Structural determination of the low-coverage phase of Al on Si(001) surface

The atomic structure of Al layer on Si(001)-(2 x 1) surface has been studied by coaxial impact collision ion scattering spectroscopy. When 0.5 monolayer (ML) of Al atoms are adsorbed on Si(001) at room temperature, it is found that Al adatoms are dimerized and Al ad-dimers are oriented parallel to the underlying Si dimers at the position of centering T3 site with a height of 1.02 Angstroms from the first layer of Si(001). The bond length of the Al dimer is 2.67 Angstroms. With increasing Al coverage up to one ML, Al ad-dimers still occupied near T3 site and the next favorable site is near HH site.     

不同覆盖度下Li原子在Si(001)表面上的吸附构型和电子结构

采用基于赝势平面波基组的密度泛函理论, 对不同Li原子覆盖度下Li/Si(001)体系的吸附构型、电子结构以及吸附Li原子对表面性质的影响进行了系统研究. 计算结果表明, 在所考察的覆盖度范围内, Li原子倾向于吸附在相邻两个Si-Si二聚体之间各种对称性较高的空穴位, 其中覆盖度为0.75 ML(monolayer)时具有最小的平均吸附能. 由能带结构分析结果可知, 随着覆盖度的增大, Si(001)表面存在由半导体→导体→半导体的变化过程. 在覆盖度为1.00 ML时, 由于表层二聚体均受到显著破坏, 使得体系带隙明显增大. 吸附后, 有较多电子从Li原子转移到底物, 导致Si(001)表面功函显著下降, 并随着覆盖度的增加表面功函呈现振荡变化. 此外, 从热力学稳定性角度上看, 覆盖度为0.75 ML的Li/Si(001)表面较难形成.     

Study of in-situ surface cleaning for Si and SiGe epitaxy on Si with a novel ion beam vapor/assisted deposition technique

Ion beam vapor deposition is a new technique to grow Si and SiGe layers on Si substrates at low temperatures. The in situ surface cleaning prior to the deposition is a crucial step in the epitaxial growth of Si and SiGe films and is achieved by Ar ion bombardment with substrates kept at ambient temperature. A high temperature annealing (800 °C) is needed to repair the damage caused by this bombardment. We studied the effects of ion beam energy and the substrate temperature during the in situ cleaning on the quality of the grown films. An ion beam energy of 150–200eV is found to be sufficient to clean the surface for epitaxial growth. While the films deposited on properly cleaned surfaces are epitaxially grown, the inadequately cleaned surface leads to the formation of polycrystalline layers especially at low substrate temperatures.     

Formation of GeO2 under Graphene on Ge(001)/Si(001) Substrates Using Water Vapor

The problem of graphene protection of Ge surfaces against oxidation is investigated. Raman, X-Ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements of graphene epitaxially grown on Ge(001)/Si(001) substrates are presented. It is shown that the penetration of water vapor through graphene defects on Gr/Ge(001)/Si(001) samples leads to the oxidation of germanium, forming GeO₂. The presence of trigonal GeO₂ under graphene was identified by Raman and XRD measurements. The oxidation of Ge leads to the formation of blisters under the graphene layer. It is suggested that oxidation of Ge is connected with the dissociation of water molecules and penetration of OH molecules or O to the Ge surface. It has also been found that the formation of blisters of GeO₂ leads to a dramatic increase in the intensity of the graphene Raman spectrum. The increase in the Raman signal intensity is most likely due to the screening of graphene by GeO₂ from the Ge(001) surface.     

Deformation-induced structural transformations in Si and the initial stage of mechanical alloying of Si and Fe

The methods of X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, and laser diffractometry are employed to study the changes in the structure and phase transformations that accompany mechanical alloying of Si and ⁵⁷Fe used in an atomic ratio of 99: 1. It is established that the process comprises the development of a nanocrystalline state of Si with crystallite sizes smaller than 10 nm; the formation of an amorphous phase of Si at particle surfaces and in near-boundary distorted zones of interfaces in Si nanostructure; the penetration of ⁵⁷Fe atoms along grain boundaries; and the formation of Si-Fe clusters, the local environment of Fe atoms in which is typical of a deformed α-FeSi₂ phase, with these clusters being located in the interfaces.     

