Carrier mobilities in multicrystalline silicon wafers made from UMG-Si

We investigate majority- and minority-carrier mobilities in multicrystalline silicon (mc-Si) made from upgraded metallurgical-grade (UMG) feedstock. Since UMG-Si contains high amounts of both boron and phosphorus, a decrease of the carrier mobility due to increased scattering at ionized impurities is expected. Minority-carrier mobilities are determined by measuring effective carrier lifetimes τ_eff on as-cut wafers, where τ_eff is limited by carrier diffusion to the unpassivated surfaces. By examining a wafer cut vertically from the mc-Si ingot, we indeed find a reduction in minority-carrier mobility μ_min with increasing dopant density. In addition, we find a further strong reduction of μ_min in the transition region from p - to n -type silicon. Similar results are obtained regarding the majority-carrier mobility μ_maj, which is investigated by combining measurements of the resistivity ρ and the equilibrium hole concentration p₀ (equilibrium electron concentration n₀ in n -type material) obtained from electrochemical capacitance-voltage measurements. Apart from an overall reduction in μ_maj compared to values measured in non-compensated p -type mc-Si, an additional pronounced decrease of the mobility with increasing compensation level is observed. This additional reduction can be explained by reduced screening of the ionized scattering centers. We propose a parameterization of the experimental data based on the Brooks-Herring equation and find excellent agreement between the two (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)