Optical Absorption in Co-Deposited Mixed-Phase Hydrogenated Amorphous/Nanocrystalline Silicon Thin Films
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1245-A09-03
Optical absorption in co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films L. R. Wienkes,1 A. Besaws,1,4 C. Anderson,2 D. C. Bobela3, P. Stradins,3 U. Kortshagen,2 and J. Kakalios1 1 2 3 4
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455 National Renewable Energy Laboratory, Golden, CO 80401 College of Menominee Nation, Keshena, WI
ABSTRACT The conductivity of amorphous/nanocrystalline hydrogenated silicon thin films (a/nc-Si:H) deposited in a dual chamber co-deposition system exhibits a non-monotonic dependence on the nanocrystal concentration. Optical absorption measurements derived from the constant photocurrent method (CPM) for similarly prepared materials are reported. The optical absorption spectra, in particular the subgap absorption, are found to be independent of nanocrystalline density for relatively small crystal fractions (< 4%). For films with a higher crystalline content, the absorption spectra indicate broader Urbach slopes and higher midgap absorption. These data are interpreted in terms of a model involving electron donation from the nanocrystals into the amorphous material. INTRODUCTION The unique properties of mixed-phase materials, in which nanoscale particles are embedded within a host semiconductor or insulator matrix, have attracted interest for such applications as high efficiency solar cells [1,2], non-volatile memory and electron emitters [3,4], bandgap engineering [5], and electroluminescent devices [5]. In recent years, interest has focused on the system of silicon nanocrystallites within hydrogenated amorphous silicon thin films (a/nc-Si:H) for photovoltaic applications. Hydrogenated amorphous silicon (a-Si:H) has long been known to exhibit light-induced defects, which degrade the electronic properties of the film and reduce the efficiency of a-Si:H-based solar cells (traditionally referred to as the Staebler-Wronski effect (SWE)) [6,7]. The interest in nanostructured a/nc-Si:H films is due in part to reports of an enhanced resistance to light-induce defect formation in a/nc-Si:H [8]. These mixed-phase materials are typically synthesized in a single chamber plasma deposition system. In this paper, we describe measurements on materials deposited in a dual chamber co-deposition system. MATERIALS PREPARATION The synthesis of these mixed-phase a/nc-Si:H films in the dual chamber co-deposition system has been described previously [9]. Briefly, the films are deposited in a system where silicon nanocrystals are grown in a separate plasma reactor, and then are entrained by an inert argon carrier gas and injected into a second plasma enhanced chemical vapor deposition (PECVD) chamber where the amorphous silicon film is grown. The silicon nanocrystallites are then incorporated into the growing a-Si:H film. Convection of the carrier gas as it enters the
second PECVD chamber can be used to deliberately vary the nanocrystallite concentratio
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