as shown in Fig.3.
Fig.1. a) to the left. Schematic cross-section of III-N nanowire LED. b) to the right. Cross-sectional SEM image taken at 80k magnification. The scale bar is 200 nm.
Fig.2. a) dependence of nanowire diameter on growth time in initial growth stages. b) dependence of m-plane length on growth time in the initial growth stages.
Fig.3. 30 deg tilted SEM on NW LEDs on different pitch/opening aperture templates: EBL 1 µm/70nm to the left, NIL 1 µm/100nm in the middle and NIL 1.5µm/140nm to the right. The scale bar is 1 µm.
The typical diameter of the NW core is dependent on the mask-opening aperture and varies between 190-220 nm for a hole size 70 to 100 nm. Nanowires up to 3µm in length could be grown with good growth uniformity. The structural quality of all internal layers in a NW-LED was characterized by transmission electron microscopy (TEM). As seen in cross-sectional TEM in Figure 4 no stacking faults or dislocations are observed in NW core/nGaN shell. This was possible to achieve through careful selection of growth conditions for each individual layer.
Fig.4. To the left: high resolution TEM on an NW nGaN core - overview. In the centre: magnification on the tip region. To the right: Electron diffraction pattern taken in zone [100], perpendicular to the NW long axis.
The side and pyramid facets were determined by electron diffraction in TEM. As expected the side of the nanowire is terminated by {1-100} facets (non-polar m-plane), while tip facets are semi-polar and have {1-101} orientation. The LED structure was extended by outer shells like InGaN QW/p-AlGaN/p-GaN grown on top of the nGaN shell. The outer shells are grown following the {1-100} and {1-101} crystallographic orientations.
Optical and electrical characterization of III-N NW LEDs.
We have grown and characterized InGaN and p-GaN layer shells using proprietary Glo technology. A uniform distribution of indium in the QWs as well as uniform growth of the QWs are important to ensure monochromatic light extraction. Indium segregation as spatial variation in indium composition is one of the issues for the QWs. To determine spatial distribution and variation of indium composition in an SQW as well as Mg dopants in the p-layers, InGaN and p-GaN shells were characterized by cathodoluminescence at a temperature of 4 K. Fig.5 shows spatially resolved CL of an NW SQW LED, taken at 7K. From spatially resolved CL spot mode we were able to determine that there is a shift in QW emission towards shorter wavelenths when moving the measurement spot from the top of the nanowire towards the bottom. The emission at the EL peak WL from SQW could from about 470nm on the top to about 430 nm at the bottom. This could be due to non-uniform indium incorporation as well as non-uniform QW thickness along the m-plane. However, no stacking faults or dislocations in the QWs were observed in TEM.
We postulate that indium incorporation could be improved by a change in growth conditions such as QW temperature and pressure as well as nanowire length, with possible improvement when using shorter nanowires. The CL data was correlated to energy dispersion spectroscopy TEM (EDS TEM), where QW thickness and indium composition have been determined. In Table 1 below EDS and TEM data on indium composition in QW and QW thickness at three spots: bottom, centre and top of the m-plane are summarized.
Fig.5. Spatially resolved cathodoluminescence on a single NW LED. Color coded are five spots along the nanowire SQW where CL spectra have been recorded and shown on the plot to the right.
Table. 1. QW thickness by TEM and indium composition by EDS.
QW top QW centre QW bottom
QW thickness (nm) 6.7 5 4.5
In composition (%) 9.3 9.4 6.5
The increase in WL from NW bottom to NW top is consistent with an increase in indium composition and QW thickness from NW base towards NW top. To confirm findings from CL, TEM and EDS we have performed high resolution X-ray diffraction measurements on NW SQWs. As shown in Fig.6. beside several Bragg peaks corresponding to AlGaN wetting layers in the substrate, there is broadening of the (101) peak corresponding to the InGaN SQW, and this broadening suggests that the QW is fully strained in the out-of-plane direction.
Fig.6. to the left. 3D isosurface of Bragg peaks originating from the wurtzite structure in the InGaN QW. To the right: Contour plot in the (001) direction.
In order to have efficient light generation low resistivity, Mg doped GaN with high hole concentration, at least 5-10 E17 cm-3, must be capping the LED structure. We characterized p-GaN shells either directly on the LED structure or on specially prepared n-p structures consisting only of nGaN core with p-GaN shell. Various characterization techniques have been employed including optical methods such as PL, CL as well as electrical characterization of single n-p nanowire structures. Low temperature PL was performed on p-GaN layers to reveal optical signatures of dopants. Fig.7 shows low temperature (4K) PL spectra taken from ensemble spectra as well as focused micro-PL for single wire spectra.
Growth of artificially miscut III-N nanowires.
The focus of this work was to realize artificially miscut m-plane sidefacets of GaN nanowires with InGaN single quantum well outer shell. Selective area epitaxy by MOVPE was chosen as a growth technique for this work. Typically the GaN surface on which the InGaN QW is grown is atomically smooth. Indium atoms which exhibit a high surface mobility can easily desorb from the flat GaN surface. A controlled number of nucleation sides like kinks and steps could improve the homogeneity of the grown layer via layer-by-layer growth, and also increase the indium surface incorporation. An increasing indium content in the QW leads to a light emission at longer wavelengths like yellow/red. A controlled layer-by-layer growth mode is desirable to avoid defects, in particular for the growth of the QW active region.
In order to increase the density of steps on GaN surface we have studied the effect of the H2/N2 ratio on n-GaN nanowire tapering, i.e. artificially changing the off-axis angle of m-plane sidefacets. We found out that varying H2/N2 in the gas phase from 0% to 2.78% increases the off-axis angle from about -0.91° (inverse tapered) to 0.6° (tapered nanowire) towards c-plane (growth direction) as shown in Fig.7. However further increase in the H2/N2 ration results in a decrease of this angle. The surface topology was characterized by atomic force microscopy (AFM). Atomic steps on the surface could be distinguished. The mean square roughness increases from 0.3 nm for the baseline with almost 0° miscut angle to 0.75 and 0.6 nm for inverse tapered and tapered nanowire, respectively, thus indicating an increased density of steps with changing artificially the miscut angle.
Figure 8. Artificially miscut GaN nanowires. From left to right: miscut angle is 0.91°,
0° and 0.6°.
The above studies are continuing with the aim to optimize the growth conditions of the active QW part of NW-LED structure. We have experienced a large variation in the EQE of the grown violet-blue NW-LEDs, and concluded this is due to a problem with nonradiative defects in the active region of the device, intimately linked to the growth conditions. At the same time there is still an obvious problem with an In-composition gradient along the growth direction.
Growth of dislocation free c-plane InGaN platelets based on InGaN nanowires.
A method to fabricate wafer-scale arrays of GaN platelets with a smooth c-plane in a submicron size was developed. Such GaN platelets are free of dislocations since they are grown based on InGaN nanowires where the dislocation propagation from the underlying GaN buffer layer can be inhibited. On the c-plane of the platelets, high quality nitride layers such as AlGaN and InGaN can be grown for electronic and optoelectronic devices.
(b)
Figure 10. Identification of GaN NWs with a dislocation propagating from the underlying GaN buffer layer. a) A TEM image showing asymmetric growth caused by deflection of a dislocation from underlying GaN buffer layer. b) A top view SEM of GaN NWs. c) A TEM image of a symmetric GaN NW which is free of dislocations.
The ability of continuously, selectively grow GaN nanowires to filter out dislocations was investigated. Typically, dislocations in the underlying buffer layer do not extend into the nanowires but when dislocations extend through the mask openings they are seen to deflect towards the side facets at early stages of the growth as shown in figure 10a. The intersection of the bent dislocation to the side facet makes the corresponding side facet grow faster than other facets, rendering the wire shape to be asymmetric, as seen from figure 10a and 10b. Six such asymmetric wires were selected for characterization by TEM and dislocations were confirmed in all of them. GaN NWs without dislocations have symmetric hexagonal shape as shown in figure 10b and 10c. In our NW growth with continuous flows, the formation of asymmetric wires is controllable.
Figure 11. Left: PL spectrum at 8 K from a single GaN platelet which was transferred to a substrate of Au/Si. Right: Decay of donor-bound exciton at 4K from a single GaN platelet which was transferred onto an Au/Si substrate. The lifetime is 400 ps.
Uniform wafer-scale arrays of GaN platelets were developed by growing a GaN shell on the dislocation-free GaN NWs mentioned above. {101 ̅1} planes are firstly formed at the wire top when the shell growth starts. Because of the extremely low growth rate, {101 ̅1} planes force the GaN to grow downwards with the {101 ̅1} planes maintained, as shown in figure 12a. With this pyramidal growth fashion, no c-plane is formed. However, we will show how the c-plane can be obtained through controlled change of the epitaxial growth mode, which leads to the formation of GaN platelets as shown in figure 12b and 12c. The size of the c-plane can be
Figure 12. Top-view (top row) and tilted-view (bottom row) SEM images of GaN shell growth on GaN NWs. a) Pyramidal shell growth without c-plane. Dotted lines show the profile of GaN NW. b) & c) show controlled platelet growth with varied c-plane size. increased by totally eliminating the {101 ̅1} planes (figure 12c). Time-resolved photoluminescence was measured on a single GaN platelet which was transferred onto an Au/Si substrate. It shows the emission of donor-bound excitons and the lifetime is 400 ps (figure 11).
The method of pyramidal growth and subsequent flattening of the pyramids was extended to the GaInN materials system with the ambition to synthesize materials for defect-free GaInN red and yellow emitting NW-LEDs. A composition of 11% In was found in the platelets.
Figure 13. Left: SEM image of InGaN pyramids grown selectively in holes opened up in a SiN growth mask. Right: PL spectrum obtained from the material upon optical excitation.
By inserting a GaInN quantum well on the c-plane we show that the wavelength of the emission can be tuned by growth parameters.
Figure 14. Left: SEM image of the GaInN platelets with embedded GaInN quantum wells for emission. Right: spectrally resolved CL from the platelets.
Figure 15. Left: a PL spectrum at room temperature on Mg-doped GaInN platelets; Right: a full LED structure based on GaInN platelets with p-GaInN at the top of pyramids.
Mg-doping on the GaInN platelets was studied. Because of the narrow band-edge emission from GaInN platelets, the emissions related to the Mg acceptor was found at the lower energy side (figure 15). Such p-GaInN growth was applied to the single QW on the GaInN platelets and a full LED structure was obtained as shown in the right SEM image in figure 6. Compared with the LEDs on c-plane GaN template, our structure offers smaller lattice mismatch between the GaInN template and the QW, which means better material quality and less quantum-confined Stark effect.
Development of III-V radial heterostructure NW materials for light emission.
III-V heterostructure growth has been developed with respect to materials composition and doping in the axial as well as radial geometry in order to produce red light emitting nanowire diodes.
TESn is the preferred n type dopant of SiH4, DTBSe and TESn with respect to dopant level achieved and control thereover.
Contactless electron off axis electron holography was used to determine shifts in the electrostatic potential, being a way to quantitatively measure doping.
By calibrating electron off axis electron holography in comparison to other methods of evaluating doping, it could be used to quantitatively determine doping level.
We found that doping by use of DEZn affects the materials composition of GaInP NWs where the GaInP materials composition was assessed by XRD, TEM and PL.
Axially homogeneous p doped GaInP NWs were grown by use of a method using in situ ramping of the TMI flow.
Parameters and growth scheme for full radial NWLED in (Al)GaInP were developed and lattice matched core shell NWLEDs were grown and characterized with respect to structural and electro optical materials properties. Phase segregation of Al and Ga during radial growth of shells was observed by use of TEM.
Hall voltage measurements was used to determine the n doping in the V-III nanowires, and doping carrier concentrations of 3x1017 cm-3 in the nanowires with highest S doped NWs were found. Lower doping levels were not possible to evaluate due to electrically non-transparent contacts.
IQE measurements of the NWLEDs with ideality factor about 3 are pending. For more details we refer to D1.6.and to the corresponding LED-section in WP3 below.
WP2. Characterization of NW-LED structures.
Structural characterization of radial GaInP/AlGaInP nanowire quantum well structures by high resolution STEM and STEM-EDX
High resolution STEM and STEM-EDX were applied to radial GaInP/AlGaInP nanowire quantum wells grown by MOCVD to study the sharpness of the heterojunctions and the compositional distribution of materials.
More than five different samples in which the Al precursor, trimethylaluminum (TMAl), was varied systematically were investigated to obtain the right concentration of Al in the AlGaInP layers. Results from one of these samples are summarized in figure 2.3-2. In all cases, Al enrichment with In and Ga deficiency was observed at the corners of the AlGaInP layers in the cross-section perpendicular to the nanowire axis. The GaInP/AlGaInP interfaces were relatively sharp.
Figure16. An example of compositional and structural characterization of radial GaInP/AlGaInP nanowire quantum well structures by the means of STEM. a) HAADF-STEM image of a FIB-prepared cross-sectional specimen. b) STEM-EDX map from the region marked with the red box in the HAADF image, showing the Al, Ga, In and P distribution. c) Atomic resolution STEM image from the AlGaInP/GaInP interface, the region marked with the small black box in the HAADF image. d) Material composition of different regions of nanowire, calculated from the EDX measurements.
Structural characterizations of NWs using X-rays: Single GaInN/GaN nanowire characterization
We present detailed nanoscale strain mapping performed on a single, 400 nm thick and 2 µm long core-shell GaInN/GaN nanowire with an x-ray beam focused down to 100 nm. A scanning electron microscopy (SEM) image of the typical NW from the same sample is shown in figure 17(b). The sample was carefully cleaved leaving the single isolated nanowire at the tip of the 60° corner of the substrate, as shown in figure 17(a). Strain mapping was performed at the X-ray nano-beam end-station [1] of the P06 beamline at the Petra III synchrotron radiation facility (Germany).
Figure 17 (a) Experimental set-up used for SXDM. A single NW at the corner of the sample was positioned in the focus of the x-ray beam and scanned in the xz plane at different angles. (b) SEM image of the NW. The white ellipse represents the FWHM of the incident beam (100x 150nm).
The strain maps were calculated from the peak positions for three different areas of the NW. From the strain maps corresponding to the GaN core (see figure 2.3-5(a)) we notice sharp strain gradients at the bottom of the NW, showing an apparent difference of the lattice constants in the NW and the underlying substrate Larger in-plane strain εxx is observed along the sides of the core, especially near the tip. Both strain components in the core are asymmetric with respect to the left and right sides of the NW. The tensile strain is larger in the left side of the core. This distribution may be caused by an asymmetric relaxation of the GaInN shell, as will be discussed next.
From the intensity map of the region B, shown in figure 18(b), we can see that it originates mostly from the narrow stripe on the left of the NW, which corresponds to the left side of the GaInN shell. The intensity is twice as strong at the top, compared to the bottom (see figure 18(b)), which is in agreement with an uneven distribution of the thickness of the shell, as measured with the cross-sectional SEM. From the strain maps in figure 18(b) we can see that the left side of the shell is compressed in the z direction in order to become coherent with the core.
