Cl insertion on Si(100)-(2x1): etching under conditions of supersaturation

We use scanning tunneling microscopy to show that Cl2 dosing of Cl-saturated Si(100)-(2x1) surfaces at elevated temperature leads to uptake beyond "saturation" and allows access to a new etching pathway. This process involves Cl insertion in Si-Si dimer bonds or backbonds, diffusion of the inserted Cl, and ultimately desorption of SiCl2. Investigations into the etch kinetics reveal that insertion occurs via a novel form of Cl2 dissociative chemisorption that is mediated by dangling bond sites. Upon dissociation, one Cl atom adsorbs at the dangling bond while the other can insert.     

State-specific enhancement of Cl+ and Cl- desorption for SiCl4 adsorbed on a Si(100) surface following Cl 2 p and Si 2 p core-level excitations

State-specific desorption for SiCl4 adsorbed on a Si(100) surface at approximately 90 K with variable coverage following the Cl 2p and Si 2p core-level excitations has been investigated using synchrotron radiation. The Cl+ yields show a significant enhancement following the Cl 2p-->8a*1 excitation. The Cl- yields are notably enhanced at the 8a*1 resonance at both Cl 2p and Si 2p edges. The enhancement of the Cl- yield occurs through the formation of highly excited states of the adsorbed molecules. These results provide some new dissociation processes from adsorbates on surfaces via core-level excitation.     

Atomic wire oxidation of H-terminated Si(100)-( 2x1): domino reaction via oxidation and H migration

We studied oxidation at a dangling bond (DB) on the H-terminated Si(100) surface by the first-principles calculations. We found that oxidation easily occurs at the exposed DB on the H-terminated Si(100) surface. The dissociated O atoms are chemisorbed at a dimer bond and a back bond, resulting in adjacent H atom migration onto the DB. As a consequence of the alternate oxidation and subsequent H atom migration processes, the atomic wire oxidation is actually found to occur on the H-terminated Si(100) surface at low temperatures without desorbing H atoms, as observed in our scanning tunneling microscopy experiment.     

Chemical interactions in noncontact AFM on semiconductor surfaces: Si(111), Si(100) and GaAs(110)

Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic force microscopy (AFM) on Si(111), Si(100) and GaAs(110) surfaces. The chemical bonding interaction between a localised dangling bond on the atom at the apex of the tip and the dangling bonds on the adatoms in the surface is shown to dominate the forces and the force gradients and, hence, to provide atomic resolution. The lateral resolution capabilities are tested in both the Si(100) and the GaAs(110) surfaces. In the first case, the two atoms in a dimer can be resolved due to the dimer flip induced by the interaction with the tip during the scan, while in the GaAs(110), we identify the anion sublattice as the one observed in the experimental images.     

Chemical reactions of chlorine on a vicinal Si(100) surface studied by ESDIAD

The reaction of Cl₂ on a vicinal Si(100) surface has been studied by ESDIAD. This type of surface possesses two types of sites: pairs of dangling bonds on Si---Si terrace dimers, and single dangling bonds on the 2-atom layer high steps. The reactivity of these two sites is compared. For low coverages the step dangling bonds are identified as the preferred Cl bonding site after 673 K-annealing. Upon higher temperature annealing and SiCl₂(g) desorption, the terrace-site Cl species are depleted more rapidly than the step-site Cl species. Extensive isothermal etching under a continuous Cl₂ flux at 800 K is found to produce a disordered surface structure. Heating to 1123 K causes a reordering of the surface.     

The mechanism of amine formation on Si(1 0 0) activated with chlorine atoms

The dissociation of NH₃ on a Si(1 0 0) surface activated with Cl atoms was investigated using X-ray photoelectron spectroscopy. Gas phase UV-Cl₂ (0.1-10 Torr Cl₂ for 10-600 s under 1000 W Xe lamp illumination) completely replaced the H-termination on aqueous-cleaned Si(1 0 0) with 0.82 ± 0.06 ML of Cl at 298 K. A single spin-orbit split Cl 2p doublet indicated that the Cl atoms were bound to Si dimer atoms, forming silicon monochloride (Cl-Si-Si-Cl). Exposing the Cl-terminated surface at 348 K to NH₃ (1-1000 Torr for 5-60 min) replaced one Cl atom with one N atom up to a coverage of 0.33 ± 0.02 ML. Cl atoms lowered the activation energy barrier for reaction to form a primary amine (Si-NH₂). Oxygen was coadsorbed due to competition by H₂O contamination. The presence of Cl on the surface even after high NH₃ exposures is attributed to site blocking and electrostatic interactions among neighboring Cl-Si-Si-NH₂ moieties. The results demonstrate a low temperature reaction pathway for depositing N-bearing molecules on Si surfaces.     

