Nondestructive Characterization of GaN Films Grown at Low and High Temperatures

Abstract

GaN films grown on GaAs and sapphire substrates by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) at both low and high temperatures (LT and HT) were characterized by Raman scattering and variable angle spectroscopic ellipsometry (VASE). Optical phonon spectra of GaN films are obtained through back-scattering geometry. Crystal quality of these films was qualitatively examined using phonon line-width. Phonon spectra showed that the HT GaN has wurtzite crystal structure, while LT GaN and GaN/GaAs have cubic-like structures. Thickness nonuniformity and defect-related absorption can be characterized by pseudo dielectric functions directly. Surface roughness also can be determined by using an effective-medium approximation (EMA) over-layer in a VASE analysis. Anisotropic optical constants of GaN, both ordinary and extraordinary, were obtained in the spectral range of 0.75 to 6.5 eV with the consideration of surface roughness, through the small and large angles of incidence, respectively. The film thickness of the GaN was accurately determined via the analysis as well.