Preparation of Low-Resistivity Ga-Doped ZnO Epitaxial Films from Aqueous Solution Using Flow Reactor

Abstract

An aqueous process based on a unique flow-reactor design was developed for the preparation of gallium-doped ZnO (ZnO:Ga) epitaxial films with a low electrical resistivity. In this process, a ZnO:Ga film was grown on a ZnO-seeded sapphire substrate heated at 80°C under a constant flow of a reaction solution. The Ga content of the resulting films was found to increase in relation to the concentration of GaCl3 used–0 to 9 mM GaCl3–resulting in epitaxial growth of ZnO containing 0–5% Ga, whereas a polycrystalline ZnO film was produced with 10 mM GaCl3. The electrical resistivity of the as-grown ZnO:Ga films varied from 0.2 to 2 Ω cm, but was reduced by two to three orders of magnitude after the films were annealed in air at 300°C. Thus, the lowest resistivity of 7 × 10–4 Ω cm was obtained with an annealed film containing 2.5% Ga, whose carrier concentration and mobility were 7 × 1020 cm–3 and 13 cm2 V–1 s–1, respectively. Furthermore, even though the non-doped ZnO film was rendered translucent by annealing, ZnO:Ga films with 1.8–4.0% Ga still exhibited transmittance as high as ∼80% in the visible spectrum.