Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

To these days ZnO is one of the most widely investigated types of transparent conductive oxides except ITO. The electronic structure of most of the native defects in ZnO was studied both theoretically and experimentally using various methods. Based on simulation and experimental results, A. Sokol, et al., , have proposed a complex model for the electronic structure of different point defects in doped and undoped ZnO [Faraday Discuss., 134, 267-282 (2007)]. There is agreement that the zinc interstitial (Zni) is a shallow donor and is mainly responsible for the n-type conductivity of intrinsic ZnO. The oxygen interstitial (Oi) is neutral in ZnO, has the energy level located about 2.8 eV below the bottom of the conduction band, and is mainly responsible for the blue-green emission at 2.5 – 2.3 eV. But the energy levels of exited Zni* and Oi are controversial. Using density functional theory calculations, Yong-Sung Kim and C. H. Park have shown that Zni* should have an energy level above the conduction band but to this day it has not been proven experimentally and the exact energy position is unknown [Phys. Rev. Lett. 102, 086403 (2009)]. In order to verify their theory we have performed detailed optical and electrical investigations of ZnO films deposited on insulating Si wafers by reactive pulsed laser deposition. The defect engineering in ZnO was performed using non-equilibrium flash lamp annealing operated in the millisecond range with different annealing ambient. Temperature dependent photoluminescence (PL) emission and excitation (PLE) were utilised to determine the radiative transitions and excitation levels in ZnO, respectively. In order to determine the carrier concentration and conductivity type of processed ZnO films, Hall Effect measurements were performed in the temperature range from 3 to 300K. According to PLE the first excited levels of Zni* and Oi are located at 0.83 eV and 0.70 eV above the conduction band, respectively. The concentration of Zni* and Oi is determined by the annealing atmosphere. The oxygen-poor atmosphere promotes the Zn-interstitial formation while annealing in oxygen suppresses the n-type defects and increases the Oi concentration. Comparison of temperature dependent PL and Hall Effect data confirms that the Zni is a main intrinsic source of n-type conduction in ZnO.