Methods of Improving Glow-Discharge-Deposited a-Si 1-x Ge x :H
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METHODS OF IMPROVING GLOW-DISCHARGE-DEPOSITED a.Sil.xGex:H Y.S. Tsuo*, Y. Xu*, E.A. Ramsay*, R.S. Crandall*, S.J. Salamon*, I. Balberg*• B.P. Nelson*, Y.
Xiao**, Y. Chen*** *Solar and Energy Research Institute, Golden, CO 80401 *University School of Colorado, Boulder, COCO 80309 ***Colorado of Mines, Golden, 80401
ABSTRACT We have studied methods of improving glow-discharge-deposited a-Sil.xGex:H alloys deposited using silane and germane gas mixtures. Material processing methods studied include (1) varying the substrate temperature from 1700 to 280°C, (2) varying the process gas composition
and pressure, (3) dilution of the feed gas by hydrogen, argon, or helium, (4) enhancing etching during deposition by adding small amounts of XeF2 vapor into the process gas, and (5) postdeposition annealing and/or hydrogenation. INTRODUCTION The development of multiple junction solar cells is important for improving the conversion
efficiency and stability of solar photovoltaic modules based on hydrogenated amorphous silicon (a-Si:H) and related alloys. Although the "same-band-gap" a-Si:H/a-Si:H tandem cell approach has achieved high efficiency and stability [1], such cells have limited potential for further
improvements in efficiency due to insufficient light absorption. To further improve the efficiency of amorphous silicon multijunction cells we need to develop high-quality low-band-gap alloys. So far, only a-SilxGex:H alloys (a-SiGe:H) have achieved high enough quality to be used in multijunction cells [2,3]. However, problems, such as preferential attachment of H to Si rather than to Ge and poor microstructure, limit the quality of a-SiGe:H. Electronic properties of a-SiGe:H deteriorate rapidly when the Ge content is increased beyond 40 at.% and when the optical band gap, E., is less than 1.5 eV. An alternative approach that has received increased attention recently is to use high-temperature-deposited a-Si:H. Although it is impossible to have a-Si:H with an E9 less than 1.5 eV, the electronic properties of a-Si:H films with an E of more than 1.60 eV do compare favorably with a-SiGe:H. In this paper we will report our studies on the optimization of a-SiGe:H. We will also compare the properties of a-SiGe:H with high-temperature a-Si:H. SUBSTRATE TEMPERATURE All radio-frequency (RF) glow-discharge-deposited films used in this study were deposited in a load-locked diode reactor system using SiH 4 and GeH4 feed gases. The substrate is mounted
on the grounded electrode. For most a-SiGe:H films, the SiH4 plus GeH 4 gas flow rate is between 40 and 50 sccm, and the reactor pressure is maintained at 0.65 torr when no feed gas dilution is used. The RF power density used is slightly above that needed to maintain the plasma. The
substrate temperature, Ts, is calibrated using a 0.005-inch-diameter thermocouple attached to the film surface with the process gas flowing. The film deposition rate decreases only slightly with increasing T., from 0.36 nm/s at 150°C to 0.29 nm/s at 3001C for a feed gas mixture of 20% GeH4 and 80% SiH4 .
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