Photoluminescence Characterization of Defects in Nitrogen-Implanted and Thermally Annealed ZnO

Abstract

Ion implantation is an important technology for modifying electrical and even optical properties of semiconductor materials. Recently, ZnO has attracted a great deal of renewed research interest due to its unique optoelectronic properties and new application potential in short-wavelength efficient light-emitting devices including laser diodes. Usually undoped ZnO exhibits N-type conductivity, which indicates the presence of native defects. It still remains a challenging issue to realize efficient P-type doping in ZnO nowadays. We thus attempt to prepare P-type ZnO by means of nitrogen implantation. In this work, we concentrate on photoluminescence characterization of defects in ZnO implanted and post-implantation annealed. ZnO samples employed in nitrogen implantation show N-type electrical conductivity and weak green emission. However, a new broad red luminescence peak was observed in all as-implanted ZnO samples, which implies that somewhat new deeper defects are induced by nitrogen implantation. Thermal treatment at different temperatures was adopted to activate the implanted ions and eliminate the implantation-induced lattice distortion. Interestingly, the defect emissions in the implanted ZnO show a strong dependence on the treatment temperature. As the treatment temperature increases, the red emission is significantly suppressed whereas the green emission greatly enhances. In particular, two sets of fine structures with a fixed energy separation of about 30 meV superimposing on the broad background were observed in the samples annealed at moderate high temperatures. The multimode Brownian oscillator (MBO) model accounting for electron-phonon coupling was employed to determine the energetic positions of defects producing the green and red emissions. Excellent agreement between theory and experiment is achieved over the entire experimental temperature range. Our experimental data favor the electronic level structure of native oxygen vacancy theoretically obtained by Janotti and Van de Walle(1) using the first principles calculation. A physical model is proposed to interpret the observed luminescence behavior of defects in ZnO. (1)Anderson Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).