Diffusion, Precipitation, and Cavity-Wall Reactions of Ion-Implanted Gold in Silicon
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DIFFUSION, PRECIPITATION, AND CAVITY-WALL REACTIONS OF ION-IMPLANTED GOLD IN SILICON S.M. MYERS AND G.A. PETERSEN Sandia National Laboratories, Albuquerque, NM 87185-1056 ABSTRACT The diffusion of Au in Si and its binding to cavities and to precipitates of the equilibrium Au-Si phase were investigated in the temperature range 1023-1123 K using ion implantation and Rutherford backscattering spectrometry. The diffusivity-solubility product for interstitial Au was found to be about an order of magnitude greater than the extrapolation of previous, indirect determinations at higher temperatures. Chemisorption on cavity walls was shown to be more stable than Au-Si precipitation by 0.1-0.3 eV in the investigated temperature range, indicating that cavities are effective gettering centers for Au impurities. INTRODUCTION The transport and thermodynamics of Au in Si have been widely investigated for several reasons: deep electronic levels associated with Au are used to reduce carrier lifetimes in Si; Au impurities can be detrimental to devices as a result of the same deep levels; and characterization of Au transport serves to illuminate the diffusion of Si self-interstitials [see, e.g., Refs. 1-3 and citations therein]. Recent studies have explored the interaction of Au with cavities in Si, these voids being formed by H ion implantation and annealing [4]; this research was motivated by the potential use of cavities for gettering of Au and other detrimental impurities. In the present paper, we report measurements of the diffusive redistribution of ionimplanted Au between layers in Si that contain either precipitates of the molten equilibrium Au-Si phase or cavities formed by He ion implantation and annealing. Resulting data are analyzed to obtain new fundamental information on underlying atomic processes including 1) the diffusivity-solubility product for mobile Au in solution, 2) evidence for the coexistence on cavity walls of ordered and disordered chemisorbed states that are more stable than the bulk equilibrium Au-Si phase, and 3) binding energies for Au in the Au-Si phase and chemisorption states. DIFFUSIVITY-SOLUBILITY PRODUCT FOR INTERSTITIAL GOLD IN SILICON The transport of Au in Si is complicated by occupation of both interstitial and substitutional sites [1-3]. The interstitial (i) is highly mobile, while the substitutional (s) atom is virtually immobile. Substitutional Au lies lower in energy, however, and in equilibrium the solution is overwhelmingly substitutional. When Au diffuses into Si from the equilibrium Au-Si phase, it migrates as an interstitial but undergoes reversible conversion to the substitutional state. The conversion is believed to occur predominantly by formation of a Si self-interstitial atom, Aui
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