The intensity and strain maps obtained for the region C, containing the diffuse signal, are shown in figure 18(c). From the intensity map it can be seen that it originates mostly from the top-right part of the GaInN shell. From the SEM image in figure 17(b) we can see that the surface of the GaInN shell is especially rough at the upper part of the NW. This explains the highly diffuse signal in the region C in reciprocal space. From the strain maps we can see that the in-plane and out-of-plane strain components have similar values, which corresponds to the relaxed InxGa1-xN with xIn = 0.32.
NW-based light emitting diodes comprise a macroscopic die with thousands of identical nanowires, each one acting as an independent light emitter. Using the single NW characterization one has to demonstrate that the single NW chosen to be measured was not a statistical outlier, but can represent an entire ensemble. Reciprocal space mapping (RSM) was performed on ensemble of nanowires at beamline I811 at Max Laboratories in Sweden, giving the average strain state in the large number of nanowires. RSMs of the (10-11) peak measured for the ensemble are compared to the single NW measurements done with the nano-focused beam in figure 2.3-6(a) and (b), respectively. In both cases we identify a signal from both the relaxed and the strained GaInN shell. This similarity allows us to extrapolate the findings based on the single nanowire to the entire sample, although in general this is not the case.
In conclusion, we have demonstrated how the combination of a nanofocused x-ray beam and SXDM can be used to map the strain distribution in a single NW with a resolution of about 100 nm. We have found that the strain distribution in the GaN core caused by the lattice mismatch at the GaN/GaInN interface is inhomogeneous. Asymmetry in the axial strain relaxation is present, with one side of the shell being fully pseudomorphic with the core and the other side plastically relaxed. In this we also demonstrate fully strained 15-28 nm thick In0.32Ga0.68N shells on GaN NWs. This exceeds the predicted critical thickness in the planar structures, which does not exceed 10 nm, demonstrating the advantages of the nanowires. However, uneven strain relaxation can lead to inhomogeneous material incorporation which has impact on the device performance. We have demonstrated here that strain mapping with the nano-focused x-ray beam has proven to be an excellent tool for measuring such nanostructures, which are too thick for alternative characterization methods.
These results are from a simpler test sample compared to a real NW-LED structure. It does however demonstrate the power of these nano-RXD measurements to discover strain inhomogeneities in NW objects. In the future there is hope that such measurements can be extended to map the strain state of the MQW active region in a NW-LED.
Improving the spatial resolution in photoluminescence from wires by using SILs
The spacing of the wires in typical arrays for LED devices is about 1µm. This is on the border of what can be resolved in our micro-PL setup. In order to improve this we are currently experimenting with various designs of solid immersion lenses (SILs). These are truncated spheres that we have fabricated ourselves as they are not commercially available. We have some preliminary results on this study. The simplest test is to study well-defined patterns and to compare with and without the SIL. This is presented in figure 20, using an array of wires with a 400nm-spacing. The left image is the image without the SIL and the right image is with the SIL. It is clear that the optical resolution is improved where the individual wires are accessible with the SIL.
Figure 20. Two white ligth images of an array of wires with a 400nm pitch. Without the SIL (left) the image is grainy and shows no periodicity. With the SIL (right) there is a clear periodicity of white spots. The improvement in resolution is obvious.
In PL imaging, we have not yet got the setup working properly. The laser needs to be spread over a larger area. Figure 21, shows some examples of PL imaging of wire arrays. The first example is InP with a 400nm pitch. Without the SIL, there is no hint of resolving the individual wires. All that is visible is the emission under the laser spot. With the SIL, there is a pattern of bright spots of the individual wires. The SIL is also used on GaInN pyramids with a pitch of 1µm. In this case, the bright spots are better resolved, as expected due to the larger pitch.
Figure 21. Left: The emission from an array of InP wires with a pitch of 400nm. In this image without the SIL, there is no structure in the bright spot excited by the laser. Centre: When the same array is imaged with the SIL, bright spots corresponding to the individual wires can be observed. Right: As the pitch is increased to 1µm, the improvement is even more pronounced, here the arrays consisted of GaInN pyramids.
WP3. NW-LED devices in the project.
Green III-nitride core-shell nanowire LEDs have been demonstrated by Glo with peak EQEs exceeding 35% and EQEs at 20 A/cm2 equivalent to the performance of commercially available planar devices. The performance characteristics of the green NW LEDs are consistent with IQE ~ 40%. The devices exhibit good p-type layer conductivity and low defect density.
Figure 22 shows the EQE of green NW-LEDs as of M18 of the project and M24 of the project, plotted against current. This plot illustrates that through optimization of the epitaxial growth process to increase the volume of green-emitting QW material on the sidewalls of the nanowire, the efficiency at high current densities can be substantially increased. In these two efficiency versus current curves, the volume of the QW is increased by a factor of more than two. This was accomplished by increasing the molar flux of TMIn and TEGa in the epitaxy reactor during the deposition of the QW.
Figure 22. External quantum efficiency (EQE) of green nanowire LEDs as of month 18 (M18) and month 24 (M24) of the project. The EQE is shown for LEDs packaged into dome lenses on Ag-coated TO-5 headers with an n=1.52 silicone dome lens.
Another element of increasing the volume of QW material is increasing the density of nanowires formed on the GaN template. In the device shown for M18, the nanowire spacing (or pitch) is 1 µm, whereas for the device shown for M24, the nanowire spacing is 0.7 µm. The nanowires are formed in a hexagonal pattern, therefore this reduction in spacing corresponds to double the number of nanowires per unit area of substrate in the M24 case. Figure 23a shows an optical microscope image of a portion of the LED active area for the M18 device and Figure 2b shows the M24 device, while the devices are operating.
Figure 23a. Optical microscope image of a portion of the M18 NW-LED while the device is operating. The bright spots are nanowires while the dark region between the bright spots is the open area on the GaN template. The nanowire pitch is 1 µm.
Figure 23b. Optical microscope image of a portion of the M24 NW-LED while the device is operating. The bright spots are nanowires while the dark region between the bright spots is the open area on the GaN template. The nanowire pitch is 0.7 µm. The individual nanowires are difficult to resolve in this image.
Full-wafer nanoimprint technology is capable of forming even denser arrays of holes in the selective area growth mask used to create the nanowires. By further reducing the nanowire pitch to e.g. 0.5 µm, the EQE of the green NW LEDs will be further improved. Our estimation is that at 0.5 µm, which corresponds to a 2x increase in number of nanowires per unit substrate area over the M24 case, the EQE at 20 A/cm2 will be improved to 22% or higher.
Processing technology for green NW LEDs has also been advanced from M18 to M24. In both cases, the LED has a anode contact (Al/Ti/Au) deposited on the n-type GaN “buffer layer” which is located underneath the selective area growth mask. The buffer layer is grown on sapphire in the cases of these LEDs, although Si has also been used. The cathode contact is a transparent conducting oxide (TCO) deposited across the surfaces of the nanowires, which are p-type. An Al/Ti/Au wire-bonding pad is deposited on the TCO to complete the cathode contact. In the M24 device, the TCO is recessed from the edges of the array of nanowires, whereas in the M18 device the TCO reaches the edge of the nanowire array. The M24 devices suffer from less resistive shunt leakage compared to the M18 devices due to this change, and thus have an increased EQE especially at low currents.
The green NW LEDs have forward voltages (Vfs) of approximately 3.2 V at 10 A/cm2 forward drive current. The Vf responds to traditional parameters for planar GaN LEDs, such as temperature for “activation” of Mg dopant atoms in the p-type GaN layer, and Mg doping concentration in the p-type GaN layer. At M18, the Vf was approximately 3.6 V and by optimizing the dopant levels and activation process parameters (such as time and temperature) the voltage was reduced by 0.4 V by M24.
NW-LED wafer process.
GLOINC demonstrated that it is possible to produce nanowire LEDs using a flip-chip process technology, thereby enabling the integration of these LEDs into systems with large numbers of devices where the otherwise-needed wire bonds are prohibitively expensive or result in yield or reliability problems of the integrated system. It is well known in the LED field that flip-chip devices require highly reflective p-type spreading contacts, in order to exhibit high optical extraction efficiency. The commonly used metals are Al and Ag. GLOINC evaluated both of these metals. Figure 24 is a cross section schematic of a flip-chip nanowire LED affixed to a submount.
Figure 24. Cross-section sketch of a flip-chip nanowire LED.
It is also a fundamental necessity of a flip-chip device that the anode and cathode contacts be formed of solderable metal. Most commonly used is AuSn. Due to the solid-phase reactions of these various metals and the potential impact thereof on device reliability, so-called barrier metals are typically provided between the reflective contact and the solder metal. A key requirement of any metal film in a flip-chip device is that the metal film is substantially free of voids. While this is typically straightforward to achieve on planar films, the high-aspect ratio topology of the nanowire LEDs tends to produce textured films (the adjective textured refers to both the commonly known morphological definition, and the term of art used by crystallographers). Figure 25 is a cross-section SEM taken of a nanowire array with a layer of TiN sputtered on the array (the top-most film); this is considered an unacceptable barrier metal surface morphology.
GLOINC selected e-beam deposited Mo as the preferred barrier metal. Additional deposition of Au solderable contacts was performed. Figure 26 is a plan-view optical microscope image of a flip-chip nanowire LED with power applied. Light from the active region, scattered off of the substrate back side, is seen surrounding the metal contacts. GLOINC has not characterized the optical extraction efficiency of this device configuration. Depending upon the results of those data, additional work on contact reflectivity may be required to realize a commercially competitive device.
Manufacturability and cost assessment
GLOINC presently estimates the manufacturing cost of GaN/InGaN nanowire LEDs to be approximately 20% higher than the manufacturing cost of GaN/InGaN planar LEDs, and anticipates that this cost differential will decrease over time as knowledge is accumulated and engineering development is done to specifically address some sources of cost increment.
The manufacturing process for a GaN/InGaN nanowire LED is, at a high level, identical to the manufacturing process for a GaN/InGaN planar LED, with two major exceptions, both of which are associated with the epitaxial growth process. The first exception is that the epitaxial growth process is done in two steps through the MOCVD reactor, as opposed to one step. In the nanowire LED process, the first MOCVD growth step is performed in order to create a GaN template layer on a substrate such as Al2O3 or Si. Then, nanoimprint lithography is used to create a selective area growth mask on the template. Finally, the wafer is loaded into the MOCVD reactor and the nanowire LED array is grown.
One source of additional manufacturing cost is associated with a longer cycle time in the MOCVD reactor, which adds depreciation cost. GLOINCs current estimate for MOCVD cycle time increase is 10%. The growth of the GaN template layer is more rapid than the growth of the GaN template used for GaN/InGaN planar LEDs, since the dislocation density of this film may be much larger for the selective area growth nanowire LED device structure. Growth of the nanowire LED array is similar in cycle time to the growth of the active layer and p-layer stacks used in planar GaN/InGaN LEDs. Because large multi-wafer production MOCVD reactors have large thermal masses, cool-down and heat-up times are typically 45 minutes to one hour long. Because two complete MOCVD cycles are required, there are four heat-up/cool-down cycles per GaN/InGaN nanowire LED wafer, as opposed to one heat-up/cool-down cycle per GaN/InGaN planar LED wafer. The net result is that the MOCVD cycle time is somewhat longer for GaN/InGaN planar LEDs.
A second source of additional manufacturing cost is the additional manufacturing steps of depositing the dielectric growth mask, performing nanoimprint lithography on that growth mask, and dry etching the growth mask to form a template for selective area growth of the nanowire array. The cost of these steps individually is relatively low. The dielectric growth mask deposition may be done in simple large scale batch furnaces which can process 100 or more wafers at a single time, for a total throughput around 100 wafers per hour. The throughput of the nanoimprint lithography step is on the order of 50 wafers per hour, depending upon the details of whether thermoplastic or UV imprint is used. The dry etch step to form the holes in the selective area growth mask may be performed on multi-wafer ICP-RIE etchers, with a throughput on the order of 50 wafers per hour. The throughput of the MOCVD step is about 5 wafers per hour, much lower than the growth mask process step.
During the early stage of ramping the volume of the GaN/InGaN nanowire LED manufacturing process, it is anticipated that the cumulative yield of the manufacturing process will be somewhat lower than is achieved today in state-of-the-art manufacturing facilities for planar GaN/InGaN LEDs. Most of this additional yield fall-out can be attributed to the selective area growth masking process, and possibly some decrease in within-wafer LED yield or across-wafer non-uniformity. Additional process control points may be implemented to screen non-conforming materials from subsequent process steps, minimizing the cost of yield fall-out. Within-wafer uniformity of GaN/InGaN nanowire LED performance is consistent with the uniformity of state-of-the-art planar GaN/InGaN LEDs today. For example, the within-wafer standard deviation of peak emission wavelength is measured to be 1.5 nm for GLOINC’s green GaN/InGaN nanowire LEDs on 50mm substrate, while it is 2.1 nm for planar GaN/InGaN LEDs on 50 mm substrates.
NW-LED performance characterization and reliability study.
GLOINC characterized the performance of a second generation of GaN/InGaN green-emitting nanowire LED in Month 24, and compared to the performance of a first generation demonstrated in Month 18. Figure 27 shows a comparison of the external quantum efficiency characteristics of these two generations of GaN/InGaN nanowire LEDs. The EQE of the second-generation devices is greater, especially at current densities greater than 3 A/cm2. This performance improvement was generated in two ways. First, the density of nanowires per unit substrate area was increased by reducing the pitch of the nanowires on the selective area growth mask and GaN template from 1.0 um to 0.7 um (a factor of 2 increase). Second, work was done on optimizing the growth conditions (principally molar flow rates of Ga and In precursor chemicals into the MOCVD reactor).
In addition to having a higher EQE, the nanowire LEDs demonstrated in Month 24 also had lower forward voltage.
The second generation green-emitting GaN/InGaN nanowire LEDs were subjected to reliability stress tests. The stress condition was 10 A/cm2 forward current in an ambient temperature of 90⁰C. After 2.000 hours of stress, the GaN/InGaN nanowire LEDs exhibited less than 10% decrease in optical output, and negligible change in forward voltage and emission spectrum. Figure 12 shows lumen maintenance data from more than 30 LEDs tested from two wafers, after 2.000 hours of stress.
Figure 28. Lumen maintenance data for green-emitting GaN/InGaN nanowire LEDs under high-temperature operating life stress conditions. At 2.000 hours of stress, all devices pass specification.
Optical properties of truncated GaInN nano-pyramids and related LEDs.
