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Optical and structural studies of compositional inhomogeneity in strain-relaxed indium gallium nitride films
2000
The structural and optical properties of indium gallium nitride (In/sub x/Ga/sub 1-x/N) films with 0.04extended X-ray absorption fine structure (EXAFS), optical transmittance, and cathodoluminescence (CL) spectroscopies. The average indium fraction (x/sub 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/sub avg/. A small amount of a high-indium (x/spl ap/0.99) phase was observed in films with x/sub avg//spl ges/0.44. From EXAFS, the composition of the indium second-neighbor shell is equal (within the 2/spl sigma/ measurement uncertainty) to x/sub 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/sub avg/ with a second-order (bowing) parameter of (-4.57/spl plusmn/0.75) eV. The magnitude of the composition fluctuations in the alloy films, denoted /spl Delta/x, 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 /spl Delta/x give similar results.
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