Analysis of laser doping of silicon using different boron dopant sources

Abstract

Implementation of selective emitter that decouples the requirements for front doping and metallization leads to improve the efficiency of crystalline silicon solar cells. Formation of such an efficient selective emitter using a laser beam with a suitable wavelength is an attractive method.The present work focuses on the analysis of laser doping of boron using different finite sources such as borosilicate glass (BSG) deposited by PECVD, spin-on solution and BCl3 gas source. KrF excimer laser (248nm) was used for the selective doping. The surface dopant concentration and depth, as measured using SIMS, were controlled by variation of the laser fluence, pulse number and dopant source thickness. Depending on the type of BSG source, sheet resistance close to 20Ω/sq was achieved at the laser fluences in the range, 2.5–5J/cm2. The PECVD-BSG layers with a relatively higher thickness resulted in a lower sheet resistance of 20Ω/sq with a junction of depth of ∼1μm at a moderate laser fluence of 2.5J/cm2. In the case of BSG deposited by spin-on source, a deeper junction of depth of ∼2.7μm with a plateau profile of 1μm was formed at a laser fluence of 3.1J/cm2 that resulted in a lower sheet resistance of ∼31Ω/sq. Redistribution of the dopant with pulse repetition was observed for the BSG deposited by BCl3 gas source.Pulse repetition at relatively lower laser fluences (>threshold energy) resulted in the best electrical results in combination with a limited laser induced damage in the silicon crystal. Also, multiple laser annealing resulted in redistribution of the dopant profiles in terms of enhanced junction depth.