The impact of surface damage region and edge recombination on the effective lifetime of silicon wafers at low illumination conditions

Abstract

The effective minority carrier lifetime of p-type silicon wafers passivated by silicon nitride and of n-type silicon wafers passivated by aluminium oxide often decreases significantly as the excess carrier concentration decreases. Several theories have been postulated to explain this effect. The main ones are asymmetric carrier lifetimes, high recombination within a surface damage region, and edge recombination. As in some cases, the effective lifetime measurements can be fitted quite well by all these effects, it is challenging to determine the main cause for the suppressed performance at low illumination. This is partly due to the fact that no study has yet included a sufficiently large set of wafers and advanced modelling to examine all these theories. The aim of this study is to determine the most likely theory based on a set of undiffused p- and n-type wafers of different sizes, passivated with both silicon nitride and aluminium oxide. Quasi-steady-state photoluminescence measurements were used in order to investigate effective lifetime at very low carrier densities, without artifact effects that commonly limit photoconductance-based measurements. Advanced modelling using Sentaurus was used to investigate the impact of different parameters—such as the fixed charge within the dielectric—on the recombination at the edge and within the surface damage region. These models were then used to simulate the measurement results. It is shown that asymmetrical surface lifetime cannot explain the observed reduction when the dielectric is highly charged (either positively or negatively). It is also shown that although edge recombination influences the effective lifetime at low excess carrier concentration, it alone cannot explain the effective lifetime reduction. It is therefore concluded that the presence of a surface damage region is the more likely explanation for the effective lifetime decrease of the studied wafers.