Characterizations of the charge transfer transitions in cobalt(II) and manganese(II) doped zinc oxide

The chapters in this dissertation describe the investigation of the physical and electronic structure of transition metal doped zinc oxide (TM2+ .ZnO) thin films. Films were made from cobalt(II)-doped or manganese(II)-doped ZnO nanoparticle targets and deposited epitaxially onto c-plane or r-plane sapphire substrates. X-ray diffraction showed that the film orientation was dependent on the orientation of the substrate. Transmission electron microscopy (TEM) showed that the ZnO films were smooth and oriented. The thin films were characterized using a variety of spectroscopic methods: electron paramagnetic resonance (EPR), X-ray absorption (XAS), specifically X-ray absorption near edge (XANES) and extended x-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (MCD) and optical MCD, electronic absorption, valence band x-ray photoemission (VB XPS), and photocurrent spectroscopies. The dopants were shown to substitionally replace the Zn2+ cations in the ZnO lattice. The ZnO exciton shifted to higher energies with increasing dopant concentration for both Mn2+ and Co2+:ZnO A sub-bandgap feature, previously identified as a metal-to-ligand charge transfer (MLCBCT) transition increased in intensity with increasing Mn 2+ concentration. The MLCBCT transition and the excitonic transition intensities followed the S = 5/2 Brillouin function, demonstrating sp-d exchange. This feature was also observed by VB XPS and photoconductivity; demonstrating that is a delocalized transition that arises from occupied Mn 2+ levels within the ZnO gap. XAS, electronic absorption, and MCD spectroscopies showed orientation dependence arising from the anisotropy of Co2+ in ZnO. Additionally, Co2+:ZnO showed sub-excitonic features due to ligand-to-metal charge transfer (LVBMCT) and ML CBCT transitions. Photoconductivity of Co2+:ZnO showed both charge transfer transitions as well as the 4A2 → 4T1(P) transition. Variable temperature and polarized photoconductivity were used to determine the mechanism of the ligand field photoconductivity and the energetic relationship with the MLCBCT transition. In both Co2+ and Mn2+-doped ZnO the characteristic features of sp-d exchange were observed by MCD spectroscopy. Despite the presence of in-gap dopant related states, Mn2+ and Co 2+ doped ZnO are archetypical doped II-VI semiconductors.