OPTICAL STUDIES OF NANOCRYSTALLINE GAAS - A REVIEW

The optical evidence that ion-implanted GaAs materials are nanocomposites of amorphous GaAs (a-GaAs) and nanocrystalline GaAs (mu-c-GaAs) is reviewed and analyzed. A key role in this assessment is played by the observation of Raman-scattering two-mode behavior for this family of materials, which provides direct evidence for the heterogeneous microstructure and which also clearly demonstrates that a-GaAs is not the L --> 0 limit of mu-c-GaAs (L is the characteristic nanocrystal size). The latter point constitutes an unusually striking contradiction of the microcrystalline model for a-GaAs, i.e., it provides a striking affirmation of the continuous-random-network model for the structure of amorphous semiconductors. Finite-size effects observed in the mu-c-GaAs Raman spectrum enable an estimate for L, and optically deduced estimates of L and of the amorphous/nanocrystalline volume ratio are given for a variety of implants. Finite-size effects observed for electronic interband excitations are also discussed and interpreted.