Surface and optical properties of indium-rich InGaN layers grown on sapphire by migration-enhanced plasma assisted metal organic chemical vapor deposition

In-rich InGaN nanoscale thin films grown on (0001) sapphire (Al₂O₃) using migration enhanced plasma-assisted metal-organic chemical vapor deposition (MEPA-MOCVD) method are characterized by high-resolution x-ray diffraction (HR-XRD), x-ray photoelectron spectroscopy (XPS), Raman scattering spectroscopy (RSS), as well as variable angle spectroscopic ellipsometry (VASE). Theoretical calculations were performed to acquire the optical constants and vibrational properties of the films. The indium composition x (In) values determined by HR-XRD and from the surface area calculated by XPS are in good agreement with each other. The valence band maxima of In_xGa_1−xN films (x = 0.65–0.80) were observed to shift to lower binding energy with the increase of indium concentration. The RSS results confirmed the one-phonon behavior for the A₁(LO) and E₂(high) modes with x (In)—independent of phonon frequencies of the substrate. The simulated refractive index n was found to increase with In composition, the band gap (E_g) shifted, however, towards the lower energy side. For a given In composition and by increasing the measurement temperature from 30 °C to 600 °C, the variations of optical constants (n and E_g) showed similar trends as in samples with increasing In composition. (paper)