Chemisorption of Fe on Au-passivated Si（001） surface

The chemisorption of one monolayer of Fe atoms on a Au-passivated Si(001) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The Fe adatom chemisorption on an ideal Si(001) surface is also considered for comparison. The chemisorption energy and layer projected density of states for a monolayer of Fe atoms on Au-passivated Si(001) surface are calculated and compared with that of the Fe atoms on an ideal Si(001) surface. The charge transfer is investigated. It is found that the most stable position is at the fourfold hollow site for the adsorbed Fe atoms, which might sit below the Au surface. Therefore there will be a Au－Fe mixed layer at the Fe/Au－Si(100) interface. It is found that the adsorbed Fe atoms cannot sit below the Si surface, indicating that a buffer layer of Au atoms may hinder the intermixing of Fe atoms and Si atoms at the Fe/Au－Si(001) interface effectively, which is in agreement with the experimental results.     

Si/Gen/Si(001)异质结薄膜的掠入射荧光X射线吸收精细结构研究

利用掠入射荧光X射线吸收精细结构(XAFS)方法研究了在400℃的温度下分子束外延生长的Si/Ge_n/Si(001)异质结薄膜(n=1，2，4和8个原子层)中Ge原子的局域环境结构.结果表明，在1至2个Ge原子层(ML)生长厚度的异质结薄膜中，Ge原子的第一近邻配位主要是Si原子.随着Ge原子层厚度增加到4ML，Ge原子的最近邻配位壳层中的Ge-Ge配位的平均配位数增加到1.3.当Ge原子层厚度增加到8ML时，第一配位壳层中的Ge-Ge配位占的比例只有55%.这表明在400℃的生长条件下，Ge原子有很强的迁移到Si覆盖层的能力.随着Ge层厚度从1 增加到2，4和8ML，Ge原子迁移到Si覆盖层的量由0.5ML分别增加到1.5，2.0和3.0ML.认为在覆盖Si过程中Ge原子的迁移主要是通过产生Ge原子表面偏析来降低表面能和Ge层的应变能. 关键词： XAFS n/Si(001)异质膜')" href="#">Si/Ge_n/Si(001)异质膜 迁移效应     

Surface kinetics driven annealing and phase segregation in non-equilibrium crystal growth

We simulate the epitaxial growth and annealing of Ge_xSi_1−x alloys on a Si(001) substrate using a stochastic Monte Carlo technique. Initiation of phase segregation into Si(001) and Ge(001) layers is obtained in our simulation. This results from the kinetically more active nature of Ge atoms as compared to Si atoms, together with an effect arising from the compressive strain in the alloy layers. We discuss our results in the light of the existing experimental data.     

Magnetic and electronic properties of Cr,Mn, and Fe adatoms on Si(001): A first-principles study

The magnetic and electronic properties of TM (TM=Cr, Mn, and Fe) adatoms adsorption on Si(001) surface are studied by means of the first-principles method. For the adsorption of a single TM atom on Si(001), we obtain decreasing spin moments and increasing adsorption energies as TM varies from Cr to Fe. In the case of TM dimers adsorption, the calculated results show that the spin coupling changes from antiferromagnetic (AFM) to ferromagnetic (FM) as the 3d electrons increased. AFM coupling is found to be preferred for Cr, while FM coupling is energetically favorable for Mn and Fe. In the case of TM wires, we find that the FM state is energetically preferred for Mn and Fe atoms on the Si(001) surface, while for Cr wires, the up–down–up state for P–M–M site Cr atoms seems to be more energy favorable. We also find that the silicon surfaces become metallic for the adsorption of TM wires.     

Thermal-hydrogen promoted selective desorption and enhanced mobility of adsorbed radicals in silicon film growth

Car-Parrinello simulations and static density-functional theory calculations reveal how hydrogen promotes growth of epitaxial, ordered Si films in plasma-enhanced chemical vapor deposition at low-temperature conditions where the exposed Si(001)-(2x1) surface is fully hydrogenated. Thermal H atoms, indeed, are shown to selectively etch adsorbed silyl back to the gas phase or to form adsorbed species which can be easily incorporated into the crystal down to T approximately 200 degrees C and start diffusing around T approximately 300 degrees C. Our results are well consistent with earlier experiments.     

