Effect of Change in Valence State of Ga During Annealing on the Structural, Optical, and Electrical Properties of GZO Crystals

In this work, the ZnO:Ga (GZO) single crystals are grown by the chemical vapor transport (CVT) method. The as‐grown crystals are annealed under an oxygen atmosphere at different temperatures. The GZO crystal's structure and its optical and electrical properties are characterized by X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, X‐ray diffraction (XRD), UV‐VIS spectrophotometry, and variable‐temperature Hall‐effect measurement. The XPS results indicate that the valence states of the majority of gallium atoms in the GZO crystals undergo transition from the metallic (Ga0) to the non‐metallic state (Gax+) with increasing annealing temperature. The Raman and XRD results show that the compressive stress along the biaxial c‐axis on GZO crystals increases gradually with annealing temperature. Meanwhile, the GZO crystal's transmittance within the range of 300 to 1000 nm is improved significantly from being opaque to about 57%. The GZO crystals exhibit a decrease in free‐carrier concentration (1020–1019 cm−3), an increase in carrier mobility (77.8–87.9 cm2/V−1s−1) and resistivity (10−4–10−2 Ω·cm). The annealed GZO crystal's carrier concentration is practically independent of temperature (90–300 K). These results show that the free‐carrier concentrations are affected by the change of valence states in gallium atoms present in the GZO crystal.