Giant band bending induced by Ag on InAs(110) surfaces at low temperature.

Abstract

We show by synchrotron radiation photoemission spectroscopy [G. Le Lay, V. Yu Aristov, J. Kanski, P. O. Nilsson, U. O. Karlsson, K. Hricovini, and J. E. Bonnet (unpublished)] (core levels and valence band) as well as by Kelvin probe measurements, both under illumination and in the dark, at low temperature (LT) T\ensuremath{\approxeq}20 K and room temperature (RT) T\ensuremath{\approxeq}300 K, that upon deposition of minute amounts of silver (about 0.01--0.1 monolayer) onto in situ well-cleaved, highly doped, n- and p-type InAs(110) surfaces, one induces a giant movement of the Fermi level ${\mathit{E}}_{\mathit{F}}$ into the conduction band. We thus create a two-dimensional (2D) electron gas at the surface (strong downwards band bending): as a matter of fact, we do observe emission of electrons from filled states up to the ${\mathit{E}}_{\mathit{F}}$ placed far above the conduction-band minimum (CBM). It is also shown that the growth is laminar at LT, while it follows a Volmer-Weber mode at RT, with indications of chemical interactions between the Ag and In atoms at both temperatures. At LT the deposited atoms interact with the dangling bonds of the In atoms and saturate them for \ensuremath{\theta}\ensuremath{\approxeq}1 monolayer (ML). At low coverages and LT, the individual Ag atoms create donorlike surface states (SS) (adsorption-induced states) and ${\mathit{E}}_{\mathit{F}}$ can be pinned by these SS in the conduction band. This leads to the formation of a 2D electron channel at the InAs surface already at \ensuremath{\theta}\ensuremath{\approxeq}0.01--0.1 ML for both types of semiconductors. At higher coverages one observes the onset of metallization for both temperatures and Ag clusters can produce another type of SS, typically metal-induced gap states near the CBM. These proposals, as well as the experimental facts, are discussed in the light of the current theoretical models of Schottky-barrier formation.