Optical and structural studies of compositional inhomogeneity in strain-relaxed indium gallium nitride films

The structural and optical properties of indium gallium nitride (In_xGa_1-xN) films with 0.04<x<0.47, grown by metal-organic chemical vapor deposition, were examined by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), optical transmittance, and cathodoluminescence (CL) spectroscopies. The average indium fraction (x_avg) was measured by wavelength-dispersive X-ray spectroscopy in an electron-probe microanalyzer (WDS/EPMA). Pure GaN and InN films were also characterized. Comparison of the 0006 XRD data with the WDS data shows that the lattice constant c is a linear function of x_avg. A small amount of a high-indium (x≈0.99) phase was observed in films with x_avg⩾0.44. From EXAFS, the composition of the indium second-neighbor shell is equal (within the 2σ measurement uncertainty) to x_avg. These results are consistent with a random-alloy structure. The optical band gap, determined from the position of the absorption edge in the transmittance spectrum, and also from the highest-energy CL peak, is described well by a quadratic function of x_avg with a second-order (bowing) parameter of (-4.57±0.75) eV. The magnitude of the composition fluctuations in the alloy films, denoted Δx, was estimated by analyzing the full widths at half maximum (FWHMs) of the XRD peaks and also the band-edge CL peaks. The two calculations of Δx give similar results