Raman study of the strain in GaN films by comparison with a free‐standing GaN

Abstract

Temperature-dependent Raman spectroscopy was used to analyze the effect of residual strain on optical phonons E2 (high) and A1 (LO) in high-quality GaN films grown on C-plane 6H-SiC, A- and C-plane sapphire substrates. By comparison with a free-standing GaN, we found that shifts in the Raman spectrum were consistent with compressive and tensile strains. Experimental results indicated that differences in the thermal expansion coefficients between GaN film and substrate have an effect on these frequency shifts. We also compared frequency shifts of the A1 (LO) mode with those of the E2 (high) mode and concluded that the strain-induced frequency shift of the A1 (LO) mode was smaller than that of the E2 (high) mode. The strain-tensor-dependences of each phonon mode are also discussed.