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AMORPHOUS HYDROGENATED SILICON CARBIDE PREPARED FROM DC-BIASED RF-PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION HSUEH YI LU AND MARK A. PETRICH Department of Chemical Engineering, Northwestern University, Evanston, Illinois 60208 ABSTRACT We present evidence that an independently applied dc bias voltage has a significant effect on the plasma deposition of amorphous hydrogenated silicon carbide. Deposition rates increase with either positive or negative dc voltages applied to the powered rf electrode. The microstructure of the films (as determined by infrared absorption) can be reduced by increasing the plasma potential (positive dc bias voltages). Negative dc biases, or excessively high positive biases, result in increased amounts of film microstructure. Film carbon content is increased when positive biases are applied, but the optical band gaps decrease suggesting increased amounts of graphitic bonding configurations. Negative biases do not change the carbon content of the films, but do increase both deposition rate and microstructure. INTRODUCTION Amorphous hydrogenated silicon carbide (a-SiC:H) with low carbon content has proved to be useful in devices such as solar cells, thin film transistors, and light emitting diodes [1-31 because it is relatively easy to tailor its physical properties to application requirements. By varying the silicon-to-carbon ratio, the optical band gap can be varied continuously between 1.75 eV and 3 eV. Unfortunately, the electrical properties of a-SiC:H degrade as carbon is added to adjust the band gap [4-11]. The photoconductivity-to-dark conductivity ratio decreases and the paramagnetic defect density, measured by electron spin resonance, increases dramatically with carbon content [5,12,13]. In plasma-enhanced chemical vapor deposition of a-SiC:H from a mixture of silane (Sil- 4 ) and methane (CH4 ), the highly complicated chemical processes start with electron impact dissociation of SiH 4 and CH4I-into neutral radicals and ions [4,14]. Neutral radicals either incorporate into the growing film or form higher molecular weight products, which may also incorporate into the film. In plasmas containing high concentrations of SiI-l 4 and C-I' 4 , SiSi, C-C, Si-C and even C=C bonds are found both in the gas phase and in the deposited film [6,13-15]. Positive ions generated in the plasma are also important in the growth chemistry. Although the portion of the film deposition resulting from ions can be neglected because of low ion concentrations [16], ionic species affect film growth because of the kinetic energy they obtain as they are accelerated by the electric field between the plasma and the substrate. Ions may transfer energy to neutral species in the plasma or to the growing film, possibly providing energy for the heterogeneous chemical processes. Modulation of ion energy has been demonstrated to induce a drastic change in growth kinetics, structure, and properties of plasmadeposited a-Si:H [17], a-SiGe:H [18], and a-C:H films [19]. In the work to be reported in this paper, we vary ion ene