Core-level spectroscopy of the clean Si(001) surface: Charge transfer within asymmetric dimers of the 2 x 1 and c(4 x 2) reconstructions

EUV-induced hydrogen desorption as a step towards large-scale silicon quantum device patterning

Atomically precise hydrogen desorption lithography using scanning tunnelling microscopy (STM) has enabled the development of single-atom, quantum-electronic devices on a laboratory scale. Scaling up this technology to mass-produce these devices requires bridging the gap between the precision of STM and the processes used in next-generation semiconductor manufacturing. Here, we demonstrate the ability to remove hydrogen from a monohydride Si(001):H surface using extreme ultraviolet (EUV) light. We quantify the desorption characteristics using various techniques, including STM, X-ray photoelectron spectroscopy (XPS), and photoemission electron microscopy (XPEEM). Our results show that desorption is induced by secondary electrons from valence band excitations, consistent with an exactly solvable non-linear differential equation and compatible with the current 13.5 nm (~92 eV) EUV standard for photolithography; the data imply useful exposure times of order minutes for the 300 W sources characteristic of EUV infrastructure. This is an important step towards the EUV patterning of silicon surfaces without traditional resists, by offering the possibility for parallel processing in the fabrication of classical and quantum devices through deterministic doping.

Roles of excess minority carrier recombination and chemisorbed O2 species at SiO2/Si interfaces in Si dry oxidation: Comparison between p-Si(001) and n-Si(001) surfaces

This study provides experimental evidence for the following: (1) Excess minority carrier recombination at SiO₂/Si interfaces is associated with O₂ dissociative adsorption; (2) the x-ray induced enhancement of SiO₂ growth is not caused by the band flattening resulting from the surface photovoltaic effect but by the electron-hole pair creation resulting from core level photoexcitation for the spillover of bulk Si electronic states toward the SiO₂ layer; and (3) a metastable chemisorbed O₂ species plays a decisive role in combining two types of the single- and double-step oxidation reaction loops. Based on experimental results, the unified Si oxidation reaction model mediated by point defect generation [S. Ogawa et al., Jpn. J. Appl. Phys., Part 1 59, SM0801 (2020)] is extended from the viewpoints of (a) the excess minority carrier recombination at the oxidation-induced vacancy site and (b) the trapping-mediated adsorption through the chemisorbed O₂ species at the SiO₂/Si interface.

A Synchrotron Radiation Photoemission Study of SiGe(001)-2×1 Grown on Ge and Si Substrates: The Surface Electronic Structure for Various Ge Concentrations

Beyond the macroscopic perspective, this study microscopically investigates Si_1−xGe_x(001)-2×1 samples that were grown on the epi Ge(001) and epi Si(001) substrates via molecular-beam epitaxy, using the high-resolution synchrotron radiation photoelectron spectroscopy (SRPES) as a probe. The low-energy electron diffraction equipped in the SRPES chamber showed 2×1 double-domain reconstruction. Analyses of the Ge 3d core-level spectra acquired using different photon energies and emission angles consistently reveal the ordered spots to be in a Ge–Ge tilted configuration, which is similar to that in epi Ge(001)-2×1. It was further found that the subsurface layer was actually dominated by Ge, which supported the buckled configuration. The Si atoms were first found in the third surface layer. These Si atoms were further divided into two parts, one underneath the Ge–Ge dimer and one between the dimer row. The distinct energy positions of the Si 2p core-level spectrum were caused by stresses, not by charge alternations.

Experimental and theoretical 2p core-level spectra of size-selected gas-phase aluminum and silicon cluster cations: chemical shifts, geometric structure, and coordination-dependent screening

We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al₁₂⁺, Si₁₅⁺, Si₁₆⁺, and Si₁₉⁺ are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed.

