A combined experimental and first-principles study is performed to study the origin of conductivity in ZnO:Al nanoparticles synthesized under controlled conditions via a reflux route using benzylamine as a solvent. The experimental characterization of the samples by Raman, nuclear magnetic resonance (NMR) and conductivity measurements indicates that upon annealing in nitrogen, the Al atoms at interstitial positions migrate to the substitutional positions, creating at the same time Zn interstitials. We provide evidence for the fact that the formed complex of AlZn and Zni corresponds to the origin of the Knight shifted peak (KS) we observe in 27Al NMR. As far as we know, the role of this complex has not been discussed in the literature to date. However, our first-principles calculations show that such a complex is indeed energetically favoured over the isolated Al interstitial positions. In our calculations we also address the charge state of the Al interstitials. Further, Zn interstitials can migrate from AlZn and possibly also form Zn clusters, leading to the observed increased conductivity.

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A novel explanation for the increased conductivity in annealed Al-doped ZnO: an insight into migration of aluminum and displacement of zinc

This work reports the study on the evolution of native defects in ZnO nanorods irradiated with hydrogen ion. ZnO nanorod arrays grown vertically on silicon substrates were irradiated by 180 keV H+ ions to a total fluence of 8.50 × 1015 ions/cm2. The X-ray diffraction spectra, photoluminescence spectra before and after irradiation and the real-time ionoluminescence spectra of the nanorod arrays during the irradiating process were measured. Formation and evolution of defects during H+ ion irradiation and effects of irradiation on the crystal structure and optical property were studied. Blue shift of exciton emission, shrink of lattice c and improvement of the crystallinity of ZnO nanorods after irradiation were observed. Simple surface passivation of the nanorods could improve the radiation resistance. Formation and evolution of the defects during H+ ion irradiation could be clarified into four stages and the related models are provided.

Evolution of native defects in ZnO nanorods irradiated with hydrogen ion.

Gamma-ray irradiation effect on ZnO bulk single crystal: Origin of low resistivity

The origins of low resistivity in H ion-implanted ZnO bulk single crystals are evaluated by elastic recoil detection analysis (ERDA), electron paramagnetic resonance (EPR), and Van der Pauw methods. The H-ion implantation (peak concentration: 5.0 × 10 15  cm −2 ) into ZnO is performed using a 500 keV implanter. The maximum of the concentration of the implanted H estimated by a TRIM simulation is at 3600 nm in depth. The resistivity decreases from ∼10 3  Ω cm for un implanted ZnO to 6.5 Ω cm for as-implanted, 2.3 × 10 −1  Ω cm for 200 °C annealed, and 3.2 × 10 −1  Ω cm for 400 °C annealed samples. The ERDA measurements can evaluate the concentration of hydrogens which move to the vicinity of the surface (surface to 300 nm or 100 nm) because of the diffusion by the annealing at 200 °C and 400 °C. The hydrogen concentration near the surface estimated using the 2.0 MeV helium beam is ∼3.8 × 10 13  cm −2 for annealed samples. From EPR measurements, the oxygen vacancy of +charge state ( V o + ) is observed in as-implanted samples. The V o + related signal ( g  = 1.96) observed under no illumination disappears after successive illumination with a red LED and appears again with a blue light illumination. The activation energy of as-implanted, 200 °C annealed, and 400 °C annealed samples estimated from the temperature dependence of carrier concentration lies between 29 meV and 23 meV, suggesting the existence of H interstitial as a shallow donor level.

Hydrogen interstitial in H-ion implanted ZnO bulk single crystals: Evaluation by elastic recoil detection analysis and electron paramagnetic resonance

Modifications in room temperature ferromagnetism by dense electronic excitations in Zn0.9Mg0.1O thin films

The origins of low resistivity in H ion-implanted ZnO bulk single crystals are studied by Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) photoluminescence (PL), and Van der Pauw methods. The H-ion implantation (peak concentration: 1.45 × 1020 cm−3) into ZnO is performed using a 500 keV implanter. The resistivity decreases from 2.5 × 103 Ω cm for unimplanted ZnO to 6.5 Ω cm for as-implanted one. RBS measurements show that Zn interstitial as a shallow donor is not recognized in as-implanted samples. From photoluminescence measurements, the broad green band emission is observed in as-implanted samples. NRA measurements for as-implanted ZnO suggest the existence of the oxygen interstitial. The origins of the low resistivity in the as-implanted sample are attributed to both the H interstitial as a shallow donor and complex donor between H and disordered O. The activation energy of H related donors estimated from the temperature dependence of carrier concentration is 29 meV.

T. Kaida

Papers

Rutherford backscattering and nuclear reaction analyses of hydrogen ion-implanted ZnO bulk single crystals

Hydrogen interstitial in H-ion implanted ZnO bulk single crystals: Evaluation by elastic recoil detection analysis and electron paramagnetic resonance