Concentrations and Reactions of Iron in Crystalline Silicon after Aluminum Gettering

Abstract

The purity of silicon starting materials and its contamination by several metal impurities are considered to be the major concern in silicon photovoltaics. Minute concentrations of impurities can have a detrimental effect on the device yield. In particular, Fe is incorporated as a highly mobile and soluble species during device processing. To improve device performance, we need to understand the properties of Fe-related defects in silicon and to design efficient processing methods such as aluminium getterring to remove iron from active device region. One of the most important techniques used to determine the electrical characteristics of Fe-related defects is known as Deep Level Transient Spectroscopy (DLTS).This thesis is divided into four parts, firstly a description of CV characteristics at different stages of treatments. This is followed by the characterization of FeB pair in Fe indiffused samples and subsequently the effects of different protocols of wet chemical etching. Secondly, DLTS is used to study the segregation of Fe from crystalline Si to an Al:Si liquid at its surface, which is the basic mechanism of aluminum gettering used in silicon photovoltaics. Simulated and experimental studies are performed to obtain a reliable segregation coefficient for Fe. The measured segregation coefficient is smaller than estimates from the binary Fe:Si and Al:Fe phase diagrams. This apparent discrepancy originates from the ternary character of the system where the solubility of Fe in Si in equilibrium with the Al-doped α-FeSi2 has to be taken as a reference. Undoubtedly in the third part, the experimental data obtained from marker methods such as platinum and gold indiffusion was able to proof that aluminum gettering injects vacancies with homogenous depth profiles. Based on our results, AlG injects vacancies into Si-wafers with non-equilibrium concentrations (1015cm-3); consequently it can strongly influence the process of impurity gettering by Al-Si alloy and should be taking into account during computer simulation of AlG. Finally, particular attention has been given to the interaction of iron and vacancies released from aluminum gettering leading to a novel deep level produced under illumination. One of the most interesting results obtained was the discovery of a new Fe-related defects which has been proofed to be a metastable complex of iron-vacancy pair. According to our knowledge, detection of FeD by DLTS was firstly described in this study and currently a long discussion has been started about FeD components and importance. Future work will be conducted to determine the roll of FeD on the minority carrier lifetime and diffusion length to optimize the efficiency of solar cell.