Carrier mobilities in multicrystalline silicon wafers made from UMG‐Si

Abstract

AbstractWe 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 τeff on as‐cut wafers, where τeff 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 μmin with increasing dopant density. In addition, we find a further strong reduction of μmin in the transition region from p ‐ to n ‐type silicon. Similar results are obtained regarding the majority‐carrier mobility μmaj, 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 μmaj 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)