Chemisorption of Au on Si（001） surface

The chemisorption of one monolayer of Au atoms on an ideal Si(001) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of the adsorption system of a Au atom on different sites are calculated. It is found that the most stable position is A site (top site) for the adsorbed Au atoms above the Si(001) surface. It is possible for the adsorbed Au atoms to sit below the Si(001) surface at the B_1 site(bridge site), resulting in a Au－Si mixed layer. This is in agreement with the experiment results. The layer projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated.     

Adsorption of Au and Pt dimers on Ge(001) and Si(001): A first-principles study

Based on first-principles total energy calculations, the adsorption of Au and Pt dimers on Ge(001) and Si(001) surfaces are investigated. We find that the Au dimer on both Ge(001) and Si(001) show a similar result with the most stable configuration C, parallel to the substrate dimer row and located in the trough between the dimer rows, and the most unstable configuration A, parallel to and on the top of the substrate dimer row. On the other hand, Pt dimer on Ge(001) prefer the configuration D, perpendicular to the substrate dimer row and located in the trough between the dimer rows, while Pt dimer on Si(001) prefer both A and D configurations. The different structural stabilities of Au and Pt dimers on Ge(001) and Si(001) surfaces are attributed to the different electronic structures of Au and Pt atoms. These results are discussed with the reported data for III, IV and V group elements on Si(001).     

Diffusion kinetics in the Pd/Cu(001) surface alloy

We use atom-tracking scanning tunneling microscopy to study the diffusion of Pd in the Pd/Cu(001) surface alloy as a function of temperature. By following the motion of individual Pd atoms incorporated in the surface, we show that Pd diffuses by a vacancy-exchange mechanism. We measure an activation energy for the diffusion of incorporated Pd atoms of 0.88 eV, which is in good agreement with our ab initio calculated energy of 0.94 eV.     

Stability and band offsets between Si and LaAlO<Subscript>3</Subscript>

The replacement of traditional SiO₂ with high-k oxides allows the physical thickness of the gate dielectric to be thinner without the tunneling problem in Si-based metal-oxide-semiconductor field-effect transistors. LaAlO₃ appears to be a promising high-k material for use in future ultra large scale integrated devices. In the present paper, the electronic properties of Si/LaAlO₃ (001) heterojunctions are investigated by first-principles calculations. We studied the initial adsorption of Si atoms on the LaAlO₃ (001) surface, and found that Si atoms preferentially adsorb on top of oxygen atoms at higher coverage. The surface phase diagrams indicate that Si atoms may substitute oxygen atoms at the LaO-terminated surface. The band offsets, electronic density of states, and atomic charges are analyzed for the various Si/LaAlO₃ heterojunctions. Our results suggest that the Si/AlO₂ interface is suitable for the design of metal oxide semiconductor devices because the valence and conduction band offsets are both larger than 1 eV.     

Nonclassical smoothening of nanoscale surface corrugations

We report the first experimental observation of nonclassical morphological equilibration of a corrugated crystalline surface. Periodic rippled structures with wavelengths of 290-550 nm were made on Si(001) by sputter rippling and then annealed at 650-750 degrees C. In contrast to the classical exponential decay with time, the ripple amplitude Alambda(t) followed an inverse linear decay, Alambda(t)=Alambda(0)/(1+klambdat), agreeing with a prediction of Ozdemir and Zangwill. We measure the activation energy for surface relaxation to be 1.6+/-0.2 eV, consistent with the fundamental energies of creation and migration on Si(001).     

Real-time view of mesoscopic surface diffusion

Photoemission electron microscopy is used to study the thermal decay of Ag islands grown epitaxially on Si(001) surfaces. (2 x 3) Ag reconstructed zones, due to migrating Ag atoms supplied to the surface by the decaying islands, surround each of the islands. The shape of these reconstructed zones depends on the degree of diffusion isotropy in the system. We demonstrate that the imaging of these reconstructed "isocoverage zones" constitutes a unique experimental method for directly observing diffusion fields in epitaxial systems. We describe the dynamics of the thermal decay of the islands and the isozones in the context of a continuum diffusion model.     

MoSi_2(001)表面的氧吸附行为

运用第一原理密度泛函理论方法,首先计算了MoSi_2各清洁表面的表面能,(001)Si-|-Si断面具有较低的表面能,是MoSi_2最可能的解理面;通过生成能及键布居分析研究了单氧原子、双氧原子及氧分子在(001)Si-|-Si断面的吸附行为,发现单氧原子在空位处吸附最稳定,此时O极易与Si结合,得到的Si-O-Si键长及键角与SiO_2的非常接近,表明低浓度下O极易与表面的Si结合生成SiO_2;双氧原子发生空位+顶位吸附时O原子除与Si有强作用外,可与Mo有一定相互作用;氧分子以平行的方式接近空位最有利于吸附,此时氧分子最易分解为氧原子,发生氧原子在空位的吸附.     

