Trapping and Detrapping of H in Si: Impact on Diffusion Properties and Solar Cell Processing
- PDF / 278,040 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 38 Downloads / 154 Views
Trapping and Detrapping of H in Si: Impact on Diffusion Properties and Solar Cell Processing Bhushan Sopori1, Y. Zhang1, R. Reedy1, K. Jones1, N. M. Ravindra2, S. Rangan3, and S.Ashok3 1 2 3
National Renewable Energy Laboratory, Golden, CO New Jersey Institute of Technology, Newark, NJ Pennsylvania State University, University Park, PA
ABSTRACT Influence of trapping and detrapping on the diffusion behavior of H in Si is investigated using both experiment and theory. Experimental H (or D) diffusion profiles, produced by plasma and ion implantation processes, are fitted with a theoretical model. This model includes three kinds of traps – stationary, process-induced, and mobile. Excellent correlation between theory and experiment is observed. Best–fit parameters provide an insight into the trapping mechanisms. We also show how some of the problems resulting from trapping can be circumvented by suitable process conditions. INTRODUCTION Hydrogenation is used in the commercial fabrication of many electronic devices such as solar cells, MOS devices, and TFTs, for passivation of impurities and defects. In solar cell applications, H must be diffused deep into the bulk using commercially compatible process conditions of temperature and time. Until recently, hydrogenation of Si solar cells was done by plasma processing at low temperatures (typically below 400°C). This approach required long hydrogenation times and worked well only for some materials. Now, there is an increasing trend to combine hydrogenation with deposition of an antireflection coating consisting of a layer of Si3N4 [1]. In this method, a layer of Si3N4 is deposited by a plasma enhanced chemical vapor deposition (PECVD) process at a low temperature, followed by a high-temperature, RTP type of process. Typically, the process parameters are empirically optimized because the detailed knowledge of the diffusion and passivation mechanisms required to predict the process conditions is still lacking. One of the problems in performing hydrogenation at low temperatures arises because H interacts not only with the Si lattice, but also with impurities and defects in Si. In the latter case, it forms semi-stable complexes where H is “trapped.” In most cases, this trapping process results in a considerably lower diffusivity of H, demanding longer hydrogenation times. Furthermore, association of H with the impurities and defects can change their electronic character, leading to unwanted changes in material properties such as resistivity. Because trapping is a temperature-dependant mechanism, higher temperature processing can detrap H, releasing it for possible interactions with desired species of impurities and defects. Therefore, it is important to understand trapping and detrapping mechanisms, and the knowledge can be used to mitigate the H diffusivity issues and optimize solar cell processing. Although, some theoretical modeling of H diffusion has been done that includes trapping in, a coherent model that explains complete diffusion profiles is lacking [2-7]. Recently,
Data Loading...
