Photoluminescence of ZnO Grown by Eclipse Pulsed Laser deposition
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
ZnO thin films and nanostructures were grown by eclipse pulsed laser deposition (EPLD) for the first time. On bare sapphire held at 600 °C, a complex nanostructured surface was formed when ablating a metallic Zn target in an oxygen ambient. Nanorods grown by a vapor-solid mechanism clumped together in well separated, micron-sized regions. Nanoscale pyramids with 6 fold symmetry formed between the nanorod clumps by vapor-liquid-solid growth. Strong photoluminescence (PL) was observed from the EPLD grown samples, an order of magnitude stronger than PLD grown nanorods formed under similar growth conditions. Low temperature PL was dominated by the I₇ exciton, which still has an unknown origin. Excitation intensity dependence of I₇ was drastically different than the rest of the nearby excitonic features, behavior which has not been previously reported for bound excitons in ZnO. I₇ also showed large, seemingly random variations in intensity across the surface of each sample compared to the other nearby recombinations, suggesting a structural connection. Introduction of a buffer layer had a profound effect on the morphology and PL from EPLD grown ZnO from a metallic Zn target. Pt has a high melting temperature, which helped suppress the vapor-liquid-solid nanostructure growth resulting in thin-film formation. For standard PLD, the ZnO film showed large grains separated by cracks on the surface. Due to the reduced growth rate in the EPLD geometry, the ZnO layer had a high density of nanoscale pores, reminiscent of the porous Pt buffer layer. Strong PL emission, which was dominated by I₇, was observed from the ZnO/Pt/Al₂O₃ which showed unusual blue/violet emission when the EPLD geometry was used for growth. Thin ZnO buffer layers deposited at reduced temperature also had a profound effect on EPLD grown ZnO, resulting in a random array of nanorods with alignment which was dependent on the growth temperature of the buffer layer. Buffer layers offer another dimension in the control over epitaxial structures and show large potential for EPLD growth of ZnO. Pb was the dominant impurity in the Zn targets used for EPLD growth, hinting at a Pb-related origin for the I7 peak. To explore this idea, hydrothermally grown bulk ZnO was ion-implanted with Pb and then annealed in oxygen at 600 °C to repair damage to the crystal. PL emission intensity was substantially reduced in the Pb-implanted ZnO but the line widths were preserved. No evidence of an I₇ feature was seen for Pb concentrations of up to 0.10%, three orders of magnitude higher than the expected level in the EPLD grown ZnO. However, this does not rule out a Pb-related complex as the origin of I₇ since Pb has complicated interactions with the impurities and native defects in ZnO. Instead of I₇, other sharp excitonic features were observed near the band edge. A bound exciton with a localization energy of 12.4 ± 0.2 meV was observed in the Pb-implanted samples and was attributed to neutral interstitial Pb donors. Pb-implantation produced a clear PL signature which is unique enough to unambiguously detect its presence in ZnO. EPLD also proved successful at depositing oxides of the noble metals. Ir, Pt, Pd, and Ru targets were ablated in oxygen and argon ambients and films were collected on room temperature substrates. Growth in argon resulted in pure metal while oxidized layers were obtained in oxygen. This was clearly evident by the semiconductor-like transmission spectra observed for the oxidized samples. The high fluence used for these growths promoted the oxidation of these resilient metals while the shadow mask blocked most of the molten particulates generated by the high fluence. EPLD is an excellent way to produce oxides from metallic targets, a technique which should be explored in more detail for many material systems.