Diffusional kinetics of SiGe dimers on Si(100) using atom-tracking scanning tunneling microscopy

Quantitative measurements of the diffusion of adsorbed mixed Ge-Si dimers on the Si(100) surface have been made as a function of temperature using atom-tracking scanning tunneling microscopy. These mixed dimers are distinguishable from pure Si-Si dimers by their characteristic kinetics-a 180 degrees rotation between two highly buckled configurations. At temperatures at which the mixed dimers diffuse, atomic-exchange events occur, in which the Ge atom in the adsorbed dimer exchanges with a substrate Si atom. Reexchange can also occur when the diffusing Si-Si dimer revisits the original site of exchange.     

Structure of quantum wires in Au/Si(557)

The structure of the Au/Si(557) surface is determined from three-dimensional x-ray diffraction measurements, which directly mandate a single Au atom per unit cell. We use a "heavy atom" method in which the Au atom images the rest of the structure. Au is found to substitute for a row of first-layer Si atoms in the middle of the terrace, which then reconstructs by step rebonding and adatoms. The structure is consistent with the 1D metallic behavior seen by photoemission.     

Two-dimensional carbon incorporation into Si(001): C amount and structure of Si(001)-c(4 x 4)

The C amount and the structure of the Si(001)-c(4 x 4) surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The c(4 x 4) phase is found to contain 1/8 monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The c(4 x 4) phase constitutes the subsurface Si(0.875)C(0.125) delta layer with two-dimensionally ordered C atoms.     

Effect of hydrogenation on B/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and the energetics of substitutional boron on the generic Si(0 0 1)-(1 × 2) surface. For a single B atom substitution corresponding to 0.5 ML coverage, we have considered two different sites: (i) the mixed Si-B dimer structure and (ii) boron substituting for the second-layer Si to form Si-B back-bond structure, which is energetically more favorable than the mixed Si-B dimer by 0.1 eV/dimer. However, when both of these cases are passivated by hydrogen atoms, the situation is reversed and the Si-B back-bond case becomes 0.1 eV/dimer higher in energy than the mixed Si-B dimer case. For the B incorporation corresponding to 1 ML coverage, among the substitutional cases, 100% interdiffusion into the third layer of Si and 50% interdiffusion into the second layer of Si are energetically similar and more favorable than the other cases that are considered. However, when the surface is passivated with hydrogen, the B atoms energetically prefer to stay at the third layer of the Si substrate.     

Dimer elongation on the monohydride-covered Ge/Si(100) surface

Using transmission ion channeling, we have made the first measurement of the Ge dimer geometry for the monohydride-covered Ge/Si(100)-2×1 surface. Comparison of calculated angular scans with experimental angular scans near the 100 and 110 directions has resulted in a measured Ge dimer bond length of 2.8 Å, which is 8% longer than the corresponding dimer bond length reported for Ge on Si(100) in the absence of H. This elongation is similar to that reported for Si dimers on the Si(100) surface. Also, relative to the (100) surface plane, the dimers change from tilted without H to untilted with H.     

单畴的单原子In纳米线阵列的制备与研究

利用Si（001）向［110］方向偏4°角的斜切表面作为衬底，成功地制备了分布均匀的单畴的单原子In链阵列.扫描隧道显微镜分析表明，沉积的In原子优先吸附在台面上沿着台阶内边缘的位置，并在两个Si的二聚体链之间形成稳定的In二聚体.In二聚体组成直的单原子链，其生长机理与Car提出的“表面聚合反应”相一致.另外，衬底具有非常窄的台面和双原子层台阶边的特殊结构是形成单畴的单原子链的关键. 关键词： 铟单原子链 硅邻近面 扫描隧道显微镜     

Stereochemistry on Si(001): angular dependence of H2 dissociation

The angular dependence of the dissociative adsorption of molecular hydrogen at terrace and step sites of vicinal single-domain Si(001) surfaces was investigated by means of molecular beam techniques and optical second-harmonic generation. A strongly anisotropic behavior was observed for terrace adsorption with polar distributions of cos3theta and cos12theta parallel and perpendicular to the dimer, respectively. The D(B)-steps show enhanced reactivity under glancing incidence in the upwards direction. The results are traced back to the directionality of the covalent surface bonds.     

