Structural and optical properties of annealed ZnO single crystals in a helium ambient

We have used X-ray diffraction and photoluminescence measurements to investigate the effects of helium annealing of ZnO single crystals grown by using the hydrothermal method. The crystal structure of the grown ZnO single crystal was hexagonal with cell parameters a = 0.3249 and c = 0.5206 nm. The crystal structure did not change after helium annealing. The full width at half-maximum (FWHM) of the (002) diffraction peak following the helium annealing treatment decreases to 0.048° from 0.093° for the as-grown sample, indicating improved crystalline quality. The PL spectra of the crystals showed near-band-edge and visible (green and yellow) emission bands. The PL intensity of the near-band edge emissions of helium-annealed sample was increased while that of the yellow emission band was decreased. From the temperature dependence of the PL spectra, the peak positions of the free exciton (FE) and the donor-bound exciton (BE) shifted to lower energy whereas the visible emission bands show an anomalous temperature-dependent behavior.