Optoelectronic Properties of High-Gap Amorphous Silicon-Carbon Alloys

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V. CHUa, J.P. CONDEb, P. BROGUEIRAb, P. MICAELOa'b, j.P. JAREGOa'b, M.F. da SILVAc and J.C. SOARESd aInstituto Engenharia de Sistemas e Computadores (INESC), Rua Alves Redol, 9, 1000 Lisboa, Portugal bInstituto Superior T6cnico (IST), Dept. of Physics, Av. Rovisco Pais, 1096 Lisboa, Portugal 'Instituto Tecnologico e Nuclear (ITN), Estrada Nacional no 10, 2685 Sacav6m, Portugal dCentro de Fisica Nuclear da Universidade de Lisboa (CFNUL), Av. Prof. Gama Pinto 2, 1699 Lisboa, Portugal ABSTRACT The dependence of the properties of hydrogenated amorphous silicon-carbon alloys deposited with a 1:10 silane (SiH4) to methane (CH 4 ) or ethylene (C2 H4 ) gas flow ratios with the hydrogen dilution, deposition pressure and power is studied. In methane-based films the carbon content shows a decrease (from =-0.75 to 0.55) with increasing hydrogen dilution from 0 to 98%, while the ethylene-based films show no dependence of the carbon fraction (=0.9) on hydrogen dilution. The photoconductivity shows an increase for hydrogen dilutions above 90% for both methane and ethylene-based films. While this increase is attributed, in the case of methane, to the observed decrease in carbon content, no corresponding decrease in carbon content is observed in the ethylene-based films, suggesting a decrease in the density of recombination centers with hydrogen dilution. The Urbach tail and the room-temperature photoluminescence peak correlate with carbon content independent of the carbon source-gas and deposition conditions used. INTRODUCTION Amorphous hydrogenated silicon-carbon alloys (a-Si,C:H) are wide-bandgap, thin-film semiconductors. Alloys with low carbon content have been intensively studied for application as transparent doped layers in single-junction solar cells 1 . For Tauc's optical bandgaps, ETauc, up to 2.2 eV, it is possible to obtain a-Si,C:H alloys with good photo-to-dark conductivity ratios 2 , but for higher carbon contents in the film the density of states in the bandgap increases rapidly with increasing carbon content, with consequent degradation of the optoelectronic properties 3 ,4 . The optoelectronic and structural properties of the a-Si,C:H alloys with high carbon fraction (Ž0.5) are not yet well understood, namely the contribution of graphitic domains to the density of states 5 , and also the relation between deposition parameters and film properties has not yet been fully explored, although a "low-power" regime, in which the power density remains below the threshold for the decomposition of methane and ethylene, has been identified 6 . High-carbon content a-Si,C:H alloys are interesting materials for their insulating properties and also because they emit visible photoluminescence at room-temperature, which led to studies of this material for application as the active layer in large-area electroluminescent devices 7 ,8. In this paper, a-Si,C:H alloys are studied with emphasis on the high carbon content samples grown with a 1:10 silane (SiH4) to methane (CH 4 ) or ethylene (C2 H 4 ) gas flow ratio. The dependence of th