Defects mediated functionalization of ZnO based materials

Abstract

ZnMgO is a wide band gap material and its band gap can be tuned from 3.37 eV to 7.8 eV. It has promising applications in ultraviolet optoelectronic devices, particularly in ultraviolet detectors and laser diodes. ZnMgO thin films at different oxygen partial pressure were prepared by pulsed laser deposition. Effect of oxygen partial pressure on structural and optical properties of ZnMgO thin films was investigated. It was found that ZnMgO films grown at oxygen partial pressure P(O2) of 0 pa, 0.5 pa and 1 pa have cubic structure, and no hexagonal ZnMgO structure was observed. Phase separation of ZnMgO into cubic and hexagonal was observed at high P(O2) of 1.5 pa. Further increase in P(O2) to 2 pa resulted in ZnMgO with hexagonal structure. Zn, Mg contents were also influenced by P(O2), and the composition of Zn in ZnMgO thin films increased with the increase in P(O2). A decrease in band gap from 5.78 eV to 4.04 eV was observed with the increase in P(O2) from 0 pa to 2 pa. ZnMgO and ZnO:Ga thin films prepared at 0 pa P(O2) was found to show a high transmittance value (~ 80%). A Transparent ZnMgO/ZnO:Ga/c-sapphire hetero-structure with an optical transmittance of 73% in the visible region was fabricated by pulsed laser deposition technique. A simple contact approach, (i.e. across a W pin on top of the ZnMgO surface and another connected to the ZnO:Ga which acts as the transparent electrode) was used to observe the bipolar resistive switching in this structure. IV measurements were performed to determine the conduction mechanisms. It was found that for low resistance state (LRS) conduction is explained by Ohmic behavior, but for high resistance state (HRS) Schottky emission model was used to explain for the conduction behavior. Moreover, the memory effect, good retention and endurance characteristics of the fabricated ZnMgO/GaZnO/c-sapphire device suggest its prospective application for the transparent resistive random-access memory devices. Magnetic measurements on un-doped ZnO thin films grown at different P(O2) were performed to explore the room temperature (RT) ferromagnetism (FM). The VZn¬-related defect in the ZnO films were studied by the coincidence Doppler broadening of the positron annihilation, revealing that VZn¬-2VO was the dominant defect in the samples. Correlations between the magnetization, grain boundary surface to volume ratio (revealed by X-ray diffraction) and VZn-2VO concentration were found in samples grown with moderate oxygen pressure. The room temperature ferromagnetism observed in these ZnO samples are thus attributed to the VZn-2VO residing on the grain boundary surface. Theoretical study shows that VZn-2VO is ferromagnetic, with the spin density mainly contributed from the p electrons on the neighboring O and the second neighboring O atoms. Temperature dependent magnetic measurements depicts that the Curie temperature of ZnO samples is well above the room temperature.