Properties and Local Structure of Plasma-Deposited Amorphous Silicon-Carbon Alloys
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PROPERTIES AND LOCAL STRUCTURE OF PLASMA-DEPOSITED AMORPHOUS SILICON-CARBON ALLOYS W. C. MOHR*, C. C. TSAI AND R. A. STREET Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA 94304. *Present address: Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. ABSTRACT Hydrogenated amorphous silicon-carbon alloy films were plasma-deposited from metnane and silane, varying gas ratio, R.F. power and substrate temperature. Carbon addition increases the optical gap, but also raises the dangling bond density while decreasing conductivity. Low C alloys can be gas-phase doped both p and n type. In the IR spectra the various Si-C stretching modes observed between 650 and 780 cm- 1 are explained by back bonding variations. A tentative method of assigning this shift to back bonding of C to the Si is given. A distribution of modes is observed for all alloys, with each mode appearing even at 2% C. The distribution is sensitive to substrate temperature, but is stable after vacuum annealing to 4000 C. INTRODUCTION The wider band gap of amorphous silicon alloyed with carbon offers many possibilities for solar cells and heterostructure device applications. The electronic effects of structural defects have prevented its widespread use. This study was designed to vary a small number of deposition parameters and then broadly characterize the resulting alloy. All the hydrogenated amorphous silicon carbon alloys (a-SiCx:H) was deposited from SiHl4 and CH4 in a single glow discharge reactor, varying the gas ratio, substrate temperature, and plasma power. Of the characterization techniques the most sensitive to local structure is Fourier transform infra-red spectroscopy (FTIR). In this paper some FTIR data is reinterpreted considering the local bonding environment. EXPERIMENTAL a-SiCX:H films were prepared in a plasma deposition system previously described [1] by decomposition of SiH4 and CH 4. Plasma RF power was either 2W or 20W over an area of 10 cm 2 . Total gas flow of 100 sccm with a gas pressure of 1 torr. The gas composition was either 25% CH 4 or 90% CH4 resulting in -2% C and -40% C films, respectively. At each deposition condition a set of substrates were prepared together including float-zone silicon, Corning 7059 and Cr evaporated Corning 7059. Final film thicknesses were between 1Lm and 8 1Lm, which required deposition times of lhr to 16hr. Optical spectra were measured with a Cary 17 on Corning 7059 substrates. FTIR spectra from Si substrates were measured with a Nicolet 20DX. OPTICAL AND ELECTRONIC PROPERTIES Applications of a-SiCF:H are due to its compatability with a-Si and its wider, tailorable band gap. Optical absorption curves versus photon energy for various conditions make up Figure 1. The optical gap increases with increasing CH 4 and decreasing substrate temperature. While not simply related to the lower RF power curves, the high RF power curves show these same tendencies. The decreasing Mat. Res. Soac Symp. Proc. Vol. 70. c 1986 Materials Research Society
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