An alternative route for the green and even red LEDs is to use GaInN-based structures. In order to avoid the problems related to homogeneity of the ternary core and to circumvent strain-related defects in wires based on a GaN core, we have investigated truncated nano-pyramids of GaInN. Using this approach, we have been able to vary the emission over a large range of emission wavelengths, as illustrated in figure 29. These spectra were recorded from truncated pyramids with different thicknesses of the active layers, but with the same layer compositions. These spectra show that it is possible to tune the emission in the range 500 to 650 nm at low temperatures. Room temperature measurements show that the emission only red-shifts with a small amount. CL imaging further reveals that most of the nano-pyramids are defect free.
The nano-pyramids have also been made into devices by putting the active layers in a pn-junction. We recently did some simple tests of one of these structures by contacting a large number of pyramids with nano-probes, (about 25 000). Figure 30 shows a photo of the one this study. The green spot is the emission from the pyramids at room temperature.
III-V nanowire LEDs (AlGaInP material system.)
Recently ULUND has strengthened its effort to demonstrate device performance of our III-V NWLEDs. As briefly mentioned in D1.6 we have modified the epi-structure of the GaInP-AlGaInP core-shell NWs in order to increase the efficiency. Several modifications were made:
The growth time for the upper segment of the p-GaInP NW core was increased to 6 min. The material composition of the ternary p-GaInP NW core was measured to be homogeneous.
On top of the p-GaInP NW core an n-GaP top segment was added in order to avoid leakage from the outer most shell directly into the core by avoiding the QW.
The p-GaInP buffer layer (inner most shell) was grown thicker (growth time 2 min instead of 15 s) because the NW core has a smaller diameter than the opening in the SiNx mask. Thus leakage from the QW directly into the substrate can be avoided.
The AlGaInP barriers were grown non-intentionally doped instead of p- and n-doped, respectively.
The bandgap of the AlGaInP barriers were increased up to 150-200 meV in order to force the carriers even more to be trapped in the quantum-well (QW).
The outer most shell n-GaInP was highly doped with tin (Sn).
The core and shells were grown lattice-matched with compositions of Ga0.47In0.53P and (AlxGa1-x)0.47In0.53P respectively.
A series of 3 samples with 3 different i-GaInP QW thicknesses were grown:
20 nm (estimated)
10 nm (estimated)
5 nm (roughly measured by TEM)
Having grown radial GaInP/AlGaInP nanowires, we processed the vertical NW arrays to define the devices and to electrically contact the NWs. Standard UV lithography was used to define 100×100 µm¬2 devices on the samples, and the samples were then contacted by sputtering 150 nm of indium-tin-oxide (ITO) on the NWs as a transparent and conductive contact, as seen in the Figure xx. Electrical measurements on the devices were carried out with a probe station. For all 3 samples bright red emission was observed (Fig 33).
Figure 31. Schematic structure of the radial GaInP-AlGaInP NWs with optimized device design.
For the 3 samples electroluminescence (EL) measurements with an optical detector were performed. We observe the highest peak intensity for a QW thickness of 10 nm. Additionally, we find a slight blue-shift for a thinner QWs (Fig.34.). This could be attributed to quantization in the QW as we decrease the QW thickness.
ULUND has not yet measured calibrated IQE/EQE but preliminary results estimate that the EQE for all three samples is below 1 %. We believe that the vertical processing has to be improved by adding a spacer.
For the III-V based NW-LED structures we foresee a similar manufacturing process as for the III-nitride NW-LEDs discussed above. A similar lift-off procedure should also be possible. The cost of the production can also be discussed in a similar way as for the III-nitrides, since a NIL procedure of the substrate is needed prior to growth of the NE-LED structure. At the moment it seems difficult to compete with the present cost of the planar III-V red LEDs.
Potential Impact:
General features and potential advantages of the NW-LED concept.
The choice of nanowires as the material to be used in LEDs has several advantages, among these are
(i) the ability to reach higher In-concentrations in the active QW-layers in a radial LED-structure enabling more long-wavelength emission,
(ii) (ii) the opportunity to design the individual nanowires into photonic cavities enabling optimal out-coupling of light, including reaching non-Lambertian angular distributions,
(iii) (iii) the ability to effectively obtain an emitting QW area that by far exceeds the planar foot-print of the LED device.
(iv) Furthermore, it is expected that it may become possible to design photonic crystal structures in the form of 2-dimensional nanowire arrays in which intended emission patterns, such as vertical emission, is enhanced while un-intended emission, such as in-plane emission, is strongly suppressed.
Today, it is possible to fabricate highly efficient blue and green NW-LEDs in the GaN-InGaN materials system, and red NW-LEDs in the GaAs-AlGaInP materials system. We expect also to very soon realize efficient yellow GaN-based NW-LEDs which will create the ideal system for color rendering optimization of R-Y-G-B (red-yellow-green-blue) individually addressable NW-LEDs. This will offer significant avenues towards application areas within advanced display applications as well as in lighting. A four-color emitter is able to reach color rendering index values exceeding 98, which is well beyond what the human eye can distinguish from the most ideal solar illumination conditions. With such a tri- or quadri-chromic illumination system that does not use any phosphors, the long-time color stability is expected to be highly superior in comparison with today’s UV/Blue-LED-pumped phosphors, also in terms of the efficiency in converting electrical in-put power to light.
In addition, the high quantum efficiency of the nanowire devices at low current densities make them especially suitable for display architectures where each nanowire or group of nanowires may form a pixel. In these architectures, the drive current per pixel is on the order of nanoamperes, necessarily meaning the current densities is below 1 A/cm2. A great and challenging opportunity, quite unique to NW-LEDs, would be the implementation of a monolithic RGB, direct-view display technology that would be exceedingly bright and power efficient in comparison with any display technologies available today.
Present shortcomings in the planar LEDs for Solid State Lighting (SSL), and how nano-LEDs may offer solutions.
The status of the SSL lamps can be summarized as follows:
• The efficacy in commercial production is about 150 lm/W for cold white lamps, somewhat less (130 lm/W) for the warm white version.
• The target for the year 2000 in the US roadmap is 200 lm/W for both versions. This appears possible to realize within the present technology, but it is questionable if this goal is in accordance with an acceptable market price for the white LED lamps. The present problem with droop in the InGaN/GaN LEDs grown on foreign substrates forces the producers to use large chip areas (of the order 1 mm2) for each planar LED. It is unavoidable that this will have a strong influence on the cost of these LED lamps, the cost could be lowered substantially of a much smaller chip size (and high drive current density) was used, which is technically feasible. The physical origin of droop is still under debate, and not definitely resolved. An interesting observation is that LEDs made on bulk GaN substrates with a low dislocation density (< 106 cm-2) demonstrate much lower droop, suggesting that dislocations may play an important role.
The presence of defects (structural defects as well as point defects) in the present planar InGaN-GaN based LED structures is a problem that is not well documented in the literature. The common practice of growing the LED structures on foreign substrates unavoidably leads to a threading dislocation density of mid 108 to 109 cm-2 throughout the structure, as a consequence of the lattice misfit between the substrate and the layers in the LED structure during the growth cycle. The common wisdom is that this has little influence on the radiative efficiency of the LED structures at low injection currents. At high currents, i e under the conditions of droop, the situation is different, and it is known that dislocations act as nonradiative defects.
Another kind of structural defects occur as a consequence of the misfit strain between the different layers in the LED structure developing in the growth cycle. Misfit dislocations and stacking faults may develop in the interfaces between the layers, both defects are causing nonradiative recombination if present in the active region of an LED structure. Such problems are a main concern for the longer wavelength LEDs, where the lattice misfit between GaN and InGaN is largest. A solution would be to use InGaN templates for the growth of the LED structures. Such planar InGaN wafers are not yet available on the market, however.
In addition to these structural defects there are also problems with point defects in the planar LEDs. It is well known that the introduction of structural defects is typically accompanied by point defects, which form more readily in the local large strain fields from structural defects. Very little definite data are known about this problem in the InGaN material, however. It has e.g. been suggested that the so called “green gap” problem is related to deep level point defects, these have been characterized in DLTS experiments, but not identified so far [1].
Another limitation in the growth of planar InGaN-GaN LED structures is the limited In composition that can be incorporated in different InGaN surfaces. The c-plane orientation is known to be superior in this respect, c-plane InGaN can be grown over the entire range from GaN to InN, and is perfectly suited for the present SSL technology on sapphire. The limitation for c-plane InGaN structures is the so called Stark effect, i e the built in polarization field that causes an excessive internal electric field in the QWs, so that the QW thickness has to be as small as 2 nm for longer wavelengths, not practical in a production environment. The In incorporation is improved if non-polar or semipolar planes are used, and wider QWs are then tolerated as the field is reduced. The strain is still very high, however, meaning that the strain-induced defects limit the radiative output of the long-wavelength LEDs beyond the green region for the standard InGaN-GaN growth technology on sapphire (or other foreign substrates).
Proof of concept research has been done on growth of planar InGaN-GaN LEDs on bulk GaN substrates. In the case of non-polar and semi-polar structures excellent results are demonstrated on small size GaN substrates of proper orientation [2]. The substrates are presently too small (about 10 mm size) for commercial production of LEDs at reasonable cost. In the case of c-plane GaN larger size substrates (2 inch diameter) exist, and industrial production of LEDs on bulk GaN has been started by Soraa Inc in USA. For the longer wavelengths the strain is still a severe limitation, as discussed above. As outlined below all-InGaN defect free structures seem to be needed for efficient long wavelength nitride LEDs. At present bulk InGaN substrates do not exist.
The above situation has led to an increased interest in nanostructures as a basis for visible LEDs. It is well known that the small dimensions of nanostructures allow at least partly a relaxation of built in strain in the structures, further the structural defects that are so dominant in the planar structures can be completely eliminated from the active region of the devices made on these nanostructures. By using a nitride buffer layer on top of a substrate, foreign substrates of any chosen dimension can be used without loss of the above properties, which means a low cost can be maintained for a high quality structure. We have previously discussed the properties of LEDs in the nanowire configuration, where the main emitting facets are the m-planes. The structures studied in this project have limitations at longer wavelengths due to the use of several GaN layers in the structure (causing high strain as discussed above), and also suffering from limited In incorporation in the m-planes during growth. The flexibility in nanostructure growth can be used to develop an all-InGaN LED structure for visible light nitride LEDs. Very regular arrays of such structures can be grown with aid of a suitable insulating mask material (i.e. SiNx) in combination with nano- imprint lithography (NIL).We shall here discuss one possible such configuration, based on nano size c-plane features.
In Fig 1 is shown several examples of geometries of GaN nanostructures, that can be grown in one single MOCVD growth run, using a NIL patterned sapphire substrate with a GaN buffer layer. The right part of the figure shows c-plane platelets of different thickness. Very similar c-plane structures can as well be prepared in InGaN (also in a single growth run), with In compositions up to about 20%. TEM investigations show that such InGaN templates are typically free of structural defects. The c-plane top face may have a slight roughness as grown, then an overgrowth will produce a perfect template for further growth of a full LED structure, in this case an all-InGaN structure. For growth of green LEDs the In composition of the template can be about 10 %, for longer wavelengths out to red a 20% In template is suitable. Quantum well structures are then grown with traditional c-plane growth conditions with InGaN barriers of similar composition as the template. An AlGaN electron blocking layer can be added before the p-layer is grown to complete the LED structure.
In Fig. 2 are shown emission spectra from samples with different configurations of the QW region. Single QWs were used for these test samples.
Figure 1. Top-view (top row) and tilted-view (bottom row) SEM images of GaN shell growth on GaN NWs. a) Pyramidal shell growth without c-plane. Dotted lines show the profile of GaN NW. b) & c) show controlled platelet growth with varied c-plane size.
Figure 2. Emission spectra at room temperature from c-plane all-InGaN nanostructures with different active region QWs.
In figure 3a green prototype emitter with this configuration is shown with electrical injection.
Figure 3. Left: picture of a green InGaN c-plane nano-emitter with electrical bias. Right: EL spectra at different current levels (room temperature).
Possible impact of all-InGaN nano-LEDs.
The above data are very preliminary and have not been optimized. It should be pointed out that in the planar technology the efficiency of nitride-based LEDs with emission above 600 nm is very low, best values reported are 1-2 % [3], which is insufficient for most applications and certainly for SSL. We want to be able to optimize these c-plane nanostructure LEDs so that the full potential of the InGaN material can be utilized. We are confident that the structures can be prepared without structural defects across all visible wavelengths, also the strain will be much reduced compared to the standard planar InGaN/GaN structures. This also means that there is a good chance that the point defects believed to be responsible for the so called green gap might be considerably reduced in concentration. A bonus with the low strain structures produced this way is that the quantum well in the active region can be kept much wider, thus simplifying the growth process.
The impact of a successful outcome of this all-InGaN approach for visible LEDs would be very large. If nitride based LEDs could be produced with similar efficiency from blue to red, there is finally a platform for the polychromatic solution for SSL, by mixing the light from efficient LEDs in all four colors blue, green yellow and red. The ability to do this in a single (InGaN) material system is definitely a technical advantage.
[1]. A. M. Armstrong et al, Applied Physics Express 7, 032101 (2014)
[2]. C.C. Pan et al, Applied Physics Express 5, 062103 (2012)
[3]. J-I Hwang et al, Applied Physics Express 7, 071003 (2014)
Wider socio-economic impact and societal implications of modern lighting solutions.
Appropriate lighting is a basic prerequisite for the quality of modern life. The electric light has enabled a great expansion of human activity. As an obvious and important example can be mentioned the value of better illumination on our roads during dark hours, improving traffic safety. Similarly, the effects of significantly improving night time illumination in pedestrian areas will lead to an important improvement in personal safety and comfort. For many of such public areas the value of the long life-times of LED-based SSL is significant, since the illumination systems can be designed without the need for frequent lamp replacements. This important advantage also holds for novel opportunities in architectural lighting, where lamp fixtures can be integrated with the buildings in many and novel ways. However, inappropriate light and light pollution also have serious side effects on human health and quality of life. Furthermore, the energy and materials used for lighting tend to have serious environmental consequences.
As a species, humans depend most heavily upon visual and endocrine regulatory information to interact with the environment. Some 80% of our sensing cortex in the brain is devoted to vision. All species, including humans, also depend upon the natural, 24-hour light-dark cycle to coordinate, literally, every physiological system in the body to live healthy and productive lives. Effective lighting is one of the important elements for a sustainable lifestyle. Thus far, most of the interest has focused on the amount of light and the efficiency of the light producing processes, often measured as lumens (lm) and lm/W. The primary aim has been to produce a sufficient amount of light at the lowest possible cost. Most of the consumer interest has focused on the cost of the light source. In a life-cycle perspective the energy cost has become a major part of the cost, in particular for lighting with ordinary incandescent light bulbs. Since the 1960s the professional market has become increasingly interested in more efficient light sources.
