Optical Studies of Wide Bandgap Semiconductors: Diamond, Gallium Nitride, and Boron Nitride.

Semiconductors such as diamond, gallium nitride and boron nitride which are characterized by large energy gaps are currently of great technological interest for novel opto-electronic applications. While the interaction of light with such "wide bandgap semiconductors" is uniformly restricted to the ultraviolet, details of the absorption and emission of light by point defects encountered in these materials vary enormously. This is the result of the remarkable variations possible in their point defects, particularly point defects involving one or more impurity atoms. In this thesis, original results are presented to explain prominent luminescence features from thin films of diamond and III-V nitride semiconductors for the first time. In CVD diamond, the dipole orientation and symmetry of a ubiquitous defect unique to CVD diamond characterized by strong emission at 738 nm is determined unequivocally to be a <110> -oriented defect with the optical transition moment oriented along the <110 >-axis. These results represent the first site symmetry analysis of a point defect in CVD diamond. In cubic GaN (c-GaN), meaningful results include the determination of the free exciton recombination energy to be 3.376 eV at 6.5 K, permitting a precise value of the c-GaN energy gap of 3.406 +/-.015 eV to be derived. In addition, a strong 6.5 K luminescence feature observed at 3.366 eV is identified with donor-bound exciton recombination. Finally, a donor-acceptor pair recombination feature at 3.31 eV accompanied by phonon sidebands from coupling to TO phonons is identified. This work constitutes the first reported detailed spectroscopic study of near-edge emission from c-GaN. Significant results of cathodoluminescence studies of boron nitride (BN) films include the first reported observations of near bandedge emission from a hexagonal BN (h-BN) film in the range from 5.5 eV to 4.90 eV. Observed emission features are attributed to recombination processes involving shallow donors and deep acceptors. This work provides a solid base for future optical studies and guidance for the design of opto-electronic devices in these wide bandgap semiconductors.