GaN layer growth in relation to buffer deposition temperature

Drastic changes in Raman spectra from GaN epitaxial layers are shown to depend on the GaN buffer layer deposition temperature: for temperatures higher than 600 °C, non intentional n-doping is evidenced by the screening of the allowed A1(LO) phonon by free carriers. Raman measurements at liquid nitrogen temperature confirm this interpretation and speak in favor of degenerate carrier gas. Partial screening of phonons with wave-vectors differing from the q = 0 transfer of incident and scattered photons is invoked to explain LO-like scattering over the whole spectral range of optical phonons. Defects correlated to three-dimensional growth and to non-radiative recombination processes in the layers are proposed as the origin of heavy n-doping and of the wave-vector non-conservation.