Ellipsometric investigation of optical constant and band gap of Ga1−xInxN/GaN (x≤0.12) heterostructures

Optical properties of Ga1−xInxN/GaN (x≤0.12) heterostructures grown on (0001)-oriented Al2O3 substrates by metalorganic chemical vapor deposition were investigated using spectroscopic ellipsometry at room temperature. The thicknesses of the Ga1−xInxN layers and the GaN buffer layers were about 40 and 1200 nm, respectively. Optical transparency of the heterostructures induces strong interference oscillations below 3.4 eV in the measured pseudodielectric function spectra. Theoretical multilayer calculations were performed for the heterostructures to estimate the optical constants and the band gap energies of the Ga1−xInxN layers. The calculated spectra with Sellmeier-type optical constants for the transparent region of the Ga1−xInxN layers show good agreement with the measured pseudodielectric function spectra. The real part of the dielectric constant of the Ga1−xInxN films obtained from the calculations is found to increase with the In composition in the transparent region. The estimated optical band gap (E0) energy of the films shifts to lower energies as the In composition increases and evolves in good agreement with optical absorption and photoreflectance results on strained Ga1−xInxN alloys.