Metallization, surface photovoltage, and quantum-well-type resonance for K-covered Si(100) observed via valence-band photoemission

The development of metal character in the surface region of K-covered Si(100) is monitored via the appearance of a Fermi edge in photoemission spectra recorded at low photon energies (ħω<6.0 eV). The Fermi-edge intensity shows an onset at a coverage close to that required to obtain the work-function minimum typical for this adsorption system. For clean, cooled (∼70 K) Si(100) substrate (n-type, 2 Ω cm), the light source used for photoemission nearly eliminates the surface band bending, but upon K deposition the band bending reappears and then has nearly the same magnitude as for the uncovered Si(100) surface left in darkness. The changeover from flat bands to full band bending occurs over a narrow monolayer coverage interval at coverages somewhat higher than required for the observation of a Fermi edge. Upon continued deposition a photoemission peak is observed which is assigned to a quantum-well-type resonance formed by alkali-metal valence electrons propagating back and forth in the overlayer between the vacuum barrier and the overlayer-substrate interface.