Super-bandgap light stimulated reversible transformation and laser-driven mass transport at the surface of As2S3 chalcogenide nanolayers studied in situ

The super-bandgap laser irradiation of the in situ prepared As-S chalcogenide films was found to cause drastic structural transformations and unexpected selective diffusion processes, leading to As enrichment on the nanolayer surface. Excitation energy dependent synchrotron radiation photoelectron spectroscopy showed complete reversibility of the molecular transformations and selective laser-driven mass transport during "laser irradiation"-"thermal annealing" cycles. Molecular modeling and density functional theory calculations performed on As-rich cage-like clusters built from basic structural units indicate that the underlying microscopic mechanism of laser induced transformations is connected with the realgar-pararealgar transition in the As-S structure. The detected changes in surface composition as well as the related local and molecular structural transformations are analyzed and a model is proposed and discussed in detail. It is suggested that the formation of a concentration gradient is a result of bond cleavage and molecular reorientation during transformations and anisotropic molecular diffusion.

Atomic and electronic structure of trilayer graphene/SiC(0001): Evidence of Strong Dependence on Stacking Sequence and charge transfer

The transport properties of few-layer graphene are the directly result of a peculiar band structure near the Dirac point. Here, for epitaxial graphene grown on SiC, we determine the effect of charge transfer from the SiC substrate on the local density of states (LDOS) of trilayer graphene using scaning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy (ARPES). Different spectra are observed and are attributed to the existence of two stable polytypes of trilayer: Bernal (ABA) and rhomboedreal (ABC) staking. Their electronic properties strongly depend on the charge transfer from the substrate. We show that the LDOS of ABC stacking shows an additional peak located above the Dirac point in comparison with the LDOS of ABA stacking. The observed LDOS features, reflecting the underlying symmetry of the two polytypes, were reproduced by explicit calculations within density functional theory (DFT) including the charge transfer from the substrate. These findings demonstrate the pronounced effect of stacking order and charge transfer on the electronic structure of trilayer or few layer graphene. Our approach represents a significant step toward understand the electronic properties of graphene layer under electrical field.

Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures

Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.

The isotype ZnO/SiC heterojunction prepared by molecular beam epitaxy--A chemical inert interface with significant band discontinuities

ZnO/SiC heterojunctions show great potential for various optoelectronic applications (e.g., ultraviolet light emitting diodes, photodetectors, and solar cells). However, the lack of a detailed understanding of the ZnO/SiC interface prevents an efficient and rapid optimization of these devices. Here, intrinsic (but inherently n-type) ZnO were deposited via molecular beam epitaxy on n-type 6H-SiC single crystalline substrates. The chemical and electronic structure of the ZnO/SiC interfaces were characterized by ultraviolet/x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. In contrast to the ZnO/SiC interface prepared by radio frequency magnetron sputtering, no willemite-like zinc silicate interface species is present at the MBE-ZnO/SiC interface. Furthermore, the valence band offset at the abrupt ZnO/SiC interface is experimentally determined to be (1.2 ± 0.3) eV, suggesting a conduction band offset of approximately 0.8 eV, thus explaining the reported excellent rectifying characteristics of isotype ZnO/SiC heterojunctions. These insights lead to a better comprehension of the ZnO/SiC interface and show that the choice of deposition route might offer a powerful means to tailor the chemical and electronic structures of the ZnO/SiC interface, which can eventually be utilized to optimize related devices.

Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure

Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene.

Electronic properties of epitaxial silicene on diboride thin films

The Si counterpart of graphene—silicene—has partially similar but also unique electronic properties that relate to the presence of an extended π electronic system, the flexible crystal structure and the large spin-orbit coupling. Driven by predictions for exceptional electronic properties like the presence of massless charge carriers, the occurrence of the quantum Hall effect and perfect spin-filtering in free-standing, unreconstructed silicene, the recent experimental realization of largely sp(2)-hybridized, Si honeycomb lattices grown on a number of metallic substrates provided the opportunity for the systematic study of the electronic properties of epitaxial silicene phases. Following a discussion of theoretical predictions for free-standing silicene, we review properties of (√3 × √3)-reconstructed, epitaxial silicene phases but with the emphasis on the extensively studied case of silicene on ZrB2(0 0 0 1) thin films. As the experimental results show, the structural and electronic properties are highly interlinked and leave their fingerprint on the chemical states of individual Si atoms as revealed in core-level photoelectron spectra as well as in the valence electronic structure and low-energy interband transitions. With the critical role of substrates and of the chemical stability of epitaxial silicene highlighted, finally, benefits and challenges for any future silicene-based nanoelectronics are being put into perspective.

