Morphological Transformations in the Crystallization of TeSe-Halide Thin Films
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MORPHOLOGICAL TRANSFORMATIONS TeSe-HALIDE THIN FILMS
IN
THE
CRYSTALLIZATION
OF
A. Blatter*, C. Ortiz and J. L. Adam ** IBM Almaden Research Center, 650 Harry Rd., San Jose, CA 95120. * Perm. address: Institute of Applied Physics, Sidlerstr. 5, CH-3012 Berne, Switzerland. ** Lab. Verres et Ceramiques, Universite de Rennes, Campus Beaulieu, 35042 Rennes, France.
ABSTRACT Amorphous Te 3Se 4 X (X = Br,I) thin films were prepared by RF sputter deposition where the halide is used to stabilize the amorphous structure. Crystallization in the films was induced by heating with pulsed laser irradiation, with an electron beam, and in a furnace. Crystallization was achieved when the halide was depleted which became more difficult the higher its mass and concentration. As a consequence, crystallization was highly anisotropic and confined to a thin surface layer with a lateral size to thickness ratio of up to 104. We observed various stages of the process. At low temperature, a spherulitic-type growth produced a low-dimensional branching morphology by non diffusive processes such as rotation. Increasing temperature favored the formation of higher dimensions with patterns typical for aggregation processes. At the highest temperatures material flow enabled the formation of three-dimensional structures. In case of the films with Iodine, facetted single crystals grew out of the surface to sizes many times larger than the original film thickness. INTRODUCTION There have been many studies on the crystallization of amorphous chalcogenide films [1,2] because of their applications in the photocopy field and in phase change optical storage. Here, we present the morphologies that develop during the crystallization of Te 3Se 4X (X = Br,I) compounds, which are different for I and Br containing films. The halide acts as an inhibitor to the crystallization [3] and must be first depleted. This was clearly shown in a previous paper [4] where the crystallization of TeSel is limited to a thin surface layer of 5nm and takes place when the halide is diffused out by the heating process. In this paper we report the evolution of the various observed microstructures and their dependence on temperature and time. We show that the characteristic features of the crystallisation vary over the whole range from one (ID) to three (3D) dimensions. EXPERIMENTAL The films were deposited by RF sputtering with an Argon pressure of 1 to 4 Pa. The targets of Te-Se-I and Te-Se-Br were cast as discs and annealed below the glass temperature of the alloys to remove internal stress. Films of thickness 40-80 nm and compositions Te 3Se 4 Br and Te3 Se 4 I were deposited using RF power levels of 30 W and 10 W, respectively. We used three different heating sources to induce the crystallization. The first one was a pulsed argon ion laser beam (A = 514.5 nm) focussed to a 10 pm FWHM spot in the experimental set up described in Ref. [5]. The pulse lengths were varied from 100 is to 3 min. This corresponds to isothermal heating conditions as the steady state was reached after
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