Optical properties of dangling-bond states at cleaved silicon surfaces

The spectral dependence of surface photovoltage and surface photoconductance both under continuous illumination as well as LEED I/V spectra were studied with cleaved Si(111)-2 × 1 surfaces at 130 K. Between 0.23 and 0.5 eV a doubly peaked absorption band was found with opposite sign compared to the SPV and SPC signals at higher photon energies. This band is due to electronic transitions from occupied to empty dangling-bond states located at the raised and the lowered rows of atoms in the 2 × 1 reconstruction, respectively. This absorption shows a pronounced dependence on the polarization of the incident light which correlates with the spatial symmetry of the dangling-bond states. Anneals at up to 500 K remove the low-energy absorption peak and equalize the 2 × 1 reconstruction: The homogeneous Si(111)-2 × 1 structure exhibits a buckling of 0.3 Å and a dangling-bond absorption with a threshold at 0.42 eV and a maximum at 0.47 eV. An anneal at 750 K, forming the 7 × 7 structure, destroys the peak of opposite sign in SPV and SPC and only leaves a broad tail with a threshold of 0.32 eV.