Propanoate grafting on (H,OH)-Si(0 0 1)-2 × 1

We have examined the reactivity of water-covered Si(0 0 1)-2 × 1, (H,OH)-Si(0 0 1)-2 × 1, with propanoic (C2H5COOH) acid at room temperature. Using a combination of spectroscopic techniques probing the electronic structure (XPS, NEXAFS) and the vibrational spectrum (HREELS), we have proved that the acid is chemisorbed on the surface as a propanoate. Once the molecule is chemisorbed, the strong perturbation of the electronic structure of the hydroxyls, and of their vibrational spectrum, suggests that the molecule makes hydrogen bonds with the surrounding hydroxyls. As we find evidence that surface hydroxyls are involved in the adsorption reaction, we discuss how a concerted or a radical-mediated reaction (involving the surface silicon dangling bonds) could lead to water elimination and formation of the ester.

Enhanced silicon oxidation on titanium-covered Si(001)

We report on a core level photoemission study of the formation of an ultrathin SiO(x) layer grown at the interface of a titanium-covered Si(001) surface. Oxygen exposure at room temperature induces a large chemical shift of the Si 2p state, predominantly assigned to Si(4+). The results indicate that a SiO(2 - δ) layer, close to the stoichiometry of SiO(2), is formed below the TiO(x) film. The thickness of the SiO(2 - δ) layer is estimated to be ∼ 0.9 nm, corresponding to three to four oxide layers. Further chemical shift caused by annealing is attributed to the formation of titanium silicate (TiSi(x)O(y)).

Growth of straight, atomically perfect, highly metallic silicon nanowires with chiral asymmetry

In the quest of nano-objects for future electronics, silicon nanowires could possibly take over carbon nanotubes. Here we show the growth by self-organization of straight, massively parallel silicon nanowires having a width of 1.6 nm, which are atomically perfect and highly metallic conductors. Surprisingly, these silicon nanowires display a strong symmetry breaking across their widths with two chiral species that self-assemble in large left-handed and right-handed magnetic-like domains.

Cycloaddition Reaction of 1,3-Butadiene with a Symmetric Si Adatom Pair on the Si(111)7×7 Surface

We first found experimentally a cycloaddition reaction of a molecule on a symmetry Si pair, 1,3-butadiene on the Si adatom pair of Si(111)7x7, while up to now only asymmetric Si pairs were reported to be involved in cycloaddition reactions on Si surfaces. As the symmetry of a Si pair is expected to influence significantly a cycloaddition product and a reaction pathway, the [4+2]-like cycloaddition product of 1,3-butadiene on the Si adatom pair is suggested to form through a concerted reaction pathway in comparison to a stepwise reaction pathway, which is favorable in the formation of the [4+2]-like cycloaddition product on the asymmetric Si pair (the Si adatom-restatom pair).

Origin of nonlocal interactions in adsorption of polar molecules on Si(001)-2 x 1

Using density functional theory slab calculations, we have investigated (i) the origin of nonlocal interactions occurring in the adsorption of small polar molecules (H2O,NH3,CH3OH,CH3NH2) on the clean Si(001)-2 x 1 surface and (ii) the nonlocal effects on two-dimensional arrangement of adsorbates. Our results show the adsorption properties are significantly altered in the presence of adsorbates on an adjacent dimer along a row. We have identified that the coverage dependent behavior arises from a combination of (i) surface polarization change, (ii) adsorbate-induced charge delocalization, (iii) adsorbate-adsorbate repulsion, and (iv) hydrogen bonding. The nucleophilic-electrophilic molecular adsorption involves charge delocalization to neighboring dimers along a row, which in turn undermines molecular adsorption on the neighboring dimers. Nonlocal effects associated with polar interactions with neighboring dimers and adsorbates vary with adsorption system. While such polar interactions are unimportant in CH3OH adsorption, hydrogen bonding and adsorbate-adsorbate repulsion play an important role in determining the adsorption structures of H2O and NH3CH3NH2, respectively. In addition, the electrostatic attraction with the buckled-up Si atoms of adjacent dimers contributes to stabilization of H2O, NH3, and CH3NH2 adsorption. We also discuss kinetic effects on two-dimensional ordering of adsorbates, in conjunction with surface phase transition and adsorption-dissociation rates.

