Early stages of cesium adsorption on the As-rich c „ 2 Ã 8 … reconstruction of GaAs„001…: Adsorption sites and Cs-induced chemical bonds C. Hogan, 1 D. Paget, 2 Y. Garreau, 3 M. Sauvage, 3 G. Onida, 4 L. Reining, 5 P. Chiaradia, 1 and V. Corradini 6 1 Dipartimento di Fisica and INFM, Universita ` di Roma ‘‘Tor Vergata,’’00133 Roma, Italy 2 Laboratoire de Physique de la Matie `re Condense ´e, Ecole Polytechnique, 91128 Palaiseau cedex, France 3 LURE, CNRS-MR-CEA, Ba ˆtiment 209d, Centre Universitaire Paris-sud, BP34, 91898 Orsay, France 4 Dipartimento di Fisica and INFM, Universita ` di Milano, Italy 5 Laboratoire des Solides Irradie ´s, UMR 7642 CNRS/CEA, Ecole Polytechnique, 91128 Palaiseau cedex, France 6 Dipartimento di Fisica and INFM, Universita ` di Modena, 1 Via G. Campi, 213/A I-41100, Modena, Italy Received 4 June 2003; published 19 November 2003 We investigate the adsorption of Cs on the As-rich c (2 8)/(2 4) reconstruction of GaAs001 at low coverages using a combination of theoretical and experimental techniques. Density-functional-theory local- density-approximation total-energy calculations and x-ray diffraction experiments find only minimal Cs- induced surface relaxation and identify three preferential adsorption sites within the partially disordered over- layer. These sites are, in order of decreasing occupation probability, the arsenic dimer bridge D site, the gallium dangling bond T 2 ' site, and the arsenic T 3 trench site. Detailed analysis of the wave functions and electronic charge densities allows us to clarify the bonding mechanisms at the three sites. At the gallium site, the bonding is strongly ionic and involves significant charge transfer to a new Cs-induced state reminiscent of the p z orbital of the gallium atom in the sp 2 configuration. In sharp contrast, at the arsenic sites, the charge transfer is minimal and the bonding rather occurs through mixing with a relatively delocalized state of the clean surface. The ionization energy decreases are estimated and compared for the three sites. DOI: 10.1103/PhysRevB.68.205313 PACS numbers: 78.40.Fy, 78.68.+m, 73.20.At I. INTRODUCTION The interaction between alkali atoms and semiconductor surfaces has long been the object of intensive fundamental studies, because of technological applications related to the lowering of the surface work function and because the ab- sence of chemical reactions at the interface makes it a model system. From an experimental point of view, such studies have mostly concerned GaAs and silicon, using Auger spec- troscopy and low-energy electron diffraction, 1–3 core level spectroscopy, 4,5 electron loss spectroscopy, 6,7 scanning tun- neling spectroscopy, 8 and x-ray diffraction. 9–11 Ab initio cal- culations have been performed using Na adsorption on GaAs110, 12 Cs adsorption on GaAs clusters simulating the 110 surface, 13 and K adsorption on Si001. 14 Among all these studies, only a very small fraction have considered very-low-coverage conditions, for which the alkali-alkali interactions are negligible as compared with alkali-substrate interactions. In this regime, fundamental as- pects of the latter interactions can be investigated in detail. In order to investigate the adsorption on a microscopic scale, it is first desirable to identify the adsorption sites and to evalu- ate the displacement of substrate atoms induced by adsorp- tion. It is of further interest to characterize the nature of the chemical bond between the adatom and the surface, to deter- mine the amount of charge transfer between the electroposi- tive alkali and the solid, and to analyze the nature of the alkali-induced surface dipole. For 001 and 011 surfaces of III-V semiconductors such as GaAs, one may think that, due to the presence of cations and anions at the surface, alkali atoms should preferentially adsorb near the empty dangling bonds of cation sites. This has been predicted using a reason- ing based on a tight-binding treatment of the hybridization of the outer s electron of the alkali and of the surface dangling bond. 15 For Cs adsorption at the 110 cleavage face, using scanning tunneling microscopy STM, 16 it has indeed been found that, at very low coverage, Cs atoms adsorb near Ga atoms. Calculations of adsorption of Na at the same surface 12 have shown that adsorption does not induce a breaking of surface chemical bonds, but results in a derelaxation of sub- strate atoms. The bonding between Na adatoms and substrate gallium atoms occurs through hybridization of the outer s alkali state and of the empty gallium dangling bond. The alkali s electron is partially transferred into the Ga dangling bonds, with only a weak perturbation of the latter. This trans- fer produces a surface dipole which, together with the alkali- induced change of surface dipole caused by the substrate derelaxation, explains the lowering of the ionization energy. For adsorption at the (2 1) reconstruction of the 001 surface of Si, the situation seems to be quite different. Using x-ray diffraction, 10 it was found that the dimer site D and the trench site T 3 are jointly populated, thus creating some dis- order in the alkali overlayer. For the former site, the silicon dimer bond seems to be broken, with a Si-Si distance close to its bulk value. These results are at variance with the ones found on the similar surface of germanium, 9 for which the Ge-Ge distances do not change by more than 8%, and in particular, the Ge dimer bonding length is essentially un- changed. Calculations performed for the K/Si system 14 con- clude that, at low coverage, the adatom region remains neu- tral, so that the amount of charge transfer is limited. The surface dipole originates from a polarization of the adatom due to the Si-K mixing. The same conclusion is also reached by a core level investigation for the same system. 17 PHYSICAL REVIEW B 68, 205313 2003 0163-1829/2003/6820/20531311/$20.00 ©2003 The American Physical Society 68 205313-1