Interface Formation Process Research Articles

Oxidation of vanadium with reactive oxygen plasma: A photoelectron spectroscopy study of the initial stages of the oxide growth process

We report on the oxidation of vanadium surface in a low-temperature oxygen plasma studied by in-situ photoelectron spectroscopy (XPS, UPS). Pure vanadium foil was exposed to the oxygen plasma for different time intervals allowing to investigate early stages of the oxide–metal interface formation process and the oxide film growth. Upon increasing the exposure time to the oxygen plasma we identify two regimes with respect to the vanadium oxidation state: formation of 4+ state on the early stages of growth and in saturated regime vanadium was found to be predominantly in the 5+ oxidation state. Angle-resolved XPS was used to perform an in-depth distribution analysis of chemical composition in outermost layers of the oxide. We found that plasma oxidation produces a well-pronounced interface with a transition layer thickness of about 1.3 nm.

半導体表面とペンタセン薄膜間の界面形成過程の微視的構造観察

Interface formation process of pentacene thin films on semiconductor surfaces has been studied using low energy electron microscopy (LEEM) and scanning tunneling microscopy. Interface interaction between clean Si surface and pentacene film is optimized by inserting bismuth thin film at the interface, resulting in the formation of monolayer islands larger than 0.2 mm in diameter. Systematic surface reactivity control on Si substrate reveals that the surface states nearby the Fermi level are the origin of the interface interaction between pentacene molecules and semiconductor surfaces. Effect of kinetic growth process on the pentacene film growth is also examined by detailed LEEM observations. The anisotropic growth speed originating from the anisotropic structure of pentacene accompanied with the kinetic growth condition could result in the formation of textured structure inside the single grain grown from a single nucleus.

Interface formation between two organic films based on phthalocyanine and perylene derivatives

The process of interface formation between two organic films composed of donor (copper phthalocyanine, CuPc) and acceptor (perylene-3,4,9,10-tetracarboxylic dianhydride, PTCDA) molecules has been studied in situ using the total current spectroscopy technique. It is established that the donor-acceptor interaction between CuPc and PTCDA molecules do not distort the energy structure of the density of electron states. The main π*, σ*1, and σ*2 bands of antibonding (unoccupied) electron states are identified, which are determined both by C-C bonds in the aromatic rings and by additional C-N and C-O bonds. The width of the interface potential barrier is evaluated and its relation to the limiting polarizability of molecules is demonstrated. The interface potential barrier is formed in the course of negative charge transfer between donor (CuPc) and acceptor (PTCDA) molecules.

A Fluorescence Spectroscopy Study of Preferential Solvation in Binary Solvents

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Singularities at the moving contact line. Mathematical, physical and computational aspects

The near-field dynamics in the moving contact-line problem described in the framework of several alternative theories is examined in the general case of finite capillary numbers against the most basic criteria to be satisfied by a mathematical model. It is shown that both types of so-called ‘slip models’ invariably fail the criteria: they lead to solutions either with a pressure singularity at the contact line or with the flow kinematics qualitatively different from that observed in experiments, or both. The same criteria applied to an early developed theory of dynamic wetting as an interface formation process are shown to be satisfied: the model leads to a singularity-free solution with the correct ‘rolling’ kinematics. The flow exhibits no spurious low-velocity region near the contact line which is unavoidable in all ‘slip models’. The pressure at the contact line remains finite, and the dynamic contact angle, being part of the solution, depends on the flow field and not only on the contact-line speed. This last feature accounts for the effect of ‘hydrodynamic assist of dynamic wetting’ reported in recent experiments. The implications of the results for numerical algorithms incorporating different models for the moving contact line are discussed.

Interface formation during molecular beam epitaxial growth of neodymium oxide on silicon

The Si/dielectric interface properties influence device performance significantly. Often the interface is not stable and changes during and/or after the growth. For a better understanding of the interface and layer formation processes of Nd2O3 on Si(001), as an example for the lanthanide oxides, well-defined experimental studies by reflection high-energy diffraction and x-ray photoelectron spectroscopy were performed under ultraclean ultrahigh vacuum conditions of molecular beam epitaxy. Complementary investigations were performed by transmission electron microscopy. We found that Nd2O3 is a candidate for replacing silicon dioxide as gate dielectric in future Si devices with suitable band gap and offset with respect to silicon. However, under ultrahigh vacuum conditions, silicide formation occurs in the initial stage of growth, which can result in large silicide inclusions and hole formation during further growth. This effect can be completely prevented by modifying the oxygen partial pressure during the interface formation and layer growth.

