Texture Evolution of Lithium Fluoride Thin Films by Nucleation
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Texture Evolution of Lithium Fluoride Thin Films by Nucleation Hakkwan Kim and Alexander H. King School of Materials Engineering, Purdue University, 501 Northwestern Avenue, West Lafayette, IN, 47907
ABSTRACT We have used a transmission electron microscope (TEM)-based method to extract grain
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size information for 111 surface normal grains in lithium fluoride (LiF) thin films, and applied this to analyze textures as a function of substrate temperature and annealing time. The size distributions of grains diffracting into the (111)+(200) and (220) rings were measured separately using dark field (DF) TEM images. From these data, we deduce the size distribution
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of 111 surface normal grains based on the assumption that only 3 principal textures (100), (110) and (111) exist in films. The (111) texture formation was also observed by x-ray diffraction (XRD). For all deposition and annealing conditions, the grain size data can be matched to lognormal distributions within an acceptable error, but at longer annealing times the distribution becomes bimodal. A novel feature of the LiF films is that the (111) texture component strengthens with annealing and substrate temperature, through the nucleation of new grains rather than the growth of existing ones. INTRODUCTION The structure of polycrystalline thin films has significant influence upon their performance in almost all applications. Grain sizes and preferred orientations influence the film properties and stability. Insulating films including polycrystalline alkali fluorides, have recently been investigated as promising materials for applications in electronics and optics [1,2]. Among the alkali halides, LiF thin films are of commercial and scientific interest for a variety of reasons including their possessing the largest band gap among solid crystals, and having point defects that can be optically active at room temperature [3,4]. In the past few years much attention has been given to the structural and optical characteristics of polycrystalline LiF thin films as a function of growth conditions [5,6]. However, the grain size distributions in ceramic thin films such as LiF, have neither been well characterized experimentally nor correlated with modeling. Techniques available for texture analysis include x-ray diffraction, which provides information about the crystal orientation distribution of a large region of the sample but does not provide microstructural information and TEM kikuchi patterns from individual grains which provide a means for relating orientation and grain size directly, but do not provide meaningful statistics about the texture or the grain size distribution. Electron back scatter pattern analysis provides both kinds of information, but requires extended data collection times. In this study, we apply a DF-TEM method to extract grain size information as a function of orientation, with meaningful statistics, for polycrystalline LiF films. The results suggest a previously unencountered mechanism of texture development in thin films.
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