High-rate and low-temperature growth of ZnO:Ga thin films by steered cathodic arc plasma evaporation

Ga-doped ZnO (GZO) thin films with various thicknesses (120–520 nm) are deposited on the glass substrate at a high growth rate of 220 nm/min and a low temperature of 120 °C by a steered cathodic arc plasma evaporation (steered CAPE). The growth mechanism, microstructure, residual stress, surface morphology, electrical and optical properties, chemical states, electron transport behaviors and thickness effect of the GZO films are investigated. The film stress is gradually relaxed from −0.516 GPa to −0.090 GPa with thickness increasing. Transmission electron microscopy (TEM) images show that the GZO microstructure consists of c-axis textured columnar grains accompanied by some embedded nanodroplets. The droplet size is significantly reduced when a high-melting-point (1975 °C) GZO ceramic target is adopted. High-resolution TEM image shows the GZO crystallites nucleated directly onto the amorphous substrate. The electrical properties improve with increasing thickness. The lowest resistivity (4.72 × 10−4 Ω cm) is achieved at the thickness of 520 nm, with a corresponding transmittance of 89% in the visible region. Temperature-dependent resistivity measurements show that metal-semiconductor transition temperature increases from 136 K to 225 K when decreasing the thickness, which is due to the increasing the localized states caused by the defects and chemisorbed oxygen in thinner film.