Thermal evolution of defects in undoped zinc oxide grown by pulsed laser deposition

published_or_final_versio

Zn-vacancy related defects in ZnO grown by pulsed laser deposition

Undoped and Ga-doped ZnO (002) films were grown c-sapphire using the pulsed laser deposition (PLD) method. Znvacancy related defects in the films were studied by different positron annihilation spectroscopy (PAS). These included Doppler broadening spectroscopy (DBS) employing a continuous monenergetic positron beam, and positron lifetime spectroscopy using a pulsed monoenergetic positron beam attached to an electron linear accelerator. Two kinds of Znvacancy related defects namely a monovacancy and a divacancy were identified in the films. In as-grown undoped samples grown with relatively low oxygen pressure P(O2)≤1.3 Pa, monovacancy is the dominant Zn-vacancy related defect. Annealing these samples at 900 oC induced Zn out-diffusion into the substrate and converted the monovacancy to divacancy. For the undoped samples grown with high P(O2)=5 Pa irrespective of the annealing temperature and the as-grown degenerate Ga-doped sample (n=1020 cm-3), divacancy is the dominant Zn-vacancy related defect. The clustering of vacancy will be discussed.published_or_final_versio

Hydrogen-mediated ferromagnetism in ZnO single crystals

We investigated the magnetic properties of hydrogen-plasma-treated ZnO single crystals by using superconducting quantum interferometer device magnetometry. In agreement with the expected hydrogen penetration depth, we found that ferromagnetic behavior is present in the first 20 nm of the H-treated surface of ZnO with magnetization at saturation up to 6 emu g−1 at 300K and a Curie temperature of Tc & 400 K. In the ferromagnetic samples, a hydrogen concentration of a few atomic per cent in the first 20 nm of the surface layer was determined by nuclear reaction analysis. The saturation magnetization of H-treated ZnO increases with the concentration of hydrogen

Emission bands of nitrogen-implantation induced luminescent centers in ZnO crystals: experiment and theory

High quality ZnO crystals with the sharp band-edge excitonic emission and very weak green emission were implanted by nitrogen ions. An additional red emission band was observed in the as-implanted ZnO crystal and investigated as a function of temperature. By employing the underdamped multimode Brownian oscillator model for the general electron-phonon coupling system, both the original green and nitrogen-implantation induced red emission bands were theoretically reproduced at different temperatures. Excellent agreement between the theory and the experiment enables us determine the energetic positions of the pure electronic levels associated with the green and red emission bands, respectively. The determined energy level of the red emission band is in good agreement with the data obtained from the deep-level transient spectroscopy measurements. © 2012 American Institute of Physics.published_or_final_versio

Deep level transient spectroscopic study of oxygen implanted melt grown ZnO single crystal

Deep level traps in melt grown ZnO single crystal created by oxygen implantation and subsequent annealing in air were studied by deep level transient spectroscopy measurement between 80 and 300 K. The E C-0.29 eV trap (E3) was the dominant peak in the as-grown sample and no new defects were created in the as-O-implanted sample. The single peak feature of the deep level transient spectroscopy (DLTS) spectra did not change with the annealing temperature up to 750 °C, but the activation energy decreased to 0.22 eV. This was explained in terms of a thermally induced defect having a peak close to but inseparable from the original 0.29 eV peak. A systematic study on a wide range of the rate window for the DLTS measurement successfully separated the Arrhenius plot data originated from different traps. It was inferred that the E3 concentration in the samples did not change after the O-implantation. The traps at E C-0.11, E C-0.16 and E C-0.58 eV were created after annealing. The E C-0.16 eV trap was assigned to an intrinsic defect. No DLTS signal was found after the sample was annealed to 1200 °C. © 2011 IOP Publishing Ltd.postprin

The Zn-vacancy Related Green Luminescence and Donor–acceptor Pair Emission in ZnO Grown By Pulsed Laser Deposition

postprin

Deep-level defects in n-type 6H silicon carbide induced by He implantation

Defects in He-implanted n -type 6H-SiC samples have been studied with deep-level transient spectroscopy. A deep-level defect was identified by an intensity with a logarithmical dependence on the filling pulse width, which is characteristic of dislocation defects. Combined with information extracted from positron-annihilation spectroscopic measurements, this defect was associated with the defect vacancy bound to a dislocation. Defect levels at 0.380.44 eV (E1 E2), 0.50, 0.53, and 0.640.75 eV (Z1 Z2) were also induced by He implantation. Annealing studies on these samples were also performed and the results were compared with those obtained from e- -irradiated (0.3 and 1.7 MeV) and neutron-irradiated n -type 6H-SiC samples. The E1 E2 and the Z1 Z2 signals found in the He-implanted sample are more thermally stable than those found in the electron-irradiated or the neutron-irradiated samples. © 2005 American Institute of Physics.published_or_final_versio

Defect studies of ZnO films prepared by pulsed laser deposition on various substrates

ZnO thin films deposited on various substrates were characterized by slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD). All films studied exhibit wurtzite structure and crystallite size 20-100 nm. The mosaic spread of crystallites is relatively small for the films grown on single crystalline substrates while it is substantial for the film grown on amorphous substrate. SPIS investigations revealed that ZnO films deposited on single crystalline substrates exhibit significantly higher density of defects than the film deposited on amorphous substrate. This is most probably due to a higher density of misfit dislocations, which compensate for the lattice mismatch between the film and the substrate