III-Nitride materials

Nitride-based semiconductor materials InN, GaN, AlN and their alloys attract great attention due to their promising applications in optoelectronic devices such as light emitting devices (LEDs) and laser diodes (LDs) [1]. Depending on the alloy composition, these systems could in principle cover a wide wavelength range from red through yellow and green to blue[2].

InGaN systems are especially promising candidates for energy-efficient solid state lighting (which combines output from red, green and blue LEDs) since the inclusion of phosphide binaries are not required to generate a white light source. Additionally, owing to quantum confinement effects, fabrication and studies of nitride-based quantum dots (QDs) have recently attracted a great deal of interest for novel applications due to their unique physical properties [3]. For example, GaN/AlN QDs have shown great promise for single-photon sources operating at much higher temperature compared to QD systems based on conventional III-V materials [3]. Despite their potential for optoelectronic applications, the electronic and optical properties of group-III-nitride QDs are still far less known than those of the more traditional III-V-semiconductor QD systems. Here at Tyndall we both model the electronic, optical and electromagnetic properties in the Photonics theory group, but also we have a growing experimental capacity both in growth and characterisation.

Electrostatic built-in fields in nitride based QDs

QDs based on III-nitride semiconductors have been grown mainly in the hexagonal phase. In comparison with conventional III-V materials, the wurtzite group III-nitrides exhibit very strong electrostatic built-in fields [4]. As a consequence the optical properties of nitride-based nanostructures are significantly modified by these contributions. For instance the built-in field gives rise to a strong spatial separation of electron and hole wave functions . Consequently, the optical recombination rate in these structures is drastically reduced.

Electron and hole charge densities in a c-plane GaN/AlN QD

To circumvent these problems, there has been a rapid increase in studies of non-polar growth of III-nitride structures, where the [0001] direction lies in the growth plane [5,6] Growth of quantum wells (QWs) along a non-polar direction can eliminate polarization-induced fields, and hence give improved radiative recombination rates [7]. However, and in contrast to the case in non-polar QWs, QDs are three dimensional objects, and therefore still retain facets along the [0001] direction, even when grown on a non-polar substrate. Therefore, it is not directly obvious how the polarization potential should behave in such a system.




External References

[1]  III–nitrides: Growth, characterization, and properties                                                                                                                                                  S. C. Jain, M. Willander and J. Narayan, Journal of Applied Physics, 87, 965 (2000),

[2]  Solid-State Lighting                                                                                                                                                                                                          C.J. Humphreys, MRS Bulletin, 33, 459 (2008)

[3]  A gallium nitride single-photon source operating at 200 K                                                                                                                                       S. Kako, C. Santori, K. Hoshino, S. Gtzinger, Y. Yamamoto and Y. Arakawa, Nature Materials, 5, 887 (2006)

[4]  Direct comparison of recombination dynamics in cubic and hexagonal GaN/AlN quantum dots                                                                      J. Simon, N. T. Pelekanos, C. Adelmann, E. Martinez-Guerrero, R. André, B. Daudin, Le Si Dang, and H. Mariette, Physical Review B, 68, 035312 (2003)

[5]  Growth of GaN quantum dots on nonpolar A -plane SiC by molecular-beam epitaxy                                                                                           S. Founta, F. Rol, E. Bellet-Amalric, E. Sarigiannidou, B. Gayral, C. Moisson, H. Mariette and B. Daudin, Physica Status Solidi B, 243, 3968 (2006)

[6]  Anisotropic strain relaxation in a-plane GaN quantum dots                                                                                                                                      S. Founta, J. Coraux, D. Jalabert, C. Bougerol, F. Rol, H. Mariette, H. Renevier, and B. Daudin, R. A. Oliver and C. J. Humphreys, T. C. Q. Noakes and P. Bailey, Journal of Applied Physics, 101, 063541 (2007)

[7]  Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes                                                                                   P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche and K. H. Ploog, Nature, 406, 865 (2000)

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