DEPENDENCE OF THE PROPERTIES OF BORON-TRIFLUORIDE-BASED AMORPHOUS-SILICON CARBIDE ON PREPARATION CONDITIONS

p-type fluorinated and hydrogenated amorphous silicon carbide (a-Si:C:F:H) thin films have been prepared by plasma-enhanced chemical vapor deposition, using mixtures of silane (SiH4), methane (CH4), and boron trifluoride (BF3). The influence of preparation conditions on the optical and electrical properties of the films has been systematically studied with the aim of confirming the validity of BF3 to make p-type amorphous silicon carbide alloys and getting a better understanding of this material for its use in the preparation of p-type window layers of amorphous silicon solar cells. It has been found that the use of a low process pressure tends to prevent film growth as a consequence of the combination of a low density of radicals in the plasma and a high probability of desorption of species. High RF-power densities favor the incorporation of relatively large amounts of fluorine and the formation of defects. These two factors determine a poor doping efficiency and film quality. It has been shown that it is possible to prevent optical gap degradation if BF3 is used instead of B2H6. The effect is attributed to the incorporation of fluorine. Optimum dopant-gas concentrations seem to be of the order of 100%, instead of 1% as is the case for B2H6. This is due to the high tightness of the B-F bond. Our best films have an optical gap of 2.1 eV, a conductivity of 2.7X10(-7).(OMEGA-cm)-1 and an extended-state conductivity activation energy of 0.49 eV. Such properties make them suitable for their use as p-type window layers for amorphous silicon solar cells.