The Effects of Hydrogen Profiling and of Light-Induced Degradation on the Electronic Properties of Hydrogenated Nanocrys
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The Effects of Hydrogen Profiling and of Light-Induced Degradation on the Electronic Properties of Hydrogenated Nanocrystalline Silicon A.F. Halverson1, J.J. Gutierrez1, J.D. Cohen1, Baojie Yan2, Jeffrey Yang2, and Subhendu Guha2 1 Department of Physics, University of Oregon, Eugene, OR 97403, U.S.A. 2 United Solar Ovonic Corporation, 1100 W. Maple Road, Troy, MI 48084, U.S.A. ABSTRACT The electronic properties of hydrogenated nanocrystalline silicon (nc-Si:H) were studied using junction capacitance methods. Drive-level capacitance profiling (DLCP) measurements revealed significant differences for nc-Si:H layers deposited under constant hydrogen dilution compared to those deposited using hydrogen profiling, with lower DLCP densities in the latter case. Transient photocapacitance (TPC) measurements revealed the mixed-phase nature of these materials. It disclosed spectra that appeared quite microcrystalline-like at lower temperatures, but more similar to a-Si:H at higher temperatures where the minority carrier collection is higher in the nanocrystalline component of these samples. This then suppresses the TPC signal from this component compared to the a-Si:H component. In contrast, because transient photocurrent signals are enhanced by the additional minority carrier collection, those spectra appear microcrystalline like at all temperatures. We also investigated the effects of light-induced degradation in these devices. This caused a dramatic decrease in hole collection, similar to that caused by reducing the measurement temperature of the samples. However, the light exposure did not appear to increase the deep defect density (dangling bonds). INTRODUCTION Varying the growth parameters used to deposit hydrogenated amorphous silicon (a-Si:H) can result in the growth of a kind of hydrogenated microcrystalline silicon with small crystallite sizes in the range of a few nm to over 20 nm. This material is also referred to as hydrogenated nanocrystalline silicon (nc-Si:H) in the literature. Such materials have been examined for possible solar cell applications for over a decade [1], and recent attempts to fabricate high efficiency a-Si:H/nc-Si:H tandem cells have become reasonably successful [2]. A couple of years ago we began experiments to try to understand the electronic properties of these nc-Si:H materials, with hopes of determining the active defects and understanding their effects, characterizing carrier densities and transport properties, and identifying whether such materials exhibit light-induced degradation similar to a-Si:H. This paper reports our progress to date toward these goals. Our approach has been to utilize several of the methods that we have previously found to be very useful in our studies of a-Si:H; namely, drive-level capacitance profiling (DLCP) [3], transient photocapacitance (TPC) spectroscopy [4], and transient junction photocurrent spectroscopy (TPI) [4]. Using these methods we were able to obtain defect distribution profiles in nc-Si:H, to deduce the fraction of photo-generated minorit
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