The Influence of High Temperature Steps on Defect Etching and Dislocations : Etch Pit Density Reduction in Multicrystalline Silicon

Abstract

This work is concerned with the study and explanation of a peculiar phenomenon that can be observed in dislocation-etched multicrystalline silicon after high temperature phosphorous diffusion steps. Dislocation etching forms visible etch pits at sites where the line-like disturbances of a crystal’s symmetry, called dislocations, intersect with the crystal surface. High temperature phosphorous diffusion gettering steps can cause the density of etch pits to reduce drastically. Yet, it is not known whether this etch pit density (EPD) reduction can be identified with a reduction in the density of dislocations or whether other effects are at play that modify the formation of etch pits. An algorithmic solution for automatic counting of etch pits has been developed that solves the distinct challenges appearing in the analysis of defect etched multicrystalline silicon (mc-Si) material. This procedure has been released to the community under a free and open source software license and is used to study the cause of etch pit density reduction directly, i.e. via the observation of the etch pit density in variation of the preparation state of the studied system. Using direct measurements of the EPD alone, hypotheses about the movement of dislocations in the observed temperature regime have been falsified and the mechanism of EPD reduction has been discerned. A complete picture of the mechanism of EPD reduction is formed by verifying a prediction of this mechanism that is outside of the scope of direct EPD measurements. Presumably, the discoveries about dislocation etching presented here apply to all types of crystalline silicon material and their implications should be considered.