Si 2p core-level shifts at the Si(001)-SiO2 interface: A first-principles study

Real-Space Pseudopotential Method for the Calculation of 1s Core-Level Binding Energies

We systematically studied a real-space pesudopotential method for the calculation of 1s core–electron binding energies of second-row elements B, C, N, and O within the framework of Kohn–Sham density functional theory (KS-DFT). With Dirichlet boundary conditions, pseudopotential calculations can provide accurate core–electron binding energies for molecular systems, when compared with the results from all-electron calculations and experiments. Furthermore, we report that with one simple additional nonself-consistent calculation as a refinement step using a hybrid exchange-correlation functional, we can generally improve the accuracy of binding energy shifts, promising a strategy for improving accuracy at a much lower computational cost. The specializations in the present approach, combined with our efficient real-space KS-DFT implementation, provide key advantages for calculating accurate core–electron binding energies of large-scale systems.

Unusual oxidation-induced core-level shifts at the HfO2/InP interface

X-ray photoelectron spectroscopy (XPS) is one of the most used methods in a diverse field of materials science and engineering. The elemental core-level binding energies (BE) and core-level shifts (CLS) are determined and interpreted in the XPS. Oxidation is commonly considered to increase the BE of the core electrons of metal and semiconductor elements (i.e., positive BE shift due to O bonds), because valence electron charge density moves toward electronegative O atoms in the intuitive charge-transfer model. Here we demonstrate that this BE hypothesis is not generally valid by presenting XPS spectra and a consistent model of atomic processes occurring at HfO₂/InP interface including negative In CLSs. It is shown theoretically for abrupt HfO₂/InP model structures that there is no correlation between the In CLSs and the number of oxygen neighbors. However, the P CLSs can be estimated using the number of close O neighbors. First native oxide model interfaces for III-V semiconductors are introduced. The results obtained from ab initio calculations and synchrotron XPS measurements emphasize the importance of complementary analyses in various academic and industrial investigations where CLSs are at the heart of advancing knowledge.

Effects of the c-Si/a-SiO2 interfacial atomic structure on its band alignment: an ab initio study

The crystalline-Si/amorphous-SiO₂ (c-Si/a-SiO₂) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO₂ interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO₂, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO₂ interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO₂ and c-Si/a-SiO₂ interfaces, the band offset significantly deviates from the experimental values by about 1 eV.

A first-principles study of As doping at a disordered Si-SiO2 interface

Understanding the interaction between dopants and semiconductor-oxide interfaces is an increasingly important concern in the drive to further miniaturize modern transistors. To this end, using a combination of first-principles density-functional theory and a continuous random network Monte Carlo method, we investigate electrically active arsenic donors at the interface between silicon and its oxide. Using a realistic model of the disordered interface, we find that a small percentage (on the order of ∼10%) of the atomic sites in the first few monolayers on the silicon side of the interface are energetically favourable for segregation, and that this is controlled by the local bonding and local strain of the defect centre. We also find that there is a long-range quantum confinement effect due to the interface, which results in an energy barrier for dopant segregation, but that this barrier is small in comparison to the effect of the local environment. Finally, we consider the extent to which the energetics of segregation can be controlled by the application of strain to the interface.

Silicon nanowire oxidation: the influence of sidewall structure and gold distribution

The oxidation behavior of Si nanowires (SiNWs) grown by the gold (Au) catalyzed vapor-liquid-solid (VLS) growth process in an electron beam evaporation (EBE) reactor is studied. The VLS SiNWs exhibit hexagonal shape with essentially {112} facets where each facet shows a saw-tooth faceting itself, consisting of alternating {111} and {113} facets. Depending on growth temperatures (450-750 degrees C) and evaporation currents (40-80 mA) that determine the silicon vapor supply, this facet formation is more or less pronounced. The diffusion of Au atoms on the faceted SiNW surfaces and the formation of Au nanoparticles on the SiNW facets during growth and during ex situ annealing are studied. Upon diffusion, the Au atoms agglomerate and form Au nanoparticles that preferably arrange themselves on {113} facets. Upon annealing in air at temperatures between 800 and 950 degrees C the gold nanoparticles agglomerate further and form bigger particles of a few tens of nm in diameter that reside at the interface between the growing silica (SiO(2)) layer and the SiNW itself, which in turn shrinks during SiNW oxidation. The oxide layer thickness and the oxide appearance depend on the annealing conditions (time and temperature) and systematically varied oxidation processing is described in this paper as investigated by cross-sectional transmission electron microscopy (TEM) including high resolution studies as well as scanning electron microscopy (SEM) studies. Our results strongly suggest that the SiNWs can be fully oxidized, thus forming silica NWs that can either keep their initial shape or, under certain annealing conditions, do not keep their initial wire shape but assume a bamboo-like shape that forms most likely as a result of locally high stresses that are related to nanocrack formation. The nanocracks form in the growing oxide layer mediated by the presence of Au nanoparticles that yield gold-enhanced SiNW oxidation and thus a faster oxidation rate.

