Crystallization of Indium Implanted Amorphous Silicon

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CRYSTALLIZATION OF INDIUM IMPLANTED AMORPHOUS SILICON E. Nygren, J.S. Williams and A. Pogany RMIT, Microelectronics Technology Centre, Melbourne 3000, Victoria, Australia R.G. Elliman CSIRO, Chemical Physics, Clayton 3168, Victoria, Australia G.L. Olson Hughes Research Laboratories, Malibu, CA, 90625 U.S.A. J.C. McCallum Melbourne University,

School of Physics, Parkville 3052, Australia

ABSTRACT The annealing behavior of In+-implanted amorphous Si layers is reviewed. Particular attention is given to the amorphous to polycrystalline transformation for peak In concentrations > 0.5 atomic percent. New data concerning the transformation rate, associated In transport and microstructure are presented and phase transformation mechanisms are discussed. INTRODUCTION The study of any metastable material such as amorphous Si (a-Si) is complicated by the degree of metastability and its influence on the properties being measured. For example, the study of diffusion in a-Si is necessarily limited to those species which diffuse a measurable amount for annealing conditions which precludes crystallization. Such measurements have been made for the diffusion in a-Si of some of the so-called crystalline fast diffusers, namely, Au, Cu, and Ag[1]. Somewhat surprisingly, the measured diffusion data agree well with previously obtained trace values for substitutional diffusion in the respective crystalline systems. Indeed, the activation energies (1.0 - 1.6 eV) and prefactors for diffusion in crystalline and amorphous Si show remarkable agreement even though the values for the amorphous systems were determined at much lower temperatures and for much higher impurity concentrations. Direct comparison of the data at the same temperature and concentration is impossible for two reasons: first, the a-Si will not withstand the high temperatures used in the crystalline experiments without undergoing crystallization itself and second the equilibrium solubility of these impurities in crystalline Si (c-Si) at the temperatures of the amorphous experiments is too low to be practically monitored. The diffusion behavior of Au, Cu, and Ag in a-Si is intriguing because it suggests that models for diffusion in crystalline Si might be applied with some success towards understanding transport in a-Si. Reported observations of In diffusion in a-Si at temperatures as low as 350 0 C[2] appear, however, to be inconsistent with this conclusion. One estimate of the indicated diffusivity at 600 0 C puts it at least 10 orders of magnitude larger than the crystalline extrapolation. It was this startling observation that inspired our investigation of the a-Si(In) system. This paper reviews the current understanding of the annealing behavior of this system. In particular, we concentrate on the intriguing transformation from amorphous to polycrystalline material observed at high In concentrations. OVERVIEW The proposed free energy diagram for Si, shown in Fig. 1, indicates that a-Si is metastable with respect to crystalline material at all temperatures Mat. Res. Soc. S