Evidence for compositional inhomogeneity in low In content (InGa)N obtained by resonant Raman scattering

Abstract

Resonant Raman scattering has been used to study hexagonal InxGa1−xN films with x≈0.1, grown by metal–organic chemical vapor deposition on sapphire substrates. To vary the energy difference between the fundamental gap energy of the (InGa)N and the photon energy of the discrete laser emission lines used for recording the spectra, the sample temperature was varied between 300 and 870 K. Raman scattering by the (InGa)N A1(LO) phonon shows a clear resonance profile when the fundamental energy gap approaches the incident photon energy, with a maximum enhancement in scattering efficiency of 10 measured relative to the scattering strength of the E2 phonon mode. The (InGa)N A1(LO) phonon was found to shift to higher frequencies with respect to the E2 mode when the experimental conditions were varied from excitation below the fundamental energy gap of (InGa)N to above-band-gap excitation. This frequency shift is explained by the presence of compositional inhomogeneity, which results in localized regions with higher In content, and thus, lower gap energy and phonon frequency, and regions with lower In content, and consequently, higher gap energy and phonon frequency.