The Enhancement of the Interdiffusion in Si/Ge Amorphous Artificial Multilayers by Additions of B and Au
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THE ENHANCEMENT OF THE INTERDIFFUSION IN Si/Ge AMORPHOUS ARTIFICIAL MULTILAYERS BY ADDITIONS OF B AND Au B. Park,) and F. Spaepen Division of Applied Sciences, Harvard University, Cambridge, MA 02138 J.M. Poate and D.C. Jacobson AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 ABSTRACT Amorphous Si/ amorphous Ge artificial multilayers were prepared by ion beam sputtering. Boron or gold impurities were introduced into the Si/Ge multilayers by ion implantation or during the sputtering deposition. Diffusion coefficients were determined by measuring the decrease in the intensity of the first order X-ray diffraction peak resulting from the composition modulation. It was found that the interdiffusion of Si and Ge in their amorphous phase can be enhanced by doping. The enhancement factor is independent of the degree of structural relaxation, as observed by the decrease of diffusivity with annealing time, of the amorphous phase. A model is proposed that describes this behavior in terms of electronic effects, introduced by the dopants, on the pre-existing structural defects governing diffusion. INTRODUCTION Atomic diffusion in amorphous covalent network formers, such as Si and Ge, is most easily thought of as governed by the presence and motion of structural defects like broken bonds, vacancies or interstitials. These non-equilibrium structural defects, which are present as a result of the growth (deposition) or implantation processes, have been used to interpret, respectively, the results of thermal diffusion [1] or ion beam-enhanced diffusion [1-3] measurements in amorphous Si/ amorphous Ge artificial multilayers. The artificial multilayer technique is known to be the most sensitive method for diffusivity measurements [4], and has been used to study the relatively slow diffusion [1,5] of the covalent network formers themselves, i.e., the Si and Ge atoms, without the risk of crystallization. Because the measurement is non-destructive, it is also suitable for the observation of the possible transients or effects of structural relaxation in the amorphous phase. The presence of dopant atoms in the amorphous phase may affect the concentration or character of the structural defects that control the diffusivity of the host atoms. Enhancement of the self-diffusivity by doping has been observed in crystalline Si and Ge, and has been explained by changes in equilibrium concentrations of defects due to a shift of the Fermi level [6,7]. It is also known that incorporation of dopants causes enhancement of the solid phase epitaxial (SPE) regrowth rate [8-12]. This is interpreted by electrically-induced generation of structural defects [13-15]. One of the problems is to understand whether these defects originate from the crystal-amorphous interface or from the bulk amorphous phase. In this paper, the effect of dopants on the interdiffusion in Si/Ge amorphous artificial multilayers are investigated. Although the atomic mechanisms for diffusion and SPE are not identical, they may be similar, since they both require bond
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