Real-time spectroscopic ellipsometry as an in-situ probe of the growth dynamics of amorphous and epitaxial crystal silic
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Real-time spectroscopic ellipsometry as an in-situ probe of the growth dynamics of amorphous and epitaxial crystal silicon for photovoltaic applications D.H. Levi, C.W. Teplin, E. Iwaniczko, Y. Yan, T.H. Wang, H.M. Branz National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401, USA ABSTRACT In this paper we report on our work using in-situ real time spectroscopic ellipsometry (RTSE) to study the dynamics of hot-wire chemical vapor deposition (HWCVD) of hydrogenated amorphous silicon (a-Si:H) and epitaxial crystal silicon (epi-Si) for photovoltaic applications. We utilize RTSE as both an in-situ diagnostic and a postgrowth analysis tool for a-Si:H/crystalline silicon heterojunction (SHJ) solar cells and epi-silicon films grown by HWCVD. RTSE enables precise thickness control of the 3 to 10 nm thick layers used in the SHJ devices, as well as monitoring crystallinity and surface roughness in real time. With the assistance of in-situ RTSE feedback we have achieved a photovoltaic energy conversion efficiency of 17% on an Al-backed p-type float-zone c-Si wafer. Open-circuit voltages above 650 mV indicate excellent passivation of the c-Si surface by the a-Si:H intrinsic layer. We have used RTSE to obtain information on the degree of crystallinity and the electronic and optical properties of films as a function of deposition conditions. RTSE has indirectly indicated the persistence of a hydrogen layer at the interface between the a-Si:H layer and the crystal silicon substrate. Absorption spectra determined by RTSE have provided guidance in device optimization. We are also applying in-situ RTSE to study the dynamics of HWCVD growth of epiSi. The goal of this work is to develop low-temperature methods for growing 2-10 µmthick layers of c-Si on c-Si seed layers on glass for solar cell applications. This study presents unique challenges for RTSE, as perfect epitaxial growth of c-Si on a c-Si wafer would produce no change at all in the RTSE spectra. We have found that by monitoring the pseudo-dielectric function in real time during growth we gain immediate feedback on the breakdown of epi-Si growth. Post-deposition analysis of the RTSE data provides quantitative information on the percent of c-Si and a-Si versus film thickness. The RTSE analysis has been confirmed by cross sectional TEM. Based on the rapid feedback provided by RTSE we have surpassed the previous HWCVD maxiumum of 200 nm of epi-Si growth, achieving a maximum thickness of 500 nm of epi-Si. TEM analysis has shown that micron-sized areas of these films achieve 1000 nm of epi-Si thickness. INTRODUCTION In-situ monitoring of material properties during thin film deposition provides researchers with a valuable tool for maximizing solar cell performance, while also enabling efficient exploration of deposition parameter space. This paper describes how our research team at NREL has utilized in-situ RTSE to maximize our productivity in two related projects. We are using HWCVD for low-temperature (90 to 350oC) deposition of very thin films of amorpho
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