Co-doping of aluminium and gallium with nitrogen in ZnO films deposited by RF magnetron sputtering

N, (N + Ga) and (N + Al) doped ZnO films were deposited on c-plane sapphire substrates by RF magnetron sputtering at room temperature. The samples were characterized by their structural, surface morphological, compositional and optical properties. The x-ray diffraction studies confirmed the co-doping of (N + Ga) and (N + Al) besides showing improvement in the crystallinity when compared with the single N doping. The surface of the films becomes rougher after co-doping. The x-ray photoelectron spectroscopy and Rutherford back-scattering analysis indicate that the co-doping changes the chemical states and varies the amount of nitrogen (N) in ZnO. The amount of 'N' has been greatly increased for (N + Ga) co-doping, indicating that it is the best co-doping pair for p-type ZnO. Additionally, co-doping has increased the average visible transmittance (40-650 nm) and the optical band gap is shifted towards shorter wavelength. In the case of (N + Al) co-doping, the band gap becomes wider than that of undoped ZnO.