RHEED Pole Figure Measurements of Biaxial Thin Film Growth Front Evolution
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RHEED Pole Figure Measurements of Biaxial Thin Film Growth Front Evolution Gwo-Ching Wang, Yu Liu, Churamani Gaire, Wen Yuan, and Toh-Ming Lu Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute 110, 8th Street, Troy, NY 12180
ABSTRACT The most frequently used characterization technique for biaxial texture formation in thin films is x-ray pole figure analysis. However, x-rays interact weakly with matter and can penetrate a few microns deep into the film. The texture obtained by x-rays is therefore an average texture from the entire thickness of the film. As the texture of a film often changes during growth, information on the basic mechanisms that control the final texture is often lost. In contrast electrons interact strongly with matter and they have very limited penetration and escape depths of a few nm. In this paper we will show how we can use our newly developed reflection high energy electron diffraction (RHEED) surface pole figure technique to probe the surface texture evolution of the growth front from the initial stage (nm thick) to the later stage. The RHEED pole figure technique is a surface-sensitive technique that allows us to obtain information on the dynamic behavior of texture evolution of the growth front during film deposition. We shall explain the principle, measurement, and construction of such RHEED surface pole figures. An example of the biaxial texture evolution of CaF2 due to the atomic shadowing effect during oblique angle deposition is described. INTRODUCTION The distribution of crystal orientation, or the texture, of nanocrystalline grains is a fundamental property and is one of the most fascinating topics in the study of nanocrystalline thin-film growth [1]. The morphology and texture of a film directly controls the optical, magnetic, mechanical, electrical, and electrochemical properties of the material [2, 3]. In particular, catalysis and energy conversion devices depend on the crystal orientation of the films. Crystal orientation combined with reduction of crystal sizes holds promise for a variety of energy applications [4]. Recently there has been increasing interest in artificially induced crystal texture orientation during film growth. This is driven by a number of applications such as high Tc superconductors, solar cells, and displays. Well-ordered crystal orientation of a surface can serve as a substrate or a buffer layer on which useful films with desired orientation can grow. A recent example is the growth of biaxially textured films as buffer layers for subsequent growth of highly oriented (both out-of-plane and in-plane) high Tc superconducting films to achieve high current density [5]. It has also been suggested that this approach can be extended to include the growth of biaxial semiconductor films on biaxially textured buffer layers on glass [6-8], amorphous substrates [9, 10] and metal sheets [11, 12] for efficient and inexpensive solar cell applications. Other potential applications include solid state lighting and optoelectronic devices
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