Interfacial Electrostatics of Self-Assembled Monolayers of Alkane Thiolates on Au(111): Work Function Modification and Molecular Level Alignments

We have isolated at T < 150 K a weakly adsorbed dimethyl disulfide (DMDS) layer on Au(111) and studied how the vibrational states, S core hole level shifts, valence band photoemission, and work function measurements evolve upon transforming this system into chemisorbed methylthiolate (MT) self-assembled monolayers (SAM) by heating above 200 K. By combining these observations with detailed theoretical electronic structure simulations, at the density functional level, we have been able to obtain a detailed picture of the electronic interactions at the interface between Au and adsorbed thiolates and disulfides. All of our measurements may be interpreted with a simple model where MT is bound to the Au surface with negligible charge transfer. Interfacial dipoles arising from Pauli repulsion between molecule and metal surface electrons are present for the weakly adsorbed DMDS layer but not for the chemisorbed species. Instead, for the chemisorbed species, interfacial dipoles are exclusively controlled by the molecular dipole, its interaction with the dipoles on neighboring molecules, and its orientation to the surface. The ramifications of these results for alignment of molecular levels and interfacial properties of this class of materials are discussed.

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.

Transition structure at the Si(100)-SiO2 interface

We characterize the transition structure at the Si(100)-SiO2 interface by addressing the inverse ion-scattering problem. We achieve sensitivity to Si displacements at the interface by carrying out ion-scattering measurements in the channeling geometry for varying ion energies. To interpret our experimental results, we generate realistic atomic-scale models using a first-principles approach and carry out ion-scattering simulations based on classical interatomic potentials. Silicon displacements larger than 0.09 A are found to propagate for three layers into the Si substrate, ruling out a transition structure with regularly ordered O bridges, as recently proposed.

Dangling bond defects at Si-SiO2 interfaces: atomic structure of the P(b1) center

Using a first-principles approach, we characterize dangling bond defects at Si-SiO2 interfaces by calculating hyperfine parameters for several relaxed structures. Interface models, in which defect Si atoms remain close to crystalline sites of the substrate upon relaxation, successfully describe P(b) and P(b0) defects at (111) and (100) interfaces, respectively. On the basis of calculated hyperfine parameters, we discard models of the P(b1) defect containing a first neighbor shell with an O atom or a strained bond. A novel model consisting of an asymmetrically oxidized dimer yields hyperfine parameters in excellent agreement with experiment and is proposed as the structure of the P(b1) center.

Origin of O 1s core-level shifts on oxygen adsorbed Si(111)-(7x7)

Density functional calculations are used to examine the chemical and structural origin of O 1s core-level shifts measured on the initial oxidation stage of Si(111)-(7x7). Our analysis of metastable core-level peaks leads to a conclusive identification of the long-sought metastable oxidation species as a tetrahedral SiO4 unit, formed by two successive O2 adsorptions on a Si adatom. The origin of a higher-binding core-level shoulder is clarified by the presence of a threefold-coordinated subsurface O atom, introduced as a decay product of the metastable SiO4 unit. The present study provides a detailed atomic-scale picture of the initial oxidation process of Si(111)-(7x7).

New structural model for Si/SiO2 interfaces derived from spherosiloxane clusters: implications for Si 2p photoemission spectroscopy

In this Letter, we investigate the Si/SiO(2) interface structure formed by the chemisorption of H8Si8O12 and other spherosiloxane clusters on Si(100). Using transition state calculations, we clearly demonstrate that the clusters do not bond to the Si(100) surface via single vertex attachment as proposed previously, but rather attach via Si-O bond cleavage. This alternative cracked cluster geometry allows us to predict the photoemission features of spherosiloxane clusters on Si(100) without invoking second nearest neighbor effects.

Bonding arrangements at the Si-SiO2 and SiC-SiO2 interfaces and a possible origin of their contrasting properties

We report ab initio calculations designed to explore the relative energetics of different interface bonding structures. We find that, for Si (001), abrupt (no suboxide layer) interfaces generally have lower energy because of the surface geometry and the softness of the Si-O-Si angle. However, two energetically degenerate phases are possible at the nominal interface layer, so that a mix of the two is the likely source of the observed suboxide and dangling bonds. In principle, these effects may be avoidable by low-temperature deposition. In contrast, the topology and geometry of SiC surfaces is not suitable for abrupt interfaces.