The role of Ga partial substitution for Al in the enhanced conductivity of transparent AZO thin film

Al-doped ZnO transparent films (AZO) with Ga partial substitution for Al were deposited on quartz at room temperature using radio frequency magnetron sputtering method. The film structure is characterized by ZnO hexagonal wurtzite phase with (002) preferred orientation, and the crystalline quality is improved with increasing Ga/Al ratio from 0 to 3.5. The Ga partial substitution leads to a great decrease in the room-temperature resistivity of AZO from 3.3 × 10−3 to 3.2 × 10−4 Ω cm, which is attributed to the increase of mobility from 2.1 to 8.8 cm2/V·s along with the large carrier concentrations in the magnitude of 1020∼1021cm−3. Combined with X-ray photoelectron spectroscopy analysis, it indicates that the addition of Ga is more effective as donors and causes less strain with respect to that of Al. Temperature dependent conductivity has been studied in the range of 80-320 K. The conductivity of the films with Ga/Al ≤ 2 demonstrates a linear relationship with temperature, which is rationalized by the weak localization mode arising from the constructive interference of scattering electrons. As the Ga/Al ratio increases to 3.5, the transport behavior turns to be metallic characterization at 255 K. This work demonstrates that the Ga partial substitution can be an effective way for obtaining AZO films having degenerate semiconducting behavior with a low resistivity in the magnitude of ∼10−4 Ω cm.