Hydrogen-induced surface metallization of beta-SiC(100)-(3x2) revisited by density functional theory calculations

Recent experiments on the silicon terminated (3 x 2)-SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of density-functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: in addition to saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, i.e., stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surface.     

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ($\text{2}\text{×}\text{2}$)R45°-H, with steps oriented parallel to the [110] and ～ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ($\text{2}\text{×}\text{2}$)R45° clean surface reconstruction to c2mm for the commensurate phase ($\text{2}\text{×}\text{2}$)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v₁ vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ～ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.     

Atomic structures of reduced SrTiO3(001) surfaces

The scanning tunneling microscope has been employed to study the reduced SrTiO₃(001) surface in ultrahigh vacuum. Topographic images show a change of structural ordering for the surface annealed under three different conditions. In addition, our STM images reveal row-like structures with spacings of 12 and 20 Å. The observations are discussed in terms of the formation of intergrowths of lamellae of Sr_{n + 1} Ti_nO_{3n + 1}; a result of accommodation of oxygen and titanium deficiencies.     

Bimodal growth of Au on SrTiO3(001)

We have investigated the vapor phase growth of Au on SrTiO3(001)-(2 x 1) substrates by UHV scanning tunneling microscopy. Submonolayer (ML) coverages below 300 degrees C wet the surface as disordered metastable 2D islands. Beyond 0.75 ML fcc nanocrystals with a (111) interface are nucleated and ripen by dewetting the surrounding layer. Some multiply twinned fivefold symmetric clusters are also created. Above 400 degrees C dewetting occurs for all coverages and the surface is only populated by nanocrystals and fivefold clusters. A planar ground state configuration for small Au clusters and a higher interface energy for crystals than for wetted 2D ML films explains these results.     

The concept of weak polarity: an application to the SrTiO3(001) surface

We propose the term “weak polarity” to characterize the electrostatic properties of some ternary (or more complex) insulating compound slabs, which are not fully ionic and in which covalency (i.e. anion-cation electron sharing) is responsible for the presence of a non-vanishing macroscopic dipole moment perpendicular to the surface. We exemplify this situation by the SrTiO₃(001) surface, which we study by means of a semi-empirical Hartree-Fock approach. The bond-breaking processes at the surface are shown to yield the charge redistribution required for the cancellation of the macroscopic electrostatic field. There is no need for strong modifications of the electronic structure such as the surface metallization predicted on the polar stoichiometric MgO(111) surface.     

SrTiO3(001)表面上Au和N原子相互作用的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势方法研究了SrTiO₃(001)表面上Au和N原子间相互作用的微观机理.通过比较分析N置换表面层O原子前后SrTiO₃(001)表面吸附Au原子体系的相关能量和电子结构,发现SrTiO₃(001)表面吸附Au原子和N替代表面层O原子的置换过程二者之间存在明显的"协同效应",即N原子置换SrTiO₃(001)表面层O原子的过程增强了相应表面吸附Au原子的稳定性,而SrTiO关键词： 表面结构 相互作用 第一性原理     

Evidence of substrate metallization by Li adsorption on the Si(001) surface

We report an evidence of substrate metallization induced by Li adsorption on the Si(001) surface, based on the combined results of electron energy-loss (EEL) and angle-resolved photoemission (ARP) measurements. The metallic surface at a low dose of Li manifests itself as a loss peak due to an intraband surface plasmon in EEL spectra and a metallic peak in ARP spectra. These peaks are coherently understood in terms of substrate metallization, where electrons from Li adatoms partially occupy the empty substrate surface bands. Furthermore, the unique negative dispersion of the plasmon reveals that local field effects may cause such an anomalous dispersion.     

H2O在SrTiO3-(001)TiO2表面上吸附和解离的密度泛函理论研究

利用密度泛函理论研究了0.25单层(ML),0.5ML,0.75ML和1ML吸附率下H2O在SrTiO3-(001)TiO2表面上的吸附行为.比较了不同吸附率下分子吸附和解离吸附的稳定性,利用微动弹性带(nudged elastic band)方法计算了H2O的解离势垒.结果表明:在低吸附率(0.25ML和0.5ML)时,H2O表现为解离吸附;在0.75ML吸附率下,分子吸附和解离吸附同时存在;而在全吸附(吸附率为1ML)时,分子吸附更稳定.基于对H2O分子与表面之间以及H2O分子之间的电荷转移和相互作用的分析,讨论了吸附率对H2O吸附和解离的影响.     

Anomalous anisotropic magnetoresistance in single-crystalline Co/SrTiO3(001) heterostructures

The anisotropic magnetoresistances (AMRs) in single crystalline Co(6 nm)/SrTiO₃(001) heterostructures from 5 K to 300 K with the current direction setting along either Co[100] or Co[110] are investigated in this work. The anomalous (normal) AMR is observed below (above) 100 K. With the current along Co[100] direction, the AMR shows negative longitudinal and positive transverse magnetoresistances at T< 100 K, while the AMR is inverse with the current along Co[110]. Meanwhile, the amplitude ratio between Co[110] and Co[100] is observed to be as large as 29 at 100 K. A crystal symmetry-adapted model of AMR demonstrates that interplay between the non-crystalline component and crossed AMR component results in the anomalous AMR. Our results may reveal more intriguing magneto-transport behaviors of film on SrTiO₃ or other perovskite oxides.     