Surface diffusion during decay of nano-island on Si(1 0 0) at high temperature

Surface diffusion during decay of a two-dimensional nano-island formed on Si(1 0 0) surface at 750-800 K is studied using STM and a kinetic Monte Carlo simulation. From a surface diffusion point of view, decay proceeds so that the total diffusion rate of atoms on a surface decreases. Atoms at step edges move more frequently than terrace atoms, which results in decay from step edges of the island. In addition, a terrace atom takes part in surface diffusion in the same way as an atom from steps of the island once it hops up on a terrace leaving a vacancy. The mass transport is not a specific atom process but terrace atoms and vacancies on the terrace are involved. Repeated upward and downward hops of atoms and their difference are combined with surface diffusion, which leads to the mass transport. Some tracks of atom using simulation show random walk with preferential diffusion along step edges, re-entering to the island, exchange of diffusing atom and filling in a vacancy on the terrace. The motion of the center of the island to the upper side of the terrace observed by STM is also well reproduced in the simulation.     

Energetics of atomic hydrogen diffusion on Si(100)

We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H₂ recombinative desorption.     

低能Cl原子刻蚀Si(100)表面的分子动力学模拟

使用分子动力学模拟方法研究了不同能量(0.3—10 eV)的Cl原子对表面温度为300 K的Si(100)表面的刻蚀过程.模拟中采用了Tersoff-Brenner势能函数来描述Cl-Si体系的相互作用.模拟结果显示,随着入射Cl原子在表面的吸附达到饱和,Si表面形成一层富Cl反应层.这和实验结果是一致的.反应层厚度随入射能量增加而增加.反应层中主要化合物类型为SiCl,且主要分布于反应层底部.模拟结果发现随初始入射能量的增加,Si的刻蚀率增大.在入射能量为0.3,1和5 eV时,主要的Si刻蚀产物为Si 关键词： 分子动力学 Cl刻蚀Si 分子动力学模拟 微电子机械系统     

Atomic and electronic structure of the Si(001) surface induced by hydrogen-adsorption

The initial stage of hydrogen (H) adsorption on the Si(001) surface is theoretically investigated to clarify the atomic and electronic structure induced by the adsorption. For this purpose, the electronic states are calculated in the density functional approach with the DV(discretized variation)-Xα-LCAO method. We also simulate the scanning tunneling microscopy (STM) image and the scanning tunneling spectroscopy (STS) spectrum in the first-principles approach. Our results of the STM image do not support the asymmetric dimer structure of the substrate with a H atom bonded to the upper Si atom. They conclude that the bright ball-like spot in the observed images comes from the free dangling bond induced on the remaining Si atom of the reacted dimer. However, the single particle picture cannot reproduce the observed features of the STS spectrum at the spot. We discuss that all the features can be well explained by the Coulomb blockade effect due to the electron correlation in the dangling bond state.     

Growth of single-domain monatomic In chain arrays on the vicinal Si(0 0 1) surface

Using the annealed vicinal Si(0 0 1) surface with 4° miscut toward the [1 1 0] direction as a substrate, single-domain monatomic In chain arrays have been fabricated. High-resolution STM images reveal that deposited In atoms preferentially form In dimers between two neighboring Si dimer rows along the step edges on the lower terrace. Formation of In dimers removes the surface dangling bonds and saturates the In valency. With increasing coverage, the In dimers develop into straight monatomic In chains along the step running direction. It is found that the ordered narrow terrace and rebonded double-layer (D_B) step edge are the keys for the formation of monatomic In chains.     

高能电子衍射研究H钝化偏角Si衬底上Si,GexSi1-x的分子束外延生长模式

用高能电子衍射(RHEED)研究H钝化偏角Si衬底上Si,Ge_xSi_1-x材料的分子束外延(MBE)生长模式,发现经低温处理的H钝化Si衬底上要经过10nm左右的Si生长才能获得比较平整的表面.Si,Ge_xSi_1-x外延时的稳定表面均以双原子台阶为主,双原子台阶与单原子台阶并存的结构.Si双原子台阶上的Si二聚体列(dimerrow)取向垂直于台阶边缘,而Ge_xSi_1-x双原 关键词：