Indium content determination related with structural and optical properties of InGaN layers

Abstract

We studied the structural and optical properties of a set of nominally undoped epitaxial single layers of InxGa1−xN (0<x⩽0.2) grown by MOCVD on top of GaN/Al2O3 substrates. A comparison of composition values obtained for thin (tens of nanometers) and thick (≈0.5 μm) layers by different analytical methods was performed. It is shown that the indium mole fraction determined by X-ray diffraction, measuring only one lattice parameter strongly depend on the assumptions made about strain, usually full relaxation or pseudomorphic growth. The results attained under such approximations are compared with the value of indium content derived from Rutherford backscattering spectrometry (RBS). It is shown that significant inaccuracies may arise when strain in InxGa1−xN/GaN heterostructures is not properly taken into account. Interpretation of these findings, together with the different criteria used to define the optical bandgap of InxGa1−xN layers, may explain the wide dispersion of bowing parameters found in the literature. Our results indicate a linear, Eg(x)=3.42−3.86x eV (x⩽0.2), “anomalous” dependence of the optical bandgap at room temperature with In content for InxGa1−xN single layers.