Optical characterization of doped zinc oxide nanowires

ZnO is a promising semiconductor material with a direct band gap energy of 3.3 eV which makes it a good candidate for UV and visible range light emitting devices. Metalorganic chemical vapour epitaxy (MOVPE) provides the possibility of industrial scale growth of ZnO, with very fine control of impurity dopants. Despite the vast recent literature on ZnO, there are very few studies of systematic intentional doping. ZnO nanowires (NWs) can be grown easily on various substrates with high crystalline quality and low defect densities and tend to exhibit reduced substrate induced strain. This enables us to perform careful spectroscopic analysis of impurity related optical transitions and identify the physical nature of various dopant species. A detailed study of low temperature photoluminescence (PL) transitions in doped ZnO NWs, thin films, and bulk crystals grown by MOVPE and chemical vapour transport (CVT) methods is presented. The standard group III donors were first investigated. Donor bound exciton (D0X) transitions previously assigned to Ga, Al, and In were confirmed in intentionally doped samples. Group IV dopants such as carbon, and tin are interesting since they can act in principle as double donors or double acceptors. We report four new shallow D0X transitions (Z-lines), at 3360.8 (Z1), 3361.2 (Z2), 3361.7 (Z3) and 3361.9 (Z4) meV, which can be greatly enhanced by co-doping with carbon tetrachloride and hydrogen. These shallow donors appear to be due to carbon impurities complexed with other unknown defects in four distinct configurations. Carbon-doped samples also exhibit two distinct acceptors with binding energies of 133 ± 5 and 181 ± 5 meV. Doping concentration and temperature dependent PL studies of unintentionally doped and Sn-doped ZnO single crystals confirmed emission from the I10 D0X transition which was recently proven to contain Sn on a Zn site. Sb-doped ZnO NWs were grown in an attempt to produce p-type material as reported by some groups. Our PL studies including Magneto PL, have shown that rather than p-doped material, the addition of small amounts of Sb-dopant resulted in a new PL transition at 3364.3 meV, which turns out to be the shallowest D0X transition so far observed in ZnO.