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

摘要

Gallium oxide (Ga_2O_3) thin films were made by sputter deposition employing a Ga_2O_3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (T_s), from 25 to 600 ℃. 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 1145.0eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga~(3+)) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ～ 531 eV, which is also characteristic of Ga-O bonds in the Ga_2O_3 phase. The granular morphology of the nanocrystalline Ga_2O_3 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 Ga_2O_3 films increases with increasing T_s due to improved structural quality and packing density of the films. The n(λ) of all the Ga_2O_3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (～200 Ω-cm) for amorphous Ga_2O_3 films grown at T_s = RT-300℃ and decreased to ～1 Ω-cm for nanocrystalline Ga_2O_3 films grown at T_s ≥ 500-600 ℃. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga_2O_3 films is derived.