Defect structures in undoped and doped ZnO films studied by solid state diffusion
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Defect structures in undoped and doped ZnO films studied by solid state diffusion Haruki Ryoken,1,2 Isao Sakaguchi,1 Takeshi Ohgaki,1 Naoki Ohashi,1 Yutaka Adachi,1 and Hajime Haneda1 1
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan 2 Kyushu University, 6-1 Kasuga-kouen, Kasuga, Fukuoka, 816-8580, Japan ABSTRACT Defect structures in ZnO thin films were studied to clarify the mechanism of charge compensation and crystallinity degradation induced by alloying. Regarding the undoped ZnO films, it was indicated that the degree of non-equilibrium behavior in the films deposited by PLD was much less than in the films prepared by the other two methods, i.e., MBE and sputtering, and, moreover, the solid-state diffusion behavior in the PLD-grown undoped ZnO was close to that of bulk ZnO. The heavily Al-doped films and alloy films with high concentrations of MgO exhibited significant non-equilibrium behavior, even for those prepared by PLD. It was indicated that the high concentration of extrinsic elements, e.g., Al and Mg, introduces non-equilibrium defects into ZnO films and those defects are the cause of the crystallinity degradation and thermal instability of the films. INTRODUCTION Zinc oxide (ZnO) is a candidate material for transparent semiconductor electronics, such as transparent field effect transistors [1,2]. To achieve high-performance transparent devices, the quality of ZnO films and control of defects, particularly in regard to defects for charge compensation, are of great importance. Undoped ZnO usually exhibits n-type conduction, and the electrons are thought to be injected from oxygen vacancies or interstitial zinc. However, theoreticians [3] have recently claimed that such native defects are not the origin of n-type conduction in ZnO. Thus, the defect chemistry of ZnO has likely returned to the starting point, where we have to think again about the origin of its intrinsic n-type conduction as well as its mechanism of charge compensation. There are several experimental techniques for characterizing the structures and properties of defects in solids. For example, electron paramagnetic resonance (EPR) has high sensitivity, and it makes it possible to do structural analyses [4]. However, the applicability of EPR is principally limited to magnetic defects. The Hall effect can only be exploited to analyze defects injecting the major carrier, and dielectric methods, such as evaluating the relationship between applied bias and capacitance at a Schottky junction, are sometimes difficult to employ because of problems with the electrodes’ stability [5] . Among these techniques, solid-state diffusion is characterized by its sensitivity to compensated defects, particularly compensated deep defects. In fact, the self-diffusion coefficient is proportional to the concentration of responsible defects. In the present study, we investigated both cation and anion diffusion in ZnO thin films prepared by pulsed laser deposition (PLD). We paid attention to two aspects: the p
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