Molecular dynamics simulations of reactive etching of SiC by energetic fluorine

Molecular dynamics simulations were performed to investigate F continuously bombarding Si-terminated 3C-SiC(001) surfaces with incident energies of 10, 100 and 200 eV at normal incidence and room temperature. For an energy of 10 eV, deposition only occurs on the surface. For energies larger than 10 eV, accompanying the saturation of F uptake, a balance between F deposition from the incident atoms and F removal from the fluorinated substrate is established, while the steady-state etching is reached. The simulated results demonstrate that Si atoms in SiC are preferentially etched, which is in good agreement with experiments. The preferential etching of Si results in formation of a C-rich interfacial layer whose thickness increases with increasing incident energy. The analysis shows that Si-containing etch products are dominant. PACS 52.65.Yy; 81.65.Cf; 52.77.Dq     

Structural and electronic properties of the interface between the high-k Oxide LaAlO3 and Si(001)

The structural and electronic properties of the LaAlO(3)/Si(001) interface are determined using state-of-the-art electronic structure calculations. The atomic structure differs from previous proposals, but is reminiscent of La adsorption structures on silicon. A phase diagram of the interface stability is calculated as a function of oxygen and Al chemical potentials. We find that an electronically saturated interface is obtained only if Al atoms substitute some of the interfacial Si atoms. These findings raise serious doubts whether LaAlO3 can be used as an epitaxial gate dielectric.     

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

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 Cl 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 (1×1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. Cl 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 valley-bridge 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.     

Compositional depth profiling of ultrathin oxynitride/Si interface using XPS

The atomic-scale compositional depth profiling of oxynitride/Si interface by applying the maximum entropy concept to the angle-resolved Si 2p photoelectorn spectra was performed after eliminating the effect of photoelectron diffraction on the Si 2p photoelectron spectra arising from Si substrate for oxynitride films containing the maximum nitrogen concentration of 3 and 6 at.% at the SiO₂/Si(1 0 0) interface. Although the distribution of intermediate oxidation states of Si depends on the amount of nitrogen atoms incorporated at the interface, total amount of intermediate oxidation states of Si does not depend on the nitrogen atoms incorporated at the interface.     

Atomic configurations during Si incorporation on GaAs(001) in As atmosphere evidenced by reflectance difference spectroscopy

The structural ordering of surface atoms during Si deposition on singular and vicinal GaAs(001) surfaces has been studied by reflectance difference (RD) spectroscopy using the difference function between the Si-covered and the bare surface. In dependence on the Si coverage the difference spectra correspond to RD spectra of the bare Si(001)-(1×2) or of the As-terminated Si(001):As(2×1) surface. This finding and the behaviour of RD transients recorded at 3.8 eV photon energy allows to define a (3×2)α phase with Si dimers in the top layer and Ga dimers in the third layer, and a (3×2)β phase with As-dimer rows on top of Si in the second layer.     

Photoemission study of CaF2 on Si(001)-2×1 during annealing

We have employed synchrotron radiation photoemission to investigate calcium fluoride deposited on Si(001)-2×1 annealed at 700 °C. The valence band spectra reveal that the initial molecules are dissociated to single Ca atoms resting on the surface. Si 2p core-level spectra manifest a negatively shifted Ca-induced line at 0.35 eV. The dissociated Ca atoms occupy only half of the surface, on top of which the CaF₂ molecules commence growth. This is in contrast to the Si(111)-7×7 counterpart, which exhibits a CaF layer in the interface. Strong elastic scattering due to the absorbed Ca atoms is evident in the photoemission spectra, thus averaging out the angle-dependent features.     

Sb/Si(001)表面的半经验研究

用基于Chadi模型和格林函数方法的一种计算表面应力的半经验方法研究了Sb吸附在Si(001) 衬底上的性质.结果显示,Sb原子在Si(001)表面形成对称的dimer,其键长为0.293nm,表 面以下层的弛豫很小.Sb/Si(001)2×1表面沿着dimer方向的张应力为1.0eV/(1×1cell),而 沿垂直于dimer方向的压应力为-1.1eV/(1×1cell).Sb/Si(001)表面应力的主要贡献来自于 最上面三层表面. 关键词： 表面应力 异质生长 格林函数方法     

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.