Structural, optical and electrical properties of aluminum doped ZnO films annealed in air and hydrogen atmosphere

Aluminum doped zinc oxide (AZO) films deposited by ion beam co-sputtering system were investigated after annealing. Films with ca. 200 nm thickness and 0.7 at.% Al were examined after 100, 200, 300, and 400 °C annealing in air and hydrogen atmosphere. The structural, optical and electrical properties of the films were measured ex situ by XRD, optical microscope, XPS, spectrometer, and Hall measurement with the Van der Pauw method, respectively. Our previous studies found that the resistivity of the film was closely related to the intensity of the (002) XRD peak, d-spacing, grain size, mean free path of the charge carrier, concentration of the carrier, and compressive stress which was released after 400 °C annealing in air. Moreover, the optical band gap was clearly correlated to the concentration of the charge carrier. Films after being annealed in air show that the release of the compressive stress decreased the grain size. After 400 °C annealing, surface cracks and the reduction of the oxygen vacancy and zinc interstitial were observed. All the observed phenomena result in the increase of the resistivity, the reduction of the transmittance, and a ca. 0.4 eV drop in optical band gap. However, films after being annealed in hydrogen atmosphere show not much change in the structure which attributed to the little variation of the optical transmittance and optical band gap, and the resistivity of the films. It clearly shows that the annealing in hydrogen is a much better way than in air to anneal the AZO films.