Determination of surface states on Si(111) by surface photovoltage spectroscopy

Clean cleaved-silicon surfaces have been investigated by surface photovoltage (SPV) spectroscopy in the photon energy range $0.50 \mathrm{eV}<h\nu <1.55\mathrm{} \mathrm{eV}$. The SPV spectra of $n$- and $p$-doped samples differ substantially from each other, while there is no significant influence of the doping level itself. In the spectral range below the energy band gap both $n$- and $p$-type samples show a signal; its sign and threshold energies, however, are different. These differences are due to different transitions between bulk and surface states. The excitation from surface states to bulk states is distinguished from the reverse process by the sign of the SPV signal; therefore, empty and occupied surface states are studied independently. From the threshold energies of the spectra, surface-band models within the gap have been deduced. The results verify the model of two bands of surface states, 0.20 eV apart, on the 2×1 reconstructed silicon surface. In contrast to published results, however, the present measurements require two surface states on the 7×7 superstructure, 0.22 eV apart, and a Fermi level at the surface in the upper half of the gap.