Nano-precipitates … a new recombination model

We have observed and reported previously that very small precipitates with sizes of the order of 2-10nm can form in silicon from slow diffusing transition metals. These nano-precipitates can act as exceptionally powerful recombination centers. In this paper we present experimental results on the recombination dynamics and show that the behavior is not well described by previous work on larger precipitates or by the conventional Shockley-Read-Hall model widely used for recombination associated with point defects. The basic reason for this is that the Fermi sea of the metal particle system allows a huge degree of freedom for charge exchange and many size dependent effects on the capture rates. These defects, which are Coulomb attractive for the minority carrier can stably capture and bind several minority carriers with large cross section. However majority carrier capture is into a repulsive centre, but the potentials associated with sub 10 nm particles are sufficiently sharp that the strength of tunneling process hugely enhances the majority carrier capture rate. In the limit of ultra-small (but still metallic) systems Coulomb blockade effects will further modify majority carrier capture. All cross sections are net charge state dependent but self-consistent three dimensional solutions to the Poisson and Schrödinger equations can successfully model capture and recombination physics. The measured and calculated trends clearly show that such precipitates may be lifetime controlling in multi-crystalline Si.