Self-aligned Submonolayer Cobalt Silicide Formation Studied by Variable Temperature Scanning Tunneling Microscopy in UHV System

Epitaxial growth of defect free metal silicides with high thermal stability is important to ultra large scale integration devices (ULSI)^[1]. Cobalt silicide has been used as an interconnect in CMOS devices due to its low resistivity and good thermal stability^[2] and excellent lattice match between cobalt silicide and silicon^[3,4]. In the present work, we have investigated epitaxial growth of ultra-thin cobalt layer (ca. 10 Å) by electron beam evaporation of cobalt on Si(111) (7×7) surface followed by thermal annealing up to 700℃ in UHV system. The morphologies and the surface structures of epitaxial silicide formation were investigated by using scanning tunneling microscopy (STM). Adlayer structures of Co silicide after annealing were observed to coexist as the closely and loosely packed clusters at 230℃ separated by a boundary. A new structure with ring-like clusters has (l×l) configuration with 3.5 A spacing between hollows of vicinal clusters at 400℃. Si-rich CoSi₂ surfaces terminated by Si bilayers showed (2×2) structures after annealing at 480℃, in which Si-rich CoSi₂ clusters were observed to be very mobile at room temperature. As the surface was annealed to above 500℃, The domain island became regular triangles, where atomic resolution of the l×l surface of CoSi₂(111) were readily discernable. CoSi₂(111) surface is suggested to be terminated by a Si-bilayer.     

Generation of clean iron nanocrystals on an ultra-thin SiO(x) film on Si(001)

Upon exposure to Fe(CO)(5), the formation of pure cubic Fe nanocrystals with dimensions up to ~75 nm is reported on ultra-thin SiO(x) films (thickness ≈ 0.5 nm) on Si(001), which have been prepared in situ under UHV conditions. The active centers for initial decomposition of Fe(CO)(5) resulting in the growth of the Fe clusters are proposed to be SiO sites. After nucleation at these sites, further crystal growth is observed due to autocatalytic dissociation of Fe(CO)(5) at room temperature. The density of the Fe clusters can be increased by irradiating the surface with a focused electron beam (15 keV) prior to gas exposure. The formation of the active SiO sites upon electron irradiation is attributed to oxygen desorption via the Knotek-Feibelman mechanism.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.     

Structural analysis of the reconstructed Si(001)-C surface

The atomic structure of reconstructed Si(001)c(4 x 4)-C surface has been studied by coaxial impact collision ion scattering spectroscopy. When the 100L of ethylene (C(2)H(4)) molecules have been exposed on Si(001)-(2 x 1) surface at 700 degrees C, it is found that C atoms cause the ordering of missing Si dimer defects and occupy the fourth layer of Si(001) directly below the bridge site. Our results provide the support for the previous model in which a missing dimer structure is accompanied by C incorporation into the subsurface.     

Structures and relative stability of medium- and large-sized silicon clusters. VI. Fullerene cage motifs for low-lying clusters Si(39), Si(40), Si(50), Si(60), Si(70), and Si(80)

We performed a constrained search, combined with density-functional theory optimization, of low-energy geometric structures of silicon clusters Si(39), Si(40), Si(50), Si(60), Si(70), and Si(80). We used fullerene cages as structural motifs to construct initial configurations of endohedral fullerene structures. For Si(39), we examined six endohedral fullerene structures using all six homolog C(34) fullerene isomers as cage motifs. We found that the Si(39) constructed based on the C(34)(C(s):2) cage motif results in a new leading candidate for the lowest-energy structure whose energy is appreciably lower than that of the previously reported leading candidate obtained based on unbiased searches (combined with tight-binding optimization). The C(34)(C(s):2) cage motif also leads to a new candidate for the lowest-energy structure of Si(40) whose energy is notably lower than that of the previously reported leading candidate with outer cage homolog to the C(34)(C(1):1). Low-lying structures of larger silicon clusters Si(50) and Si(60) are also obtained on the basis of preconstructed endohedral fullerene structures. For Si(50), Si(60), and Si(80), the obtained low-energy structures are all notably lower in energy than the lowest-energy silicon structures obtained based on an unbiased search with the empirical Stillinger-Weber potential of silicon. Additionally, we found that the binding energy per atom (or cohesive energy) increases typically >10 meV with addition of every ten Si atoms. This result may be used as an empirical criterion (or the minimal requirement) to identify low-lying silicon clusters with size larger than Si(50).     

Morphological evolution of Ge islands on the Si(100) surface: from huts to pits

The morphological evolution on the size and shape of Ge island on the Si(100) surface by depositing and subsequent annealing processes is studied in situ by using scanning tunneling microscopy at ultrahigh vacuum environment. A slower growth rate is maintained when the islands grow to larger sizes beyond the wetting layers. While at room temperature, the epitaxial strain is relieved by the formation of three‐dimensional islands (so‐called ‘hut’ clusters). When the sample is annealed at 200 °C, the strain is relieved by forming pits, having the circular cone shape but with their apex pointing down, with Ge clusters formed at the rim of pits. Copyright © 2016 John Wiley & Sons, Ltd.     

