Deep level transient spectroscopy studies of electrically active centers in solar-grade Si

In this work we have employed deep level transient spectroscopy (DLTS) for studies of electrically active centers in solar-grade Si. The purpose of the work is to develop robust experimental routines for detecting and identifying the dominant recombination centers. DLTS measurements have been performed prior and after heat treatments corresponding to such technological processes as contact firing and rapid thermal processing (RTP). Multi-crystalline (mc-Si) and Cz-Si with p-type doping and the resistivity in the 1-6 Ohm range have been investigated. Firstly, the effect of Fe has been investigated. In order to ensure that the carrier recombination is determined by Fe, the samples have been intentionally enriched by Fe via in-diffusion from the surface. It has been observed by DLTS that a heat treatment corresponding to the contact firing process leads to activation of Fe in the interstitial form (Fe_i) with a donor state at 0.43 eV above the valence band. Secondly, RTP has received a considerable attention recently due to increasing control in diffusion profile for the emitter formation in solar cells. However, electrically active defects and impurities may be introduced or activated by RTP due to the high temperatures of the treatment followed by rapid cooling, when the impurities can be quenched with concentrations above the solubility level. It is found that RTP at 1000°C for 2 minutes results in formation of two dominant hole traps at ~0.3 and ~0.4 eV above the valence band edge with concentrations in the range 5×10¹²-5×10¹³ cm^-3. The observed levels exhibit Poole-Frankel effect suggesting the acceptor nature of the centers.