Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.     

Simulations of iodine adsorbed Ge(0 0 1) surface and its STM images

A layer of iodine at Ge(0 0 1) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(0 0 1) surface covered by 0.25 monolayer of iodine. The p(2×4), p(2×2), c(2×4) and p(1×4) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I₂ molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data.     

The formation of a surface iodide on Ni{100} and adsorption of I2 at low temperatures

Iodine adsorption on clean Ni[100] has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At temperatures below 340 K. a saturated surface of adsorbed iodine atoms in a c(2 × 2) structure is observed. Adsorption of iodine on clean Ni{100} at temperatures in exces of 370 K forms a structure identified as a single layer of the layered compound NiI₂ on the metal substrate. Solid iodine is shown to grow epitaxially on both the c(2 × 2) chemisorbed surface and the surface iodide at temperatures less than 185 K. Heating to 185 < T < 226 K leaves a physisorbed molecular iodine layer, while on returning to room temperature the original c(2 × 2) or iodide is restored.     

Reversible superstructural transitions on the GaAs(001) surface under the selective effect of iodine and cesium

The selective interaction of the iodine and cesium atoms with the GaAs(001) surface, which leads to a decrease in the bond energy of the Ga and As surface atoms, respectively, owing to the redistribution of the electron density in the near-surface region under the effect of electronegative and electropositive adsorbates, has been experimentally investigated. This selective interaction makes it possible to remove alternately the Ga and As monolayers in the iodine and cesium adsorption followed by heating at T ≤ 450°C and, thus, to implement reversible low-temperature transitions between the Ga-and As-stabilized superstructures, as well as the atomic layer etching of the semiconductor with the physically ultimate monolayer accuracy.     

Ti原子与氮掺杂金刚石(001) 界面结合强度的第一性原理研究

摘 要：金属钛原子在金刚石表面的结合强度直接影响金刚石真空介电窗口的使用性能和寿命. 本文通过基于密度泛函理论的第一性原理方法研究了Ti原子与不同氮掺杂位置的金刚石（001）界面的结合能、电荷分布和稳态几何结构. 结果表明：Ti原子与N原子取代掺杂在第二层C原子处金刚石表面的结合能比未掺杂和掺杂在第三层的结合能都高,达到-7.293 eV,使得金刚石表面形成的界面结构更加稳定,结合强度更好;通过电荷分布分析,N原子掺杂在第二层金刚石表面的Ti原子上的电荷转移最明显,对金刚石表面碳原子吸附最强,也具有更好的结合强度. 与未掺杂金刚石表面形成的Ti-C键键长相比,N掺杂在第二层和第三层C原子处金刚石表面形成的Ti-C键键长比前者分别长0.051 Å和0.042 Å,略有增加.     

Subject index

Lead iodide is known to dissociate under band gap illumination. We investigated the decomposition products from sintered samples by quadrupole mass spectrometry and found that neutral atomic and molecular iodine species desorb from the illuminated surface. The desorption rate was measured as a function of light intensity and surface temperature. Atomic iodine desorption seems to be thermally activated throughout the temperature range (300–500 K), while molecular iodine has that feature only up to 400 K. We performed measurements of the dark and photoconductivity of PbI₂ as functions of temperature and light intensity. Correlations of those results with the photolysis mass-spectrometric data indicate a possible model for the dissociation process. It suggests a diffusion-controlled mechanism of iodine desorption as well as of the dark conductivity, which seems to be ionic in that temperature range. The molecular iodine is apparently formed in surface sites, such as grain boundaries, which have a longer life-time for adsorbed iodine atoms as well as a dominant role in surface electronic conduction.     

Scanning tunneling microscopy identification of atomic-scale intermixing on Si(100) at submonolayer Ge coverages

The positions of Ge atoms intermixed in the Si(100) surface at very low concentration are identified using empty-state imaging in scanning tunneling microscopy. A measurable degree of place exchange occurs at temperatures as low as 330 K. Contrary to earlier conclusions, good differentiation between Si atoms and Ge atoms can be achieved by proper imaging conditions.     

Potential energy landscape of monolayer-surface systems governed by repulsive lateral interactions: the case of (3 x 3)-I-Pt(111)

Combined density functional theory (DFT) and Monte Carlo (MC) approach is applied to study the potential energy landscape of four iodine atoms adsorbed on the Pt(111) surface in a (3 x 3) unit cell. Three critical points were identified: (3 x 3)-sym and (3 x 3)-asym, corresponding to structures well known from experimental studies, while the third one (3 x 3)-zigzag is a new structure not reported before. An interaction model fitted to DFT calculations allows us to explain the difference between arrangements of iodine monolayer in vacuum, air, and solution environments as a result of different repulsion regimes.     

