Multiscale Modeling of CdTe Thin Film Deposition Process
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Multiscale Modeling of CdTe Thin Film Deposition Process Alexey Gavrikov1, Andrey Knizhnik1, Dmitry Krasikov1, Boris Potapkin1, Svetlana Selezneva2, Timothy Sommerer2 1 Kintech Lab Ltd., 1, Kurchatov Sq., Moscow 123182, Russia 2 GE Global Research, Niskayuna, NY 12309 U.S.A. ABSTRACT Deposition of semiconductor films is a key process for production of thin-film solar cells, such as CdTe or CIGS cells. In order to optimize photovoltaic properties of the film a comprehensive model of the deposition process should be build, which can relate deposition conditions and film properties. We have developed a multiscale model of deposition of CdTe film in close space sublimation (CSS) process. The model is based on kinetic Monte Carlo method on the rigid lattice, in which each site can be occupied by either Cd or Te atom. The model tabulates the energy of the site as a function of its local environment. These energies were obtained from first-principles calculates and then approximated with analytical formulas. Based on determined energies of each site we performed exchange (diffusion) processes using Metropolis algorithm. In addition the model included adsorption and desorption processes of Cd and Te2 species. The results of the model show that a steady-state structure of the surface layer is formed during film growth. The model can reproduce transition from film deposition to film etching depending on external conditions. Moreover, the model can predict deposition rates for non-stoichiometric gas compositions. INTRODUCTION CdTe is a very promising material for low-cost solar cells. Solar cells based on CdTe have been able to reach efficiencies as high as 18.3% [1]. Close space sublimation is a relatively inexpensive technique for deposition of polycrystalline thin-films due to the moderate operating pressure (0.01–15 Torr) and simple configuration [2], [3]. This has created much interest in using CSS for deposition of low-cost polycrystalline CdTe solar cells. The method is able to deposit at very high rates (up to ~15 µm/min) [2]. The combination of high growth rates and simple equipment creates an opportunity to use the CSS large-scale production of low cost solar cell. For CSS growth technique, some models were proposed for diffusion-limited and sublimation limited cases [4]. However, these models do not take into account the stoichiometry of gas phase. But to control of the intrinsic defects concentration the gas phase composition may be essential; besides the difference in gas diffusion coefficient may give rise to deviation from P(Cd)/P(Te2)= 2/1 ratio both at source and substrate surface. Therefore, there is a need for a model that describes CdTe growth/evaporation in nonstoichiometric conditions. This article presents such growth/evaporation model built from the first principles. CdTe (111) surface orientation is often observed in CdTe films grown by CSS technique, for example [2], [3]. So the present work mainly deals with CdTe (111)B surface. FIRST PRINCIPLES STUDY Chemical processes on the surface of CdTe (111)B,
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