Abstract
The epitaxial growth of silicon films by chemical vapor deposition (CVD) is strongly affected by temperature and hydrogen (H) termination. We report measurements of $p$-polarized optical second-harmonic (SH) spectra generated in reflection from clean $2\ifmmode\times\else\texttimes\fi{}1$-reconstructed and H-terminated epitaxial Si(001) surfaces with no intentional doping by Ti:sapphire femtosecond laser pulses for SH photon energies $3.0l~2\ensuremath{\Elzxh}\ensuremath{\omega}l~3.5\mathrm{eV}$ near the bulk ${E}_{1}$ resonance. Temperatures were varied from 200 to 900 K and H coverages from 0 to 1.5 monolayers (ML). Increases in temperature at fixed H-coverage redshift and broaden the ${E}_{1}$ resonance, as observed in linear bulk spectroscopy. Increases in H coverage from 0 to 1 ML at fixed temperature strongly quench, redshift, and distort the lineshape of the ${E}_{1}$ resonance even though reflection high-energy electron diffraction shows that the surface maintains the dimerized $2\ifmmode\times\else\texttimes\fi{}1$ reconstruction. The latter spectroscopic variations cannot be explained by vertical strain relaxation in the selvedge region, nor by bulk electric-field-induced SH (EFISH) effects. We instead attribute these variations to a monohydride-induced surface chemical modification, which we parametrize as a surface EFISH effect because submonolayer H strongly alters surface electric fields by redistributing charge from surface dimers into the bulk. The effects of vertical strain relaxation are weakly evident as a blueshift of the ${E}_{1}$ resonance accompanying dihydride termination (1.0--1.5 ML), which breaks the surface dimer bond. This modification is parametrized as a separate field-independent alteration to the surface dipole susceptibility ${\ensuremath{\chi}}_{\mathrm{surface}}^{(2)}.$ Finally, guided by these SH spectroscopic studies, we demonstrate dynamic real-time (100-ms resolution) SH monitoring of H coverage (5% accuracy) during temperature programmed hydrogen desorption and CVD epitaxial growth of silicon from disilane.
Published Version
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