Self-assembled nanostructured materials for electronic and optoelectronic applicatons

Nature of the result: The most fundamental issue for stacked quantum dots is the answering question of when (i.e. how close) and how (i.e. via electrons or holes) they become electronically coupled. These questions have been answered during the project. Potential applications of the result: The result can be applied to further research on quantum dot molecules with relevance for quantum computing. It can also be applied to laser devices in which stacks of dots are used to increase the gain and coupling should be avoided, or for fine tuning of the emission wavelength, when coupling is exploited. End-users of the result: The end users of this result are the scientific community and industry (semiconductor laser manufacturers). Main innovative features/benefits: The result offers, for the first time, a clear answer to a question of fundamental importance in self-assembled quantum dots. Analysis of market or application sectors: The result is a principally scientific one, so such an analysis is not applicable. Potential barriers: There are no potential barriers to the use of this result.

Nature of the result: The result involves the development of a methodology for the calculation of phonon modes and the exciton-phonon interaction in stacked self-assembled quantum dots (SAQDs). A theoretical approach, based on the multimode dielectric continuum model for phonons in confined systems, is extended to calculate phonon modes and the exciton-phonon interaction in stacked layers of SAQDs. The optical-absorption and luminescence spectra of InAs stacked SAQDs are analysed within the framework of the non-adiabatic theory of phonon-assisted optical transitions in quantum dots. In addition, Raman spectroscopy and photoluminescence measurements on local phonon modes in stacked SAQDs enable an insight in exciton-phonon interaction and the influence of stacking on the local phonon properties. Potential applications of the result: The result can be used for the characterization of stacked SAQDs and other semiconductor nanostructures using experimental data on optical-absorption, luminescence and Raman spectra. End-users of the result: The end users of this result are all those involved in the research and development of semiconductor nanostructures, including members of the project consortium and the wider scientific community. Main innovative features/benefits: The optical-phonon spectra in stacked quantum dots acquire a more complicated structure than those in a single quantum dot. In particular, in a stack of N quantum dots each interface-phonon frequency of a single quantum dot splits into N branches due to the electrostatic interaction between the optical polar vibrations of different quantum dots. The result offers the possibility for an advanced understanding of the vibrational spectra and optical properties of self-assembled semiconductor nanostructures. Analysis of market or application sectors: The result is a principally scientific one, so such an analysis is not applicable. Potential barriers: There are no barriers to the use of this result.

Nature of the result: The result involves a joint theoretical and experimental understanding of the properties of charged excitons in quantum wells. Potential applications of the result: The result can be applied to the excitonic properties of quantum wells. Some aspects of the result can be applied to the excitonic properties of other semiconductor nanostructures. End-users of the result: The end users of this result are the scientific community. Toshiba Research Europe Limited has a highly relevant (scientific) research programme. Main innovative features/benefits: The result resolves the main scientific issues related to the properties of charged excitons. Analysis of market or application sectors: The result is an entirely scientific one, so such an analysis is not applicable. Potential barriers: There are no barriers to the use of this result, which has been widely disseminated.

Nature of the result: The result involves the development of a methodology for the determination of the confinement and excitonic properties of self-assembled quantum dots (QDs) and other self-assembled semiconductor nanostructures. These issues are at the core of the physics of self-assembled nanostructured materials, and are therefore of crucial importance not only for our fundamental understanding of their properties, but also for their exploitation as (opto)electronic devices. Potential applications of the result: The result can be applied to understanding the electronic and optoelectronic properties of all semiconductor nanostructures, and therefore has a very general applicability. End-users of the result: The end users of this result are all those involved in the research and development of semiconductor nanostructure (devices). This includes members of the project consortium, the wider scientific community and industry. Main innovative features/benefits: The result offers the possibility for an advanced understanding of the properties of self-assembled semiconductor nanostructures, offering substantially improved insight over the previous state of the art. This will benefit further research and improve the knowledge/design of semiconductor nanostructure based devices. Analysis of market or application sectors: The result is a principally scientific one, so such an analysis is not applicable. Potential barriers: Full use of the result requires specialist knowledge and/or infrastructure, however a more general understanding of the broad features of the result should still have wide applicability.