In-situ imaging of the nucleation and growth of epitaxial anatase TiO2(001) films on SrTiO3(001)

The growth of TiO₂ anatase films on Nb‐doped SrTiO₃(001) molecular beam epitaxy has been studied in-situ by scanning tunneling microscopy. We show that the initial growth follows the Stranski–Krastanov mode, where islands form on top of a wetting layer consisting of two monolayers (ML) of TiO₂. The epitaxial islands subsequently nucleate and coalesce into large commonly oriented crystallites. The reconstruction observed by reflection high-energy electron diffraction (RHEED) is shown to result from the coexistence of individual (4 × 1) and (1 × 4) reconstructions present on different crystallite surfaces. The anatase grows in units of bilayers, resulting in a step height of 2 ML (~ 0.5 nm). This result explains the fact that the measured period of the RHEED specular-beam intensity oscillations corresponds to the time required for deposition of 2 ML. Ar ion sputtering and UHV annealing results in a transformation to coexisting (4 × 1) and (1 × 4) reconstructed terraces on individual crystallites, as commonly observed by ex-situ STM studies.     

Growth of SrTiO3 on Si(001) by hybrid molecular beam epitaxy

We report the heteroepitaxial growth of SrTiO₃ thin films on Si(001) by hybrid molecular beam epitaxy (hMBE). Here, elemental strontium and the metal‐organic precursor titanium tetraisopropoxide (TTIP) were co‐supplied in the absence of additional oxygen. The carbonization of pristine Si surfaces during native oxide removal was avoided by freshly evaporating Sr into the hMBE reactor prior to loading samples. Nucleation, growth and crystallization behavior as well a structural properties and film surfaces were characterized for a series of 46‐nm‐thick SrTiO₃ films grown with varying Sr to TTIP fluxes to study the effect of non‐stoichiometric growth conditions on film lattice parameter and surface morphology. High quality SrTiO₃ thin films with epitaxial relationship (001)SrTiO₃ || (001)Si and [110]SrTiO₃ || [100]Si were demonstrated with an amorphous layer of around 4 nm thickness formed at the SrTiO₃/Si interface. The successful growth of high quality SrTiO₃ thin films with atomically smooth surfaces using a thin film technique with scalable growth rates provides a promising route towards heterogeneous integration of functional oxides on Si. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Understanding the mechanism of conductivity at the LaAlO3/SrTiO3(001) interface

The observation of conductivity at (001)-oriented interfaces of the 2 band insulators LaAlO₃ and SrTiO₃ is both fascinating and potentially useful for next-generation electronics. The paradigm commonly used to explain this phenomenon is an electronic reconstruction resulting from the instability created by forming an interface of polar and nonpolar perovskites, leading to the formation of a two-dimensional electron gas. This explanation has typically been conceptualized within the framework of an atomically abrupt interface. However, a significant and growing body of data now exists which reveals that the interface is not abrupt, and that all four cations diffuse across the interface. Yet, the potential roles of the resulting defects and dopants in alleviating the polar catastrophe and promoting conductivity are rarely considered. The purpose of this prospective is to take an overview of the field from outside the reigning paradigm and consider ways in which dopants and defects might affect the electronic structure.     

Hydrogen-induced metallization of the 3C-SiC(0 0 1)-(3 × 2) surface

A surprising metallization of the SiC(0 0 1)-(3 × 2) surface induced by hydrogen adsorption was discovered in recent experiments. The effect was ascribed to dangling bonds created on the third layer of the surface system by H adsorption and stabilized by steric hindrance. We have investigated the surface metallization by density functional calculations. Our total-energy minimizations show that dangling bonds on the third layer are very unstable. Instead, H adatoms form angular Si-H-Si bonds on the third layer after the asymmetric dimers on the top layer have been saturated by H forming monohydrides. The novel Si-H-Si bonds on the third layer give rise to a metallic surface, indeed. But the mechanism for metallization is very different from the one suggested originally. Likewise, H atoms can also occupy bridge positions in angular Si-H-Si bonds on the second layer and induce metallization, as well. In addition to monohydrides on the top-layer dimers, we have also investigated dihydride surfaces with additional H on the second and/or third layer. The dihydride surface structure with H adsorbed on both the second and third layers is energetically most favorable and is also metallic. In all three cases the new Si-H-Si bonds are the origin of the surface metallization while its nature is somewhat more intricate, as will be discussed.     

Water adsorption on SrTiO3(001): I. Experimental and simulated STM

Density functional theory-based simulations of scanning tunneling micrographs were used for comparison to published experimental images of reconstructed SrTiO₃(001) surfaces. It was found that the addition of dissociatively adsorbed H₂O to the presently accepted structural solution of the 2 × 1 reconstruction is more consistent with the experimental data. A proposed model for the c(4 × 4) reconstruction, based on the hydrated 2 × 1 structure, agrees well with experiment and is consistent with a formation process consisting of the simple dehydration of a wet 2 × 1 structure.     

Hydrogen-induced self-organized nanostructuring of the Ir(100) surface

We show that the Ir(100) surface forms a new nanostructure in a self-organized way when its reconstructed equilibrium surface is exposed to hydrogen. Scanning tunneling microscopy and quantitative low-energy electron diffraction retrieve that a long-range ordered superlattice of defect-free Ir chains with average lateral spacing of 1.36 nm and micrometer lengths develops. This can be used as a template for the formation of other nanostructures as is demonstrated.     

Hydrogen-induced instability of the Ge(105) surface

The structure and stability of the hydrogen-terminated (105) surface of Ge deposited on Si(105) substrates are investigated by scanning tunneling microscopy (STM). Investigations combining STM, electron energy loss spectroscopy, and theory reveal that Si incorporation into the surface Ge layer of hydrogen-terminated Ge/Si(105) drastically destabilizes the surface. The STM images obtained on this surface are well explained by the recently established rebonded-step structure model.     

Unintentional F doping of SrTiO3(001) etched in HF acid-structure and electronic properties

We show that the HF acid etch commonly used to prepare SrTiO₃(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of TiO₂. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~ 13% of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO₂ terminated.