Amorphous Silicon Selenium Alloy Film Deposited Under Hydrogen Dilution
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AMORPHOUS SILICON SELENIUM ALLOY FILM DEPOSITED UNDER HYDROGEN DILUTION MUZHI HE, GUANG H. LIN, AND J. O'M. BOCKRIS Department of Chemistry, Texas A$M University, TX 77843
College Station,
ABSTRACT Amorphous silicon selenium alloy films were prepared by plasma enhanced chemical vapor deposition with hydrogen dilution. The flow rate ratio of hydrogen to silane was about 8:1. Amorphous silicon selenium alloy was found to have an optical bandgap ranging from 1.7 eV to 2.0 eV depending on the selenium concentration in the films. The light to dark conductivity ratios of the alloy films are - 104. The optical and electrical properties, Urbach tail energy and sub-bandgap photo response spectroscopy of the alloy film were investigated. The film quality of the alloy deposited with hydrogen dilution is greatly improved comparing to that of the alloy film deposited without hydrogen dilution. The electron spin resonance experiment shows that selenium atom is a good dangling bond terminator. INTRODUCTION Hydrogenated amorphous silicon selenium alloy (a-Si:Se:H) has been studied regarding potential applications in the tandem photovoltaic devices [1-3]. The alloy films exhibited a wider optical bandgap with increasing Se incorporation. The electrical and optical properties of the alloy films indicated that amorphous silicon selenium alloy was a promising material for photovoltaic applications. Selenium atom can be bonded in a double Si vacancy in a Si tetrahedral structure [4]. No unpaired electrons are formed in this position. Unpaired dangling bonds will be removed by Se atoms [3,4]. For alloy films having light to dark conductivity ratio greater than 103, the dangling bond density was close to that of amorphous silicon film [3]. Thus, structural defects, e.g., the amorphous nature of the lattice, will control the film quality. The hydrogen dilution technique has been used in preparing high quality amorphous silicon germanium and amorphous silicon carbon alloys [5-8]. The hydrogen ion etches out the weaker bonding in the surface and consequently leading to a denser network. Thus, film quality is improved by hydrogen addition during the film deposition. In this paper, research leading to electrical and optical properties of amorphous silicon-selenium alloy, prepared with hydrogen dilution in plasma enhanced chemical vapor deposition (PECVD), is described. Defects of the alloy films were investigated by electron spin resonance (ESR) sub-bandgap photo response spectrum (SPRS) and Urbach tail energy measurement. A significant improvement of film quality was achieved. EXPERIMENTAL Amorphous
silicon selenium alloy films were deposited in
Mat. Res. Soc. Symp. Proc. Vol. 219. D1991 Materials Research Society
a
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two chamber system by PECVD method using hydrogen dilution. Rf power with frequency 13.6 MHz was capacitively coupled into the reaction chamber of a parallel plate reactor. A grounded stainless steel mesh was used as an electrostatic shield surrounding the reactor which confined the rf plasma between the two electrode
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