Synthesis And Properties Of TM (Co, Cr, Cu, And Mn) Doped ZnO Thin Films

Diluted magnetic semiconductors (DMS) have been attracting much interest of almost a decade now due to their potential to manipulate charge and spin degrees of freedom in a single material. Dietl et al. predicted high-temperature ferromagnetism in transition-metal (TM) doped wide-band-gap semiconductors particularly in ZnO and GaN. This fact has motivated many researchers to study the properties of TM doped semiconductors. Recent reports on the observation of room-temperature (RT) ferromagnetism in TM-doped ZnO have been welcomed with great enthusiasm by the scientific community. ZnO-based DMS have some advantages over others because of their unique characteristics, such as having a large band gap (~3.4 eV), large exciton binding energy at RT (~60 meV), high optical gain (300 cm–1), and very short luminescence lifetime, which are required for various optoelectronic and magneto-optical devices.This thesis is focused on the hotspots and challenges in the field of ZnO materials research. The purpose of this thesis is to investigate the room ferromagnetic behavior and the orgin of ferromagnetism in ZnO based DMS films. TM (Here Co, Cr, Cu, and Mn) doped ZnO thin films were synthesized via magnetron sputtering method and its microstructure, optical properties, and magnetic properties were investigated. The following is the major results:1. Structural analyses suggest that Co, Cr, Cu, and Mn occupied the Zn sites successfully and did not change the wurtzite structure of ZnO at low doping content. However, a small amount of Co3O4 clusters could be found in Zn1-xCoxO films when the doping level x reached 0.313. In addition, ZnCr2O4 could also be found in Zn1-xCrxO films when the doping level x reached 0.098. A point worth emphasizing is that nanoscale columnar grain arrays were found in the cross-sectional images of Mn- and Cu-doped ZnO films. As the magnetron sputtering method can produce economically feasible large area films with well-controlled composition, we suggest that the method in our experiment may be applied to future large-scale manufacturing of aligned Mn- and Cu-doped ZnO nanoscale columnar grains and nanorod arrays.2. The near band edge (NBE) emissions at around 375 nm were observed in the photoluminescent spectra of the Co- and Mn-doped ZnO films, which suggest the possibility of their application in ultraviolet light-emitting devices. Interestingly, when Co concentration was further increased up to 31.3 atom% and 37.7 atom%, a broad intense emission band centered at 332 nm (deep ultraviolet emission) was observed. The deep ultraviolet emission should be related to the O2-→Co2+ charge-transfer process in Co3O4 clusters embedded in Zn1-xCoxO films. The intense deep ultraviolet emissions of the Co3O4/ Zn1-xCoxO composite structures suggest their potential applications for short wavelength magneto-optical devices, such as light-emitting diodes and laser diodes.3. The ZnO films doped with moderate TM (Co, Cr, Cu, or Mn) exhibit obvious ferromagnetic ordering, which origins from TM-ZnO matrix rather than impurities and can be ascribed to originate from the bound magnetic polarons (BMP) model. The ferromagnetism of these TM-doped ZnO films also suggests their potential applications for future spintronics. In addition, the magnetic moment per TM atom decreased as the TM concentration further increased. It is proposed that the decrease in magnetic moment per TM atom as the TM concentration increases is due to an increase in the number of TM atoms that occupy adjacent cation lattice positions with an attendant increase in antiferromagnetic interaction between those TM atoms.