Effect of epitaxial growth on electrical properties of Ga-doped ZnO thin films

The effect of epitaxial growth on the electrical properties of Ga-doped ZnO (GZO) thin films was studied. GZO thin films were grown on Al2O3 (0001) substrates using pulsed laser deposition (PLD) at various substrate temperatures. Pole figure measurement revealed that the GZO films were grown epitaxially with a 30° in-plane rotation relative to Al2O3 (GZO [112¯0]||Al2O3 [011¯0]) to reduce the lattice mismatch. An interesting difference in the variation trend of the resistivity by substrate temperature compared with that of the polycrystalline GZO films, which were prepared for comparison, was observed. The resistivity of the polycrystalline GZO decreased at 200 °C, decreased slightly more at 400 °C, and abruptly increased at 600 °C. However, the resistivity of the epitaxial GZO decreased sharply at up to 400 °C, and decreased continuously at 600 °C. Consequentially, much lower resistivities were obtained at high temperatures of 400 and 600 °C. Considering little difference in the rate of decrease in the carrier concentration between the two kinds of films at each temperature, this different tendency in the resistivity can be explained by the fact that the rate of increase in the Hall mobility of the epitaxial GZO at above 200 °C is much higher than that of the polycrystalline GZO. The higher Hall mobility of the epitaxial GZO is attributed to high crystallinity caused by the epitaxial growth that can induce lower grain boundary scattering. It was proved by using an atomic force microscope (AFM) and a high resolution X-ray diffractometer (HRXRD) that the epitaxial GZO grown at higher temperature showed larger grain size and higher crystallinity, that is, the grain boundary scattering can decrease more, and the Hall mobility can increase enough to compensate for the decrease in the carrier concentration even at high temperature differently from the polycrystalline GZO films.