High-resolution photoemission studies of the interfacial reactivity and interfacial energetics of Au and Cu Schottky barriers on methyl-terminated Si(1 1 1) surfaces

The Schottky junction formation by the stepwise evaporation of gold and copper, respectively, onto methyl-terminated silicon, CH₃-Si(1 1 1), was investigated by synchrotron X-ray photoelectron spectroscopy. During the junction formation process, interface reactions occurred as revealed by the appearance of chemically shifted Si 2p components. Upon deposition of Au, the formation of about one monolayer of gold silicide, SiAu₃, with a Si 2p chemical shift of +0.75(2) eV, was observed. The SiAu₃ floated on top of the growing gold layer. Similarly, for the deposition of Cu, the methyl termination layer was partially disrupted, as indicated by the appearance of a −0.28(2) eV chemically shifted Si 2p component attributable to an interfacial copper silicide phase, SiCu₃. Hence, the termination of the Si(1 1 1) surface by methyl groups did not completely prevent interfacial reactions, but did reduce the amount interfacial reaction products as compared to bare Si(1 1 1)-(7 × 7) surfaces.Electron Schottky barrier heights of 0.78(8) eV (Au) and 0.61(8) eV (Cu) were measured. Within the experimental uncertainty the observed Schottky barriers were identical to those ones obtained on non-passivated, (7 × 7)-reconstructed Si(1 1 1) surfaces. Thus, the modification of the electronic properties of the silicon-metal contact requires the complete absence of interfacial reactions.     

Hydrocarbon film growth by energetic CH3 molecule impact on SiC (0 0 1) surface

Classic molecular dynamics (MD) calculations were performed to investigate the deposition of thin hydrocarbon film. SiC (1 0 0) surfaces were bombarded with energetic CH₃ molecules at impact energies ranging from 50 to 150 eV. The simulated results show that the deposition yield of H atoms decreases with increasing incident energy, which is in good agreement with experiments. During the initial stages, with breaking Si-C bonds in SiC by CH₃ impacting, H atoms preferentially reacts with resulting Si to form Si-H bond. The C/H ratio in the grown films increases with increasing incident energy. In the grown films, CH species are dominant. For 50 eV, H-Csp3 bond is dominant. With increasing energy to 200 eV, the atomic density of H-Csp2 bond increases.     

Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr-O valence electron matching to the number of dangling bonds per surface Si atom. The Zr-O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C-O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.     

Effect of hydrogenation on B/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and the energetics of substitutional boron on the generic Si(0 0 1)-(1 × 2) surface. For a single B atom substitution corresponding to 0.5 ML coverage, we have considered two different sites: (i) the mixed Si-B dimer structure and (ii) boron substituting for the second-layer Si to form Si-B back-bond structure, which is energetically more favorable than the mixed Si-B dimer by 0.1 eV/dimer. However, when both of these cases are passivated by hydrogen atoms, the situation is reversed and the Si-B back-bond case becomes 0.1 eV/dimer higher in energy than the mixed Si-B dimer case. For the B incorporation corresponding to 1 ML coverage, among the substitutional cases, 100% interdiffusion into the third layer of Si and 50% interdiffusion into the second layer of Si are energetically similar and more favorable than the other cases that are considered. However, when the surface is passivated with hydrogen, the B atoms energetically prefer to stay at the third layer of the Si substrate.     

High resolution scanning tunneling spectroscopy of ultrathin Pb on Si(1 1 1)-(6 × 6) substrate

The electronic structure of Si(1 1 1)-(6 × 6)Au surface covered with submonolayer amount of Pb is investigated using scanning tunneling spectroscopy. Already in small islands of Pb with thickness of 1 ML Pb_(1 1 1) and with the diameter of only about 2 nm we detected the quantized electronic state with energy 0.55 eV below the Fermi level. Similarly, the I(V) characteristics made for the Si(1 1 1)-(6 × 6)Au surface reveal a localized energy state 0.3 eV below the Fermi level. These energies result from fitting of the theoretical curves to the experimental data. The calculations are based on tight binding Hubbard model. The theoretical calculations clearly show prominent modification of the I(V) curve due to variation of electronic and topographic properties of the STM tip apex.     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

Effect of hydrogenation on P/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and energetics of substitutional phosphorus (P) on the generic Si(0 0 1)-(1 × 2) surface. For the 0.5 ML coverage of P, we have considered three different substitutional sites: (i) the mixed Si-P dimer structure (i.e., the P-nondiffused case), (ii) P-interdiffused to the second layer Si (i.e., intermixed P-Si bond structure) and (iii) P-interdiffused to the third layer Si. We have found that the mixed Si-P dimer structure is 0.79 eV/dimer energetically more favorable than the P-interdiffused case. However, for the hydrogenation of above cases, we have found that the situation is reversed and the interdiffused case is 0.3 eV/dimer energetically more favorable than the P-nondiffused case. Reductions in the number of P-Si is identified as a contributing factor which determines energetically the stable structures during P on Si(0 0 1).     

