Effects of the Post-Annealing Treatment on the Properties of Ga-Doped SnO_x Thin Films

We investigated the effects of the post-annealing treatment (PAT) on the properties of Ga-doped tin-oxide (Ga-SnO_x) thin films grown at room temperature by using a radio-frequency magnetron sputtering technique. On the basis of X-ray photoelectron spectroscopy (XPS), dynamic secondaryion mass spectrometry, X-ray diffraction (XRD), and Hall Effect measurements, we conclude that n-type SnO₂ is the dominant phase in all samples regardless of PAT at low temperatures (25–200 °C). The Sn²⁺ area decreased to 32.5% with increasing temperature up to 150 °C, with a simultaneous increase in the Sn⁴⁺ area to 59%. This was attributed to a decrease and an increase in the Ga and the oxygen contents in the samples, respectively, which also caused a decrease in the number of oxygen vacancies in the samples treated at higher temperatures. In contrast, XPS on the samples post-annealed at temperatures higher than 150 °C showed results opposite to those of the samples treated at temperatures lower than 150 °C. This indicates that the Ga ions in Ga-doped SnO_x films act as hole acceptors and that heat treatment is useful for controlling the number of oxygen vacancies, Sn²⁺ ions, and Sn⁴⁺ ions in Ga-doped SnO_x films. In addition, XRD showed that post-annealing did not affect the amorphous phase in the samples.