Micro-Raman scattering and microphotoluminescence of GaN thin films grown on sapphire by metal-organic chemical vapor deposition

Abstract

Microscale optical spectroscopic probe techniques, mainly micro-Raman scattering (μ-RS) and microphotoluminescence (μ-PL) spectroscopy, are employed to study a variety of GaN thin film materials, undoped and Si and Mg doped, grown on c-sapphire substrates by low-pressure metal-organic chemical vapor deposition. The optical properties and correlated structural features of these epitaxial materials grown under different conditions are studied. Visible micro-Raman measurements show different mode behaviors under different laser incidence configurations. Raman line shape analysis through two sets of equations leads to the monitoring of the sample quality and stress, and the optical determination of electrical properties of GaN films. An approximate 3 cm - 1 shift in the E 1 (LO) mode frequency is observed across the GaN film from the sapphire/GaN interface to the GaN surface, indicating the spatial variation of strains along the film growth direction within a few micrometers thickness range. Combined Raman and PL spectra are obtained under UV excitation, and interesting high-order resonance Raman features are observed using a sensitive UV-Raman-PL microscope. A new way to characterize the p-type GaN is introduced. These fruitful results demonstrate the capabilities of μ-RS-PL technology for the research and development of GaN and other materials.