Effects of misfit strain on properties of ZnO films grown by pulsed laser deposition
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Effects of misfit strain on properties of ZnO films grown by pulsed laser deposition M.C. Park, W.H. Yoon, D.H. Lee, and J.M. Myoung Information & Electronic Materials Laboratory, Dept. of Metallurgical Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120-749, Korea S.H. Bae, S.Y. Lee, and I. Yun Dept. of Electrical and Electronic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120-749, Korea ABSTRACT A series of ZnO films with various thicknesses were prepared on (0001) sapphire substrate by pulsed laser deposition (PLD). Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis were utilized to investigate the effects of misfit strain on the surface morphology and the crystallinity. The electrical and optical properties of the films were also investigated as a function of the film thickness. It was found that the crystalline quality, electrical and optical properties of the films depended on the film thickness and were improved with increasing the film thickness. This is attributed to the fact that the films thinner than 400 nm are under the severe misfit strain, which decreases as the film thickness increases further. INTRODUCTION ZnO is a very attractive material for optoelectronic device applications, such as blue-, violet-, and ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs), as it has a direct and wide band gap of 3.3 eV at room temperature [1]. Moreover, owing to the large exciton binding energy (~60 meV), ZnO thin films exhibit very strong emissions by excitons even at room temperature [2]. Despite these advantages, successful realization of optical devices has not been reported to date as it is difficult to achieve p-type ZnO films [3]. Because of these difficulties, it has only been used for other applications such as transparent conducting electrodes in display devices, surface acoustic wave (SAW) devices, and solar cell windows [4,5]. However, recent progress in the crystal growth technique has led to the realization of high-quality epitaxial layers exhibiting room-temperature excitonic lasing, high-temperature excitonic stimulated emission, and homostructural p-n junctions [6,7]. The PLD technique is very unique for the growth of oxide materials because the oxygen plasma created by the pulsed laser is very energetic and its density is easily controllable by changing the oxygen pressure. To date, there are several reports regarding the effect of oxygen pressure and growth temperature on the properties of ZnO films grown by PLD [8]. However, there has been no report on the effect of misfit strain on properties of ZnO grown by PLD. In this paper, we report on the effect of misfit strain on the surface morphology and structural, electrical, and optical properties of ZnO thin films grown on c-plane sapphire by PLD. It is found that as the film thickness increases, the crystallinity and optical and electrical properties of ZnO thin films are improved. Furthermore, ZnO films thicker than 400 nm exhibit the near-bulk properties.
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