A Structural Study of the Amorphous to Crystalline Transformation in In 2 O 3 Thin Films
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A Structural Study of the Amorphous to Crystalline Transformation in In2O3 Thin Films Burag Yaglioglu, Hyo-Young Yeom, Eric Chason, and David C. Paine Brown University, Division of Engineering, Box D Providence, RI 02910 ABSTRACT We have investigated the structure of sputter deposited amorphous and crystalline indium oxide films by electron diffraction. Selected area diffraction patterns were recorded for both states from which radial density functions were derived. The comparison of the crystalline radial density function to the amorphous one shows that the first nearest neighbor distance corresponding to the In-O bond length is 2.2Å and is the same for both states. A model density function for crystalline structure was used to explore the difference in higher order peak positions. We report that the In-In separation in amorphous state has a single characteristic distance of 3.6Å compared to the crystalline state which has two non-equivalent In sites and consequently different separations as In(1)-In(2) at 3.4Å and In(2)-In(2) at 4.3Å. INTRODUCTION Indium oxide is widely used in optoelectronic applications that require optical transparency in the visible spectrum combined with reasonable conductivity. These properties are the result of indium oxide being an n-type semiconductor with a band gap of between 3.5 and 4 eV [1,2]. Understanding the electrical and optical properties of these films requires structural characterization on the atomic scale [3]. Previous work on indium oxide films has focused primarily on the crystalline state. Indium oxide has the bixbyite crystal structure which has an 80 atom unit cell with the Ia3 space group and a 10Å lattice parameter in an arrangement that is based on the stacking of MO6 coordination groups. The 80 atom unit cell consists of 48 oxygen atoms and 32 indium atoms. Indium is distributed in two non-equivalent sites with ¼ of the indium atoms, In(1) atoms, positioned at the center of a trigonally distorted oxygen octahedron (diagonally missing O) and the remaining ¾, In(2) atoms, positioned at the center of a more distorted and less symmetric octahedron that forms with the removal of two oxygen atoms from the face of the octahedron. By studying the short range structure in both amorphous and crystalline samples, we aim to characterize the change in local atomic ordering during the a/c transformation [4]. In general, the radial density function (rdf) is a curve that oscillates about zero and has maxima that represent frequently occurring atom-atom separations [5]. The theoretical and experimental framework for the calculation of the rdf’s from electron diffraction data is explained in the following sections. Its interpretation is complicated for materials which have more than one type of atom. However, in amorphous In2O3, the rdf would be expected to have a peak at 2.2Å that corresponds to the In-O separation. Higher order peaks, however, have contributions from In-In, In-O and O-O separations as well [6]. To evaluate these effects we used the hard sphere model of crys
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