Crystal Polarity and Electrical Properties of Heavily Doped ZnO Films
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Crystal Polarity and Electrical Properties of Heavily Doped ZnO Films
Yutaka Adachi, Naoki Ohashi, Isao Sakaguchi and Hajime Haneda National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan
ABSTRACT In this study, ZnO films heavily doped with Al or Ga were grown on a polarity-controlled buffer layer using pulsed laser deposition. The films prepared using a 1 mol% Al-doped target with the buffer layer grown at 700 °C had the c(+)-face, whereas the films with the buffer layer grown at 400 °C had the c(-)-face, which means that the polarity control can be successfully carried out using the buffer layer. However, the films prepared using targets doped with more than 1 mol% Al or Ga had the c(+)-face regardless of the polarity of the buffer layer. The 1 mol% Al-doped ZnO film with the c(+)-face had lower electron concentration and higher growth rate than the film with the c(-)-face. This result indicates that the Al content in the film with the c(-)-face was larger than that in the film with the c(+)-face. INTRODUCTION ZnO has attracted considerable attention for its application to transparent electrodes for solar cells owing to its transparency in the range of visible light and its low resistivity [1]. ZnO has the wurtzite structure, which has no inversion symmetry; thus, it has the spontaneous polarization along the c-axis, which results in two types of surface, the c(+) (Zn-terminated) face and the c(-) (O-terminated) face. It is known that the polarity affects the properties of ZnO, for example, its electronic structure [2], chemical stability [3] and interfacial properties [4]. ZnO films tend to have the c-axis orientation. Therefore, for device applications it is important to investigate how the polarity affects the properties of ZnO films. Recently, we have found that Al doping into ZnO films results in the inversion of polarity [5]. ZnO films grown on sapphire using pulsed laser deposition (PLD) with a nominally undoped ZnO target exhibited c(-)-polarity, while films deposited with a 1 mol% Al-doped ZnO target exhibited c(+)-polarity. This inversion of polarity occurs not only on sapphire but also on SiO2 glass substrates [6]. These results indicate that impurity doping affects the polarity of a ZnO film. Doping efficiency is also affected by the polarity of ZnO. Maki et al. reported that nitrogen doping is favored by the c(+)-face of ZnO [7]. Recently, we found that the Mg content in PLDgrown MgZnO films depends on the film polarity [8]. The Mg content in MgZnO films grown on the c(-)-face of a ZnO single crystal was larger than that in films grown on the c(+)-face of a single crystal, despite the same growth conditions used. For the application to transparent electrodes for solar cells, Al or Ga, which both act as a donor in ZnO, is heavily doped into ZnO films. As described above, the polarity of ZnO is expected to affect the incorporation of impurities, which would affect the electric properties,
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especially the electron concentration, of the films. In this study, w
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