Photoemission study of cobalt interaction with the oxidized Si(1 0 0)2 × 1 surface

The interaction of cobalt atoms with an oxidized Si(1 0 0)2 × 1 surface was studied by photoelectron spectroscopy with synchrotron radiation at room and elevated temperatures. The SiO_x layer grown in situ on the crystal surface was ∼0.3 nm thick, and the amount of deposited cobalt was varied within eight atomic layers. It was found that Co atoms could penetrate under the SiO_x layer even at room temperature in the initial growth. As the Co amount increased, a ternary Co-O-Si phase was formed at the interface, followed by a Co-Si solid solution. Silicide synthesis associated with the decomposition of these phases started under the SiO_x layer at ∼250 °C, producing cobalt disilicide with a stable CaF₂-type of structure.     

Core-level shifts at the Pt/W(1 1 0) monolayer bimetallic interface

We have measured W and Pt 4f_7/2 core-level photoemission spectra from interfaces formed by ultrathin Pt layers on W(1 1 0), completing our core-level measurements of W(1 1 0)-based bimetallic interfaces involving the group-10 metals Ni, Pd, and Pt. With increasing Pt coverage the sequence of W spectra can be described using three interfacial core-level peaks with binding-energy (BE) shifts (compared to the bulk) of −0.220 ± 0.015, −0.060 ± 0.015, and +0.110 ± 0.010 eV. We assign these features to 1D, 2D pseudomorphic (ps), and 2D closed-packed (cp) Pt phases, respectively. For ∼1 ps ML the Pt 4f_7/2 BE is 71.40 ± 0.02 eV, a shift of +0.46 ± 0.09 eV with respect to the BE of bulk Pt metal. The W 4f_7/2 core-level shifts induced by all three adsorbates are semiquantitatively described by the Born-Haber-cycle based partial-shift model of Nilsson et al. [39]. As with Ni/W(1 1 0), the difference in W 4f_7/2 binding energies between ps and cp Pt phases has a large structural contribution. The Pt 4f lineshape is consistent with a small density of states at the Fermi level, reflective of the Pt monolayer having noble-metal-like electronic structure.     

Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

Ultrathin ZrO2 films on Si-rich SiC(0 0 0 1)-(3 × 3): Growth and thermal stability

The growth and thermal stability of ultrathin ZrO₂ films on the Si-rich SiC(0 0 0 1)-(3 × 3) surface have been explored using photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The films were grown in situ by chemical vapor deposition using the zirconium tetra tert-butoxide (ZTB) precursor. The O 1s XAS results show that growth at 400 °C yields tetragonal ZrO₂. An interface is formed between the ZrO₂ film and the SiC substrate. The interface contains Si in several chemically different states. This gives evidence for an interface that is much more complex than that formed upon oxidation with O₂. Si in a 4+ oxidation state is detected in the near surface region. This shows that intermixing of SiO₂ and ZrO₂ occurs, possibly under the formation of silicate. The alignment of the ZrO₂ and SiC band edges is discussed based on core level and valence PES spectra. Subsequent annealing of a deposited film was performed in order to study the thermal stability of the system. Annealing to 800 °C does not lead to decomposition of the tetragonal ZrO₂ (t-ZrO₂) but changes are observed within the interface region. After annealing to 1000 °C a laterally heterogeneous layer has formed. The decomposition of the film leads to regions with t-ZrO₂ remnants, metallic Zr silicide and Si aggregates.     

Characterization of cubic phase MgZnO/Si(1 0 0) interfaces

The microstructural properties of the Mg_xZn_1−xO/Si(1 0 0) interface were investigated using transmission electron microscopy (TEM) and chemical states of the heterostructure were studied by high resolution X-ray photoelectron spectroscopy (XPS). By analyzing the valence band spectra of thin Mg_xZn_1−xO/Si(1 0 0) heterostructures, the valence band offset between such Mg_0.55Zn_0.45O and Si(1 0 0) was obtained to be 2.3 eV. Using the cubic ternary thin films as insulators, metal-insulator-semiconductor (MIS) capacitors have been fabricated. Leakage current density lower than 3 × 10⁻⁷ A/cm² is obtained under the electrical field of 600 kV/cm by current-voltage (I-V) measurement. Frenkel-Poole conduction mechanism is the main cause of current leakage under high electrical field.     