Structural study of lanthanum nickelate thin films deposited on different single crystal substrates

Fabrications of La₂NiO_4+δ thin film layers by liquid-injection metalorganic chemical vapor deposition were tried on different single crystals substrates: (001)Si, (001)MgO, (001)LaAlO₃ and (001)SrTiO₃. As results of structural characterizations, polycrystalline dendritic layers of La₂NiO_4+δ tetragonal (or orthorhombic) phase were observed on (001)Si substrates while layers of a perovskite-like cubic structure were observed on the other single crystal substrates. From a high-resolution TEM study of a layer deposited on (001)MgO, such a perovskite-like cubic structure exhibits many planar structural faults likely similar to planes of oxygen vacancies of the La₂NiO_4+δ orthorhombic structure. A thin intermediate epitaxial layer of NiO phase was also identified. Using a X-ray texture diffractometer, the layer structure on (001)MgO, (001)LaAlO₃ and (001)SrTiO₃ was confirmed to be of cubic structure with 〈100〉 axes parallel to those of the substrate. The T dependence of the resistivity of a layer deposited on (001)MgO substrate was found to be of a semi-conducting behavior.     

Striking Size and Doping Effects of Ti−Si−O Clusters on Methane Conversion Reactions

The size and doping effects in methane activation by Ti−Si−O clusters have been explored by using a combination of gas-phase experiments and quantum chemical calculations. All [Ti_mSi_nO_2(m+n)]^.+ (m+n=2, 3, 8, 10, 12, 14) clusters can extract a hydrogen from methane. The associated energies and structures have been revealed in detail. Moreover, the doping and size effects have been discussed involving generalized Kohn-Sham energy decomposition analysis, natural population analysis, Wiberg bond indexes (WBI), molecular polarity index (MPI) and ionization potential (IP). It suggested that Ti−Si−O clusters with a low Ti : Si ratio is beneficial to adsorbing methane and inclination to the hydrogen atom transfer (HAT) process, while the clusters with a high Ti : Si ratio favors the generation of a terminal oxygen radical and results in high reactivity and turnover frequency. On the other hand, a cluster size of m+n=12 is recommended considering both the ionization potential and the turnover frequency of the reaction. Hopefully, these finding will be instructive for the design of high-performance Ti−Si−O catalyst toward methane conversion.     

Cubic aluminum silicides RE8Ru12Al49Si9(Al(x)Si12-x) (RE = Pr, Sm) from liquid aluminum. Empty (Si,Al)12 cuboctahedral clusters and assignment of the Al/Si distribution with neutron diffraction.

Two new quaternary aluminum silicides, RE8Ru12Al49Si9(Al(x)Si12-x) (x approximately 4; RE = Pr, Sm), have been synthesized from Sm (or Sm2O3), Pr, Ru, and Si in molten aluminum between 800 and 1000 degrees C in sealed fused silica tubes. Both compounds form black shiny crystals that are stable in air and NaOH. The Nd analog is also stable. The compounds crystallize in a new structural type. The structure, determined by single-crystal X-ray diffraction, is cubic, space group Pm3m with Z = 1, and has lattice parameters of a = 11.510(1) A for Sm8Ru12Al49Si9(Al(x)Si12-x) and a = 11.553(2) A for Pr8Ru12Al49Si9(Al(x)Si12-x) (x approximately 4). The structure consists of octahedral units of AlSi6, at the cell center, Si2Ru4Al8 clusters, at each face center, SiAl8 cubes, at the middle of the cell edges, and unique (Al,Si)12 cuboctohedral clusters, at the cell corners. These different structural units are connected to each other either by shared atoms, Al-Al bonds, or Al-Ru bonds. The rare earth metal atoms fill the space between various structural units. The Al/Si distribution was verified by single-crystal neutron diffraction studies conducted on Pr8Ru12Al49Si9(Al(x)Si12-x). Sm8Ru12Al49Si9(Al(x)Si12-x) and Pr8Ru12Al49Si9(Al(x)Si12-x) show ferromagnetic ordering at Tc approximately 10 and approximately 20 K, respectively. A charge of 3+ can be assigned to the rare earth atoms while the Ru atoms are diamagnetic.     

