Origins of carrier traps in p-type multicrystalline silicon

Abstract

Carrier trapping effects in multicrystalline silicon wafers are investigated by measuring the dependence of their photoconductance on carrier injection level and by fitting a theoretical model to the data. The main information thus obtained is the density of trapping centres, which may be a useful indicator of the quality of the material. We have found that the trap density can vary significantly for different cast multicrystalline silicon ingots. Typically, the concentration of traps also changes for wafers from different positions within an ingot, and we have found a correlation between the concentrations of trapping centers and dislocations in the mc-Si wafers. Additional experiments based on cross-contaminating ultra pure single-crystal silicon wafers with multicrystalline silicon specimens revealed that some of the traps are mobile and probably due to transition metal contaminants. The density of trapping centers in p-type float zone wafers of varying resistivities was found to be linearly proportional to the background boron concentration, suggesting that the trapping effects are caused by boron-impurity pairs. Two possible physical origins for the trapping centers in multicrystalline silicon have been identified: crystallographic defects (dislocations) and mobile impurities.