Low temperature growth of gallium oxide thin films via plasma enhanced atomic layer deposition

Abstract

Herein we describe an efficient low temperature (60–160 °C) plasma enhanced atomic layer deposition (PEALD) process for gallium oxide (Ga₂O₃) thin films using hexakis(dimethylamido)digallium [Ga(NMe₂)₃]₂ with oxygen (O₂) plasma on Si(100). The use of O₂ plasma was found to have a significant improvement on the growth rate and deposition temperature when compared to former Ga₂O₃ processes. The process yielded the second highest growth rates (1.5 Å per cycle) in terms of Ga₂O₃ ALD and the lowest temperature to date for the ALD growth of Ga₂O₃ and typical ALD characteristics were determined. From in situ quartz crystal microbalance (QCM) studies and ex situ ellipsometry measurements, it was deduced that the process is initially substrate-inhibited. Complementary analytical techniques were employed to investigate the crystallinity (grazing-incidence X-ray diffraction), composition (Rutherford backscattering analysis/nuclear reaction analysis/X-ray photoelectron spectroscopy), morphology (X-ray reflectivity/atomic force microscopy) which revealed the formation of amorphous, homogeneous and nearly stoichiometric Ga₂O₃ thin films of high purity (carbon and nitrogen <2 at.%) under optimised process conditions. Tauc plots obtained via UV-Vis spectroscopy yielded a band gap of 4.9 eV and the transmittance values were more than 80%. Upon annealing at 1000 °C, the transformation to oxygen rich polycrystalline β-gallium oxide took place, which also resulted in the densification and roughening of the layer, accompanied by a slight reduction in the band gap. This work outlines a fast and efficient method for the low temperature ALD growth of Ga₂O₃ thin films and provides the means to deposit Ga₂O₃ upon thermally sensitive polymers like polyethylene terephthalate.