The coexistence of silicon borides with boron-saturated silicon: Metastability of SiB 3

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The coexistence of silicon borides with boron-saturated silicon: Metastability of SiB3 T. L. Aselage Sandia National Laboratories, Albuquerque, New Mexico 87185-1421 (Received 9 July 1997; accepted 22 December 1997)

The silicon-rich end of the Si-B phase diagram, defining the silicon boride(s) that coexist in equilibrium with boron-saturated silicon, is poorly known. Understanding this equilibrium has implications for the processing of p 1 silicon wafers, whose boron concentrations are near the solubility limit. Additionally, silicon boride precipitates produced by boron-ion-implantation and annealing of crystalline silicon have recently been shown to be efficient internal getters of transition metal ions. The experiments described in this paper probe the stability of these silicon borides. A phase with a boron-carbide-like structure, SiB3 , grows from boron-saturated silicon in both the solid and the liquid state. However, SiB3 is not found to be stable in either circumstance. Rather, SiB3 is a metastable phase whose formation is driven by the relative ease of its nucleation and growth. The silicon boride that exists in stable equilibrium with boron-saturated silicon is SiB6 . A qualitative understanding of the metastability of SiB3 comes from recognizing the conflict between the bonding requirements of icosahedral borides such as SiB3 and the size mismatch between silicon and boron atoms.

I. INTRODUCTION

Substitution of boron atoms for silicon atoms in silicon crystals is a well-known means of introducing positive charge carriers. The solubility of boron in the silicon lattice is relatively small: about 1020 boron atomsycm3 (ø0.2 at. %) at 1000 ±C.1,2 Heavily doped, p 1 silicon wafers with boron concentrations near this solubility limit are extensively manufactured. When the concentration of boron exceeds its solubility limit, precipitates of a silicon boride can appear within the silicon crystal. For example, silicon boride precipitates were produced by implantation of boron ions to local concentrations of about 1021ycm3 , followed by either a 2 h anneal at 1000 ±C, a 2 h anneal at 1100 ±C, or a 24 h anneal at 1200 ±C.3,4 Strikingly, transition metal impurities such as Cu, Fe, Co, and Au are found to segregate preferentially within these silicon boride precipitates.3,4 This observation suggests that silicon boride precipitates might function as very efficient internal getters of transition metal ions in silicon. Silicon boride precipitates formed by ion implantation and subsequent anneals as described above are amorphous.3,4 High spatial resolution, quantitative EELS spectra obtained from these precipitates suggest their B : Si ratio is approximately 2 : 1.4 Silicon boride precipitates have also been formed in supersaturated silicon having as-grown, uniform boron concentrations of 3 3 1020ycm3 .1 Annealing these samples at 1100 ±C for long times, 240 h, produced crystalline silicon boride precipitates. Electron diffraction patterns of these precipitates 1786

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