The evolving technology, primarily in Light Emitting Diodes (LED), is beginning to enable new tailor-made and dynamic forms of lighting. The commercial LED technology is approaching the same level of efficiency as fluorescent tubes. Incandescent lamps mainly produce heat, fluorescent tubes produce the light in an indirect way and LEDs produce the light in a direct, and thereby more effective and versatile way. The purchase price for high quality LED-based light sources is still relatively high, but the life-cycle cost is becoming more competitive. There is keen interest in light quality to enhance plant growth within intensive greenhouse production systems.
So far there has not been much public awareness of the importance of the quality of light. Since the 1990s, there have been significant breakthroughs in knowledge about the positive and negative effects of different kinds of light, on human health and comfort. There is also an evolving interest in light pollution, i.e. too much light and unsuitable kinds of light.
Plants and algae use light energy absorbed by chlorophyll in photosynthesis to produce carbo-hydrates from carbon dioxide and water. Besides this, the information carried by light is used to optimize plant growth and behavior, to guarantee the best chances of survival and reproduction. Different pigments other than chlorophyll absorb light for these processes and allow control of e.g. gas exchange, flowering, shade avoidance and production of phytochemicals. The light emitted from high-pressure sodium lamps used traditionally in greenhouses is quite poorly adjusted to what the plants use, which results in unnecessarily high energy costs. With LED light in greenhouses, the energy consumption can be drastically lowered, since the different wavelengths can be more balanced. Furthermore, light can be varied to stimulate specific processes such as the synthesis of specific plant products important to human health. One potentially very important application of this technology may be the use of the separate spectral components in novel applications such as green-house lighting systems, where indeed sharp spectral features in the blue and in the red regions can be designed to optimally induce growth of plants. This can be expected to lead to a new way of feeding our population by eco-friendly and efficient local food production, hence reducing expensive and polluting long-distance transportation. For local production of food in northern regions of Europe having short daylight periods during the winter season, it could become quite realistic to keep producing vegetables based on a combination of heat stored in the ground and highly efficient, and spectrally optimized, illumination using such LEDs.
In addition the versatility in regulating the emission wavelengths and power of these new light sources allow significant advances in other research fields, like visual science, medicine and psychology.
Main dissemination activities in the project.
As part of dissemination activities the partner groups have participated in other joint symposia with research groups with similar interest. One such event was the Lund-Tokyo-Copenhagen-Beijing Joint workshop on nanostructure quantum devices held in Lund-Copenhagen March 24-25 2013. At this event our development of regular Hall measurements on single nanowires was reported and demonstrated (Kristian Storm).
Another example of such an event was the second Industry-Academia Workshop on Nanophotonics for Energy Efficiency 11-12 Nov 2013 in Stockholm. This was a separately EC-supported activity with participation from many European countries. Bo Monemar gave a presentation of our work in NWs4LIGHT.
Another international event with connection to European Lighting Industry was “Symposium on Materials Science for Light Emitting Diodes”, organized jointly by Lund University and the local organization “Invest in Skåne” in Lund 14-15 Oct 2013. There were several participants from Japan, such as H Amano, Nagoya University, and S Kamiyama, Meijo University, they reported about LED developments in Japan, Lars Samuelson, Jonas Ohlsson and Bo Monemar gave presentations on our work at Lund University, such as the NWs4LIGHT. Several industries with interests in lighting also gave presentations, one example is IKEA.
On Feb 5 2015 The Lund University Open Innovation Center arranged a symposium “Multidisciplinary Lighting seminar”, with participation of researchers from social science, medical science, technology and several industrial partners. The event had several international invited speakers covering different aspects of future lighting solutions. Bo Monemar participated representing the Solid State Lighting Center at ULUND.
The above is just a few examples of many such events with useful interaction and contact with relevant lighting industry as well as with foreign research activities outside the project. It should be mentioned that Dr N Gardner at GLO has participated in important committees related to the SSL roadmap in the USA. This may be important for the future positioning of NW-LEDs in this application area.
An important element in the international and European dissemination work in this project has been plenary and invited presentations, given worldwide and primarily by the coordinator Lars Samuelson at ULUND. These were mostly angled at the development of growth technology and basic understanding of the NW properties, but included also results on the performance of the NW-LEDs produced in the projects. Several such presentations were also given by representatives of the GLO partner, mainly by Nate Gardner. In that case new information on growth of NW-LED structures and related device processing was demonstrated from a semi-industrial perspective, and relevant performance data were also transmitted. These presentations have showed the international research community where the leading edge in this field lies, and clearly indicated prospects for future applications, both in Solid State Lighting and in other fields like display screens. A complete list of these talks is given separately in the report D5.7.
In addition to these plenary and invited presentations there were many ordinary contributed oral and poster presentations at conferences and symposia in the field of nanostructures as well as light-emitting structures and devices. These contributions are listed in full in the report D5.7.
Exploitation activities in the project.
The exploitation of the foreground produced in the project has mainly relied on the industrial partner Glo AB in Lund, Sweden (GLO) and its subsidiary Glo-USA Inc. in Sunnyvale, USA (GLOINC). Via this partner several patents related to nano-wire LED technology have been issued during the project duration, as discussed separately in Sec. 4.2. GLOINC has set up a functioning processing plant in order to demonstrate the viability of the NW-LED approach. GLOINC demonstrated that it is possible to produce nanowire LEDs using a flip-chip process technology, thereby enabling the integration of these LEDs into systems with large numbers of devices where the otherwise-needed wire bonds are prohibitively expensive or result in yield or reliability problems of the integrated system. The produced devices have a performance close to the corresponding planar LEDs on the market, and also exhibit similar excellent data in reliability tests.
GLO has an extensive contact area with possible customers for NW-LEDs. It seems like the SSL market is not yet ready for the NW-LED introduction, a breakthrough for long wavelength nitride NW-LED structures would certainly change this situation. That is why we recently have put some substantial effort at ULUND to advance this part of the growth activities (see above). This part is presently producing new foreground that will be protected and hopefully lead to a game-changing production of polychromatic NW-LEDs with a much higher efficiency and lower droop than the present SSL solutions on the market.
In addition, the demonstrated high quantum efficiency of the nanowire devices at low current densities make them especially suitable for display architectures where each nanowire or group of nanowires may form a pixel. In these architectures, the drive current per pixel is on the order of nano-amperes, necessarily meaning the current densities is below 1 A/cm2. A great and challenging opportunity, quite unique to NW-LEDs, would be the implementation of a monolithic RGB, direct-view display technology that would be exceedingly bright and power efficient in comparison with any display technologies available today.
Project website.
The website is found under Lund University at the address:
http://www.ftf.lth.se/research/links_to_some_projects/nws4light
List of Websites:
The website is found under Lund University at the address:
http://www.ftf.lth.se/research/links_to_some_projects/nws4light
Contact details:Prof. Lars Samuelson, Solid State Physics Division, Physics Department, Lund University, SE-223 62 Lund, Sweden
Visiting address: Professorsgatan 1, email: lars.samuelson@ftf.lth.se
Final report
Executive summary:
This collaborative project concerns materials research for future solid state lighting solutions, i. e. illumination with white light indoors as well as outdoors. The project is coordinated by Lund University, Sweden, (ULUND), Prof. Lars Samuelson. Other academic partners are: The Technical University of Denmark, (DTU); Copenhagen University, (UCPH); Stiftung Deutsches Electronen Synchrotron, DESY, Hamburg, (DESY); and Forschungszentrum Juelich GMBH, Juelich,(JUELICH). Two industrial partners are involved: Glo AB, Lund, Sweden (GLO), and Glo-USA Inc. Sunnyvale, USA, (GLOINC). The partners have specific roles in the project: ULUND, JUELICH, GLO and GLOINC are producing the materials for the LEDs, and also perform characterization of these materials; DESY and UCPH provide high spatial resolution synchrotron based structural studies, DTU assists with high resolution electron microscopy and related material analysis. GLO and GLOINC grow, produce and characterize LED devices based on the grown materials.
White LEDs are just entering the market as the future solution for lighting. Today’s LED lamps are based on violet GaN-InGaN LEDs with a cover of a phosphor powder emitting in yellow (and red). This concept was awarded the Nobel Prize in Physics 2014. These planar LEDs are typically grown on sapphire, leading to a high defect density. The nano-wire based LEDs (NW-LEDs) can be grown defect free (even on foreign substrates), and contain a quantum well (QW) based active region just like the planar LEDs. The NW-LEDs are typically rods a couple of µm long and about 400 nm diameter, with a hexagonal cross section. The different layers in a NW-LED are stacked radially, with a thin core in the middle, buffer layers and active QW layers further out, blocking layer and p-layer outmost, in a so called core-shell configuration. As substrate sapphire or silicon can be used, with a thin GaN nucleation layer and an insulating layer (e.g SiNx) on top. A nano-imprint lithography (NIL) process is preferably used to expose the GaN nucleation layer in a regular pattern, leading to growth of NWs in the same regular way. The growth is done with the Metal Organic Chemical Vapor Deposition (MOCVD) method, well established for planar LEDs.
This project aims at developing efficient LEDs in four colours, blue, green, yellow and red. By combining and driving four such LEDs in the same package white light emission of higher efficiency, and with higher color rendering. than in the planar case is foreseen. We have produced NW-LEDs in both InGaN-GaN and AlGaInP materials systems. Blue and green InGaN –based NW-LEDs give comparable output to the planar versions. For the red-emitting AlGaInP based NW-LEDs the output is still not optimized.
We have also developed InGaN-based LEDs based on nano-templates with a c-plane InGaN surface. Growing a planar LED structure on top of such templates allows NW-LEDs which are defect free, have low built in strain and can cover the entire visible spectrum with InGaN QWs.
Project Context and Objectives:
Summary description of project context and objectives.
The invention of new efficient white light sources based on light emitting diodes (LEDs) was awarded the 2014 Nobel Prize in Physics. This can be seen as a recognition of an achievement in new material development involving the III-nitride materials (like GaN and InGaN) and the industrial process to bring the white LEDs to the market. The present white LED lamps are based on violet-emitting InGaN LEDs assembled together with phosphor materials emitting at longer wavelengths. The mix of these emissions is perceived as white light by the human eye, although with rather poor color rendering properties. These lamps are available on the market with efficiencies exceeding these of the fluorescent lamps, like the popular compact versions (CFLs), they typically produce of the order 130 – 150 lm per W of incoming electric effect. These nitride structures are usually grown on foreign substrate materials (like Sapphire, SiC or silicon) to reduce production cost. Still the cost of these white lamps must come down nearly an order of magnitude to dominate the market for lighting.
The drawback of these white LEDs is a loss of about 25% of the energy of the primary violet emission in the down conversion from violet to longer wavelengths, this energy is lost to heating in the phosphor material. In addition there is a rather strong droop (decrease of efficiency at high currents), which can be as high as 40% at high currents. This droop is entirely a property of the primary violet LEDs, and is in production handled by using a much larger emitting chip area that otherwise necessary. This is obviously a factor that contributes to the high cost of the present white lamps. Further, so called warm white light similar to the sunlight is difficult to obtain with high efficiency using the presently available phosphors. Future research will therefore need to address these major topics. There is some evidence that the droop has to do with the growth on foreign substrates, which creates a high dislocation density throughout the LED structure (up to 109 cm-2). Growth on low defect density GaN substrates (or preferably InGaN substrates) is presently unrealistic in LED production due to limited availability of such material, in combination with a high cost.
Avoiding structural defects is therefore one base line in the materials research related to future LEDs. It is well established that in nanostructures like nanowires the materials can be grown free of structural defects, mainly due to the limited dimensions. Defect free nanostructures may be obtained even for growth on a foreign substrate, provided a suitable buffer layer is used. These facts make the nanowire concept very interesting for development of a new generation of LEDs for solid state lighting (SSL).
In the present development work we want to explore whether the use of phosphors can also be avoided for white LEDs. The general goal of this project has been to establish new material technologies for efficient LEDs for four key light wavelengths in the visible region, i.e. blue, green, yellow and red. White light can then be efficiently produced by mixing of the output of all four LED colours produced by combining four different colour LEDs in the same LED package (the so called polychromatic solution). This solution would then potentially offer a much higher efficiency for conversion of electricity into visible light (up to 260 lm/W is predicted in LED production for this concept). The significant novel approach in this project is that this will be done based on nanowires as the active elements for light emission. An essential development is then that the nanowire based LEDs can be produced without any structural defects. The emphasis is on materials development and related characterization of relevant properties, but realistic device prototypes are also demonstrated via an industrial partner during the project duration. The bold long term ambition in the project is that NW-based LEDs will be demonstrated that are equally efficient than the corresponding commercially available LEDs from the present planar technology, for all four colours blue, green, yellow and red. The objectives expressed in the EC call for project applications in 2011 had a 10 years perspective for the fulfilment of somewhat more ambitious goals.
The growth development at ULUND and GLO is focused on two different material systems, InGaN and AlInGaP. The InGaN alloy system is the one employed for the planar violet-blue- green LEDs today available commercially. While such planar blue InGaN LEDs are very good, there are serious problems with the radiative efficiency in the planar green LEDs. In the planar InGaN LED technology there are severe limitations towards extension of the emitting wavelengths towards the red, mainly due to strain-induced defect creation in the In-rich active quantum well (QW) regions during growth. Such defects are known to cause non-radiative recombination in LEDs, reducing the light output, while also expected to reduce the life-time and reliability of the devices. It is expected that such strain related problems are somewhat less severe in our NW-based LED structures, where the active regions are positioned closer to an outer surface. In addition to regular nano-wire LED structures with concentric device layers around a vertical core (the so called core-shell configuration) we are presently investigating different solutions for the active region, including InGaN c-plane nano-templates, to explore the limits for obtaining efficient InGaN-based NW-LEDs towards the red spectral range. Obviously it would be a great advantage for LED processing to obtain all LEDs for different emission colors in one and the same material system.
The new development of c-plane InGaN nanotemplates may potentially resolve many of the presently discovered problems with InGaN-based LEDs. There are many reports on excessive strain in the present planar longer wavelength (green and longer) InGaN LED structures. Structural defects like misfit dislocations and stacking faults are observed as a result of the strain induced by the strong difference in lattice parameters between GaN barriers and the InGaN QWs. There is also a suspicion that this built-in strain also promotes a high concentration of point defects in the same regions of the LED structure. These problems may be eliminated by using InGaN templates as a base for growth of the LED structure. Defect free c-plane InGaN nano-templates with In composition potentially up to 20% can be produced in a single growth step, these platelets are typically strain-free and also free of structural defects. For green LEDs a template In composition about 10% is suitable, for longer wavelength the In content in the template has to be increased accordingly. Using this technology InGaN LEDs with emission from blue to red has been demonstrated in the project, although not yet optimised.
