Plasma Enhanced Chemical Vapor Deposition of Silicon Sulfide and Phosphorus Sulfide thin Films
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Figure 1. E(O), E(1), and E(2 ) sites in silicon sulfide glasses and films, as inferred from 29Si MAS NMR. 587 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Structural diversity is an exciting and sought after aspect for such glass systems; since it offers the potential opportunity of altering the properties of the glass, by altering the structural speciation. Traditionally these glasses are synthesized from the elements at high temperature and extended periods of time. PECVD offers a nontraditional approach to chalcogenide glass synthesis; to date, it has only been exploited by Ribes et al for the synthesis of germanium selenide glasses from the precursors, germane and hydrogen selenide. 12 Our interest in using PECVD stems from the fact that with melt quench methods, one is limited to the frozen in thermodynamic structure of the melt. This combined with the fact that some of these glass systems have relatively narrow glass forming regions leads to the result that melt quenching techniques offer very little, or no, opportunity for compositional and structural tailoring. By using PECVD we open up more synthetic pathways by virtue of the large number of adjustable parameters intrinsic to the method, not to mention the advantages of thin film processing. 13 This mode of synthesis may lead to unique structural speciation not attainable via melt quenching. Moreover, PECVD is ideally suited to compositional variation; typically, film stoichiometry is governed by flow rate ratios of the precursors. This contribution reports the PECVD synthesis of amorphous materials in the silicon-sulfur system and preliminary results in the phosphorus-sulfur system. Our studies, which focus on the chalcogen-rich side of the composition diagram are complementary to the work of other groups, who have reported on PECVD sulfur-doped a-Si:H films with sulfur concentrations up to a maximum of 25 at. %.14-19 EXPERIMENTAL Bulk amorphous silicon sulfide (a-SiS2) was prepared from the elements using standard literature procedures. 6 The a-SiSx:H samples were synthesized using the remote-PECVD apparatus shown schematically in Figure 2. The deposition chamber and the discharge tube are constructed
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of fused silica, 2.5-inch o.d., 10-inch
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length, and 0.5-inch o.d., 11 inchlength, respectively. In a typical SiSx:H deposition run an argon (Liquid Carbonic, chromatography grade)
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was sustained (PAr = 300-600
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,
mTorr) with a 2.45 GHz Raytheon microwave power generator. Silane (Liquid Carbonic, semiconductor grade) mixed with hydrogen sulfide (Aldrich, 99.5+%) was introduced into the downstream tail of this plasma. The apparatus and the substrate were not externally heated and the samples were deposited directly on the walls
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vacuum system
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and on an aluminum foil substrate positioned to 3 Typical cm downstream of deposition the plasma0.5tail.
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