Study of arsenic doping ZnO thin films grown by metal-organic chemical vapor deposition via x-ray photoelectron spectroscopy

Arsenic-doped ZnO (ZnO:As) films were grown by metal-organic chemical vapor deposition (MOCVD) on GaAs layers, which were deposited on sapphire substrates by sputtering using a GaAs target. The As doping was obtained by thermal diffusion. The ZnO: As films were annealed in nitrogen (ZnO:As:N-2) and oxygen (ZnO:As:O-2) atmosphere, respectively. X-ray photoelectron spectroscopy (XPS) measurements showed that annealing in oxygen facilitated even As doping in ZnO films and the content of As remains around 3.4% after Ar+ bombardment. However, annealing in N-2 leads to the aliquation of As at the surface for ZnO:As films. Core level results show that there are two chemical states of As in ZnO:As:O-2 films including As-Zn-2V(Zn) and As-Zn, while four types exist in ZnO:As:N-2 films including AsZn, As-Zn-2V(Zn), A(Si), and As-O. The contribution centered at around 43.9 eV of the As 3d peak is ascribed to As-Zn-2V(Zn). The valence band maximum (VBM) spectrum taken by ultraviolet photoelectron spectroscopy (UPS) indicates that the Fermi energy of ZnO:As:O-2 films shifts toward the VBM by 0.37 eV compared with undoped ZnO films, which proves As-Zn-2V(Zn) is an acceptor in ZnO:As:O-2 films, but the ZnO:As films still show n- type conductivity, which is possibly due to the compensation of As- related donor and native defects. Detailed analysis of the chemical states of As may help point toward paths for growing high-quality ZnO:As films.