Raman spectroscopy study of wurtzite crystals: Gallium nitride and zinc oxide

Raman spectroscopic techniques have been used to study wurtzite crystals gallium nitride (GaN) and zinc oxide (ZnO). These semiconductors have attracted considerable interest in the past few years for their applications in optoelectronics and high-temperature microelectronics. The sensitivity of Raman spectroscopy to lattice strain and doping have motivated several studies of these materials, particularly GaN and its alloys with indium nitride and aluminum nitride. However, results from different authors are inconsistent, and the theoretical interpretation of the data is questionable.; Much of the experimental work discussed in this dissertation was performed with a special high-resolution Raman instrument that makes it possible to obtain accurate line shapes and peak positions. This capability, combined with a theoretical analysis that takes into account the special symmetry properties of wurtzite crystals and utilizes results from state-of-the-art ab initio calculations, has lead to a much better understanding of lattice dynamics in GaN and ZnO and to the resolution of many of the puzzles in the literature. Highlights of the work presented here are the explanation of the anomalously large linewidth of the A₁ longitudinal optical phonon in GaN, the discovery and explanation of dispersive effects in the phonon self-energy in GaN and ZnO and a critical study of the reliability of Raman spectroscopy for the determination of doping levels and mobilities in GaN films.