Defect specific photoconductance: Carrier recombination through surface and other extended crystal imperfections

Abstract

The analysis of carrier recombination through defects using a unified approach, common to all extended defects independent of their dimensionality is presented. The considered crystal defects include surface, grain boundaries, dislocations, and clusters of crystal defects and impurities. It is shown that, at low carrier injection levels, carrier recombination through extended defects is controlled by the electrostatic potential barrier formed at the defects. The barrier model is applied to the analysis of the photoconductance in the defect controlled regions. Unlike in the case of recombination through point defects in the bulk, which is characterized by the linear dependence of the steady-state photoconductance on the light intensity and exponential decay of the photoconductance after cessation of the illumination, the photoconductance associated with recombination through extended defects under depletion conditions exhibits logarithmic dependence on the light intensity and characteristic logarithmic decay after cessation of the illumination. The predictions of the barrier-controlled recombination model are compared with the measurements of photoconductance in electronic grade silicon. Differentiation between time dependences of the photoconductance decay associated with recombination through the point defects and through the extended crystal imperfections is used as a basis of the defect specific photoconductance (DSPC) methodology. It is shown that DSPC allows for the separation of the bulk and surface characteristics without any additional surface treatments, enabling electrical characterization of both bulk and thin-film wafer structures critical for photovoltaic (PV) solar cell and high-brightness light emitting diode (HB-LED) manufacturing. The comparison of the "barrier-controlled recombination" and standard diffusion model based on surface recombination velocity is discussed. © 2012 American Institute of Physics.