Raman Scattering Study of In_xGa_1−xN Alloys with Low Indium Compositions

In_xGa_1−xN alloys with low indium composition x in the range 0.13≤x≤0.23 are systematically studied mainly based on a Raman scattering technique. Scanning electron microscopy and x−ray diffraction results show that our samples can be divided into two groups: pseudomorphic (0.13≤x≤0.16) and relaxed (0.18≤x≤0.23). The prominent enhancement of A₁ longitudinal−optical (LO) mode is found with 325 nm laser excitation. For pseudomorphic samples, the frequencies of A₁ (LO) phonons agree well with the theoretical predictions, which verifies that the samples are fully strained. For relaxed In_xGa_1−xN samples, a linear dependence of the A₁ (LO) mode frequency is obtained: Ω₀ (x)=(740.8±3.3)−(143.1±16.0)x, which is the evidence of one-mode behavior in In_xGa_1−xN ternary alloys. Residual strains in these partially relaxed samples are also evaluated.