Optical investigations on the wide bandgap semiconductors diamond and aluminumnitride

Abstract

New results about optical defects and intrinsic properties of diamond, AlN and AlGaN alloys have been obtained. Different aspects of intentional and background doping of diamond were discussed. In phosphorus-doped CVD diamonds a novel spectrum only 26 meV below the optical gap has been studied. This is the highest energy luminescence ever reported in diamond, consisting of a triplet structure characteristic of isoelectronic bound excitons. High excitation PL experiments on diamond led to the first observation of a condensed electron-hole liquid phase in diamond. The study of this liquid represents a major part of the thesis. From a lineshape analysis of the temperature-dependent EHD luminescence spectra, the exciton-versus-liquid phase diagram was deduced. Several important parameters like the critical density, the critical temperature, and the low-temperature density inside the drops were evaluated. Cathodoluminescence investigations on epitaxial wurtzite AlN layers grown on sapphire, SiC, and Si substrates, have shown that although the material is generally of good optical quality, deep level luminescence are still dominating the spectra. Temperature dependent CL measurements and numerical line decompositions reveal complicated substructures in the excitonic lines. The temperature dependence of the energy positions and broadening parameters of the transition have been studied and compared with the other materials. Epitaxial AlGaN/AlN heterostructures grown on sapphire, and SiC substrates were investigated by cathodoluminescence at 10 K. The energy positions of the near band-edge transitions follow the Al mole fractions of the solid solution with an evident bowing whose numerical value was determined. The alloy broadening of the excitonic resonances was discussed in terms of the statistical disorder of a random alloy.