Abstract

A number of recent second harmonic (SH) spectroscopic studies of Si/SiO₂ interfaces and clean reconstructed Si surfaces have reported interfacial E_t critical point two-photon resonances at energies (3.2 to 3.3 eV) that are strongly red- shifted from the bulk E₁ energy (3.37 to 3.40 eV).¹ Although these red shifts have been qualitatively interpreted exclusively in terms of subsurface tensile strain, the shifts are larger than known microscopic models of vertical Si—Si bond stretching near the interface can explain.² In fact one recent calculation³ of the 2 X 1-recon- structed Si(001) surface even indicates slight vertical compression of the surface layers compared with bulk, apparently contrary to observed Ej red shifts. In these previous spectroscopic studies, the near-surface strain was static. Moreover little quantitative attention was focused on the lineshape or nearby nonresonant background of the E_t resonance. In the current work we track the magnitude, lineshape, peak position, and nonresonant background of the two-photon Ej resonance of Si(001)-2 X 1 while the reconstruction-induced strain is systematically relaxed by controlled hydrogen adsorption. The results reveal that the well-known nonlinear optical effect of optical interference between resonant (R) and nonresonant (NR) contributions to SHG significantly red shifts the peak position and distorts the line shape.

© 1996 Optical Society of America