Electronic Properties of RF Glow Discharge Intrinsic Microcrystalline Silicon near the Amorphous Silicon Phase Boundary

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Electronic Properties of RF Glow Discharge Intrinsic Microcrystalline Silicon near the Amorphous Silicon Phase Boundary James J. Gutierrez1, Adam F. Halverson1, Eric D. Tweeten1, J. David Cohen1, Baojie Yan2, Jeffrey C. 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 microcrystalline silicon have been characterized for the first time using transient photocapacitance spectroscopy (TPC) and drive-level capacitance profiling (DLCP). These methods were applied to microcrystalline films deposited by the RF glow discharge method at United Solar. The DLCP method allowed the shallow doping density to be profiled and the deep defect densities to be estimated. The TPC spectra were found to reveal that both a microcrystalline as well as an amorphous component are present in these samples. By varying the measurement temperature for these TPC spectra we were also able to directly monitor the degree of minority carrier collection in these films. Significant effects due to light soaking on the TPC spectral properties were also observed. INTRODUCTION It has long been known that simple variations in the growth parameters used to deposit hydrogenated amorphous silicon (a-Si:H) result in the growth of a form of hydrogenated microcrystalline silicon with crystallite sizes less than 10nm (here we will refer to this material as hydrogenated nanocrystalline silicon, nc-Si:H). Such materials have been studied seriously for solar cell applications for over a decade [1]; however, only fairly recently have attempts to fabricate high efficiency a-Si:H/nc-Si:H tandem cells shown a reasonable degree of success.[2] Compared to a-Si:H, however, the range of materials called micro- or nano-crystalline silicon is enormous. Not only is there the important variable of crystallite size, but also issues such as the degree of a-Si:H contained between the crystallites, the role of hydrogen in passivating grain boundaries, oxygen contamination [3], and the variation of these properties as a function of film thickness [4] are expected to play a crucial role in determining the electronic properties of these materials. Understanding such properties includes identifying the active defects and understanding their effects, characterizing carrier densities and transport properties, and identifying under what conditions such materials may exhibit types light-induced degradation similar to a-Si:H. A primary method used to date to characterize the electronic structure of such materials has been sub-band-gap absorption measurements primarily via the constant photocurrent method (CPM) [5-8]. In the study reported below we explore, for the first time, the applicability of two additional methods that have previously been very useful in the study of a-Si:H: drive-level capacitance profiling (DLCP) [9], and transient photocapacitance (TPC) spectroscopy [10]. These methods are employed to address issues