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

Abstract

Ga-doped ZnO (GZO) thin films with various thicknesses (120–520nm) are deposited on the glass substrate at a high growth rate of 220nm/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 −0.516GPa to −0.090GPa 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 (4.72×10−4Ωcm) is achieved at the thickness of 520nm, with a corresponding transmittance of 89% in the visible region. Temperature-dependent resistivity measurements show that metal-semiconductor transition temperature increases from 136K to 225K when decreasing the thickness, which is due to the increasing the localized states caused by the defects and chemisorbed oxygen in thinner film.