Nature of the result: The result involves the development of a methodology for the determination of the confinement and excitonic properties of stacked layers of self-assembled quantum dots (QDs) and other self-assembled semiconductor nanostructures. Such issues are at the core of the physics of self-assembled nanostructured materials, and are therefore of crucial importance not only for our fundamental understanding of their properties, but also for their exploitation as (opto) electronic devices. Potential applications of the result: The result can be applied to understanding the electronic and optoelectronic properties of all stacked semiconductor nanostructures, and therefore has general applicability. End-users of the result: The end users of this result are all those involved in the research and development of semiconductor nanostructure (devices). This includes members of the project consortium, the wider scientific community and industry. Main innovative features/benefits: The result offers the possibility for an advanced understanding of the properties of stacked layers of self-assembled semiconductor nanostructures, offering substantially improved insight over the previous state of the art. This will benefit further research and improve the knowledge/design of semiconductor nanostructure based devices. Analysis of market or application sectors: The result is a principally scientific one, so such an analysis is not applicable. Potential barriers: Full use of the result requires specialist knowledge and/or infrastructure, however a more general understanding of the broad features of the result should still have wide applicability.

Nature of the result The result involves the development of a methodology for the calculation of phonon modes and the exciton-phonon interaction in single layers of self-assembled quantum dots (SAQDs). Calculations are based on the multimode dielectric continuum model for phonons in confined systems with given geometric and material parameters. Optical absorption spectra of single SAQDs are calculated using the non-adiabatic theory of phonon-assisted optical transitions in quantum dots. The proposed approach is applied to analyse the optical-absorption and photoluminescence (PL) spectra of single InAs/GaAs SAQDs and quantum-dot quantum wells. It provides a basis also for the calculation of the PL excitation (PLE) and Raman spectra of SAQDs. The developed methodology of calculations of optical spectra of SAQDs has been used to analyse some experimental data obtained at RSU and TUB. Approximate values of an effective value of the electron-phonon interaction can be obtained directly from PL spectra whenever multiphonon structures clearly appear in the spectra, provided artifacts due to the simultaneous contributions of ground state phonon replica and excited states are taken into account. In the case of broad PL spectra, low temperature PLE and resonant PL (RPL) gives rise to a fluorescence narrowing and to a partial recovery of phonon structures. Acoustic phonons in self-organized InAs/GaAs quantum dots were investigated through spectral hole burning experiments. Potential applications of the result: The result can be used for the characterization of single layers of SAQDs and other semiconductor nanostructures using experimental data on optical-absorption, luminescence and Raman spectra. End-users of the result: The end users of this result are all those involved in the research and development of semiconductor nanostructures, including members of the project consortium and the wider scientific community. Main innovative features/benefits: As distinct from the models of bulk phonons and the dielectric continuum model, within the developed approach the phonon modes in a quantum dot are hybrids of bulk-like and interface vibrations. The result offers the possibility for an advanced understanding of the vibrational spectra and optical properties of self-assembled semiconductor nanostructures. Analysis of market or application sectors: The result is a principally scientific one, so such an analysis is not applicable. Potential barriers: There are no barriers to the use of this result, which has been widely disseminated.

Nature of the result: Photoluminescence at room temperature of InAs QWR have been achieved at 1.55µm. Using the knowledge adquired in tuning the wavelength emission of QWR and stacked layer of QWR several laser have been grown and processed. asing at 1.45µm at 100K from a single layer QWR device has been obtained, and lasing up to 250K from a three layer QWR device with low threshold current. This result is directly related to D21 (and supersedes D3). The development and performance of these lasers from CSIC is state-of-the-art at this wavelength, and already shows Jth 4x lower than for comparative quantum well (QW) devices at the same wavelength. Potential applications of the result: This result is an important stepping stone to achieving 1.55µm lasers with nanostructures as the active material. As such strong competition with the current InP based QW lasers is expected for telecomms applications. End-users of the result: Telecommunications components manufacturers, telecommunications systems Main innovative features/benefits: 1.55µm is the minimum attenuation point in standard optical fibre, the type of which is widely deployed around the planet. Dispersion however is non-zero, and as such any transmitter should have low or zero chirp to enable long distances to be spanned without regeneration. Reducing the dimensionality i.e. moving from quantum wells (current laser technology) to quantum wires (this work) is expected to lead to a concentration of the density of states at the lasing energy and a more symmetric gain profile within the device, resulting in a reduction in chirp. Quantum confinement leads to an increase in the exciton binding energy and an increase in the oscillator strength enabling operation up to higher temperatures. If the laser can operate at high temperatures (85°C) then there is no need for a thermoelectric cooler. This represents a cost saving in terms of parts, assembly, control electronics and system cooling. Analysis of market or application sectors: 1.55µm reduced chirp direct modulation QWR laser for 10GBits-1 over 80km reach. Potential barriers: There is currently no literature on high frequency operation of QWRs lasers at 1.55µm to confirm whether direct modulation at 10GBs-1 is realistic. The poor electron confining potential has to be improved before high temperature operation can be successful. Investigation of the stacking of the QWRs is needed to retain the correct emission wavelength as established for the single layer devices. A less quantifiable but probably more significant barrier is the ongoing advancement of competing technologies. If significant progress can be made in improving the current QW based devices, then this market window may close.

