Second-harmonic spectroscopy of a Si(001) surface during calibrated variations in temperature and hydrogen coverage

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 $3.0<~2\ensuremath{\Elzxh}\ensuremath{\omega}<~3.5\mathrm{eV}$ 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.