Origin of fine structure in si photoelectron spectra at silicon surfaces and interfaces

Using a first-principles approach, we investigate the origin of the fine structure in Si 2p photoelectron spectra at the Si(100)-(2 x 1) surface and at the Si(100)-SiO2 interface. Calculated and measured shifts show very good agreement for both systems. By using maximally localized Wannier functions, we clearly identify the shifts resulting from the electronegativity of second-neighbor atoms. The other shifts are then found to be proportional to the average bond-length variation around the Si atom. Hence, in combination with accurate modeling, photoelectron spectroscopy can provide a direct measure of the strain field at the atomic scale.

Origin of p(2 x 1) phase on Si(001) by noncontact atomic force microscopy at 5 k

The controversial issue of the origin of the p(2 x 1) reconstruction of the Si(001) surface observed in recent low temperature scanning tunneling microscopy experiments is clarified here using 5 K noncontact atomic force microscopy. The c(4 x 2) phase is observed at separations corresponding to weak tip-surface interactions, confirming that it is the ground state of the surface. At larger frequency shifts the p(2 x 1) phase of symmetric dimers is observed. By studying the interaction of a reactive Si tip with the c(4 x 2) Si(001) surface using an ab initio method, we find that the observed change in the surface reconstruction is an apparent effect caused by tip induced dimer flipping resulting in a modification of the surface structure and appearance of the p(2 x 1) phase in the image. Using an appropriate scanning protocol, one can manipulate the surface reconstruction at will, which has significance in nanotechnology.

The structure of the Si9H12 cluster: A coupled cluster and multi-reference perturbation theory study

Full geometry optimizations using both singles and doubles coupled cluster theory with perturbative triple excitations, CCSD(T), and second order multi-reference perturbation theory, MRMP2, have been employed to predict the structure of Si9H12, a cluster commonly used in calculations to represent the Si(100) surface. Both levels of theory predict the structure of this cluster to be symmetric (not buckled), and no evidence for a buckled (asymmetric) structure is found at either level of theory.

Electronic Excited States of Si(100) and Organic Molecules Adsorbed on Si(100)

The electronically excited states of the Si(100) surface and acetylene, benzene, and 9,10-phenanthrenequinone adsorbed on Si(100) are studied with time-dependent density functional theory. The computational cost of these calculations can be reduced through truncation of the single excitation space. This allows larger cluster models of the surface in conjunction with large adsorbates to be studied. On clean Si(100), the low-lying excitations correspond to transitions between the pi orbitals of the silicon-silicon dimers. These excitations are predicted to occur in the range 0.4-2 eV. When organic molecules are adsorbed on the surface, surface --> molecule, molecule --> surface, and electronic excitations localized within the adsorbate are also observed at higher energies. For acetylene and benzene, the remaining pipi* excitations are found to lie at lower energies than in the corresponding gas-phase species. Even though the aromaticity of 9,10-phenanthrenequinone is retained, significant shifts in the pipi* excitations of the aromatic rings are predicted. This is in part due to structural changes that occur upon adsorption.

