Author : Zheng Yang
Release : 2009
Genre : Photoluminescence
Kind : eBook
Book Rating : /5 ( reviews)
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Book Synopsis Doping in Zinc Oxide Thin Films by : Zheng Yang
Download or read book Doping in Zinc Oxide Thin Films written by Zheng Yang. This book was released on 2009. Available in PDF, EPUB and Kindle. Book excerpt: Doping in zinc oxide (ZnO) thin films is discussed in this dissertation. The optimizations of undoped ZnO thin film growth using molecular-beam epitaxy (MBE) are 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 x 10 to 1.8 x 10 cm -3, the dominant PL line at 9 K changes from I 1 (3.368 - 3.371 eV), to I DA (3.317 - 3.321 eV), and finally to I 8 (3.359 eV). The dominance of I, 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 I DA has characteristics of a donor acceptor pair transition, and use a detailed, quantitative analysis to argue that it arises from Ga Zn donors paired with Zn-vacancy (V Zn) acceptors. In this analysis, the Ga Zn 0/+ energy is well-known from two-electron satellite transitions, and the V Zn 0/- energy is taken from a recent theoretical calculation. Typical behaviors of Sb-doped p -type ZnO are presented. The Sb doping mechanisms and preference in ZnO are discussed. Diluted magnetic semiconducting ZnO:Co thin films with above room-temperature T C 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 ~10 19 cm -3, indicating a carrier-mediated mechanism for ferromagnetism. The anomalous Hall effect was 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.
