Amorphous GaN and GaN/AlN Superlattices

The vast majority of commercial semiconducting devices are based on silicon, but its band structure does not render it useful in any sophisticated opto-electronic applications. For these applications the materials of choice are the III-V compounds, particularly GaAs. The relatively small band gap of that material renders it useful only for red light, and so the recent development of opto-electronic devices (e.g. blue/green lasers and light emitting diodes) based on crystalline GaN and related compounds [1,2] has generated much excitement in the international community.[3]

Most of the work on semiconductors has concentrated on crystalline materials, although solar cells based on amorphous silicon (a-Si) have been on the market since the early eighties. Amorphous semiconductors have a number of potential advantages over their crystalline cousins, in particular their cheapness of manufacture and suitability for deposition on arbitrary substrates. However there are general limitations which have prevented large-scale adoption of amorphous semiconductors in applications:
a) localised states in the band gap, arising from imperfect bonding configurations, and 
b) limited electron mobility.

It is because of these ‘known’ limitations that interest has been excited by the publication of preliminary experimental [4,5] and theoretical[6] results showing that a-GaN has remarkable promise as an opto-electronic material. In particular, the absence of a low density of defect states in the band gap suggests that optical and electrical properties may be far superior to typical amorphous materials (e.g. a-Si, a-Ge). a-GaN is a wide band gap semiconductor and so, like its crystalline counterpart, it may be particularly interesting as an emitter of UV/blue/green light.

We intend to perform a detailed study of the growth of amorphous III-N semiconductors (a-GaN, the wider-band-gap a-AlN, and the narrower band gap a-InN) and their fundamental optical and electronic properties of both bulk materials and superlattices grown by a reactive evaporation technique. [7]

Iin collaboration with Prof Joe Trodahl at Victoria University of Wellington,  Dr Tony Bittar of Industrial Research, and Dr Andreas Markwitz at the Insitute of Geological and Nuclear Sciences,  we have recently received funding for this project from NERF.

References

  1. H. Morkoc et al, J. Appl. Phys. 76, 1363 (1994).
  2. S. Nakamura et al, Jap. J. Appl. Phys 34, L1332 (1995); ibid35, L74 (1996); ibid 35, L217 (1996); Appl. Phys. Lett. 68, 2105 (1996); ibid 68, 3269 (1996).
  3. The level of interest, range of applications and number of interesting physical problems can be appreciated from the enormous volume: Mat. Res. Soc. Symp. Proc. 449, (1997)
  4. S. Nonomura et al, J. Non-Cryst. Solids 198-200, 174 (1996).
  5. T. Yaji et al, p911 in ‘Silicon Carbide and related materials’, IOP Publishing, (1995).
  6. P. Stumm and D. Drabold, Phys. Rev. Lett. 79, 677 (1997).
  7. See, for example, G. Williams, A. Bittar and H. J. Trodahl, J. Appl. Phys. 64, 5148 (1988).