PHYSICAL REVIEW B VOLUME 47, NUMBER 20 1S MAY 1993-II Adsorption of potassium on the Si(111)+3 X +3R 30':B surface: Observation of an insulating surface at submonolayer coverage T. M. Grehk Materials Science, The Royal Institute of Technology, S-100 44 Stockholm, Sweden L. S. O. Johansson Department of Synchrotron Radiation Research, Institute of Physics, University of Lund, Solvegatan 14, S-223 62 Lund, Sweden U. O. Karlsson and A. S. Flodstrom Materials Science, The Royal Institute of Technology, S-100 44 Stockholm, Sweden (Received 15 September 1992) The Si(111)&3 X &3R 30':B surface with submonolayer coverages of potassium has been studied with angle-resolved photoelectron spectroscopy. Deposition of potassium leads to the formation of a state 0.7 eV below the Fermi level. The energy position of this state showed only a minor dependence on the ac- tual potassium coverage and the state displayed a small dispersion of 0. 1 eV when probed along the I -M' direction in the &3 X &3R 30'-surface Brillouin zone. These results support the idea that a mainly covalent bonding exists between the potassium atoms and the Si surface. Interface systems of alkali metals on silicon surfaces have in recent years been studied extensively as model systems for metal-semiconductor interfaces. ' Despite the simple electronic structure of the alkali metals, with one s electron in their outer electronic shell, there still exists some controversy about the binding mechanisms associ- ated with the formation of the silicon-alkali-metal inter- faces and the absolute coverage at saturation. ' Two main, di6'ering pictures of the alkali-metal bond have been proposed. In the ionic-bond picture, there is a complete (or nearly complete) charge transfer of the alkali-metal valence electron to the semiconductor sur- face, causing a strong dipole-induced electrical field at the surface and, consequently, a strong reduction in the work function. The metallization of the surface occurs through the partial filling of the surface-state bands and hence the surface is metallic over a wide coverage range. In the polarized-covalent-bond picture, a small, fractional charge is transferred from the adsorbate atom to the substrate surface and changes in the work function are caused by a weak, polarized covalent bond between the alkali metal and the semiconductor substrate. The metallization occurs when the valence-electron orbital of the metal atoms overlaps. Therefore, in contrast to the ionic-bond picture, a critical adsorbate concentration is needed for metallization. In some studies' '" a mixed bonding model has been favored with almost ionic bind- ings at low coverage, whereas an increase in the adsor- bate coverage led to a higher degree of covalence. To get a better understanding of the electronic struc- ture associated with alkali-metal silicon bonding, the ad- sorption system, potassium on the Si(111)V'3X &3R 30':8 has been studied. This surface has the advantage of pos- sessing only one type of Si adatom and it is well estab- lished that the 8 atom on the Si(111)&3 X &3R 30:8 sur- face occupies the B-S& position directly below the Si ada- tom. ' ' The Si adatom in this configuration forms three bonds to its nearest, first-layer Si neighbors and donates one electron to the boron atom directly below it. Experimentally, an empty dangling-bond state, two filled back bond states, and one filled surface state, connected with boron bonded beneath the surface top layer, have been observed on the Si(111)+3 X v'3R 30:8 surface. ' ' ' In a recent high-resolution core-level x-ray photoemis- sion and photoabsorption study (XPS and XAS) of K ad- sorption on the Si(111)&3 X &3R30:8 surfaces, it was argued that the adsorbate substrate bonding is deter- mined primarily by the hybridization of the K 4s orbitals with an empty surface state. ' Due to the existence of an empty surface state on the Si(111)&3X&3R30':8 sur- face, a larger amount of charge is transferred from the potassium atoms to the surface than would be the case for a surface with half-filled surface state. It was further proposed' that one potassium atom interacts with each &3X&3 surface cell and that the resulting band would be half-filled, i.e. , a metallic surface is formed. In this paper, data from an angle-resolved photoemis- sion spectroscopy (ARPES) study of the Si(111)V3 X&3R30':8 surface, with difFerent amount of deposited potassium, is presented. The K-saturated sur- face has also been investigated along the I -M' direction, in the &3 X &3 surface Brillouin zone (SBZ). It was found that the filled surface bands of the Si(111)&3X &3R30':8 surface were una(Fected by the K adsorption. Instead, a new state was observed developing 0. 7 eV below the Fermi level. Surprisingly, no density of states was observed in the vicinity of the Fermi level. Only after exposure well beyond the saturation of the work-function change could the development of a metal- lic Fermi edge be seen. The ARPES spectra and the K 3p data were collected simultaneously using planar polarized synchrotron radia- tion of 21 eV and second-order light of 42 eV energy, re- spectively, at beamline 41 at the MAX-LAB synchro- 47 13 887 1993 The American Physical Society