The III-V material system AlInGaP has been developed extensively for planar red-emitting LEDs during the last decade, and there is good hope that efficient yellow and red emission can be achieved using NW-based structures for these same materials. Our ongoing work explores the feasibility of growing these core-shell structures based on InGaP NW cores. The first red-emitting such NW-LEDs were recently demonstrated in the project.
In addition to these efforts there is a parallel activity at JUELICH to grow core-shell nano- crystals (NCs, about 5 nm size) in the InP-ZnS materials system. The idea is to test the suitability of such dots as light emitting agents in combination with the InGaN based NW-LED emitters, i e the dots will be optically excited by violet, blue, or green NW-LEDs and emit in the yellow-to-red wavelengths, when suitably applied in the NW-LED structure. So far the emission in the green range from the fabricated core-shell NCs is quite good (above 50 % efficiency), and recent work on the red-emitting NCs indicate an efficiency about 40%.
The materials characterization efforts have been conducted as planned, in close connection with the growth efforts. The beneficiaries working at the large scale synchrotron facilities have developed new world class techniques for specific studies at the nano scale, not possible in ordinary materials research laboratories. The much shorter wavelengths produced in the synchrotron facilities allow focusing of a nano-beam of such radiation to a specific part of a single NW-LED object, and then also produce a high resolution image of the result, such as composition and strain. This way strain inhomogeneities have been discovered in NW-LED samples which by ordinary inspection in a SEM picture appear perfectly symmetric. Low temperature cathodo-luminescence (CL) measurements have proven to be an invaluable tool to evaluate the emission properties in cross section for an individual NW-LED. This way important gradients in composition in the active QW layers vs wire length have been discovered, such properties do affect the radiative emission properties in a negative way. Upgraded equipment for the optical studies at ULUND (including a streak camera setup) has been installed at ULUND, and the first results on NW luminescence dynamics were presented. The excellent facilities for electron microscopy at DTU have produced important data (via electron holography) on profiling of the potential across the produced doped NWs intended for NW-LED structures.
A properly equipped pilot production line for NW-LEDs has been established at the GLOINC partner. This allows evaluation of fully processed NW-LEDs according to the work plan. Recently quite promising blue as well as green LEDs were demonstrated, with an external quantum efficiency (EQE) of close to 30% for blue NW-LEDs and more than 40% for green NW-LEDs. In parallel, work at ULUND develops fully contacted single NW LEDs, and characterization techniques for those (electrical and optical). More details from this device-related work is presented in the section about S&T results below.
Project Results:
The main S&T results in the project.
The overall goal with this project has been to develop nanowire structures for LED applications, notably for visible light emission, i.e. so called Solid State Lighting (SSL). This involves research on suitable growth procedures for the complex LED configurations involved. The nanowire topology means that the characterization of the relevant physical properties partly requires novel experimental techniques, adapted to the nanowire geometry. A complete characterization would ideally involve sub-nm resolution of the measured properties, not yet realistic. To verify that the results in growth and characterization lead to the projected properties of the nano-wire based LEDs (NW-LEDs), a pilot production facility has been established at one of the industry partners (GLOINC). The results obtained from the evaluation of the produced NW-LEDs form a relevant basis for the evaluation of the progress as well as remaining problems in the application of NW-LEDs for SSL purposes.
The most important details of the results obtained will be described in what follows, following the structure of the different work packages. In the growth area it is clear that the MOCVD technique has proven to be a useful vehicle to produce complex core-shell NW-LED structures with nm precision. This does not mean that problems are not present. It is clear that the obtained irregular results of the efficiency of blue NW-LED emitters involve a defect problem that is not directly revealed by the experimental investigations. Likewise the rather strong droop observed for green emitters is not expected from the presently accepted theory, it may be viewed as another evidence for nm size structural defects related to the complex geometry of these NWs. More research is obviously needed before such problems are under control.
In the characterization work package important developments have taken place in optical methods, electron microscopy and XRD techniques. The electron holography techniques can now be used with confidence since the influence of electron irradiation on defect creation during the experiments has been understood. The potential profile across a single NW-LED structure can now be mapped out, which is very valuable to understand other data, such as electrical measurements. Photoluminescence (PL) and cathodoluminescence (CL, both at temperatures down to 5 K) are valuable to understand the nature of the radiative transitions in the LEDs, the latter technique also has the spatial resolution to study variations of emission across a single NW-LED. A suitable platform for electrical measurements on single NW-LEDs has been developed and used in the project.
The third work package in focused on demonstrating a device processing technology for the studied NW-LEDs, by using a wafer process flow running on a commercial wafer fabrication line. One important task is to compare the predicted performance of the NW-LEDs with reality, in terms of lumen and color maintenance, wall-plug efficiency, reliability, and predicted production cost. GLOINC has demonstrated that it is possible to produce nanowire LEDs using a flip-chip process technology, thereby enabling the integration of these LEDs into systems with large numbers of devices. The reliability tests also show that these devices behave very similarly to the commercial planar LEDs. As mentioned above there are still some areas where more work is needed to understand the behavior, e.g the lower average emission intensity from the blue NW-LEDs (as compared to the planar violet-blue LEDs) and the excessive droop of the green NW-LEDs.
Below some important results in the project will be described in more detail.
WP1. MOCVD growth technology for III-nitride and III-V NW-LED structures.
The III-N NW LED structure consists typically of a Si-doped GaN core in the shape of a pillar grown perpendicular to the wafer surface. The GaN core is of wurtzite crystal structure and acts as a template for a radially grown InGaN quantum well acting as an active region for the LED device, an electron blocking AlGaN shell and a Mg-doped GaN: p-GaN shell as a source of holes injected to the active layer, where they can recombine with electrons and generate light. The vertical form of nanowire is beneficial for enhancing light extraction. A schematic representation of the NW LED cross-section along a cut perpendicular to the m-plane can be seen in Fig. 1a. A complementary cross-section view from SEM on a typical NW LED is shown in Fig. 1b.
The growth template consists of a silicon substrate, of (111) orientation, or a c-plane sapphire wafer. An AlGaN/AlN wetting layer is grown before the main buffer layer, which is a 2-4 µm Si-doped thick GaN layer, with Si doping in the range 1-5 E18 cm-3. A silicon nitride mask was deposited on the GaN layer, with a thickness ranging from 30 to 50nm. Electron beam lithography (EBL) or nanoimprint lithography (NIL) have been used to make openings in the silicon nitride mask down to the GaN layer. The typical opening aperture ranges between 70 to 140 nm, however in this project we have also tested openings up to 300 nm. The openings form a honeycomb pattern, with a pitch distance between the nearest neighbors varying between 1 and 1.5 µm. These openings define the positions and dimensions of the grown nanowires. To determine the surface morphology scanning electron microscopy was performed on nanowire samples. A Thermal Field Emission Scanning Electron Microscope (SEM) LEO 1560 was used, with 10 kV electron acceleration voltage. Cathodoluminescence (CL) was performed at Lund University (ULUND), using a Cambridge Instruments S250 M3 SEM, with a GaAs photomultiplier tube (PMT) recording the CL spectra, and with a cooling stage capable of cooling the sample to below 5 K. Photoluminescence (PL) data was recorded using a micro-PL setup at Lund University, with an excitation laser at 375nm, 9mW.
In order to ensure controlled growth of NW LEDs, nanowire cores must be grown uniformly from the mask openings. Since different pitch and different hole size was used, it is important to use proper nucleation conditions. In the initial nucleation step the flows of TEGa as Ga precursor and NH3 as N precursor were optimized for various pitch/hole sizes. Effects of thermal pre-annealing as well as nucleation and nanowire core growth temperature were taken into account. After nucleation, TEGa and NH3 were continuously flown to ensure the targeted nanowire length. The results of nanowire diameter in respect to growth time in the case of EBL 1 µm /70nm opening are shown in Figure 2a, and the dependence of m-plane length on growth time are shown in Fig. 2b.
As seen in Fig 2a. the diameter is enlarged in the first 30 seconds of growth, after which it has stabilized at about 160 nm. The m-plane length increases linearly with the growth time. Based on the initial growth stage findings and by applying volume growth rate calculations to different pitch/opening size we have obtained uniform growth of NW LEDs on available substrates as shown in Fig.3.
Fig.1. a) to the left. Schematic cross-section of III-N nanowire LED. b) to the right. Cross-sectional SEM image taken at 80k magnification. The scale bar is 200 nm.
Fig.2. a) dependence of nanowire diameter on growth time in initial growth stages. b) dependence of m-plane length on growth time in the initial growth stages.
Fig.3. 30 deg tilted SEM on NW LEDs on different pitch/opening aperture templates: EBL 1 µm/70nm to the left, NIL 1 µm/100nm in the middle and NIL 1.5µm/140nm to the right. The scale bar is 1 µm.
The typical diameter of the NW core is dependent on the mask-opening aperture and varies between 190-220 nm for a hole size 70 to 100 nm. Nanowires up to 3µm in length could be grown with good growth uniformity. The structural quality of all internal layers in a NW-LED was characterized by transmission electron microscopy (TEM). As seen in cross-sectional TEM in Figure 4 no stacking faults or dislocations are observed in NW core/nGaN shell. This was possible to achieve through careful selection of growth conditions for each individual layer.
Fig.4. To the left: high resolution TEM on an NW nGaN core - overview. In the centre: magnification on the tip region. To the right: Electron diffraction pattern taken in zone [100], perpendicular to the NW long axis.
The side and pyramid facets were determined by electron diffraction in TEM. As expected the side of the nanowire is terminated by {1-100} facets (non-polar m-plane), while tip facets are semi-polar and have {1-101} orientation. The LED structure was extended by outer shells like InGaN QW/p-AlGaN/p-GaN grown on top of the nGaN shell. The outer shells are grown following the {1-100} and {1-101} crystallographic orientations.
Optical and electrical characterization of III-N NW LEDs.
We have grown and characterized InGaN and p-GaN layer shells using proprietary Glo technology. A uniform distribution of indium in the QWs as well as uniform growth of the QWs are important to ensure monochromatic light extraction. Indium segregation as spatial variation in indium composition is one of the issues for the QWs. To determine spatial distribution and variation of indium composition in an SQW as well as Mg dopants in the p-layers, InGaN and p-GaN shells were characterized by cathodoluminescence at a temperature of 4 K. Fig.5 shows spatially resolved CL of an NW SQW LED, taken at 7K. From spatially resolved CL spot mode we were able to determine that there is a shift in QW emission towards shorter wavelenths when moving the measurement spot from the top of the nanowire towards the bottom. The emission at the EL peak WL from SQW could from about 470nm on the top to about 430 nm at the bottom. This could be due to non-uniform indium incorporation as well as non-uniform QW thickness along the m-plane. However, no stacking faults or dislocations in the QWs were observed in TEM.
We postulate that indium incorporation could be improved by a change in growth conditions such as QW temperature and pressure as well as nanowire length, with possible improvement when using shorter nanowires. The CL data was correlated to energy dispersion spectroscopy TEM (EDS TEM), where QW thickness and indium composition have been determined. In Table 1 below EDS and TEM data on indium composition in QW and QW thickness at three spots: bottom, centre and top of the m-plane are summarized.
Fig.5. Spatially resolved cathodoluminescence on a single NW LED. Color coded are five spots along the nanowire SQW where CL spectra have been recorded and shown on the plot to the right.
Table. 1. QW thickness by TEM and indium composition by EDS.
QW top QW centre QW bottom
QW thickness (nm) 6.7 5 4.5
In composition (%) 9.3 9.4 6.5
The increase in WL from NW bottom to NW top is consistent with an increase in indium composition and QW thickness from NW base towards NW top. To confirm findings from CL, TEM and EDS we have performed high resolution X-ray diffraction measurements on NW SQWs. As shown in Fig.6. beside several Bragg peaks corresponding to AlGaN wetting layers in the substrate, there is broadening of the (101) peak corresponding to the InGaN SQW, and this broadening suggests that the QW is fully strained in the out-of-plane direction.
Fig.6. to the left. 3D isosurface of Bragg peaks originating from the wurtzite structure in the InGaN QW. To the right: Contour plot in the (001) direction.
In order to have efficient light generation low resistivity, Mg doped GaN with high hole concentration, at least 5-10 E17 cm-3, must be capping the LED structure. We characterized p-GaN shells either directly on the LED structure or on specially prepared n-p structures consisting only of nGaN core with p-GaN shell. Various characterization techniques have been employed including optical methods such as PL, CL as well as electrical characterization of single n-p nanowire structures. Low temperature PL was performed on p-GaN layers to reveal optical signatures of dopants. Fig.7 shows low temperature (4K) PL spectra taken from ensemble spectra as well as focused micro-PL for single wire spectra.
Growth of artificially miscut III-N nanowires.
The focus of this work was to realize artificially miscut m-plane sidefacets of GaN nanowires with InGaN single quantum well outer shell. Selective area epitaxy by MOVPE was chosen as a growth technique for this work. Typically the GaN surface on which the InGaN QW is grown is atomically smooth. Indium atoms which exhibit a high surface mobility can easily desorb from the flat GaN surface. A controlled number of nucleation sides like kinks and steps could improve the homogeneity of the grown layer via layer-by-layer growth, and also increase the indium surface incorporation. An increasing indium content in the QW leads to a light emission at longer wavelengths like yellow/red. A controlled layer-by-layer growth mode is desirable to avoid defects, in particular for the growth of the QW active region.
In order to increase the density of steps on GaN surface we have studied the effect of the H2/N2 ratio on n-GaN nanowire tapering, i.e. artificially changing the off-axis angle of m-plane sidefacets. We found out that varying H2/N2 in the gas phase from 0% to 2.78% increases the off-axis angle from about -0.91° (inverse tapered) to 0.6° (tapered nanowire) towards c-plane (growth direction) as shown in Fig.7. However further increase in the H2/N2 ration results in a decrease of this angle. The surface topology was characterized by atomic force microscopy (AFM). Atomic steps on the surface could be distinguished. The mean square roughness increases from 0.3 nm for the baseline with almost 0° miscut angle to 0.75 and 0.6 nm for inverse tapered and tapered nanowire, respectively, thus indicating an increased density of steps with changing artificially the miscut angle.
Figure 8. Artificially miscut GaN nanowires. From left to right: miscut angle is 0.91°,
0° and 0.6°.
The above studies are continuing with the aim to optimize the growth conditions of the active QW part of NW-LED structure. We have experienced a large variation in the EQE of the grown violet-blue NW-LEDs, and concluded this is due to a problem with nonradiative defects in the active region of the device, intimately linked to the growth conditions. At the same time there is still an obvious problem with an In-composition gradient along the growth direction.
Growth of dislocation free c-plane InGaN platelets based on InGaN nanowires.
A method to fabricate wafer-scale arrays of GaN platelets with a smooth c-plane in a submicron size was developed. Such GaN platelets are free of dislocations since they are grown based on InGaN nanowires where the dislocation propagation from the underlying GaN buffer layer can be inhibited. On the c-plane of the platelets, high quality nitride layers such as AlGaN and InGaN can be grown for electronic and optoelectronic devices.
