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Photochemical etching of silicon: The influence of photogenerated charge carriers

1989; American Physical Society; Volume: 39; Issue: 14 Linguagem: Inglês

10.1103/physrevb.39.10120

1095-3795

Frances A. Houle,

Silicon Nanostructures and Photoluminescence

Low-intensity cw band-gap excitation enhances the etch rate of silicon by ${\mathrm{XeF}}_{2}$. It has been proposed that the enhancement mechanism involves participation of photogenerated charge carriers in the fluorination reaction itself. A new study has been made of this system by molecular beam mass spectrometry. The results show that for both n- and p-type silicon the ${\mathrm{SiF}}_{3}$ free radical is the primary etch product at Ar-ion laser powers exceeding 40 W/${\mathrm{cm}}^{2}$. ${\mathrm{SiF}}_{4}$ is also observed, but its formation is independent of light intensity. The data, including measurements of most probable translational energies, are consistent with a photochemical process being responsible for ${\mathrm{SiF}}_{3}$ formation. Surface heating, which is minimal, cannot account for the experimental results. Since ${\mathrm{SiF}}_{3}$ is the principal adsorbate on the surface, it is argued that etching is a result of desorption of ${\mathrm{SiF}}_{3}$ stimulated by a chemical reaction involving two charge carriers. This is distinct from the photodesorption mechanism usually invoked for semiconductor surfaces, which involves single charge capture by a surface adsorbate. Evidence pertaining to participation of charge carriers in other stages of the fluorination reaction---adsorption of ${\mathrm{XeF}}_{2}$ and diffusion of ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$---has also been obtained. The data indicate that photogenerated charge carriers inhibit chemisorption of ${\mathrm{XeF}}_{2}$. Field-assisted diffusion, which has been invoked as a rate-determining process in photoassisted etching of semiconductors, is not found to be so for this system.