Influence of chromium on minority carrier properties in intentionally contaminated n-type mc-Si wafers

Abstract

In n-type silicon the recombination strength of most metallic impurities is neatly smaller than in p-type, due to asymmetric capture cross-sections of these impurities for charge carriers. That is true for several impurities (Fe, Ni, Co, Al) but not for Cr. In this paper we try to demonstrate that Cr is not so detrimental. N-type multicrystalline silicon (mc-Si) wafers were voluntarily contaminated by ion implantation or by deposition of a Cr layer and annealing at 850 °C for 90 min. In contaminated samples τ p and L p were around 20 μs and 120 μm, while the initial values were 100 μs and 400 μm, respectively. After phosphorus diffusion at 850 °C for 30 min, the initial values are practically recovered, due to a marked gettering effect. It appears that Cr is easily removed by heterogeneous precipitation and external gettering. Additional thermal treatments might release Cr atoms segregated at extended crystallographic defects or included in micro-precipitates. However, as Cr atoms are volatilised or are segregated at the top of the multicrystalline ingots during the directional solidification, the Cr concentration is sufficiently low to be not detrimental.