Raman Analysis of Electron-Phonon Interactions in GaN Films

Abstract

ABSTRACTRaman analysis of film quality, phonon-plasmon coupling, and phonon-exciton interaction in GaN films with varying Si doping levels is presented. The films exhibit a small stress component ∽0.1 GPa, calculated from the frequency shift of the E2 mode. No correlation between the stress and the doping concentration was found. No forbidden Raman lines were detected in the spectra, implying high quality oriented films. The Raman lineshape of the E2 mode is a Lorentzian with similar linewidths at room temperature and at 10K indicating a homogeneous lifetime broadening mechanism which is not significantly affected by the change in temperature. The linewidth is also independent of Si concentration. The phonon-plasmon mixed frequency modes were calculated to be at ω−.=86 cm−1 and at ω+=741 cm−1 The modes are not present in the spectra and the only effect of the plasmons is a change of the Al(LO) lineshape. Analysis of the A1(LO) line indicated a uniform spatial doping distribution. A resonance effect was observed for the symmetry-allowed A1(LO) mode at T=10K with sub bandgap excitation light. The resonance interaction is consistent with the free exciton model.