Effects of substrates roughness on c-axis preferred orientation of ZnO films deposited by rf magnetron sputtering
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Effects of substrates roughness on c-axis preferred orientation of ZnO films deposited by rf magnetron sputtering Jae Bin Lee, Sanghyon Kwack, and Hyeong Joon Kim School of Material Science & Engineering, Seoul National University, Seoul 151-742, Korea ABSTRACT We investigated the effect of substrate surface roughness on c-axis preferred orientation of ZnO films deposited by radio frequency (rf) magnetron sputtering. We used as substrates a bare Si(100), evaporated Au/Si(100), evaporated Al/Si(100), and sputtered Al/Si(100), of which rms roughness by atomic force microscope (AFM) were 0.127, 1.71, 2.11, and 6.5~11.8 nm, respectively. The crystallinity and the c-axis preferred orientation of ZnO films strongly depended on the surface roughness of the used substrates. INTRODUCTION Recently, extensive studies have been carried out to manufacture film acoustic wave devices using ZnO thin films. In order to get similar properties to single crystal ZnO necessary for optimum acoustic wave devices, polycrystalline ZnO thin films must meet two requirements; 1) c-axis preferred orientation, and 2) unvoided grain boundary. Especially, the c-axis preferred orientation of ZnO film is the most important factor for applications of acoustic wave devices. Till now, numerous articles have been reported for the preparation and characterization of ZnO thin films having high c-axis preferred orientation. These articles include the effects of working pressure, input power, oxygen partial pressure [1,2], substrate temperature [2], position [2], Zn/ZnO ratio [1], oblique deposition [3,4], electric field distribution [4], thickness [2,5], and so forth. In many published articles concerned with the c-axis preferred orientation, most of ZnO films were deposited on smooth substrates such as Si, sapphire, and glass wafer. We have investigated the influence of the various substrates, especially surface roughness, upon the quality of ZnO films deposited on them. EXPERIMENTAL
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The rf magnetron sputtering was used in the experiment. Among many deposition methods, sputtering is the most favorable and popular deposition method since it is possible to obtain well c-axis oriented and uniform ZnO film even on amorphous substrates at high deposition rate [6]. We prepared four substrates; Si (100), evaporated Au/Si (100), evaporated Al/Si (100), and sputtered Al/Si (100). We used the sintered ZnO target (4”, 99.99 %, Cerac Co. Ltd). The distance from ZnO target to the substrate was 7.6 cm. The sputtering chamber was pumped down to 2X10-5 Torr using a turbo molecular pump prior to introduction of Ar-O2 gas mixture. The oxygen content varied from 0 to 70 % and the total working pressure was 10 mTorr. The substrate temperature and the rf input power were 25oC and 150 W, respectively. During sputtering, the substrate temperature was elevated unintentionally up to about 100 oC. The substrate was rotated by 40 rpm. The crystallographic texture was characterized by x-ray diffraction (XRD) measurements. Normal XRD diffraction spectrum provides informati
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