Band-edge emission, defects, morphology and structure of in-doped ZnO nanocrystal films

In-doped ZnO films grown by ultrasonic spray pyrolysis have been studied by means of the scanning electron microscopy (SEM), energy dispersive X ray spectroscopy (EDS) and X ray diffraction (XRD) methods. The photoluminescence (PL), transmittance and absorbance have been controlled as well. It was shown that the ZnO optical band gap demonstrates the blue high energy shift to 3.31 eV at 300 K and the PL intensity of near band edge (NBE) emission enlarges at In doping 0.5-2.5 at%. Simultaneously, the positions of XRD peaks and their intensities vary insignificantly owing to the difference in the In3+ and Zn2+ ionic radii. Meanwhile, intensity decreasing the green PL band confirms the occupation of the zinc vacancies by In ions with the formation of substitutional In-Zn defects and ZnO crystal quality improving. At higher In contents the new PL band (3.034eV) appears in PL spectra and its peak shifts to lower energy with In content increasing. This PL band was attributed to the emission via the complex defects, formed by In-i interstitial atoms. Simultaneously, the PL intensity and ZnO film crystallinity falling down, the ZnO crystal lattice parameters increase and the ZnO optical band gap demonstrates the red low energy shift. To reveal a nature of the optical transition responsible for the new PL band, PL spectra have been studied in the temperature range 11-290 K. The dependence of the In-i complex defect formation versus In contents in ZnO NC films is analyzed and discussed. The optimal In concentration range to fabricate the ZnO films with high optical parameters has been estimated.