Investigation into the adsorption of atomic nitrogen on an Al2O3 (0001) surface

Computer simulation of sapphire nitridation used to obtain nitride-based heterostructures (GaN) on an Al₂O₃ substrate has been performed. The adhesion of atomic nitrogen to the sapphire (0001) surface is investigated ab initio. The possibility of replacing surface-layer oxygen atoms with nitrogen atoms has been examined. The calculated results indicate that adsorbed nitrogen atoms occupy the most stable positions above surface oxygen atoms at different nitrogen concentrations. The changes in the total system energy after replacement of surface oxygen atoms with nitrogen atoms have been calculated. It turns out that oxygen replacement is energetically unfavorable for a single nitrogen adatom. However, this process becomes energetically favorable if the concentration of nitrogen atoms increases. This outcome, obtained for the first time, enables better understanding of the atomic-scale mechanism of sapphire nitridation.     

Ab initio study of the fcc-WC(1 0 0) surface and its interaction with cobalt monolayers

The WC(1 0 0) surface has been studied by using ab initio methods of the density functional theory and pseudopotentials. Calculations have shown that surface and undersurface atoms move from their bulk positions. Namely, carbon atoms moved outward, while tungsten atoms moved inward. Five geometric cases for Co/WC(1 0 0) system were compared: (A) Co atoms are above C atoms; (B) Co atoms are above W atoms; (C) Co atoms are in the four-fold sites above WC pairs; (D and E) Co atoms are above the W-W-C and C-C-W three-fold sites, respectively - and the (A) case has been found to be energetically preferable. In all cases, Co layers have been found to be ferromagnetic. The densities of states for the bulk fcc-WC, the WC(1 0 0) surface, and the WC/Co system were compared.     

A saturable absorber for the iodine laser

An absorber for iodine laser pulses has been developed using iodine atoms in the ground state as the absorbing species. The iodine atoms have been produced by dissociation of gaseous molecular iodine in a heated cell. The absorption is easily saturated at moderate energy densities.     

Cesium adsorption on the β2-GaAs(001) surface

The results of first-principles calculations of the cesium adsorption energy on the β2-GaAs(001) surface performed within approaches of the density functional theory are presented for two possible terminations of the surface. It is shown that, among the considered high-symmetry positions, the energy-preferred position for cesium is position T ₃ when the surface layer contains arsenic and position T ₄ for gallium terminated surface. Cesium introduces insignificant perturbations in the positions of surface-layer atoms, and surface dimers do not break even in the case of adsorption at the dimer bridge and top positions. It is shown that cesium bonding to the GaAs (001) substrate can be explained by sp hybridization of arsenic and gallium orbitals as well as by formation of cesium states mixed with delocalized states of a clean surface. At low coverage, more preferable adsorbate sites are those with nearest neighbor arsenic atoms for both surface terminations.     

Photoelectron holography analysis of W(1 1 0)(1×1)–O surface

By applying photoelectron holography analysis to a W(1 1 0)(1×1)–O surface, the real space image was reconstructed around the oxygen atoms. The peak positions showed that the oxygen atoms lie within the top layer, which does not contradict the results of the R-factor X-ray photoelectron diffraction analysis of Daimon et al. [Surf. Sci. 408 (1998) 260]. The holographic analysis of the calculated photoelectron diffraction pattern indicated that holographic reconstruction reveals atomic positions up to the third nearest-neighbor scatterers. Our study suggests the possibility that the distance among the adsorbate oxygen atoms is slightly larger than the lattice constant of the substrate.     

Cu(100) (2×22)R45°-O的表面结构与电子态的密度泛函研究

用第一性原理的总能计算研究了Cu(100))面的表面结构、弛豫以及氧原子的(2×22)吸附状 态.计算给出了Cu(100) (2×22)R45°-O吸附表面的结构参数，并得到了上述结构下氧吸附 的Cu(100)表面氧原子和各层Cu原子的电子态密度.计算得到的吸附表面功函数为4.58 eV ，与清洁Cu(100)表面功函数(～4.53 eV)几乎相同.吸附氧原子与最外层铜原子之间的垂直 距离约为0.02 nm，其能带结构体现出一定的金属性，同时由于Cu-O的杂化作用在费米能以 下约6.4 eV附近出现了局域的表面态.可以认为，在Cu(100) (2×22)R45°的氧吸附表面结 构下，吸附氧原子和衬底之间的结合主要来源于表面最外层铜原子与氧原子的相互作用. 关键词： Cu(100)(2×22)R45°-O表面 缺列再构 表面电子态     

Early stages of vanadium deposition on Si(1 1 1)-7 × 7

We investigate the low-coverage regime of vanadium deposition on the Si(1 1 1)-7 × 7 surface using a combination of scanning tunnelling microscopy (STM) and density-functional theory (DFT) adsorption energy calculations. We theoretically identify the most stable structures in this system: (i) substitutional vanadium atoms at silicon adatom positions; (ii) interstitial vanadium atoms between silicon adatoms and rest atoms; and (iii) interstitial vanadium - silicon adatom vacancy complexes. STM images reveal two simple vanadium-related features near the Si adatom positions: bright spots at both polarities (BB) and dark spots for empty and bright spots for filled states (DB). We relate the BB spots to the interstitial structures and the DB spots to substitutional structures.     