This result is directly related to D22, but also includes (and supercedes) D1. USFD, in very close collaboration with BHM, have developed state-of-the-art 1.3µm QD lasers with record low threshold current densities compared to the published literature. Nature of the result 1.3µm is the minimum dispersion wavelength of standard optical fibres, the type used widely by TDM and CWDM systems. Current laser products are based on InP substrates, which are more fragile, more expensive to fabricate and generally smaller than their GaAs counterparts. In addition carrier confinement in current InP based QW systems is poor, resulting in high laser temperature sensitivity. The growth of InGaAs QWs emitting above -1.1µm on GaAs substrates is prevented by strain effects. Self-assembled QDs enable this strain limitation to be overcome, providing a cost effective GaAs based system. Using QDs, projected yields and costs can be significantly improved by moving to a GaAs based platform. The ultra-low threshold current densities predicted for QD lasers have taken some time to materialise in practise, primarily due to the extremely high demands placed upon the optical quality of the QDs. The result achieved on this programme, which followed extensive growth optimisation, represents current state of the art. Most significantly a lower threshold current density has been achieved compared to any QW based device. A low threshold current is important in that the current used in reaching threshold is essentially wasted, and merely serves to heat the chip. Potential applications of the result Direct modulation QD lasers for uncooled 1.3µm data transmission at 10GBs-1. End-users of the result Telecommunications components manufacturers, telecommunications systems Main innovative features/benefits Fibre-optic wavelength laser based on a GaAs substrate. High temperature operation possible with low threshold current density. Analysis of market or application sectors The main market for 1.3µm lasers is data transmission in optical fibres. Since new 1.3µm systems will run at 10Gbit/s, suitable lasers will need to exhibit 3dB bandwidths of -12GHz (to allow for error correction data etc). Direct modulation of the laser is desirable for short haul applications since a separate modulator would dramatically increase chip real estate or necessitate costly coupling optics. There are currently established 1.3µm lasers based on InGaAsP QWs grown on InP substrates. A potential market is provided by the fact that these established devices are not suitable for high temperature operation without a costly and power hungry thermo-electric cooler. Additionally, QD lasers are predicted to suffer less than QW lasers from optical feedback effects, and so may not require the same level of optical isolation. Potential barriers There is currently a very limited body of results obtained from 1.3µm single frequency lasers based on QDs. In particular much more work is needed on the dynamic properties of QD lasers to ensure that suitable link reach at 10GBs-1 can be achieved. A more serious barrier to the eventual uptake of QD based 1.3µm lasers may be that the limitations of the incumbent technology are presently being addressed by several device manufacturers via the development of the AlInGaAs material system on InP and GaInAs(Sb)N on GaAs. It is not yet clear which technology (QDs, AlInGaAs-InP and GaInAs(Sb)N-GaAs) will be the most successful.

Nature of the result: Spectral hole burning as a tool to circumvent inhomogeneous broadening in SAQD ensembles is demonstrated. Resonant optical excitation is used to selectively saturate the absorption of SAQDs and allows addressing SAQDs with identical ground state transition energies. This high-resolution saturation spectroscopy experiment enables to investigate e.g. the binding energy of charged trion complexes as well as the temperature dependence of the coupling to acoustic phonons. In addition, persistent spectral hole burning presents a basis for future wavelength-selective memory devices. The result suggests that more than 103 parallel bits in just one spatial data unit are addressable in future SAQD memory structures via wavelength-domain multiplexing. Potential applications of the result: The result can be used for further investigations of memory devices based on SAQDs. Moreover, the understanding of optical, phononic and exitonic properties of semiconductor nanostructures is largely enhanced, and therefore the result has also a general applicability. End-users of the result: The end users of the result are all those involved in the basic research area and also in the development of new semiconductor nanostruture devices, especially in the area of future memory device development. This includes members of the project consortium, the wider scientific community and industry. Main innovative features/benefits: The result represents the first spectral hole burning experiment at low temperatures with a storage time of 1 ms with high spectral resolution below 200µeV. This encouraging values present a basis for further research on spectral hole burning and for further investigations of SAQDs for memory devices. Analysis of market or application sectors: The result is in principle a more scientific one, so a detailed analysis is not applicable. In the long term the result will be useful for the development of novel semiconductor memory devices. Potential barriers: There are no barriers to the use of this result.

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