(b)
Figure 10. Identification of GaN NWs with a dislocation propagating from the underlying GaN buffer layer. a) A TEM image showing asymmetric growth caused by deflection of a dislocation from underlying GaN buffer layer. b) A top view SEM of GaN NWs. c) A TEM image of a symmetric GaN NW which is free of dislocations.
The ability of continuously, selectively grow GaN nanowires to filter out dislocations was investigated. Typically, dislocations in the underlying buffer layer do not extend into the nanowires but when dislocations extend through the mask openings they are seen to deflect towards the side facets at early stages of the growth as shown in figure 10a. The intersection of the bent dislocation to the side facet makes the corresponding side facet grow faster than other facets, rendering the wire shape to be asymmetric, as seen from figure 10a and 10b. Six such asymmetric wires were selected for characterization by TEM and dislocations were confirmed in all of them. GaN NWs without dislocations have symmetric hexagonal shape as shown in figure 10b and 10c. In our NW growth with continuous flows, the formation of asymmetric wires is controllable.
Figure 11. Left: PL spectrum at 8 K from a single GaN platelet which was transferred to a substrate of Au/Si. Right: Decay of donor-bound exciton at 4K from a single GaN platelet which was transferred onto an Au/Si substrate. The lifetime is 400 ps.
Uniform wafer-scale arrays of GaN platelets were developed by growing a GaN shell on the dislocation-free GaN NWs mentioned above. {101 ̅1} planes are firstly formed at the wire top when the shell growth starts. Because of the extremely low growth rate, {101 ̅1} planes force the GaN to grow downwards with the {101 ̅1} planes maintained, as shown in figure 12a. With this pyramidal growth fashion, no c-plane is formed. However, we will show how the c-plane can be obtained through controlled change of the epitaxial growth mode, which leads to the formation of GaN platelets as shown in figure 12b and 12c. The size of the c-plane can be
Figure 12. Top-view (top row) and tilted-view (bottom row) SEM images of GaN shell growth on GaN NWs. a) Pyramidal shell growth without c-plane. Dotted lines show the profile of GaN NW. b) & c) show controlled platelet growth with varied c-plane size. increased by totally eliminating the {101 ̅1} planes (figure 12c). Time-resolved photoluminescence was measured on a single GaN platelet which was transferred onto an Au/Si substrate. It shows the emission of donor-bound excitons and the lifetime is 400 ps (figure 11).
The method of pyramidal growth and subsequent flattening of the pyramids was extended to the GaInN materials system with the ambition to synthesize materials for defect-free GaInN red and yellow emitting NW-LEDs. A composition of 11% In was found in the platelets.
Figure 13. Left: SEM image of InGaN pyramids grown selectively in holes opened up in a SiN growth mask. Right: PL spectrum obtained from the material upon optical excitation.
By inserting a GaInN quantum well on the c-plane we show that the wavelength of the emission can be tuned by growth parameters.
Figure 14. Left: SEM image of the GaInN platelets with embedded GaInN quantum wells for emission. Right: spectrally resolved CL from the platelets.
Figure 15. Left: a PL spectrum at room temperature on Mg-doped GaInN platelets; Right: a full LED structure based on GaInN platelets with p-GaInN at the top of pyramids.
Mg-doping on the GaInN platelets was studied. Because of the narrow band-edge emission from GaInN platelets, the emissions related to the Mg acceptor was found at the lower energy side (figure 15). Such p-GaInN growth was applied to the single QW on the GaInN platelets and a full LED structure was obtained as shown in the right SEM image in figure 6. Compared with the LEDs on c-plane GaN template, our structure offers smaller lattice mismatch between the GaInN template and the QW, which means better material quality and less quantum-confined Stark effect.
Development of III-V radial heterostructure NW materials for light emission.
III-V heterostructure growth has been developed with respect to materials composition and doping in the axial as well as radial geometry in order to produce red light emitting nanowire diodes.
TESn is the preferred n type dopant of SiH4, DTBSe and TESn with respect to dopant level achieved and control thereover.
Contactless electron off axis electron holography was used to determine shifts in the electrostatic potential, being a way to quantitatively measure doping.
By calibrating electron off axis electron holography in comparison to other methods of evaluating doping, it could be used to quantitatively determine doping level.
We found that doping by use of DEZn affects the materials composition of GaInP NWs where the GaInP materials composition was assessed by XRD, TEM and PL.
Axially homogeneous p doped GaInP NWs were grown by use of a method using in situ ramping of the TMI flow.
Parameters and growth scheme for full radial NWLED in (Al)GaInP were developed and lattice matched core shell NWLEDs were grown and characterized with respect to structural and electro optical materials properties. Phase segregation of Al and Ga during radial growth of shells was observed by use of TEM.
Hall voltage measurements was used to determine the n doping in the V-III nanowires, and doping carrier concentrations of 3x1017 cm-3 in the nanowires with highest S doped NWs were found. Lower doping levels were not possible to evaluate due to electrically non-transparent contacts.
IQE measurements of the NWLEDs with ideality factor about 3 are pending. For more details we refer to D1.6.and to the corresponding LED-section in WP3 below.
WP2. Characterization of NW-LED structures.
Structural characterization of radial GaInP/AlGaInP nanowire quantum well structures by high resolution STEM and STEM-EDX
High resolution STEM and STEM-EDX were applied to radial GaInP/AlGaInP nanowire quantum wells grown by MOCVD to study the sharpness of the heterojunctions and the compositional distribution of materials.
More than five different samples in which the Al precursor, trimethylaluminum (TMAl), was varied systematically were investigated to obtain the right concentration of Al in the AlGaInP layers. Results from one of these samples are summarized in figure 2.3-2. In all cases, Al enrichment with In and Ga deficiency was observed at the corners of the AlGaInP layers in the cross-section perpendicular to the nanowire axis. The GaInP/AlGaInP interfaces were relatively sharp.
Figure16. An example of compositional and structural characterization of radial GaInP/AlGaInP nanowire quantum well structures by the means of STEM. a) HAADF-STEM image of a FIB-prepared cross-sectional specimen. b) STEM-EDX map from the region marked with the red box in the HAADF image, showing the Al, Ga, In and P distribution. c) Atomic resolution STEM image from the AlGaInP/GaInP interface, the region marked with the small black box in the HAADF image. d) Material composition of different regions of nanowire, calculated from the EDX measurements.
Structural characterizations of NWs using X-rays: Single GaInN/GaN nanowire characterization
We present detailed nanoscale strain mapping performed on a single, 400 nm thick and 2 µm long core-shell GaInN/GaN nanowire with an x-ray beam focused down to 100 nm. A scanning electron microscopy (SEM) image of the typical NW from the same sample is shown in figure 17(b). The sample was carefully cleaved leaving the single isolated nanowire at the tip of the 60° corner of the substrate, as shown in figure 17(a). Strain mapping was performed at the X-ray nano-beam end-station [1] of the P06 beamline at the Petra III synchrotron radiation facility (Germany).
Figure 17 (a) Experimental set-up used for SXDM. A single NW at the corner of the sample was positioned in the focus of the x-ray beam and scanned in the xz plane at different angles. (b) SEM image of the NW. The white ellipse represents the FWHM of the incident beam (100x 150nm).
The strain maps were calculated from the peak positions for three different areas of the NW. From the strain maps corresponding to the GaN core (see figure 2.3-5(a)) we notice sharp strain gradients at the bottom of the NW, showing an apparent difference of the lattice constants in the NW and the underlying substrate Larger in-plane strain εxx is observed along the sides of the core, especially near the tip. Both strain components in the core are asymmetric with respect to the left and right sides of the NW. The tensile strain is larger in the left side of the core. This distribution may be caused by an asymmetric relaxation of the GaInN shell, as will be discussed next.
From the intensity map of the region B, shown in figure 18(b), we can see that it originates mostly from the narrow stripe on the left of the NW, which corresponds to the left side of the GaInN shell. The intensity is twice as strong at the top, compared to the bottom (see figure 18(b)), which is in agreement with an uneven distribution of the thickness of the shell, as measured with the cross-sectional SEM. From the strain maps in figure 18(b) we can see that the left side of the shell is compressed in the z direction in order to become coherent with the core.
The intensity and strain maps obtained for the region C, containing the diffuse signal, are shown in figure 18(c). From the intensity map it can be seen that it originates mostly from the top-right part of the GaInN shell. From the SEM image in figure 17(b) we can see that the surface of the GaInN shell is especially rough at the upper part of the NW. This explains the highly diffuse signal in the region C in reciprocal space. From the strain maps we can see that the in-plane and out-of-plane strain components have similar values, which corresponds to the relaxed InxGa1-xN with xIn = 0.32.
NW-based light emitting diodes comprise a macroscopic die with thousands of identical nanowires, each one acting as an independent light emitter. Using the single NW characterization one has to demonstrate that the single NW chosen to be measured was not a statistical outlier, but can represent an entire ensemble. Reciprocal space mapping (RSM) was performed on ensemble of nanowires at beamline I811 at Max Laboratories in Sweden, giving the average strain state in the large number of nanowires. RSMs of the (10-11) peak measured for the ensemble are compared to the single NW measurements done with the nano-focused beam in figure 2.3-6(a) and (b), respectively. In both cases we identify a signal from both the relaxed and the strained GaInN shell. This similarity allows us to extrapolate the findings based on the single nanowire to the entire sample, although in general this is not the case.
In conclusion, we have demonstrated how the combination of a nanofocused x-ray beam and SXDM can be used to map the strain distribution in a single NW with a resolution of about 100 nm. We have found that the strain distribution in the GaN core caused by the lattice mismatch at the GaN/GaInN interface is inhomogeneous. Asymmetry in the axial strain relaxation is present, with one side of the shell being fully pseudomorphic with the core and the other side plastically relaxed. In this we also demonstrate fully strained 15-28 nm thick In0.32Ga0.68N shells on GaN NWs. This exceeds the predicted critical thickness in the planar structures, which does not exceed 10 nm, demonstrating the advantages of the nanowires. However, uneven strain relaxation can lead to inhomogeneous material incorporation which has impact on the device performance. We have demonstrated here that strain mapping with the nano-focused x-ray beam has proven to be an excellent tool for measuring such nanostructures, which are too thick for alternative characterization methods.
These results are from a simpler test sample compared to a real NW-LED structure. It does however demonstrate the power of these nano-RXD measurements to discover strain inhomogeneities in NW objects. In the future there is hope that such measurements can be extended to map the strain state of the MQW active region in a NW-LED.
Improving the spatial resolution in photoluminescence from wires by using SILs
The spacing of the wires in typical arrays for LED devices is about 1µm. This is on the border of what can be resolved in our micro-PL setup. In order to improve this we are currently experimenting with various designs of solid immersion lenses (SILs). These are truncated spheres that we have fabricated ourselves as they are not commercially available. We have some preliminary results on this study. The simplest test is to study well-defined patterns and to compare with and without the SIL. This is presented in figure 20, using an array of wires with a 400nm-spacing. The left image is the image without the SIL and the right image is with the SIL. It is clear that the optical resolution is improved where the individual wires are accessible with the SIL.
Figure 20. Two white ligth images of an array of wires with a 400nm pitch. Without the SIL (left) the image is grainy and shows no periodicity. With the SIL (right) there is a clear periodicity of white spots. The improvement in resolution is obvious.
In PL imaging, we have not yet got the setup working properly. The laser needs to be spread over a larger area. Figure 21, shows some examples of PL imaging of wire arrays. The first example is InP with a 400nm pitch. Without the SIL, there is no hint of resolving the individual wires. All that is visible is the emission under the laser spot. With the SIL, there is a pattern of bright spots of the individual wires. The SIL is also used on GaInN pyramids with a pitch of 1µm. In this case, the bright spots are better resolved, as expected due to the larger pitch.
Figure 21. Left: The emission from an array of InP wires with a pitch of 400nm. In this image without the SIL, there is no structure in the bright spot excited by the laser. Centre: When the same array is imaged with the SIL, bright spots corresponding to the individual wires can be observed. Right: As the pitch is increased to 1µm, the improvement is even more pronounced, here the arrays consisted of GaInN pyramids.
WP3. NW-LED devices in the project.
Green III-nitride core-shell nanowire LEDs have been demonstrated by Glo with peak EQEs exceeding 35% and EQEs at 20 A/cm2 equivalent to the performance of commercially available planar devices. The performance characteristics of the green NW LEDs are consistent with IQE ~ 40%. The devices exhibit good p-type layer conductivity and low defect density.
Figure 22 shows the EQE of green NW-LEDs as of M18 of the project and M24 of the project, plotted against current. This plot illustrates that through optimization of the epitaxial growth process to increase the volume of green-emitting QW material on the sidewalls of the nanowire, the efficiency at high current densities can be substantially increased. In these two efficiency versus current curves, the volume of the QW is increased by a factor of more than two. This was accomplished by increasing the molar flux of TMIn and TEGa in the epitaxy reactor during the deposition of the QW.
Figure 22. External quantum efficiency (EQE) of green nanowire LEDs as of month 18 (M18) and month 24 (M24) of the project. The EQE is shown for LEDs packaged into dome lenses on Ag-coated TO-5 headers with an n=1.52 silicone dome lens.
Another element of increasing the volume of QW material is increasing the density of nanowires formed on the GaN template. In the device shown for M18, the nanowire spacing (or pitch) is 1 µm, whereas for the device shown for M24, the nanowire spacing is 0.7 µm. The nanowires are formed in a hexagonal pattern, therefore this reduction in spacing corresponds to double the number of nanowires per unit area of substrate in the M24 case. Figure 23a shows an optical microscope image of a portion of the LED active area for the M18 device and Figure 2b shows the M24 device, while the devices are operating.
Figure 23a. Optical microscope image of a portion of the M18 NW-LED while the device is operating. The bright spots are nanowires while the dark region between the bright spots is the open area on the GaN template. The nanowire pitch is 1 µm.
Figure 23b. Optical microscope image of a portion of the M24 NW-LED while the device is operating. The bright spots are nanowires while the dark region between the bright spots is the open area on the GaN template. The nanowire pitch is 0.7 µm. The individual nanowires are difficult to resolve in this image.
Full-wafer nanoimprint technology is capable of forming even denser arrays of holes in the selective area growth mask used to create the nanowires. By further reducing the nanowire pitch to e.g. 0.5 µm, the EQE of the green NW LEDs will be further improved. Our estimation is that at 0.5 µm, which corresponds to a 2x increase in number of nanowires per unit substrate area over the M24 case, the EQE at 20 A/cm2 will be improved to 22% or higher.
