Date of Graduation
2004
Document Type
Dissertation/Thesis
Abstract
ZnO is a wide band-gap semiconductor material presently being developed for device applications in the ultraviolet range. In this work, photoluminescence (PL), photoluminescence excitation (PLE), and optical absorption spectroscopies were used to study undoped, N2-doped, thermally annealed, and Li-diffused ZnO bulk crystals. The bulk ZnO crystals were grown at Eagle-Picher Technology by the seeded-chemical-vapor-transport method. The annealing and diffusion treatments of the bulk ZnO crystals were performed at WVU. The free exciton transition energy (thus the band-gap energy) is established from 5 K to room temperature (RT) using PLE spectroscopy. The commonly observed UV emission band near 3.26 eV at RT is shown to be the first longitudinal-optical phonon replica of the free exciton emission. At 5 K, a bound exciton emission at 3.363 eV quenches when ZnO samples are annealed. The optical signatures of copper, lithium, and nitrogen impurities in ZnO were established. Copper can be present in two different charge states either Cu+ or Cu2+ in ZnO depending on the position of the Fermi level. There are two distinct green luminescence bands in bulk ZnO crystals, which are both most likely related to Cu. The structured green band is due to a localized excitation of Cu2+ ions. The commonly observed unstructured green band comes from donor-acceptor pair (DAP) recombination involving Cu + acceptors and shallow donors. Lithium doping is very efficient to convert the n-type bulk ZnO crystal to a semi-insulating state. A bound exciton line at 3.347 eV most likely is an acceptor bound exciton related to lithium acceptors. The Li-related yellow emission band at about 590 nm normally appears as a broadening to longer wavelengths of the Cu2+-green band, if using above band-gap excitation. If a below-band-gap excitation is used, the Li-related PL band is clearly resolved. Temperature dependent PL experiments on the undoped as-grown and N2-doped as-grown ZnO crystals revealed the nitrogen-related electron-to-acceptor (e, A0) at 3.232 eV and the nitrogen related DAP at 3.216 eV. The ionization energy of the nitrogen acceptor in ZnO crystal was determined to be 209 meV at 5 K by the lineshape analysis on the nitrogen (e, A0) transition.
Recommended Citation
Wang, Lijun, "Photoluminescence study of as-grown and thermally annealed bulk zinc oxide crystals." (2004). Graduate Theses, Dissertations, and Problem Reports. 9969.
https://researchrepository.wvu.edu/etd/9969