Electrical characterization of hydrogen-vacancy-related defects in monocrystalline silicon

Abstract

Electrical characterization of point defects in silicon (Si) is carried out to study fundamental defects present in Si solar cell structures. The interactions between various defect complexes is investigated, while paying special attention to the interactions with hydrogen. As hydrogen is readily incorporated during manufacturing processes, it is important to learn about the defect dynamics of hydrogen related defects Firstly, hydrogen implantation into p- and n-type based pn-junctions has been investigated in order to learn about the fundamental nature of the defect reactions between vacancy related defects and hydrogen. Samples have been electrically characterized by capacitance–voltage (CV), deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS), which reveals the presence of multiple hydrogen-related levels. A firm correlation between the reported donor and acceptor states of the vacancy–oxygen–hydrogen (VOH) complex is established in p-type samples, providing data on formation and annealing of both states. In the n-type samples, there is a rapid formation of a hydrogen-related level at E_c − 0.36 eV, at the expense of VO, without any formation of VOH after heat treatments at 200 C. This complex is not observed in the p-type samples, leading to the conclusion that the formation barrier for VOH must be higher in n-type than in p-type material. Further, an observation of a hydrogen-related defect in p-type samples, achieved by MCTS, provides preliminary data that may challenge the accepted model for the di–vacancy–hydrogen complex (V₂H). The level observed in p-type samples,corresponds to a well-known DLTS peak in hydrogen contaminated n-type Si and is often ascribed to the acceptor level of V₂H. However, the theoretically predicted single donor level of V2H is not observed in the investigated samples, thereby urging additional studies on the validity of the model of V₂H. Finally, a study of interfaces between tin doped indium oxide (ITO) and silicon has been conducted. Thin films of ITO were sputtered onto crystalline pand n-type Si wafers, and a lithography process was applied to make individual diodes of various areas. By combining current–voltage (IV), CV and DLTS with secondary ion mass spectrometry (SIMS), a clear understanding of the composition and behavior of the interface is achieved. Heat treatments of the samples up to 600 C provide information of the defect evolution and the subsequent change in properties of the junction. The ITO/n-Si samples are found to be rectifying at all investigated temperatures, while the ITO/p-Si samples are rectifying up to 400 C, after which the junctions are transformed into an Ohmic behavior. Correlating these observations with the DLTS measurements reveals that the dominant hole traps anneal out as the rectification of the ITO/p-Si samples is at its highest. Likewise, the annealing of the dominant electron trap is followed by a significant increase in the rectification of the ITO/n-Si samples.