Direct monitoring of minority carrier density during light induced degradation in Czochralski silicon by photoluminescence imaging

Abstract

In this paper, we present a new method for studying the light induced degradation process, in which the minority carrier density is monitored directly during light soaking by photoluminescence imaging. We show experimentally that above a certain minority carrier concentration limit, Δnlim, the boron oxygen (B-O) defect generation rate is fully independent of the injected carrier concentration. By simulation, we determine Δnlim for a range of p-type Czochralski silicon samples with different boron concentrations. The normalized defect concentrations, Nt*, are determined for the same samples by time-resolved Quasi Steady State Photoconductance measurements. After 10 min of light degradation, no correlation between Δnlim, and Nt* is observed. These results indicate that the role of the excess carriers during the rapid decay is to first change the charge state of the defects by shifting the electron quasi-Fermi level across the energy level of the defect centre in its passive state (Elat = EV + (635 ± 18) meV) and that, subsequently, another rate-determining step proceeds before the defect centre becomes recombination active.