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A Novel Approach to the Fabrication of Nanoscale Light Emitting Diodes

Final Report Summary - NANOLEDS (A novel approach to the fabrication of nanoscale light emitting diodes)

Research in nanoscience and nanotechnology (N&N) will drive the scientific and technological development of future years with revolutionary perspectives in many different aspects of our society, from materials to telecommunications and energy, from medicine to the environment. Future large-volume applications will require fast and flexible methods to fabricate electronic and photonic components. The main objective of the NANOLEDS project was to develop a novel approach to the fabrication and miniaturisation of photonic components by exploiting hydrogen implantation and laser-writing techniques. The proposed fabrication approach is based on the laser-driven diffusion of H atoms in III-V p-i-n light-emitting diode (LED) structures to create nanoscale channels for the current flow, resulting in the electrical and optical activation of sub-micrometre regions of the LED structure. This approach is fast and flexible, can be easily implemented in different material systems, and has the potential for large-volume applications and for making miniaturised components of photonic devices. The NANOLEDS project also aimed to enrich the knowledge of the physics of hydrogen in a novel class of semiconductor alloys (i.e. Ga(AsN), Ga(AsBi), and In(AsN)), which is of great interest for several technological applications. An additional, important objective of the fellowship (Intra-European fellowships for career development of the European Union (EU)'s People / Marie Curie actions) was to provide the fellow with the opportunity to acquire new and complementary skills, and to acquire research competencies needed for his professional development.

The NANOLEDS project investigated the effect of hydrogen introduction and laser-driven diffusion in different p-i-n LED structures and semiconductor alloys. First, studies on laser-induced hydrogen diffusion were performed in hydrogenated Ga(AsN) alloys as a function of the laser wavelength, laser power density, and exposure time. These studies revealed a resonant laser absorption by N-H complexes and their consequent photodissociation, thus resulting in a strong modification of the electronic properties of the system (i.e. a decrease of the fundamental band gap energy - up to approximately 150 MeV for sample with (N) round 1 % - after the exposure to a focused laser beam). The possibility of laser writing the electronic properties in several dilute nitride material systems (i.e. Ga(AsN), Ga(PN), and (InGa)(AsN)) was demonstrated, and in-plane band gap profiles over a micrometre scale obtained. The in-plane laser profiling of the band-gap energy offers flexibility in the control and exploitation of the electronic properties of a semiconductor without the implementation of lithographic / etching techniques. The versatility of hydrogen makes this direct laser writing approach of general interest and relevant to the development of fast and easy fabrication approaches to nanotechnologies.

In (GaMn)As- and GaAs:C-based LED structures, the incorporation of hydrogen has shown a general improvement of the electrical properties of the devices. A reduction of the leakage current over several orders of magnitude (which represents an important result from applicative and technological point of views) as well as an increase in the resonance quality factor of resonant tunnel diodes has been observed following the post-growth hydrogenation. This results from the hydrogen passivation of structural defects and impurities present in the intrinsic region of the p-i-n structures.

Laser-annealing experiments indicated that the effects related to the hydrogen introduction can be locally controlled by laser-induced hydrogen diffusion and / or complex dissociation, thus allowing the realisation of nano-LEDs in Ga(AsN)-based devices. This work is still in progress, but has already revealed very promising results. On the other hand, the formation of hydrogen complexes with impurities and defects present in the device’s contact layer hampered the formation of nanoscale channels for the current. This effect has shifted the focus of the project and has been employed to fabricate the first movable light emitting area in an inorganic LED. Indeed, the H-C complex formation and relative drop of conductivity in the p-type GaAs:C layer of the device has been used to engineer the voltage drop across a p-i-n GaAs/AlAs-based resonant tunnelling LED (rectangular-type (RT)-LED), thus acting to localise the light emission on a micrometer-size area whose spatial position is controlled by the applied voltage; namely, allowing it to obtain a 'movable RT-LED'. In particular, it has been shown that the spatial position of the light-emitting area can be linearly shifted over tens of µm by varying the applied voltage by only a few hundred mV. An additional, important outcome of tailoring the resistivity in the p-type GaAs:C contact layer with hydrogen is the possibility - under given bias conditions - of achieving simultaneous resonant injection of both electrons and holes into the quantum well states of the RT-LED. This results in a tenfold increase of the electroluminescence (EL) intensity and a threefold increase in the peak-to-valley EL intensity ratio versus applied voltage, a figure of merit of a RT-LED. The developed approach to the simultaneous carrier resonant injection has the potential to be extended to several innovative device concepts in emerging nanotechnologies that exploit the quantum tunnelling of charge carriers into a quantum state in several fields of research, which span from THz resonators to high-efficiency single-photon detectors and room-temperature negative differential-resistance devices.

The investigation of hydrogen effects has been extended also to the case of Ga(AsBi) and In(AsN) alloys. These studies have demonstrated the passivation effect of hydrogen on the native Bi-induced acceptors in Ga(AsBi) alloys and indicated that the narrowing of the band gap and the p-type conductivity, which are observed in Ga(AsBi) alloys with increasing the Bi concentration, are two uncorrelated effects arising from different Bi-induced electronic levels. Moreover, in the In(AsN) system, this study has shown that following the formation of N-H donor complexes with energy levels well above the Fermi energy - and far from resonance with the conduction electrons - is possible to achieve simultaneously high electron concentrations and mobilities. The peculiar response to hydrogen incorporation unveiled in Ga(AsBi) and In(AsN) alloys make these material systems highly promising for future implementation in p-i-n structures for near-infrared LED and laser devices.

In summary, the NANOLEDS project has shed light on new effects of hydrogen in semiconductor alloys that promises to be of great interest for technological applications (i.e. Ga(AsN), Ga (AsBi), and (InAsN)). A new, fast and flexible laser writing technique of the in-plane electronic properties has been demonstrated in hydrogenated III-N-V alloys and nano-LEDs obtained in Ga(AsN)-based devices. Also, the project developed an innovative approach for the fabrication of movable-LEDs in inorganic materials, which employs the hydrogen passivation of dopants on the device's contact layer. This enables an accurate control of the position of the light-emitting area and an increase of the light intensity by the simultaneous resonant injection of both electrons and holes in the active region of a resonant tunnelling diode.

Finally, the number of new experimental techniques and research experiences acquired by the fellow during the course of the NANOLEDS project, together with the excellent scientific results obtained in this period, represent a key outcome for the development of his professional career and his new appointment at the IFN-CNR in Rome (Italy).

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