UC Riverside

Doping in zinc oxide (ZnO) thin films were discussed in this dissertation.The optimizations of undoped ZnO thin film growth using molecular-beam epitaxy (MBE) were discussed. The effect of the oxygen ECR plasma power on the growth rate, structural, electrical, and optical properties of the ZnO thin films were studied. It was found that larger ECR power leads to higher growth rate, better crystallinity, lower electron carrier concentration, larger resistivity, and smaller density of non-radiative luminescence centers in the ZnO thin films. Low-temperature photoluminescence (PL) measurements were carried out in undoped and Ga-doped ZnO thin films grown by molecular-beam epitaxy. As the carrier concentration increases from 1.8 × 1018 to 1.8 ×1020 cm-3, the dominant PL line at 9 K changes from I1 (3.368 - 3.371 eV), to IDA (3.317 -3.321 eV), and finally to I8 (3.359 eV). The dominance of I1, due to ionized-donor bound excitons, is unexpected in n-type samples, but is shown to be consistent with the temperature-dependent Hall fitting results. We also show that IDA has characteristics of a donor-acceptor-pair transition, and use a detailed, quantitative analysis to argue that it arises from GaZn donors paired with Zn-vacancy (VZn) acceptors. In this analysis, the GaZn0/+ energy is well-known from two-electron satellite transitions, and the VZn0/- energy

is taken from a recent theoretical calculation. Typical behaviors of Sb-doped p-type ZnO

were presented. The Sb doping mechanisms and preference in ZnO were discussed.

Diluted magnetic semiconducting ZnO:Co thin films with above room-temperature TC

were prepared. Transmission electron microscopy and x-ray diffraction studies indicate

the ZnO:Co thin films are free of secondary phases. The magnetization of the ZnO:Co

thin films shows a free electron carrier concentration dependence, which increases

dramatically when the free electron carrier concentration exceeds ~1019 cm-3, indicating a

carrier-mediated mechanism for ferromagnetism. The anomalous Hall effect is observed

in the ZnO:Co thin films. The anomalous Hall coefficient and its dependence on

longitudinal resistivity were analyzed. The presence of a side-jump contribution further

supports an intrinsic origin for ferromagnetism in ZnO:Co thin films. These observations

together with the magnetic anisotropy and magnetoresistance results, supports an intrinsic

carrier-mediated mechanism for ferromagnetic exchange in ZnO:Co diluted magnetic

semiconductor materials. Well-above room temperature and electron-concentration

dependent ferromagnetism was observed in n-type ZnO:Mn films, indicating long-range

ferromagnetic order. Magnetic anisotropy was also observed in these ZnO:Mn films,

which is another indication for intrinsic ferromagnetism. The electron-mediated

ferromagnetism in n-type ZnO:Mn contradicts the existing theory that the magnetic

exchange in ZnO:Mn materials is mediated by holes. Microstructural studies using transmission electron microscopy were performed on a ZnO:Mn diluted magnetic

semiconductor thin film. The high-resolution imaging and electron diffraction reveal that

the ZnO:Mn thin film has a high structual quality and is free of clustering/segregated

phases. High-angle annular dark field imaging and x-ray diffraction patterns further

support the absence of phase segregation in the film. Magnetotransport was studied on

the ZnO:Mn samples, and from these measurements, the temperature dependence of the

resistivity and magnetoresistance, electron carrier concentration, and anomalous Hall

coefficient of the sample is discussed. The anomalous Hall coefficient depends on the

resistivity, and from this relation, the presence of the quadratic dependence term supports

the intrinsic spin-obit origin of the anomalous Hall effect in the ZnO:Mn thin film.