PHOTOCAPACITY STUDY OF GRAIN BOUNDARY RECOMBINATION IN SILICON

Key features of the grain boundary recombination in semiconductors derive from the limitation of the electron and hole flows to the interface, due to diffusion through the potential barrier of the boundary. There follows a feedback effect in the process of recombination, as the electrostatic potential in the barrier is determined in turn by the charge state of the boundary traps. To our knowledge, no satisfactory account has been given so far of this feedback effect on the kinetics of grain boundary recombination. To obtain a consistent picture of the electronic processes taking place at the interface, one has thus to write down a set of coupled equations describing (i) the diffusion of the carriers through the potential barrier of the boundary ; (ii) the recombination of electrons and holes at the boundary traps (Shockley-Read recombination statistics) ; and (iii) the relationship between the variation of the electrostatic potential in the barrier and the occupancy of the boundary traps (Poisson equation). These equations allow one to determine the magnitude of the boundary charge and the recombination current as a function of the density of injected carriers. Last, by computing the current density through for a small ac applied voltage, one obtains the expression for the complex impedance of the boundary, that one needs to interpret the transport properties of a bicrystal under light illumination