Interaction of oxygen with an Si(001) surface coated with bismuth

Ionization and Auger spectroscopy show that a submonolayer coating of bismuth on Si(001) reduces the initial attachment coefficient for molecular oxygen by comparison with a clean Si(001) surface by two orders of magnitude, while exposure to >10⁵ Langmuir of O₂ causes bismuth to stimulate the formation of surface silicon oxide having stoichiometry close to SiO₂. Taras Shevchenko Kiev University, 64 ul. Vladimirskaya, Kiev 17, Ukraine 252601. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 2, pp. 124–127, March–April, 1997.     

Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicides Li(15)Si(4), Li(13)Si(4), and Li(7)Si(3) have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the (6)Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li(7)Si(3) and Li(13)Si(4) at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Busmann concept for the electronic structure of these materials. The (29)Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li(13)Si(4) their identification aids in the assignment of individual lithium sites via(29)Si{(7)Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static (7)Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent (6)Li MAS-NMR as well as (6)Li{(7)Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li(15)Si(4) the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li-Si anodes for high-performance batteries. A comparison of all the (6)Li and (7)Li NMR spectroscopic data obtained for the three different lithium silicides and of Li(12)Si(7) previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.     

Phosphine dissociation and diffusion on Si(001) observed at the atomic scale

A detailed atomic-resolution scanning tunneling microscopy (STM) and density functional theory study of the adsorption, dissociation, and surface diffusion of phosphine (PH(3)) on Si(001) is presented. Adsorbate coverages from approximately 0.01 monolayer to saturation are investigated, and adsorption is performed at room temperature and 120 K. It is shown that PH(3) dissociates upon adsorption to Si(001) at room temperature to produce both PH(2) + H and PH + 2H. These appear in atomic-resolution STM images as features asymmetric-about and centered-upon the dimer rows, respectively. The ratio of PH(2) to PH is a function of both dose rate and temperature, and the dissociation of PH(2) to PH occurs on a time scale of minutes at room temperature. Time-resolved in situ STM observations of these adsorbates show the surface diffusion of PH(2) adsorbates (mediated by its lone pair electrons) and the dissociation of PH(2) to PH. The surface diffusion of PH(2) results in the formation of hemihydride dimers on low-dosed Si(001) surfaces and the ordering of PH molecules along dimer rows at saturation coverages. The observations presented here have important implications for the fabrication of atomic-scale P dopant structures in Si, and the methodology is applicable to other emerging areas of nanotechnology, such as molecular electronics, where unambiguous molecular identification using STM is necessary.     

Atomic arrangement of Al-induced clusters on Si(0 0 1) surface at high temperature

The atomic structure of the Al-induced clusters on Si(0 0 1) surface formed by the annealing of 0.5 ML Al/Si(0 0 1) at 500 °C has been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). CAICISS results proposed that the Al atoms occupy the cave site (T4 site) and off-centered T4 site. To determine the structure of the Al-induced clusters definitely, classical ion-scattering trajectory simulations using scattering and recoiling imaging code (SARIC) have been performed for the recently proposed most possible four different cluster models (Bunk, Zotov, Kotlyar, and Zavodinsky model). Our CAICISS spectra and simulation results show that the Bunk model is the best plausible one among the models. As the results of the simulations, it is found that Al–Si dimers has been oriented on the topmost layer of the Si(0 0 1) surface with a bonding length (Δz) of 1.00 ± 0.05 Å.     

Si衬底上生长的铁硅化合物结构和取向分析

采用分子束外延法分别在650-920 ℃的Si（110）和920 ℃的Si（111）衬底表面生长出铁的硅化物纳米结构,并主要分析了920 ℃高温下纳米结构的形貌、组成相及其与Si 衬底的取向关系. 扫描隧道显微镜（STM）研究表明,920 ℃高温下,Si（110）衬底上生长的铁硅化合物完全以纳米线的形式存在,且其尺寸远大于650 ℃低温下外延生长的纳米线尺寸;Si（111）衬底上生长出三维岛和薄膜两种形貌的铁硅化合物,其中三维岛具有金属特性且直径约300 nm、高约155 nm,薄膜厚度约2 nm. 电子背散射衍射研究表明920 ℃高温下Si（110）衬底上生长的纳米线仅以β-FeSi₂的形式存在,且β-FeSi₂相与衬底之间存在唯一的取向关系：β-FeSi₂（101）//Si（111）;β-FeSi₂ [010]//Si[110];Si（111）衬底上生长的三维岛由六方晶系的Fe₂Si 相组成,Fe₂Si 属于164 空间群,晶胞常数为a=0.405 nm,c=0.509 nm;与衬底之间的取向关系为Fe₂Si（001）∥Si（111）和Fe₂Si[120]//Si[112].