On using photoconductivity decay to determine Si free carrier recombination lifetime: Possibilities and challenges

Abstract

Free carrier recombination lifetime (τ) in Si single crystals is a parameter, that characterizes the quality of the material. The primary procedure for τ measurement in ingots is photoconductivity decay (PCD) analyses, including contactless μ-PCD and eddy current methods. One of the main problem in this methods is a surface recombination influence on the PCD curve. It is usually assumed that in ingots the influence of surface recombination can be neglected. Well-known dependence of effective lifetime (τeff ) on τ, surface recombination velocity (S) and sample thickness (d) describes the maximum value of a lifetime. Based on the numerical calculations of the one dimensional continuity equation we have shown that if the sample thickness exceed 5LD , the maximum lifetime cannot be achieved before PCD signal decline to 5% of maximum, so the question arises what is the interval of the decay curve where the experimental value of the τeff has to be determined. We used the range from 45-5% of the maximum PCD signal following the recommended SEMI MF 1575 standard (τSEMI ). The dependencies of τSEMI on τ, d and S were calculated for different versions of PCD measurements and measurement equipment. The recommended thickness for lifetime measurements of non-passivated samples is in the range (1-5)LD . The hardware capabilities of contactless methods are compared.