Photoeffects on the fluorination of silicon. I: Influence of doping on steady-state phenomena

The effect of visible light on the reaction between silicon and XeF2 has been studied as a function of pressure, light intensity, and dopant. Steady-state etch rates are sensitive to all three variables. Two major light-induced processes have been identified. The reaction can be suppressed by diffusion of fluorine into unreactive sites and enhanced by an increased rate of addition of fluorine to silicon centers relative to the rate of desorption of the intermediate fluorosilyl species. Experiments indicate that these unreactive sites are located beneath the surface. The data have been interpreted in terms of a model in which adsorption of electronegative fluorine causes band bending in the semiconductor, the direction of which depends on whether the material is doped n or p type. Photogenerated charge carriers induce faster fluorine insertion rates into Si–Si bonds, and enhance diffusion below the surface. N- and p-type silicon differ mainly in their relative ability to stabilize subsurface fluorine. The data have important implications for the mechanism of the spontaneous reaction of fluorine with silicon. They may also be relevant to photo-oxidation of clean semiconductor surfaces and issues related to the stability of amorphous Si:H:F alloys.