Cl Doping Effect by Thermal Treatment with KCl for ZnO Single Crystals

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Cl Doping Effect by Thermal Treatment with KCl for ZnO Single Crystals Akira Fujimoto and Yoshiyuki Harada Nanomaterials Microdevices Research Center, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan ABSTRACT 0B

We investigated the optical and electrical properties of ZnO single crystals coated with KCl by examining the photoluminescence (PL) at 9 K and the temperature dependence of the carrier concentration. The band-edge PL intensity of the KCl-coated sample was much larger than that of the uncoated sample. Moreover, a substantial increase in Hall electrons was observed in the coated sample. X-ray photoelectron spectroscopy revealed bonding between chlorine and zinc atoms in the coated sample. Therefore, coating the surface of ZnO single crystals with KCl enhances the donor concentration and improves the surface state. INTRODUCTION 1B

Oxide semiconductors such as zinc oxide (ZnO), In2O3, TiO2, and Ga2O3 have been widely investigated recently for the purpose of developing new devices [1, 2]. ZnO is an environmentally friendly II-VI compound semiconductor; it produces highly effective exciton emission at room temperature (RT) and is expected to be applicable in attractive blue-ultraviolet (UV) laser devices owing to its large exciton binding energy of 60 meV. In ZnO crystals, bandedge and visible photoluminescence (PL) appear, depending on the growth methods (e.g., chemical vapor deposition, sputtering, molecular beam epitaxy, and the sol-gel method.) Although it is very important to enhance the band-edge PL intensity for optical applications, it is desirable to decrease the visible PL intensity related to deep defects such as oxygen vacancies at the ZnO surface [3]. Moreover, ZnO single crystals of superior quality have been grown by the hydrothermal method, and good X-ray rocking curves and PL spectra for the samples have been obtained [4]. In an effort to improve the surface states of ZnO single crystals, we investigated the optical and electrical properties of the ZnO samples coated with potassium chloride (KCl). Harada et al. reported the optical properties of ZnO single crystals [5] and nanoparticles [6, 7] coated with an alkali halide. However, the surface state of ZnO coated with KCl is not well understood. In this paper, we report the electrical properties, such as the temperature dependence of the resistivity and the carrier concentration obtained by Hall measurements, to clarify the chlorine (Cl) doping effect and the ZnO surface improvement by KCl. Cl doping will be applied to some electronic devices in the future. For example, the diffusion layers and insulating layers in ZnO field effect transistors can be formed concurrently without using Knudsen cells and ion beams. In addition, X-ray photoemission spectroscopy (XPS) was performed in order to examine the respective bonding states of potassium (K) and Cl in the ZnO single crystals.

KCl powder

ZnO single crystal

quartz plate

KCl thin film

Annealing

Figure 1 Schematic diagram of the method of coating ZnO single crys

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