Theory of adsorption: Ordered monolayers from Na to Cl on Si(001) and Ge(001)

Abstract

Abstract. First principles calculations of clean and adsorbate-covered surfaces of Si(001) and Ge(001) are reported. Chemical trends in the adsorption of ordered Na, K, Ge, As, Sb, S, Se and CI overlayers are discussed. The calculations are based on the local-density approximation and employ non-local, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method. From total-energy minimization calculations we obtain optimal surface reconstructions which show asymmetric dimers for Si(001), Ge(001) and Ge:Si(001). For As:Si(001), Sb:Si(001) and Sb:Ge(001), we find symmetric adatom dimers in the equilibrium geometries. S or Se adlayers are found to be adsorbed in bridge positions forming a (1 x 1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. C1 atoms adsorb on top of the dangling bonds of symmetric Si dimers residing in the first substrate-surface layer. Our calculations for Na:Si(001) and K:Si(001) confirm valleybridge site adsorption for half monolayer coverage. For full monolayer alkali-metal coverage, adsorption in pedestal and valley-bridge positions is found to be energetically most favourable. The calculated optimal adsorption configurations are in excellent agreement with a whole body of recent experimental data on surface-structure determination. For these structural models, we obtain electronic surface band structures which agree very good with a wealth of data from angle-resolved photoemission spectroscopy investigations. The adsorption of adlayers at semiconductor surfaces has been under intensive study in recent years. These investigations are motivated by fundamental physical reasons and important technological applications. In basic research, semiconductors are interesting substrates for adsorption systems due to the unsaturated directional bonds at their surfaces. This allows the formation of chemical compounds which are neither stable as small, isolated molecules nor as extended solids. The technological interest ranges from homoand heteroepitaxy and growth, via surface passivation and doping to dry etching and photo-surface cleaning. For example, the quality of the heteroepitaxial growth of GaAs on Si is determined by the initial stage of this process which consists in formation of As adlayers. Furthermore, surface passivation may involve As, S or Se atoms. In the fabrication of electronic devices, dry etching and photo-surface cleaning is carried out by the reaction of C1 on Si. The adsorption of alkali metals in the range of monolayer coverage leads to a drastic reduction of the ionization energy. These systems have important applications as high-efficiency emitters. They are also widely discussed in the context of the Schottky-barrier problem [1].

Atomic and electronic structure of the Si(001)2×1–K surface

“…This is in agreement with the experimental RHEED results [15], as well as with the theoretical predictions of Morikawa et al [9] and Kr€ uger and Pollmann [21]. Our calculated dimer length of 2.51 A is very similar to the values of 2.54 and 2.56 A determined by Morikawa et al [19] and Kr€ uger and Pollmann [21], respectively. Our theoretically predicted relaxation of the first layer (dimer) silicon atoms of )0.08 A is somewhat smaller than the RHEED result of )0.…”

“…Our calculations predict the S 1 and S 0 1 surface state bands to have a splitting of $0.15 eV at the K point of the SBZ, with these two bands gradually merging along the K-J 0 symmetric direction. This is consistent with the calculations of both Morikawa et al [19] and Kr€ uger and Pollmann [21]. The experimental SR-ARPES data, however, determined these two bands to be essentially degenerate both at the K point and along the K-J 0 symmetric direction [17].…”

“…Despite all of this theoretical and experimental work, however, some of the features of the surface electronic structure induced by this double-layer of K adatoms on the Si(0 0 1)2 · 1 surface still remain unresolved. The valence band electronic structure for the DL model determined by Kobayashi et al [18], Morikawa et al [19], Ishida and Terakura [20] and Kr€ uger and Pollmann [21] has been shown to be in good agreement with the experimental data. The inverse photoemission data for the unoccupied electronic surface states obtained by Johansson and Reihl [22], however, reveals some interesting and somewhat surprising features.…”

“…A derived from the calculations of Morikawa et al [9], and the considerably smaller values of 3.08 and 3.01 A obtained by Kr€ uger and Pollmann [21].…”

“…Kr€ uger and Pollmann [21] also determined two dominant occupied surface state bands close to the Fermi energy which they labeled as D and D Ã . In agreement with the above discussion, these were shown to be hybrid states formed by the interaction of the silicon dangling bonds and the valence electrons of the potassium atoms.…”