Xe原子吸附对GaAs(110)表面重构的影响

基于第一性原理的自洽场密度泛函理论(DFT)和广义梯度近似(GGA),利用缀加平面波加局域轨道(APW+lo)近似方法，建立了五层层晶超原胞模型，模拟了GaAs(110)表面结构和单个Xe原子在其表面的吸附.利用牛顿动力学方法，对GaAs(110)表面原子构形的弛豫和Xe原子在GaAs(110)表面的吸附进行了计算.从三种不同的初始构形出发，即Xe原子分别在Ga原子的顶位，As原子的顶位以及桥位，都发现Xe原子位于桥位时体系能量最低.由此，认为Xe原子在GaAs(110)表面的吸附位置在桥位，并且发现吸附Xe原子后GaAs(110)表面有趋向于理想表面的趋势，表面重构现象趋于消失，表面原子间键长有一定的恢复,这与理论预言相符合. 关键词： 密度泛函理论 表面结构 APW 表面原子吸附     

Tensor LEED analysis for the electrodeposited Pt(1 1 1)-(3×3)–Ag,I surface structure

A crystallographic analysis is reported using low-energy electron diffraction (LEED) in the tensor LEED approach for the electrodeposited coadsorption (3×3) structure with 4/9 monolayer (ML) of silver and 4/9 ML of iodine on the Pt(1 1 1) surface. The structure approximates a two-layer slice of bulk AgI cut parallel to its (1 1 1) plane and superimposed on the substrate with the Ag atoms in contact with the topmost Pt(1 1 1) layer, and the I atoms forming an overlayer on the Ag atoms. There are two types of Ag atoms in the (3×3) unit mesh; one type bonds to a single Pt atom, while the other type bonds to three Pt atoms. The average Ag–Pt bond distances are close to 2.48 and 2.82 Å respectively for the one and three-coordinate Ag atoms, but both types of Ag atoms bond to three I atoms with an average Ag–I distance of 2.67 Å. No significant corrugation is observed for either the I layer or the Ag layer.     

LEED,Auger and work function study of iodine adsorbed on W(110)

The adsorption of iodine on a W(110) surface has been studied by LEED, Auger and work function changes. LEED has revealed several phases which desorb in different temperature regimes and are accordingly designated γ, α, β₁, β₂, and β₃. The γ and α phases exhibited p(2 × 2) and p(2 × 1) surface nets respectively with coherently positioned antiphase boundaries which produced a splitting of selected LEED beams. The separation between the antiphase boundaries of the α phase increased with decreasing coverage. Both the γ and α phases were associated with molecularly absorbed iodine. The three β phases were associated with dissociatively adsorbed iodine which formed chain structures on the surface with the arrangement of iodine atoms within each chain being unique to the particular phase. Continuous changes in coverage then occurred by sheets of these chains shearing to produce packing faults at the resulting shear lines. This shearing process occurred coherently in the β₁ and β₃ phases and incoherently in the β₂ phases. In the former cases, the effect was seen by the continuous movement of coherent LEED beams with changing coverage. A phase diagram was constructed to describe the relative coverages and thermal stability of the phases. The characteristic Auger electron emission of iodine was observed at 495 eV and used to estimate the surface coverage. The work function was found to decrease by 0.4 eV with the adsorption of the first half monolayer and remained unchanged with further adsorption.     

Simultaneous imaging of the In and As sublattice on InAs(110)-(1×1) with dynamic scanning force microscopy

Distance-dependent dynamic scanning force microscopy (SFM) measurements of InAs(110)-(1×1) acquired in ultrahigh vacuum at low temperatures are presented. On this surface, the atoms of the As sublattice are lifted by 80 pm with respect to the In sublattice and terminate the surface. Thus, since in most dynamic SFM images only protrusions with the periodicity of one sublattice are observed, these protrusions are correlated with the positions of the As atoms. However, under certain conditions, an additional contrast is visible which can be attributed to an interaction between the foremost tip atoms and the In atoms. Possible contrast mechanisms are discussed in terms of tip–sample distance and tip structure.     

Effects of oxygen concentration and irradiation defects on the oxidation corrosion of body-centered-cubic iron surfaces: A first-principles study

Oxidation corrosion of steels usually occurs in contact with the oxygen-contained environment, which is accelerated by high oxygen concentration and irradiation. The oxidation mechanism of steels is investigated by the adsorption/solution of oxygen atoms on/under body-centered-cubic (bcc) iron surfaces, and diffusion of oxygen atoms on the surface and in the near-surface region. Energetic results indicate that oxygen atoms prefer to adsorb at hollow and long-bridge positions on the Fe(100) and (110) surfaces, respectively. As the coverage of oxygen atoms increases, oxygen atoms would repel each other and gradually dissolve in the near-surface and bulk region. As vacancies exist, oxygen atoms are attracted by vacancies, especially in the near-surface and bulk region. Dynamic results indicate that the diffusion of O atoms on surfaces is easier than that into near-surface, which is affected by oxygen coverage and vacancies. Moreover, the effects of oxygen concentration and irradiation on oxygen density in the near-surface and bulk region are estimated by the McLean's model with a simple hypothesis.