| Summary: | GaN and related materials are of paramount interest because of the possible applications for optoelectronic devices operating in the blue and UV range. GaN can be grown in a stable wurtzite and a metastable cubic phase, which has attracted attention only recently because it is much more difficult to grow. However, theoretical calculations predict essential advantages of the cubic phase. In this work the optical properties of both, cubic and hexagonal GaN were analysed and compared. Raman spectroscopy was used to characterize the quality and structural purity of the samples. The individual optical transitions occurring in both phases were analysed with photoluminescence and reflectance spectroscopy. The influence of temperature, excitation intensity and delay time after pulse excitation were used to clarify the nature of the transitions. Some of the transitions were already discussed in literature, however in most of the cases there is not a commonly accepted interpretation. Therefore, in addition to completely new transitions, already discussed transitions were studied to achieve a complete picture of the optical processes. Since most of the transitions that appear in semiconductors are related to the band gap, the temperature dependency of the gap was determined via the free exciton resonances. From the broadening of the resonances it could be clearly shown that coupling to both, optical and acoustic phonons, is present, in contrast to reports in literature. The emission spectra of GaN are often dominated by transitions related to either intrinsic or extrinsic impurities. Si and Mg are used to achieve n-type and p-type conductivity in GaN, respectively. Si-doping leads in both phases to broad near-band-edge emissions which can be ascribed to transitions from a Si-donor band to the valence band. The donor band develops from a Si donor level with increasing Si concentration. The temperature behaviour of the emission shows that bandgap-renormalization is very important. Also doping with Mg leads in both phases to similar emissions. Slight Mg-doping introduces an acceptor level that is the origin of a free-to-bound emission. Heavy Mgdoping creates in addition to a rather shallow acceptor level compensating deep donor levels. The photoluminescence spectra are then dominated by broad emission bands, which are composed of various donor-acceptor-pair transitions. In almost all non-intentionally-doped samples an emission occurs at round 2.2eV. This so-called yellow band shows clearly donoracceptor- pair behaviour, although in one sample additional levels can change the recombination kinetics of the emission. The emission is suppressed in p-type doped hexagonal which shows residual p-type conductivity. The formation energy of one of the defects involved in yellow band emission seems to be very high in p-type material. Oxygen doping introduces a donor level in hexagonal GaN which gives rise to a donorbound- exciton and a bound-to-fee emission. At the moment the structural quality of hexagonal GaN is much higher and the emissions related to the band gap are therefore much straighter. However with the theoretical advantages of the cubic phase in mind it will be only a matter of time until the quality of cubic GaN will equal the quality of hexagonal GaN.
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