Stress-induced surface characterization by wavelet and fractal analysis in Ga-doped ZnO thin films
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Stress-induced surface characterization by wavelet and fractal analysis in Ga-doped ZnO thin films Chenlei Jing1, Yang Hu1 and Wu Tang1 1 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China ABSTRACT The Ga-doped ZnO (GZO) were deposited by magnetron reactive sputtering on glass substrates at room temperature with different deposited times to obtain various thickness. The root-mean-square (RMS) roughness obtained from the atomic force microscopy (AFM) images is observed to shift linearly with the deposited time, the fractal geometry and multi-resolution signal decomposition (MRSD) based on wavelet transform were applied on the surface profiles and the results does not synchronously changes as the thickness, which is related to the profile's frequency. The calculated compressive in-plane stress of highly c-axis oriented GZO films also shows an irregular variation as the increase of film thickness, what’s more, the in-plane stress and fractal dimension exhibit a polynomial relationship and the two parameters can be used for describing the surface morphology. INTRODUCTION As a wide band gap semiconductor, zinc oxide has attracted many researchers in the last few decades. With the optimum electrical and optical properties, the application of ZnO films is expanded to solar cells, transistors and other optoelectronic applications [1]. Many techniques can be applied to fabricate thin films and among them RF magnetron sputtering is regarded as the robust technique and easily scalable for large area deposition [2, 3]. Since the surface morphology significantly affects the film physical properties, and with different sputtering methods and under various preparation conditions the surface fluctuation shows diversity [4], and the investigation of thin film surface evolution can offer an insight on the film growth [5], the interface morphology analysis for various thicknesses thin films is necessary. As the thin films deposited under non-equilibrium circumstances, the traditional measurements such as rootmean-square (RMS) roughness and average roughness which only gives the height information may not sufficient to describe the irregularity and complication of surface fluctuation, hence the multi-resolution signal decomposition (MRSD) based on the wavelet transform and fractal geometry are applied as a description of surface morphology. What’s more, the stress is another factor related to physical properties [6, 7], to analyze the surface with different stresses, the inplane stress and the fractal dimension which characterizes the fractal feature were calculated. EXPERIMENTAL DETAILS The Ga doped ZnO thin films which are analyzed in this article were deposited through magnetron reactive sputtering on glass substrates at room temperature under the pressure of 7×10-4Pa. The GZO thin films samples were obtained with the sputtering power of 150W under high purity Ar gas and the sputtering pressure is 0.5Pa, the applied GZO ceramic
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