Ab initio molecular dynamics: concepts, recent developments, and future trends

The methodology of ab initio molecular dynamics, wherein finite-temperature dynamical trajectories are generated by using forces computed "on the fly" from electronic structure calculations, has had a profound influence in modern theoretical research. Ab initio molecular dynamics allows chemical processes in condensed phases to be studied in an accurate and unbiased manner, leading to new paradigms in the elucidation of microscopic mechanisms, rationalization of experimental data, and testable predictions of new phenomena. The purpose of this work is to give a brief introduction to the technique and to review several important recent developments in the field. Several illustrative examples showing the power of the technique have been chosen. Perspectives on future directions in the field also will be given.

First-principles study of the adsorption of cesium on Si(001)(2×1) surface

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed on cesium adsorption on Si(001)(2 x 1) surface. The optimized geometries and adsorption energies have been obtained and the preferred binding sites have been determined for the coverage (Theta) of one monolayer and half a monolayer. At Theta = 0.5 ML the most stable adsorption site is shown to be T3 site. At Theta = 1 ML two Cs atoms are adsorbed at HH and T3 sites, respectively. It was found that the saturation coverage of Cs for the Si(001)(2 x 1)-Cs surface is one monolayer instead of half a monolayer. This finding supports the majority of experimental observations but does not support recent coaxial impact collision ion scattering spectroscopy investigations [Surf. Sci. 531, L340 (2003)] and He(+) Rutherford backscattering spectroscopy studies [Phys. Rev. B 62, 4545 (2000)]. Mulliken charge and overlap population analysis showed that the Cs-Si bond is indeed ionic rather than polarized covalent as generally assumed for alkali metal (AM) on Si(001)(2 x 1) surface. Geometrical structure analysis seems to have limitations in determining the nature of AM-substrate bond. We also found that the silicon surface is metallic and semiconducting for the coverages of 0.5 and 1 ML, respectively.

Reaction Mechanism of cis-1,3-Butadiene Addition to the Si(100)-2 × 1 Surface

A set of 40 finite temperature ab initio molecular dynamics trajectories is employed to investigate the distribution of addition products and underlying microscopic mechanism of the addition of 1,3-butadiene to the Si(100)-2 x 1 surface. The product yields are in good agreement with recent STM measurements and include a Diels-Alder [4 + 2] adduct with a surface dimer acting as the dienophile, a [4 + 2]-like adduct that bridges two dimers within a row, a [4 + 2]-like adduct that bridges two dimers in adjacent rows, and an interdimer [2 + 2]-like adduct. The trajectories indicate that a common mechanism underlies the distribution and is predominantly a nonconcerted stepwise mechanism that proceeds via an intermediate zwitterion composed of a carbocation bonded to a negatively charged surface dimer.

Reaction Pathway of the [4 + 2] Diels−Alder Adduct Formation on Si(100)-2×1

Despite a long history of experimental and theoretical investigation, the mechanism of the Diels-Alder (DA) reaction has been controversial since its discovery 80 years ago. From these investigations, two schools of thought have emerged, namely that the reaction can proceed via a concerted, symmetric or asymmetric mechanism or via a nonconcerted mechanism involving a zwitterion or diradical as an intermediate. Here, we employ finite temperature ab initio molecular dynamics simulations, employing forces computed "on the fly" from electronic structure calculations, to investigate the microscopic mechanism of DA adduct formation between 1,3-butadiene and the Si(100)-2x1 surface. Free energy profiles and nonequilibrium trajectories strongly suggest a nonconcerted mechanism that forms a zwitterionic intermediate state. This mechanism, which begins with a nucleophilic attack of the C=C double bond on the positive member of a charge-asymmetric buckled Si-Si dimer, was previously shown to be common to the formation of a wide range of adducts that can form on the surface.

Low-energy electron diffraction study of the phase transition of Si(001) surface below 40 K

The phase transition of Si(001) surface below 40 K was studied by low-energy electron diffraction (LEED). The temperature dependence of the intensities and widths of the quarter order diffraction spots and LEED intensity versus electron energy curves (I-V curves) were obtained in the temperature region from 20 to 300 K. While the spot intensities of the quarter order spots decrease and the widths broaden, the I-V curves do not change so much below 40 K. This clearly shows that a phase transition occurs from an ordered phase above 40 K to a disordered phase below 40 K.