Characterization of grain boundaries in silicon

Abstract

Results from several research activities on properties of grain boundaries in silicon materials are reported. Zero-bias conductance and capacitance measurements at various temperatures were used to study trapped charges and potential barrier height at the boundaries. Deep-level transient spectroscopy (DLTS) was applied to measure the density of states at the boundary. The result is consistent with the model in which the density of states increases as the states become deeper. Anomalous phenomena have been observed by DLTS and other methods, which can be only explained by a new model in which the spatial distribution of the localized states is not uniform along the boundary in the microscopic scale. A study of photoconductivity of grain boundaries in p-type silicon demonstrated the applicability of the technique in the measurement of minority carrier recombination velocity at the grain boundary. The data are consistent with the concept of recombination velocity increasing with boundary-state density and light intensity. Enhanced diffusion of phosphorus at grain boundaries in three cast polycrystalline photovoltaic materials was studied. Enhancements for the three were the same, indicating that the properties of boundaries are similar, although grown by different techniques. Grain boundaries capable of enhancing the diffusion were found always to have strong recombination activities; the phenomena could be related to dangling bonds at the boundaries. The present study gives the first evidence that incoherent second-order twins of (111)/(115) type are diffusion-active.