Photoemission From Si(100)/Na,K Metallization, Surface Photovoltage and Quantum Well States

Abstract: Si(100) covered by Na and K has been studied with photoemission using low photon energies (hw<6eV). With photons in this energy range little information is obtained regarding the electronic states of the substrate. However, the low photon energies turn out to be convenient for observation of alkali metal adsorption induced changes such as the interface metallization, the work function change, the change of the surface photovoltage and for observation of quantum well type states formed by the alkali metal valence electrons confined to ultra thin alkali metal overlayers. The main advantage with low photon energies is the large cross section for emission of alkali metal valence electrons which is ascribed to the surface photoelectric effect. The results of main interest are as follows:

(a) The work function change measured for the cooled substrate (100 K or 70 K) shows the coverage dependence typical of alkali adsorption on metal surfaces with a work function minimum in the monolayer coverage range.

(b) The interface obtains metal character at around half of full monolayer and remains metallic also at higher coverages.

(c) The surface band bending is not much affected by alkali metal adsorption. This result is contrary to some previous observations.

(d) The surface photovoltage shows a dramatic alkali metal coverage dependence with a sharp decrease at the coverage marking the onset of metal character for the interface.

(e) Quantum well type states for alkali metal overlayers on a semiconductor surface may be nicely resolved by photoemission. Thin films with sufficient thickness homogeneity are obtained only when the substrate is cooled. In the present work the energy separation between such states have been used to determine film thickness in a novel manner.