| Large-grained, multi crystalline silicon for solar cell applications is a very inhomogeneous material with localized regions of high dislocation density and large impurity and precipitate concentrations which limit solar cell efficiency by acting as carrier recombination sites. Due to slow dissolution of precipitates in multi crystalline silicon, these regions cannot be improved by conventional gettering treatments for removal of metal impurities which give good results for single crystal silicon. Extended, high-temperature aluminum gettering is shown to successfully improve the minority carrier diffusion lengths in these localized, poor quality regions and to homogenize the electrical properties of multicrystalline silicon wafers.; Cold is a substitutional-interstitial impurity in silicon, whereby its diffusion creates non-equilibrium concentrations of the native point defects in silicon, self-interstitials and vacancies. The diffusion. of gold is therefore controlled by the relaxation of these non-equilibrium point defect concentrations by dislocations or surfaces. Deliberate gold contamination of single-crystal, dislocation-free silicon wafers has been performed followed by aluminum gettering of the gold in silicon. It is shown that the aluminum gettering process can successfully getter gold in silicon, and that the outdiffusion of gold from the silicon is controlled by the proximity to the wafer surfaces rather than by proximity to the aluminum gettering layer.; The indiffusion of gold in silicon has been widely demonstrated by experiments and modeling to be dominated by the Kick-Out mechanism, whereby the indiffusion of gold causes supersaturation of self-interstitials, with a minimal contribution from the Frank-Turnbull mechanism, whereby the indiffusion of gold causes undersaturation of vacancies. Fitting of the above-mentioned experimental results on aluminum gettering of gold has been done to show that the opposite holds true for outdiffusion of gold, i.e., the Frank-Turnbull mechanism has the dominant contribution. The fundamental physical reason for this phenomenon is identified, viz., that the diffusion mechanism that causes supersaturation of a point defect is likely to dominate over the one that causes undersaturation of the other point defect as the concentration gradient for outdiffusion of the supersaturated species is much larger than the concentration gradient for indiffusion of the undersaturated species. |
