The impact of Ga and In co-doping on structural, optical, and electrical characteristics of Mg0.20Zn0.80O films for TCO applications

Abstract

Mg_xZn₁₋_xO alloys are important semiconductors for ultraviolet (UV) devices. However, a key limitation to the application of Mg_xZn₁₋_xO films is their insufficient electrical conductivity. In this work, we investigate the synergistic effect of co-doping with two different donor elements on the electrical conductivity of Mg_0.20Zn_0.80O films, along with the impact of this co-doping on structural, photoluminescence and optical characteristics. The Mg_0.20Zn_0.80O films grown by spray pyrolysis on quartz and (100)-oriented p-type monocrystalline Si substrates and co-doped with a fixed In concentration (2 at%) and varying Ga concentrations (0.5–2.5 at%) were investigated. The films exhibit a wurtzite crystal structure, the transmittance of 80–95% in the spectral range 300–800 nm, and the well-defined absorption edges. It was found that co-doping with Ga and In, whose ionic radii are smaller (Ga: 0.64 Å) and larger (In: 0.80 Å), respectively, than those of Mg (0.72 Å) and Zn (0.74 Å), shifts the onset of the self-compensation process toward higher donor concentrations and significantly reduces the electrical resistivity of the films to 7.65 × 10⁻⁴ Ω·cm. Simultaneously, this co-doping induces the structural reorientation of the grains in the films and increases the intensity of the near band edge (NBE) photoluminescence. The obtained Mg_0.20Zn_0.80O:Ga:In films are promising transparent conducting oxides (TCOs) for ZnO-based optoelectronic and photoelectronic devices, including displays, touch panels, photodiodes, solar cells and other transparent electronic applications.