Raman scattering study of compositional homogeneity in InGaN ternary compounds

本文从理论上和定量上研究了<1-x>用MOVPE法在蓝宝石衬底上直接生长In_xGa_ N三元化合物的组分均匀性。首先，我们从热力学分析中发现，即使In的摩尔分数x增加，InGaN中的Ga-N和In-N键长也保持了纯GaN和InN的键长。因此，这表明InN和GaN的混合晶体不是均匀的。接下来，我们通过使用SEM图像和TED图像来评估InxGal-xN的宏观性质，作为结果，整个样品显示为厚的单晶。另外，我们发现随着x的增加，PL谱的展宽增强。最后，利用空间相关模型分析了拉曼谱E_2声子模的非对称展宽。这是增强在理论上提出的不溶性区域通过增加铟的摩尔分数。计算了E_2声子模的关联长度。它在大约10 nm的数量级上，这与川口等人通过TEM分析提出的柱状结构的尺寸在同一数量级上。[J.晶体生长189/190（1998）24]。它显示了一个很好的协议适用于由Azuhata等人给出的色散关系。[Physica B 219/220（1996）493]。此外，我们试图分析<2iow>强烈反映样品成分均匀性的E_声子模，但我们无法观察到。这是因为该模式的态密度较低，且在激发光源附近存在本征能。

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.