Electronic structures of the monohydride (2×1):H and the dihydride(1×1)∷2HSi(001) surfaces studied by angle-resolved electron-energy-loss spectroscopy

We have identified the monohydride Si(001)-(2×1):H surface and the dihydride $\mathrm{Si}\mathrm{}\left(001\right)\mathrm{}-(1\mathrm{}\times 1)\colon\colon 2\mathrm{H}$ surface by angle-resolved electron-energy-loss spectroscopy and elastic low-energy electron diffraction. On the monohydride (2×1):H surface the $S_3$ transition from the back-bond surface state was distinctly observed although the $S_1$ transition from the dangling-bond surface state disappeared, while on the dihydride $(1\mathrm{}\times 1)\colon\colon 2\mathrm{H}$ surface both the $S_3$ and the $S_1$ transitions completely disappeared. These facts show that on the monohydride surface the subsurface strain due to dimerization remains; on the other hand, on the dihydride surface the strain is healed out. The hydrogen-induced transitions for the two surfaces were clearly distinguished; the transition energy for the (2×1):H surface (S${\mathrm{H}}_1$) was 8.0 eV and that for the $(1\mathrm{}\times 1)\colon\colon 2\mathrm{H}$ surface (S${\mathrm{H}}_2$) increased from 7.0 to 7.5 eV with increasing $\left|k_{\parallel }\right|$ due to dispersion. The present results support and develop the models for the monohydride and the dihydride surfaces proposed by Sakurai and Hagstrum.