Strong improvement of diffusion length by phosphorus and aluminum gettering

Abstract

The minority carrier diffusion length in polycrystalline silicon has been strongly improved by using several gettering processes. These processes include different surface treatments followed by conventional thermal annealing (CTA) performed at temperatures between 800 and 950 °C. The n+p structures with a back lapped surface exhibit a maximum increase of the diffusion length from 35 to 140 μm for 45 min annealing duration at temperatures of 900 and 950 °C. The realization of a back surface field (BSF) on the lapped surface by aluminum deposition followed by a CTA cycle at 950 °C, with slow cooling rate (2 °C/min) induces a strong additional gettering, resulting in the increase of the minority carrier diffusion length up to 350%.

Building intuition of iron evolution during solar cell processing through analysis of different process models

“…The exact profile shape depends on the metal paste being used, and here we use a typical profile, which is the same as that used in [67]. We focus on the effect of the thermal step on the iron dynamics in the presence of the phosphorus-rich layer and do not include in our analysis other aspects that may have a secondary effect (such as the potential hydrogenation due to the silicon nitride layer [89] or gettering if an aluminum back surface field is present [90,91]). We use a no-flux boundary condition for the phosphorus in these firing simulations to describe the fact that there is no additional phosphorus source present at the surface during firing, and we simulate a phosphorus-rich emitter on only one side of the wafer.…”

Recent Progresses in Understanding Gettering in Silicon

“…A layer of Al-Si liquid on the Si wafer surface will also getter Au because its solubility is very high in the Al-Si layer and extremely low in Si. Moreover, there is a synergistic effect of using Al and P indiffusion together in the improvement of solar cell efficiencies [2][3][4][5], meaning that the gettering effectiveness exceeds that of sequentially performing the two gettering schemes. We have modeled gettering of Au by P indiffusion, by the use of an Al-Si liquid layer, and by the simultaneous use of the two [17,19].…”

“…Gettering of metallic contaminants away from the device active regions has already become an essential part of the IC manufacturing technology using Czochralski (CZ) Si wafers [1], and is experiencing an increasing attention in Si solar cell fabrication for improving cell efficiency [2][3][4][5]. Because of its indirect bandgap, the Si carrier lifetime component due to band-band activities is on the order of 1 s with the usually observed lifetimes of much less than 1 ms due to electrically active impurities (metals) and defects.…”

Polycrystalline silicon S-diode fabricated using phosphorus thermal diffusion along grain boundaries

“…Recently it was found the correlation of the phosphorus diffusion and enhanced recombination activity of GB in the multicrystalline silicon solar cells [4]. Strong improvement of diffusion length by phosphorus gettering was observed by Loghmarti and co-authors [5]. The rapid phosphorus diffusion and its availability to passivate the dangling bonds at GB can be used in order to form n-channels along GB in poly-Si devices, such as transistors [6].…”