Diffusion of Au in Amorphous Si Measured by the Artificial Multilayer Technique
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Diffusion of Au in Amorphous Si Measured by the Artificial Multilayer Technique E. Nygren), B. Parkb), L.M. Goldman, D.T. Wu, A.V. Wagner and F. Spaepen Division of Applied Sciences, Harvard University, Cambridge, MA 02138 ABSTRACT The diffusivity of Au in amorphous Si (a-Si) has been determined in the temperature range 200-260'C using and Ar-ion sputter deposited artificial multilayered films of a-Si and a-Si(O.7at%Au) with repeat lengths between 44 and 48 A. Diffusion on lengths scales of the order of interatomic spacings have been investigated. The Au diffusivity exhibits an Arrhenius temperature dependence with an activation enthalpy of 1.2eV and shows good agreement with extrapolations of higher temperature Au diffusion data obtained by Rutherford backscattering spectrometry (RBS) in ion-implanted, CVD, and sputter deposited a-Si. The measured Au diffusivity also shows a significant time dependence. INTRODUCTION Atomic diffusion in crystalline silicon has been studied extensively for decades, and can be understood in detail by the concentration and motion of structural defects in thermal equilibrium [1,2]. Experimental investigations are usually restricted to high temperatures because limited solubilities and low diffusivities hamper detection. By contrast, there have been relatively few studies of atomic transport in the amorphous phase of silicon [3-8], in which non-equilibrium defects, introduced by the growth or implantation processes, can play an important role. Difficulties with these investigations arise due to the metastabilities of the material. Measurements must be made at temperatures sufficiently low or for times sufficiently short to prevent crystallization and/or precipitation. These restrictions necessitate the use of very sensitive techniques. Au is a so-called 'fast diffuser' in crystalline silicon (c-Si). It also diffuses fast enough in amorphous silicon (a-Si) to permit study by conventional techniques, such as Rutherford backscattering spectrometry (RBS), without complications from phase transformations [4,5]. Diffusion of Cu and Ag in a-Si have also been studied this way [4]. However, c-Si 'slow diffusers', such as In, As, Sb and Bi, show no detectable diffusion in RBS measurements on a-Si. The artificial multilayer technique is the most sensitive technique available for the measurement of diffusion. In the technique, the intensity of radiation (x-ray) diffracted from the composition modulation of a multilayer film with repeat length of a few nanometers is monitored as a function of annealing time. The multilayer film functions as a collection of parallel diffusion couples and the change in diffracted x-ray intensity can be related to the amount of interdiffusion. By this method, diffusivities as low as 10- cm2/sec and diffusion or mixing lengths smaller than an interatomic distance can be routinely measured [9-11]. The technique is non-destructive so diffusivity can be measured as a function of time. In this paper we report the first measurements of diffusion, on a length scale on the ord
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