Processing technology for green NW LEDs has also been advanced from M18 to M24. In both cases, the LED has a anode contact (Al/Ti/Au) deposited on the n-type GaN “buffer layer” which is located underneath the selective area growth mask. The buffer layer is grown on sapphire in the cases of these LEDs, although Si has also been used. The cathode contact is a transparent conducting oxide (TCO) deposited across the surfaces of the nanowires, which are p-type. An Al/Ti/Au wire-bonding pad is deposited on the TCO to complete the cathode contact. In the M24 device, the TCO is recessed from the edges of the array of nanowires, whereas in the M18 device the TCO reaches the edge of the nanowire array. The M24 devices suffer from less resistive shunt leakage compared to the M18 devices due to this change, and thus have an increased EQE especially at low currents.
The green NW LEDs have forward voltages (Vfs) of approximately 3.2 V at 10 A/cm2 forward drive current. The Vf responds to traditional parameters for planar GaN LEDs, such as temperature for “activation” of Mg dopant atoms in the p-type GaN layer, and Mg doping concentration in the p-type GaN layer. At M18, the Vf was approximately 3.6 V and by optimizing the dopant levels and activation process parameters (such as time and temperature) the voltage was reduced by 0.4 V by M24.
NW-LED wafer process.
GLOINC demonstrated that it is possible to produce nanowire LEDs using a flip-chip process technology, thereby enabling the integration of these LEDs into systems with large numbers of devices where the otherwise-needed wire bonds are prohibitively expensive or result in yield or reliability problems of the integrated system. It is well known in the LED field that flip-chip devices require highly reflective p-type spreading contacts, in order to exhibit high optical extraction efficiency. The commonly used metals are Al and Ag. GLOINC evaluated both of these metals. Figure 24 is a cross section schematic of a flip-chip nanowire LED affixed to a submount.
Figure 24. Cross-section sketch of a flip-chip nanowire LED.
It is also a fundamental necessity of a flip-chip device that the anode and cathode contacts be formed of solderable metal. Most commonly used is AuSn. Due to the solid-phase reactions of these various metals and the potential impact thereof on device reliability, so-called barrier metals are typically provided between the reflective contact and the solder metal. A key requirement of any metal film in a flip-chip device is that the metal film is substantially free of voids. While this is typically straightforward to achieve on planar films, the high-aspect ratio topology of the nanowire LEDs tends to produce textured films (the adjective textured refers to both the commonly known morphological definition, and the term of art used by crystallographers). Figure 25 is a cross-section SEM taken of a nanowire array with a layer of TiN sputtered on the array (the top-most film); this is considered an unacceptable barrier metal surface morphology.
GLOINC selected e-beam deposited Mo as the preferred barrier metal. Additional deposition of Au solderable contacts was performed. Figure 26 is a plan-view optical microscope image of a flip-chip nanowire LED with power applied. Light from the active region, scattered off of the substrate back side, is seen surrounding the metal contacts. GLOINC has not characterized the optical extraction efficiency of this device configuration. Depending upon the results of those data, additional work on contact reflectivity may be required to realize a commercially competitive device.
Manufacturability and cost assessment
GLOINC presently estimates the manufacturing cost of GaN/InGaN nanowire LEDs to be approximately 20% higher than the manufacturing cost of GaN/InGaN planar LEDs, and anticipates that this cost differential will decrease over time as knowledge is accumulated and engineering development is done to specifically address some sources of cost increment.
The manufacturing process for a GaN/InGaN nanowire LED is, at a high level, identical to the manufacturing process for a GaN/InGaN planar LED, with two major exceptions, both of which are associated with the epitaxial growth process. The first exception is that the epitaxial growth process is done in two steps through the MOCVD reactor, as opposed to one step. In the nanowire LED process, the first MOCVD growth step is performed in order to create a GaN template layer on a substrate such as Al2O3 or Si. Then, nanoimprint lithography is used to create a selective area growth mask on the template. Finally, the wafer is loaded into the MOCVD reactor and the nanowire LED array is grown.
One source of additional manufacturing cost is associated with a longer cycle time in the MOCVD reactor, which adds depreciation cost. GLOINCs current estimate for MOCVD cycle time increase is 10%. The growth of the GaN template layer is more rapid than the growth of the GaN template used for GaN/InGaN planar LEDs, since the dislocation density of this film may be much larger for the selective area growth nanowire LED device structure. Growth of the nanowire LED array is similar in cycle time to the growth of the active layer and p-layer stacks used in planar GaN/InGaN LEDs. Because large multi-wafer production MOCVD reactors have large thermal masses, cool-down and heat-up times are typically 45 minutes to one hour long. Because two complete MOCVD cycles are required, there are four heat-up/cool-down cycles per GaN/InGaN nanowire LED wafer, as opposed to one heat-up/cool-down cycle per GaN/InGaN planar LED wafer. The net result is that the MOCVD cycle time is somewhat longer for GaN/InGaN planar LEDs.
A second source of additional manufacturing cost is the additional manufacturing steps of depositing the dielectric growth mask, performing nanoimprint lithography on that growth mask, and dry etching the growth mask to form a template for selective area growth of the nanowire array. The cost of these steps individually is relatively low. The dielectric growth mask deposition may be done in simple large scale batch furnaces which can process 100 or more wafers at a single time, for a total throughput around 100 wafers per hour. The throughput of the nanoimprint lithography step is on the order of 50 wafers per hour, depending upon the details of whether thermoplastic or UV imprint is used. The dry etch step to form the holes in the selective area growth mask may be performed on multi-wafer ICP-RIE etchers, with a throughput on the order of 50 wafers per hour. The throughput of the MOCVD step is about 5 wafers per hour, much lower than the growth mask process step.
During the early stage of ramping the volume of the GaN/InGaN nanowire LED manufacturing process, it is anticipated that the cumulative yield of the manufacturing process will be somewhat lower than is achieved today in state-of-the-art manufacturing facilities for planar GaN/InGaN LEDs. Most of this additional yield fall-out can be attributed to the selective area growth masking process, and possibly some decrease in within-wafer LED yield or across-wafer non-uniformity. Additional process control points may be implemented to screen non-conforming materials from subsequent process steps, minimizing the cost of yield fall-out. Within-wafer uniformity of GaN/InGaN nanowire LED performance is consistent with the uniformity of state-of-the-art planar GaN/InGaN LEDs today. For example, the within-wafer standard deviation of peak emission wavelength is measured to be 1.5 nm for GLOINC’s green GaN/InGaN nanowire LEDs on 50mm substrate, while it is 2.1 nm for planar GaN/InGaN LEDs on 50 mm substrates.
NW-LED performance characterization and reliability study.
GLOINC characterized the performance of a second generation of GaN/InGaN green-emitting nanowire LED in Month 24, and compared to the performance of a first generation demonstrated in Month 18. Figure 27 shows a comparison of the external quantum efficiency characteristics of these two generations of GaN/InGaN nanowire LEDs. The EQE of the second-generation devices is greater, especially at current densities greater than 3 A/cm2. This performance improvement was generated in two ways. First, the density of nanowires per unit substrate area was increased by reducing the pitch of the nanowires on the selective area growth mask and GaN template from 1.0 um to 0.7 um (a factor of 2 increase). Second, work was done on optimizing the growth conditions (principally molar flow rates of Ga and In precursor chemicals into the MOCVD reactor).
In addition to having a higher EQE, the nanowire LEDs demonstrated in Month 24 also had lower forward voltage.
The second generation green-emitting GaN/InGaN nanowire LEDs were subjected to reliability stress tests. The stress condition was 10 A/cm2 forward current in an ambient temperature of 90⁰C. After 2.000 hours of stress, the GaN/InGaN nanowire LEDs exhibited less than 10% decrease in optical output, and negligible change in forward voltage and emission spectrum. Figure 12 shows lumen maintenance data from more than 30 LEDs tested from two wafers, after 2.000 hours of stress.
Figure 28. Lumen maintenance data for green-emitting GaN/InGaN nanowire LEDs under high-temperature operating life stress conditions. At 2.000 hours of stress, all devices pass specification.
Optical properties of truncated GaInN nano-pyramids and related LEDs.
An alternative route for the green and even red LEDs is to use GaInN-based structures. In order to avoid the problems related to homogeneity of the ternary core and to circumvent strain-related defects in wires based on a GaN core, we have investigated truncated nano-pyramids of GaInN. Using this approach, we have been able to vary the emission over a large range of emission wavelengths, as illustrated in figure 29. These spectra were recorded from truncated pyramids with different thicknesses of the active layers, but with the same layer compositions. These spectra show that it is possible to tune the emission in the range 500 to 650 nm at low temperatures. Room temperature measurements show that the emission only red-shifts with a small amount. CL imaging further reveals that most of the nano-pyramids are defect free.
The nano-pyramids have also been made into devices by putting the active layers in a pn-junction. We recently did some simple tests of one of these structures by contacting a large number of pyramids with nano-probes, (about 25 000). Figure 30 shows a photo of the one this study. The green spot is the emission from the pyramids at room temperature.
III-V nanowire LEDs (AlGaInP material system.)
Recently ULUND has strengthened its effort to demonstrate device performance of our III-V NWLEDs. As briefly mentioned in D1.6 we have modified the epi-structure of the GaInP-AlGaInP core-shell NWs in order to increase the efficiency. Several modifications were made:
The growth time for the upper segment of the p-GaInP NW core was increased to 6 min. The material composition of the ternary p-GaInP NW core was measured to be homogeneous.
On top of the p-GaInP NW core an n-GaP top segment was added in order to avoid leakage from the outer most shell directly into the core by avoiding the QW.
The p-GaInP buffer layer (inner most shell) was grown thicker (growth time 2 min instead of 15 s) because the NW core has a smaller diameter than the opening in the SiNx mask. Thus leakage from the QW directly into the substrate can be avoided.
The AlGaInP barriers were grown non-intentionally doped instead of p- and n-doped, respectively.
The bandgap of the AlGaInP barriers were increased up to 150-200 meV in order to force the carriers even more to be trapped in the quantum-well (QW).
The outer most shell n-GaInP was highly doped with tin (Sn).
The core and shells were grown lattice-matched with compositions of Ga0.47In0.53P and (AlxGa1-x)0.47In0.53P respectively.
A series of 3 samples with 3 different i-GaInP QW thicknesses were grown:
20 nm (estimated)
10 nm (estimated)
5 nm (roughly measured by TEM)
Having grown radial GaInP/AlGaInP nanowires, we processed the vertical NW arrays to define the devices and to electrically contact the NWs. Standard UV lithography was used to define 100×100 µm¬2 devices on the samples, and the samples were then contacted by sputtering 150 nm of indium-tin-oxide (ITO) on the NWs as a transparent and conductive contact, as seen in the Figure xx. Electrical measurements on the devices were carried out with a probe station. For all 3 samples bright red emission was observed (Fig 33).
Figure 31. Schematic structure of the radial GaInP-AlGaInP NWs with optimized device design.
For the 3 samples electroluminescence (EL) measurements with an optical detector were performed. We observe the highest peak intensity for a QW thickness of 10 nm. Additionally, we find a slight blue-shift for a thinner QWs (Fig.34.). This could be attributed to quantization in the QW as we decrease the QW thickness.
ULUND has not yet measured calibrated IQE/EQE but preliminary results estimate that the EQE for all three samples is below 1 %. We believe that the vertical processing has to be improved by adding a spacer.
For the III-V based NW-LED structures we foresee a similar manufacturing process as for the III-nitride NW-LEDs discussed above. A similar lift-off procedure should also be possible. The cost of the production can also be discussed in a similar way as for the III-nitrides, since a NIL procedure of the substrate is needed prior to growth of the NE-LED structure. At the moment it seems difficult to compete with the present cost of the planar III-V red LEDs.
Potential Impact:
General features and potential advantages of the NW-LED concept.
The choice of nanowires as the material to be used in LEDs has several advantages, among these are
(i) the ability to reach higher In-concentrations in the active QW-layers in a radial LED-structure enabling more long-wavelength emission,
(ii) (ii) the opportunity to design the individual nanowires into photonic cavities enabling optimal out-coupling of light, including reaching non-Lambertian angular distributions,
(iii) (iii) the ability to effectively obtain an emitting QW area that by far exceeds the planar foot-print of the LED device.
(iv) Furthermore, it is expected that it may become possible to design photonic crystal structures in the form of 2-dimensional nanowire arrays in which intended emission patterns, such as vertical emission, is enhanced while un-intended emission, such as in-plane emission, is strongly suppressed.
Today, it is possible to fabricate highly efficient blue and green NW-LEDs in the GaN-InGaN materials system, and red NW-LEDs in the GaAs-AlGaInP materials system. We expect also to very soon realize efficient yellow GaN-based NW-LEDs which will create the ideal system for color rendering optimization of R-Y-G-B (red-yellow-green-blue) individually addressable NW-LEDs. This will offer significant avenues towards application areas within advanced display applications as well as in lighting. A four-color emitter is able to reach color rendering index values exceeding 98, which is well beyond what the human eye can distinguish from the most ideal solar illumination conditions. With such a tri- or quadri-chromic illumination system that does not use any phosphors, the long-time color stability is expected to be highly superior in comparison with today’s UV/Blue-LED-pumped phosphors, also in terms of the efficiency in converting electrical in-put power to light.
In addition, the high quantum efficiency of the nanowire devices at low current densities make them especially suitable for display architectures where each nanowire or group of nanowires may form a pixel. In these architectures, the drive current per pixel is on the order of nanoamperes, necessarily meaning the current densities is below 1 A/cm2. A great and challenging opportunity, quite unique to NW-LEDs, would be the implementation of a monolithic RGB, direct-view display technology that would be exceedingly bright and power efficient in comparison with any display technologies available today.
Present shortcomings in the planar LEDs for Solid State Lighting (SSL), and how nano-LEDs may offer solutions.
The status of the SSL lamps can be summarized as follows:
• The efficacy in commercial production is about 150 lm/W for cold white lamps, somewhat less (130 lm/W) for the warm white version.
• The target for the year 2000 in the US roadmap is 200 lm/W for both versions. This appears possible to realize within the present technology, but it is questionable if this goal is in accordance with an acceptable market price for the white LED lamps. The present problem with droop in the InGaN/GaN LEDs grown on foreign substrates forces the producers to use large chip areas (of the order 1 mm2) for each planar LED. It is unavoidable that this will have a strong influence on the cost of these LED lamps, the cost could be lowered substantially of a much smaller chip size (and high drive current density) was used, which is technically feasible. The physical origin of droop is still under debate, and not definitely resolved. An interesting observation is that LEDs made on bulk GaN substrates with a low dislocation density (< 106 cm-2) demonstrate much lower droop, suggesting that dislocations may play an important role.
The presence of defects (structural defects as well as point defects) in the present planar InGaN-GaN based LED structures is a problem that is not well documented in the literature. The common practice of growing the LED structures on foreign substrates unavoidably leads to a threading dislocation density of mid 108 to 109 cm-2 throughout the structure, as a consequence of the lattice misfit between the substrate and the layers in the LED structure during the growth cycle. The common wisdom is that this has little influence on the radiative efficiency of the LED structures at low injection currents. At high currents, i e under the conditions of droop, the situation is different, and it is known that dislocations act as nonradiative defects.
