Structure of Au-Al 2 O 3 Granular Metal Films
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STRUCTURE OF Au-A1203 GRANULAR METAL FILMS B. ABELES*, Q. ZHANG*, H.H. HUNG*, K.S. LIANG*, H. OHTANI*, S.K BEHAL*, M.M. DISKO*, W.A. LANFORD**, M. GADENNE*** and P. GADENNE*** *Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801 **State University of New York, Albany, NY 12246 ***Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris, Cedex 05, France ABSTRACT We have investigated the structural properties of sputter deposited granular Au-A1203 films. The films exhibit an abrupt percolation transi-, tion from insulating to metallic behavior at the composition of Xc-40 vol.% Au. STM measurements yield images of individual grains and clusters of grains consistent with the TEM images. The surface roughness increases abruptly from 30A to 50A as the Au concentration increases from 35 to 45 vol.%. The roughening of the films in the metallic regime is consistent with behavior of the X-ray reflectivity. INTRODUCTION Granular metals are composites of finely dispersed immiscible metals and insulators with interesting physical properties and many technological applications [1,2]. The granular metal Au-A1203 is a model system for studying structural and electronic properties of this class of materials. The focus of this work were the structural changes near the percolation transition from metallic to insulating behavior. Results are presented for STM and X-ray reflectivity measurements, techniques which had previously not been applied to granular metals. SAMPLE PREPARATION The films were deposited by RF sputtering (2] using the composite Au-A1 2 03 target configuration shown in Fig. 1. The composition was varied by changing the diameter of the four small A120 3 discs. This particular target configuration provided the compositional and thickness uniformity required for the X-ray reflectivity measurements measurements, i.e., -1% uniformity over an area of 3 x 3 cm. The atomic percent of Au and Al in the films were determined by RBS. The volume percent of Au was computed from the atomic percent using [2] 3.0 g/cm3 for the density of sputtered A120 3 . RESISTIVITY MEASUREMENTS The resistivity was measured on films deposited on quartz substrates. The same samples were also used for the X-ray reflectivity measurements measurements. Resistivity measurements in the plane of the films were made using copper stripe electrodes pressed on the films. As can be seen in Fig. 2, the present measurements on thick films agree very well with those previously published [2]. Fig. 2b shows that the resistivities of the films, with the exception of the film at the percolation threshold with 40 vol.% Au, are essentially independent of film thickness, indicating that the films are 3-D. The film with 40 vol.% Au is 3-D for thicknesses larger than 500A. Below this thickness the resistivity
Mat. Res. Soc. Symp. Proc. Vol. 237. @1992 Materials Research Society
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