Grain growth and texture development in lithium fluoride thin films
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We have studied grain-growth and texture development in polycrystalline lithium fluoride thin films using dark-field transmission electron microscopy. We demonstrate that we can isolate the size distribution of 〈111〉 surface normal grains from the overall size distribution, based on simple and plausible assumptions about the texture. The {111} texture formation and surface morphology were also observed by x-ray diffraction and atomic force microscopy, respectively. The grain-size distributions become clearly bimodal as the annealing time increases, and we deduce that the short-time size distributions are also a sum of two overlapping peaks. The smaller grain-size peak in the distribution corresponds to the {111}-oriented grains, which do not grow significantly, while all other grains increase in size with annealing time. A novel feature of the LiF films is that the {111} texture component strengthens with annealing, despite the absence of growth for these grains, through the continued nucleation of new grains.
I. INTRODUCTION
The structure of polycrystalline thin films has significant influence upon their performance in almost all applications. Porosity, grain shape, grain size, grain-size distributions, grain-boundary or triple-junction networks, and grain orientations all influence the film properties and stability. These characteristics of grain structure are developed via grain growth occurring during thin-film deposition and postformation processing such as annealing; extensive research has been done on grain growth in thin films.1–4 Insulating films including polycrystalline alkali fluorides have recently been investigated as promising materials for applications in electronics and optics.5,6 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.7,8 In the past few years, attention has been given to the structural and optical characteristics of polycrystalline LiF thin films as a function of growth conditions.9,10 In a recent study,11 we have identified the growth conditions required to make fully dense films. Grain growth has been studied in bulk12 and thin-film LiF,13 and prior investigators have determined that the grain-growth exponent is anoma-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0049 452
J. Mater. Res., Vol. 23, No. 2, Feb 2008
lously low for ceramics14 and nanocrystalline materials.15,16 Although they have suggested several possible reasons for these exceptionally low grain-growth exponent values, they are not fully understood yet. Techniques available for texture analysis include x-ray diffraction (XRD), which provides information about the crystal orientation distribution of a large region of the sample, but does not provide microstructural information,17,18 and transmission electron microscopy (TEM) Kikuchi patterns from individual grains provid
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