Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (T-s), from 25 to 600 degrees C. The effect of T-s on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p(3/2) and Ga 2p(1/2) peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE similar to 531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing T-s. The SE analyses indicate that the index of refraction (n) of Ga2O3 films increases with increasing T-s due to improved structural quality and packing density of the films. The n(lambda) of all the Ga2O3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (similar to 200 Omega-cm) for amorphous Ga2O3 films grown at T-s = RT-300 degrees C and decreased to similar to 1 Omega-cm for nanocrystalline Ga2O3 films grown at T-s >= 500-600 degrees C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga2O3 films is derived. (C) 2014 AIP Publishing LLC.