Another kind of structural defects occur as a consequence of the misfit strain between the different layers in the LED structure developing in the growth cycle. Misfit dislocations and stacking faults may develop in the interfaces between the layers, both defects are causing nonradiative recombination if present in the active region of an LED structure. Such problems are a main concern for the longer wavelength LEDs, where the lattice misfit between GaN and InGaN is largest. A solution would be to use InGaN templates for the growth of the LED structures. Such planar InGaN wafers are not yet available on the market, however.
In addition to these structural defects there are also problems with point defects in the planar LEDs. It is well known that the introduction of structural defects is typically accompanied by point defects, which form more readily in the local large strain fields from structural defects. Very little definite data are known about this problem in the InGaN material, however. It has e.g. been suggested that the so called “green gap” problem is related to deep level point defects, these have been characterized in DLTS experiments, but not identified so far [1].
Another limitation in the growth of planar InGaN-GaN LED structures is the limited In composition that can be incorporated in different InGaN surfaces. The c-plane orientation is known to be superior in this respect, c-plane InGaN can be grown over the entire range from GaN to InN, and is perfectly suited for the present SSL technology on sapphire. The limitation for c-plane InGaN structures is the so called Stark effect, i e the built in polarization field that causes an excessive internal electric field in the QWs, so that the QW thickness has to be as small as 2 nm for longer wavelengths, not practical in a production environment. The In incorporation is improved if non-polar or semipolar planes are used, and wider QWs are then tolerated as the field is reduced. The strain is still very high, however, meaning that the strain-induced defects limit the radiative output of the long-wavelength LEDs beyond the green region for the standard InGaN-GaN growth technology on sapphire (or other foreign substrates).
Proof of concept research has been done on growth of planar InGaN-GaN LEDs on bulk GaN substrates. In the case of non-polar and semi-polar structures excellent results are demonstrated on small size GaN substrates of proper orientation [2]. The substrates are presently too small (about 10 mm size) for commercial production of LEDs at reasonable cost. In the case of c-plane GaN larger size substrates (2 inch diameter) exist, and industrial production of LEDs on bulk GaN has been started by Soraa Inc in USA. For the longer wavelengths the strain is still a severe limitation, as discussed above. As outlined below all-InGaN defect free structures seem to be needed for efficient long wavelength nitride LEDs. At present bulk InGaN substrates do not exist.
The above situation has led to an increased interest in nanostructures as a basis for visible LEDs. It is well known that the small dimensions of nanostructures allow at least partly a relaxation of built in strain in the structures, further the structural defects that are so dominant in the planar structures can be completely eliminated from the active region of the devices made on these nanostructures. By using a nitride buffer layer on top of a substrate, foreign substrates of any chosen dimension can be used without loss of the above properties, which means a low cost can be maintained for a high quality structure. We have previously discussed the properties of LEDs in the nanowire configuration, where the main emitting facets are the m-planes. The structures studied in this project have limitations at longer wavelengths due to the use of several GaN layers in the structure (causing high strain as discussed above), and also suffering from limited In incorporation in the m-planes during growth. The flexibility in nanostructure growth can be used to develop an all-InGaN LED structure for visible light nitride LEDs. Very regular arrays of such structures can be grown with aid of a suitable insulating mask material (i.e. SiNx) in combination with nano- imprint lithography (NIL).We shall here discuss one possible such configuration, based on nano size c-plane features.
In Fig 1 is shown several examples of geometries of GaN nanostructures, that can be grown in one single MOCVD growth run, using a NIL patterned sapphire substrate with a GaN buffer layer. The right part of the figure shows c-plane platelets of different thickness. Very similar c-plane structures can as well be prepared in InGaN (also in a single growth run), with In compositions up to about 20%. TEM investigations show that such InGaN templates are typically free of structural defects. The c-plane top face may have a slight roughness as grown, then an overgrowth will produce a perfect template for further growth of a full LED structure, in this case an all-InGaN structure. For growth of green LEDs the In composition of the template can be about 10 %, for longer wavelengths out to red a 20% In template is suitable. Quantum well structures are then grown with traditional c-plane growth conditions with InGaN barriers of similar composition as the template. An AlGaN electron blocking layer can be added before the p-layer is grown to complete the LED structure.
In Fig. 2 are shown emission spectra from samples with different configurations of the QW region. Single QWs were used for these test samples.
Figure 1. Top-view (top row) and tilted-view (bottom row) SEM images of GaN shell growth on GaN NWs. a) Pyramidal shell growth without c-plane. Dotted lines show the profile of GaN NW. b) & c) show controlled platelet growth with varied c-plane size.
Figure 2. Emission spectra at room temperature from c-plane all-InGaN nanostructures with different active region QWs.
In figure 3a green prototype emitter with this configuration is shown with electrical injection.
Figure 3. Left: picture of a green InGaN c-plane nano-emitter with electrical bias. Right: EL spectra at different current levels (room temperature).
Possible impact of all-InGaN nano-LEDs.
The above data are very preliminary and have not been optimized. It should be pointed out that in the planar technology the efficiency of nitride-based LEDs with emission above 600 nm is very low, best values reported are 1-2 % [3], which is insufficient for most applications and certainly for SSL. We want to be able to optimize these c-plane nanostructure LEDs so that the full potential of the InGaN material can be utilized. We are confident that the structures can be prepared without structural defects across all visible wavelengths, also the strain will be much reduced compared to the standard planar InGaN/GaN structures. This also means that there is a good chance that the point defects believed to be responsible for the so called green gap might be considerably reduced in concentration. A bonus with the low strain structures produced this way is that the quantum well in the active region can be kept much wider, thus simplifying the growth process.
The impact of a successful outcome of this all-InGaN approach for visible LEDs would be very large. If nitride based LEDs could be produced with similar efficiency from blue to red, there is finally a platform for the polychromatic solution for SSL, by mixing the light from efficient LEDs in all four colors blue, green yellow and red. The ability to do this in a single (InGaN) material system is definitely a technical advantage.
[1]. A. M. Armstrong et al, Applied Physics Express 7, 032101 (2014)
[2]. C.C. Pan et al, Applied Physics Express 5, 062103 (2012)
[3]. J-I Hwang et al, Applied Physics Express 7, 071003 (2014)
Wider socio-economic impact and societal implications of modern lighting solutions.
Appropriate lighting is a basic prerequisite for the quality of modern life. The electric light has enabled a great expansion of human activity. As an obvious and important example can be mentioned the value of better illumination on our roads during dark hours, improving traffic safety. Similarly, the effects of significantly improving night time illumination in pedestrian areas will lead to an important improvement in personal safety and comfort. For many of such public areas the value of the long life-times of LED-based SSL is significant, since the illumination systems can be designed without the need for frequent lamp replacements. This important advantage also holds for novel opportunities in architectural lighting, where lamp fixtures can be integrated with the buildings in many and novel ways. However, inappropriate light and light pollution also have serious side effects on human health and quality of life. Furthermore, the energy and materials used for lighting tend to have serious environmental consequences.
As a species, humans depend most heavily upon visual and endocrine regulatory information to interact with the environment. Some 80% of our sensing cortex in the brain is devoted to vision. All species, including humans, also depend upon the natural, 24-hour light-dark cycle to coordinate, literally, every physiological system in the body to live healthy and productive lives. Effective lighting is one of the important elements for a sustainable lifestyle. Thus far, most of the interest has focused on the amount of light and the efficiency of the light producing processes, often measured as lumens (lm) and lm/W. The primary aim has been to produce a sufficient amount of light at the lowest possible cost. Most of the consumer interest has focused on the cost of the light source. In a life-cycle perspective the energy cost has become a major part of the cost, in particular for lighting with ordinary incandescent light bulbs. Since the 1960s the professional market has become increasingly interested in more efficient light sources.
The evolving technology, primarily in Light Emitting Diodes (LED), is beginning to enable new tailor-made and dynamic forms of lighting. The commercial LED technology is approaching the same level of efficiency as fluorescent tubes. Incandescent lamps mainly produce heat, fluorescent tubes produce the light in an indirect way and LEDs produce the light in a direct, and thereby more effective and versatile way. The purchase price for high quality LED-based light sources is still relatively high, but the life-cycle cost is becoming more competitive. There is keen interest in light quality to enhance plant growth within intensive greenhouse production systems.
So far there has not been much public awareness of the importance of the quality of light. Since the 1990s, there have been significant breakthroughs in knowledge about the positive and negative effects of different kinds of light, on human health and comfort. There is also an evolving interest in light pollution, i.e. too much light and unsuitable kinds of light.
Plants and algae use light energy absorbed by chlorophyll in photosynthesis to produce carbo-hydrates from carbon dioxide and water. Besides this, the information carried by light is used to optimize plant growth and behavior, to guarantee the best chances of survival and reproduction. Different pigments other than chlorophyll absorb light for these processes and allow control of e.g. gas exchange, flowering, shade avoidance and production of phytochemicals. The light emitted from high-pressure sodium lamps used traditionally in greenhouses is quite poorly adjusted to what the plants use, which results in unnecessarily high energy costs. With LED light in greenhouses, the energy consumption can be drastically lowered, since the different wavelengths can be more balanced. Furthermore, light can be varied to stimulate specific processes such as the synthesis of specific plant products important to human health. One potentially very important application of this technology may be the use of the separate spectral components in novel applications such as green-house lighting systems, where indeed sharp spectral features in the blue and in the red regions can be designed to optimally induce growth of plants. This can be expected to lead to a new way of feeding our population by eco-friendly and efficient local food production, hence reducing expensive and polluting long-distance transportation. For local production of food in northern regions of Europe having short daylight periods during the winter season, it could become quite realistic to keep producing vegetables based on a combination of heat stored in the ground and highly efficient, and spectrally optimized, illumination using such LEDs.
In addition the versatility in regulating the emission wavelengths and power of these new light sources allow significant advances in other research fields, like visual science, medicine and psychology.
Main dissemination activities in the project.
As part of dissemination activities the partner groups have participated in other joint symposia with research groups with similar interest. One such event was the Lund-Tokyo-Copenhagen-Beijing Joint workshop on nanostructure quantum devices held in Lund-Copenhagen March 24-25 2013. At this event our development of regular Hall measurements on single nanowires was reported and demonstrated (Kristian Storm).
Another example of such an event was the second Industry-Academia Workshop on Nanophotonics for Energy Efficiency 11-12 Nov 2013 in Stockholm. This was a separately EC-supported activity with participation from many European countries. Bo Monemar gave a presentation of our work in NWs4LIGHT.
Another international event with connection to European Lighting Industry was “Symposium on Materials Science for Light Emitting Diodes”, organized jointly by Lund University and the local organization “Invest in Skåne” in Lund 14-15 Oct 2013. There were several participants from Japan, such as H Amano, Nagoya University, and S Kamiyama, Meijo University, they reported about LED developments in Japan, Lars Samuelson, Jonas Ohlsson and Bo Monemar gave presentations on our work at Lund University, such as the NWs4LIGHT. Several industries with interests in lighting also gave presentations, one example is IKEA.
On Feb 5 2015 The Lund University Open Innovation Center arranged a symposium “Multidisciplinary Lighting seminar”, with participation of researchers from social science, medical science, technology and several industrial partners. The event had several international invited speakers covering different aspects of future lighting solutions. Bo Monemar participated representing the Solid State Lighting Center at ULUND.
The above is just a few examples of many such events with useful interaction and contact with relevant lighting industry as well as with foreign research activities outside the project. It should be mentioned that Dr N Gardner at GLO has participated in important committees related to the SSL roadmap in the USA. This may be important for the future positioning of NW-LEDs in this application area.
An important element in the international and European dissemination work in this project has been plenary and invited presentations, given worldwide and primarily by the coordinator Lars Samuelson at ULUND. These were mostly angled at the development of growth technology and basic understanding of the NW properties, but included also results on the performance of the NW-LEDs produced in the projects. Several such presentations were also given by representatives of the GLO partner, mainly by Nate Gardner. In that case new information on growth of NW-LED structures and related device processing was demonstrated from a semi-industrial perspective, and relevant performance data were also transmitted. These presentations have showed the international research community where the leading edge in this field lies, and clearly indicated prospects for future applications, both in Solid State Lighting and in other fields like display screens. A complete list of these talks is given separately in the report D5.7.
In addition to these plenary and invited presentations there were many ordinary contributed oral and poster presentations at conferences and symposia in the field of nanostructures as well as light-emitting structures and devices. These contributions are listed in full in the report D5.7.
Exploitation activities in the project.
The exploitation of the foreground produced in the project has mainly relied on the industrial partner Glo AB in Lund, Sweden (GLO) and its subsidiary Glo-USA Inc. in Sunnyvale, USA (GLOINC). Via this partner several patents related to nano-wire LED technology have been issued during the project duration, as discussed separately in Sec. 4.2. GLOINC has set up a functioning processing plant in order to demonstrate the viability of the NW-LED approach. GLOINC demonstrated that it is possible to produce nanowire LEDs using a flip-chip process technology, thereby enabling the integration of these LEDs into systems with large numbers of devices where the otherwise-needed wire bonds are prohibitively expensive or result in yield or reliability problems of the integrated system. The produced devices have a performance close to the corresponding planar LEDs on the market, and also exhibit similar excellent data in reliability tests.
GLO has an extensive contact area with possible customers for NW-LEDs. It seems like the SSL market is not yet ready for the NW-LED introduction, a breakthrough for long wavelength nitride NW-LED structures would certainly change this situation. That is why we recently have put some substantial effort at ULUND to advance this part of the growth activities (see above). This part is presently producing new foreground that will be protected and hopefully lead to a game-changing production of polychromatic NW-LEDs with a much higher efficiency and lower droop than the present SSL solutions on the market.
In addition, the demonstrated high quantum efficiency of the nanowire devices at low current densities make them especially suitable for display architectures where each nanowire or group of nanowires may form a pixel. In these architectures, the drive current per pixel is on the order of nano-amperes, necessarily meaning the current densities is below 1 A/cm2. A great and challenging opportunity, quite unique to NW-LEDs, would be the implementation of a monolithic RGB, direct-view display technology that would be exceedingly bright and power efficient in comparison with any display technologies available today.
Project website.
The website is found under Lund University at the address:
http://www.ftf.lth.se/research/links_to_some_projects/nws4light
List of Websites:
The website is found under Lund University at the address:
http://www.ftf.lth.se/research/links_to_some_projects/nws4light
Contact details:Prof. Lars Samuelson, Solid State Physics Division, Physics Department, Lund University, SE-223 62 Lund, Sweden
Visiting address: Professorsgatan 1, email: lars.samuelson@ftf.lth.se
