Laser-induced oxidation of the Si(111) surface

Pulsed laser induced oxidation of clean Si(111) surfaces has been studied by Auger electron spectroscopy and electron energy loss spectroscopy. The short duration time of the pulse has allowed a precise investigation of the first stages of the oxidation. About 1–2 oxide monolayers first grow in less than 10 s. Their stoichiometry evolves from SiO_x towards SiO₂ with increasing beam energy densities. Once this superficial layer has formed, no evolution is seen with further irradiation, suggesting that oxygen diffusion during the pulse duration cannot sustain the oxide growth.     

Phase transitions on the (111) surface of diamond type semiconductors

The reconstruction and the phase transition are discussed on the (111) surfaces of Si and Ge by the use of the lattice gas model on a triangular net. A Monte Carlo simulation calculation is carried out of the lattice gas model. It is shown that the characteristic diffuse scattering of electrons in the (1x1) phase of Ge(111) can be reproduced by the model. Moreover, the underlying mechanism is discussed in terms of particle arrangements divided into irregular hexagon-like (2x2) domains which yield the diffuse patterns in the (1x1) phase. For the Si(111) surface, on the other hand, the calculation concludes the absence of stacking faults in the (1x1) state. The interrelation between the reconstructions of the surfaces is also discussed.     

金刚石延(111)面生长的第一性原理研究

利用密度泛函理论(DFT)对碳原子在镍(111)表面吸附结构进行了计算，得到了吸附能以及态密度 (density of state, DOS)分布，分析了吸附在镍(111)面的碳原子和金刚石(111)面的碳原子的分波态密度(PDOS)，结果表明吸附在镍表面的碳原子具有与金刚石表面碳原子相类似的电子结构特点，即两者都存在孤对的和成键的sp³杂化电子,进而发现吸附在镍表面的碳原子极易与金刚石表面相互作用形成稳定的类金刚石几何结构. 关键词： 密度泛函理论 化学吸附 电子结构 金刚石生长     

Oxygen adsorption on a Pd(111) surface

The interaction of oxygen with Pd(111) has been studied by means of AES, ELS, thermal desorption (TD), electron stimulated desorption (ESD) and work function measurements. It was found that a very small part ( ～ 2–3%) of the available adsorption sites are contributing to the O⁺ electron stimulated yield, the population of the latter being accompanied by enormously large work function changes (up to ～ 0.9 eV). A mechanism of adsorption and depopulation of these sites involving oxygen bulk and surface diffusion has been proposed.     

Molecular SCF calculations to model (111) surface states and relaxation of diamond

Ab-initio SCF-MO computations in an STO-3G Gaussian basis on a molecular system, designed to model a carbon atom on the (111) surface of diamond, predict that the danglingbond carbon relaxes about 0.10 Å toward the bulk from its position on an extension of the diamond lattice. The dangling bond carbon cation relaxes Inward 0.30 Å, while the anion relaxes outward 0.10 A. Tentative ways to describe the dangling bond in terms of computed quantities are presented.     

Adsorption of hydrogen on a Pt(111) surface

The H₂/Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H₂ was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H₂/D₂ exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I/V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at ?15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β₂-) state desorbing with second order kinetics below 400 K and a low temperature (β₂-) state that fills up, in the main, after the first peak saturates. The β₂-state is associated with an activation energy for desorption E1 of 9.5 kcal/mole. The decrease E1 with increasing coverage and the formation of the β₁-state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δ? was found to have a small positive maximum (～ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of ?230 mV at saturation. The variation of Δ? with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δ? compared well with the results of the analysis of the thermal desorption spectra.     

Adsorption of potassium on a Si(111) surface

The adsorption of potassium on several sites (T₁, T₄, H₃) of a Si(111) surface is studied using the extended Hückel method and cluster models. The results show that the chemisorption energy on the T₄ site is almost equal to that on the H₃ site for an ideal surface. However, the analysis of chemical bonding of adatoms on a Si surface suggests that the chemisorption of adatoms might induce a kind of reconstruction on the Si(111) surface. Rough calculations for the adsorption on this reconstructed surface are carried out. In this paper, charge transfer, density of states and characterization of the bonds between the adatom and substrate Si atoms are also investigated in detail.     

The Tamm surface state on Cu(111) and Ag(111)

It is shown, that the well-known d-electron Tamm surface-state emission observed in photoelectron spectra from Cu(111) at the M̄ point in the surface Brillouin zone, is indeed due to such a surface state and not a bulk band transition as recently suggested L. Wallden, Solid State Commun. 59, 205 (1986). A similar surface state on Ag(111) is reported.     

Hydrogen adsorption on the silicon (001) surface and on a step on the (111) surface

Adsorption of atomic hydrogen on an ideal (001) silicon surface is investigated in the present paper. Saturation of one of the two dangling bonds of a silicon atom on this surface by hydrogen removes the interaction (hybridization) between them, resulting in the appearance of a bonding and an antibonding chemisorption state associated with the attacked dangling bond, and in the shift of the peak of the remaining unsaturated dangling bond to the energy typical of a surface state of the (111) surface. Further saturation leads to the disappearance of this peak from the energy spectrum. An analogous situation occurs for the silicon atom with two dangling bonds on a step on the (111) surface, when hydrogen is chemisorbed. Both examples testify to the local chemical nature of Shockley surface states in silicon.The authors thank A. N. Sorokin for useful discussions.     

Early stages of laser graphitization of diamond

A mechanism for photographitization of a free diamond surface is proposed. The quantum-kinetic rate of this process is determined. The graphitization rate is close to zero if the activation energy of the graphitization process is taken as being equal to the binding energy of a carbon atom with the surface (i.e. equal to the sublimation energy of a carbon atom). On the contrary, if the activation energy is close to the energy of C–C bonds, the graphitization process may occur at a noticeable rate and be observed under ‘relatively smooth’ experimental conditions. The temperature rise leads to a considerable increase in the graphitization rates. Preliminary experimental data on the low-rate laser ablation of diamond are presented to support the proposed model of photographitization. An early stage of laser-induced graphitization in the bulk of diamond is also considered. It is found that the nucleation of a ‘tiny graphite drop’ is possible in the bulk of the diamond inside the focal area of a laser beam; the ‘graphite drop’ growth causing the appearance of mechanical stresses in the surrounding regions. The maximum size of the graphite drop is determined, which, when exceeded, leads to mechanical damage of the sample and to a change in the mechanism of laser graphitization. An evident mechanical criterion for laser-induced damage of diamond is proposed. Received: 2 October 2002 / Accepted: 5 October 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. E-mail: stvn@stankin.ru