Raman scattering study of compositional homogeneity in InGaN ternary compounds

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We have studied theoretically and quantitatively the compositional homogeneity of In_xGa_<1-x>N ternary compounds directly grown on sapphire substrates with the aid of low-temperature-grown AIN buffer layer by MOVPE method. First, we found that Ga-N and the In-N bond.length in InGaN kept the bond length of pure GaN and InN from thermo-dynamical analyses even though the In molar fraction x is increasing. Therefore, it suggested that the mixed crystal of InN and GaN did not be homogeneous. Next, we evaluated the macroscopic nature of InxGal-xN by using the SEM image and the TED image, As the results, the whole samples were shown as the thick single crystals. Also, we found the enhancement of the broadening of the PL spectrum as increasing of x. Finally, we analyzed the asymmetric broadening of the E_2 phonon mode of the Raman spectra with the aid of the Spatial Correlation model. That is enhanced in the region of immiscibility suggested theoretically by increasing the indium molar fraction. The correlation length, which corresponds to the decay length of E_2 phonon mode, was estimated. It was on the order of about lOnm, which is on the same order magnitude as the size of the columnar structure suggested by TEM analyses by Kawaguchi et al.[J. Cryst. Growth 189/190 (1998) 24]. It showed a good agreement for applying to the dispersion relation given by Azuhata et al.[Physica B 219/220 (1996) 493]. Furthermore, we tried to analyze the E_<2iow> phonon mode which was strongly reflected the compositional homogeneity of the samples, however, we could not observe. This reason is that the mode has the low density of states and the existence of the eigen-energy around the excited light source.