Study of Ni/Si(1 0 0) solid-state reaction with Al addition

The characteristics of Ni/Si(1 0 0) solid-state reaction with Al addition (Ni/Al/Si(1 0 0), Ni/Al/Ni/Si(1 0 0) and Al/Ni/Si(1 0 0)) is studied. Ni and Al films were deposited on Si(1 0 0) substrate by ion beam sputtering. The solid-state reaction between metal films and Si was performed by rapid thermal annealing. The sheet resistance of the formed silicide film was measured by four-point probe method. The X-ray diffraction (XRD) was employed to detect the phases in the silicide film. The Auger electron spectroscopy was applied to reveal the element profiles in depth. The influence of Al addition on the Schottky barrier heights of the formed silicide/Si diodes was investigated by current-voltage measurements. The experimental results show that NiSi forms even with the addition of Al, although the formation temperature correspondingly changes. It is revealed that Ni silicidation is accompanied with Al diffusion in Ni film toward the film top surface and Al is the dominant diffusion species in Ni/Al system. However, no Ni_xAl_y phase is detected in the films and no significant Schottky barrier height modulation by the addition of Al is observed.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Methanol adsorption on silicon (0 0 1)

Using a first-principles pseudopotential technique, we have investigated the adsorption of CH₃OH on the Si(0 0 1) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (0 0 1) is as follows: the gas phase CH₃OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si-OCH₃ and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5 ML, i.e., one molecule per Si-Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.     

Thermal stability of atomic-layer-deposited ultra-thin niobium oxide film on Si (1 0 0)

Ultra-thin Nb₂O₅ films with excellent uniformity have been grown on Si (1 0 0) by atomic-layer-deposition using Nb(OC₂H₅)₅ and H₂O precursors, and the corresponding thermal stability has been studied through atomic force microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The results indicate that the ultra-thin (∼3 nm) Nb₂O₅ film is gradually built up into distributed large islands with increasing rapid thermal annealing (RTA) temperature. Meanwhile, both crystalline and amorphous phases are formed in the matrix of Nb₂O₅ annealed at 700 °C. In terms of the as-prepared sample, an interfacial layer (IL) with a thickness of around 1.5 nm is observed, that is composed of niobium silicate (Nb-O-Si). Further, the high temperature RTA leads to a thickened IL, which is attributed to the formation of more Nb-O-Si bonds and new silicon oxide (Si-O-Si) adjacent to the Si (1 0 0).     

Thickness dependent stripe structure stability of Ag films on Si(1 1 1)-(4 × 1)-In substrate

The thickness dependent stripe structure stabilization of Ag films on Si(1 1 1)-(4 × 1)-In substrate is thermodynamically considered. It is found that for the stability of the structure, there is a competition between the sum of elastic energy and stacking fault energy in the film and the film-substrate interface energy. The presence of equilibrium of them leads to a critical film thickness. Beyond it, the stripe structure will transform into a flat one. Our prediction for n_c of Ag films shows reasonable agreement with experimental data. In addition, according to the established model, it is predicted that Au could also form the above stripe structure on this substrate with a similar n_c value of Ag.     

Effect of erbium interlayer on nickel silicide formation on Si(1 0 0)

To reveal the influence of erbium interlayer on the formation of nickel silicide and its contact properties on Si substrate, Er(0.5-3.0 nm) and Ni(20 nm) are successively deposited onto Si(1 0 0) substrate and are treated by rapid thermal annealing in pure N₂ ambient. The NiSi formation temperature is found to increase depending on the Er interlayer thickness. The formation temperature of NiSi₂ (700 °C) is not influenced by Er addition. But with 2 nm Er interlayer, the formed NiSi₂ is observed textured with preferred orientation of (1 0 0). During the formation of NiSi, Er segregates to the surface and little Er remains at the NiSi/Si(1 0 0) interface. Therefore, the Schottky barrier height of the formed NiSi/n-Si(1 0 0) contact is measured to be 0.635 ∼ 0.665 eV which is nearly invariable with different Er addition.     

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T₄) or threefold coordinated (H₃), the ontop (T₁) sites or substituted for a Si atom in the S₅ position located underneath the Si adatom in the T₄ site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T₄ position whereas at 1 ML, it occupies an H₃ site. This contrasts with the clean surface where the most energetically favored site is the S₅ at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.