Defect evolution in ZnO nanocrystal films at doping by group IIIA elements

The variation of the optical, structural, and electrical parameters of the ZnO nanocrystal films versus Ga or In atom contents has been investigated. ZnO films were prepared by ultrasonic spray pyrolysis with different concentrations of Ga or In atoms. The films have been studied by means of energy dispersive X-ray spectroscopy, scanning electron microscopy, photoluminescence X-ray diffraction, as well as X-ray photoelectronic spectroscopy. Three stages in the process of ZnO parameter variation with donor doping levels were revealed. In the first doping stage, the Ga (In) ions are located on the Zn sites of the ZnO crystal lattice and the substitutional donor defects are formed. Simultaneously, the ZnO crystallinity improves, the PL intensity of near band edge (NBE) emission and the bandgap energy of ZnO increase, and the electrical resistivity decreases significantly. In the second stage, the increase in Ga (>1.5 at%) or In (>1.0 at%) contents stimulates the initiation of the self-compensating process. This process manifests itself by appearing in an NBE emission band, related to electron-hole recombination through Ga (In) donor-acceptor pairs (DAPs). The appearance of DAPs is a “fingerprint” of a self-compensating process. Simultaneously, the effective charges of the Ga (In) ions decrease and the rate of reduction in electrical resistivity slows. At the highest Ga (In) contents (>4.0 at% Ga or>2.5 at% In), Ga (In) nanoclusters appear in the ZnO films. The effective charges of Ga(In) ions continue to decrease, the PL intensity of the NBE band and ZnO crystallinity downturn, the crystal lattice parameters of ZnO increase significantly, but the change in electrical resistivity is saturated. The compressive stresses in the ZnO:In films are favored for the generation of compensating acceptor defects and the early initiation of the self-compensating process versus In contents, compared to its initiation in ZnO:Ga films.