ELECTRONIC-STRUCTURE OF THE ULTRATHIN CS SI(100)-(2X1) AND CS/SI(111)-(7X7) INTERFACES IN THE THRESHOLD ENERGY REGION/

The electronic band structure of the Cs/Si(100)-(2 x 1) and Cs/Si(111) -(7 x 7) interfaces has been studied near the Fermi level at submonola yer coverages. The technique of threshold photoemission spectroscopy w ith Linearly polarized light excitation has been employed. Surface pho toemission spectra reveal one Cs-induced band which can be either belo w the VBM or at the Fermi level, depending on the substrate. Parameter s of the Cs band and the change in ionization energy and work function are obtained as a function of Cs coverage. Experimental data provide direct evidence that the Cs/Si(100)(2 x 1) interface exhibits semicond ucting character in a great part of the submonolayer range, in contras t to the metallic-like Cs/Si(111)-(7 x 7) interface.