Synchrotron light in semiconductor research: Three decades of revolution

Since the late 1960s, synchrotron-based experimental techniques have been playing a key role in semiconductor research. Major milestones include the observation of intrinsic and extrinsic surface and interface states, the study of interface formation processes, band structure mapping and, more recently, the study of exotic correlation effects in low-dimensional systems. We briefly review these developments with special emphasis on the contributions of the University of Wisconsin Synchrotron Radiation Center (SRC) and on the corresponding environment.

A fluorescence spectroscopy study of preferential solvation in binary solvents

Published data are reviewed on the dynamics of nonspecific preferential solvation in binary solvents, as obtained by fluorescence spectroscopy (in particular, by magnetic spin fluorescence method) using probe molecules in which strong charge redistribution yielding dipoles and ion pairs takes place upon excitation. The dynamics of preferential solvation of the excited probe in a binary solvent is described by a sequence of consecutive equilibrium reactions, in which each step is the absorption of a molecule of the polar component by the solvation shell of the excited probe, where transport from the bulk solution occurs via translational diffusion. The characteristic effect of the time saturation of solvation is attributed to the energetically unfavorable process of interface formation between the solvation shell, which a polar nanosized cluster, and the homogeneous binary mixture. Such polar clusters may affect the course of photochemical reactions involving charged species or particles with a large dipole moment. Simple model concepts, such as the Frenkel theory of heterophase fluctuations, the Onsager model, and the Hildebrand theory of solubility used for the description of the physical picture of preferential solvation, are considered.

Capillary breakup of liquid threads: a singularity-free solution

The process of capillary breakup of a thread of Newtonian liquid is considered theoretically in the simplest case where the thread is surrounded by an inviscid, dynamically passive gas. The goal is to remove the singularities inherent in the known solutions to the problem obtained in the framework of the standard model and explain some puzzling qualitative features of the process observed in experiments. The analysis is based on the idea that, since the known solutions indicate that the rate at which fresh free-surface area is created tends to infinity as breakup is approached, one has that the surface tension, whose relaxation to equilibrium is always associated with a small but finite relaxation time, is bound to deviate from its equilibrium value in the process of breakup. This gives rise to a surface-tension gradient which starts to pull the liquid thread apart (the flow-induced Marangoni effect), whilst the role of the capillary pressure-driven squeezing of the liquid out of the neck diminishes as the surface tension in the minimal cross-section decreases. An earlier developed theory incorporating the interface formation process is applied without any ad hoc alterations and analysed in the framework of the slender-jet approximation. The resulting solution is singularity-free and allows one to describe some previously unexplained features of experiment by Kowalewski (1996, Fluid Dyn. Res., 17, 121).

Ultraviolet photoelectron spectroscopy investigation of interface formation in an indium–tin oxide/fluorocarbon/organic semiconductor contact

It has been demonstrated that hole-injection in organic light-emitting devices (OLEDs) can be enhanced by inserting a UV-illuminated fluorocarbon (CFx) layer between indium–tin oxide (ITO) and organic hole-transporting layer (HTL). In this work, the process of interface formation and electronic properties of the ITO/CFx/HTL interface were investigated with ultraviolet photoelectron spectroscopy. It was found that UV-illuminated fluorocarbon layer decreases the hole-injection barrier from ITO to α-napthylphenylbiphenyl diamine (NPB). Energy level diagrams deduced from the ultraviolet photoelectron spectroscopy (UPS) spectra show that the hole-injection barrier in ITO/UV-treated CFx/NPB is the smallest (0.46 eV), compared to that in the ITO/untreated CFx/NPB (0.60 eV) and the standard ITO/NPB interface (0.68 eV). The improved current density–voltage (I–V) characteristics in the UV-treated CFx-coated ITO contact are consistent with its smallest barrier height.

Engineering the nm-thick Interface Layer Formed Between a High-k Film and Silicon

ABSTRACTThe interface layer between thin sputter-deposited tantalum oxide (TaOx) high-k film and silicon substrate was engineered with the Hf doping method and the insertion of a thin 5Å TaNx interface. The following results have been obtained: 1) the Hf dopant in the TaOx film was involved in the interface formation process, e.g., forming a new, thinner high-k HfSixOy interface layer rather than the SiOx layer, 2) when the TaNx interface was inserted, the interface layer composition was even more complicated, e.g., including TaOxNy and HfSixOy structures. No hafnium nitride or oxynitride was detected, 3) the interface layer structure was changed, e.g., from single-zone to multi-zone with different compositions, 4) when a low concentration of Hf existed in the TaOx film, the high-k dielectric properties, such as the k value, fixed charge density, dielectric strength, were improved, and 5) when the thin TaNx interface layer was inserted, the above electric properties were further improved. However, the fixed charge density and interface states were increased due to the insertion of the TaNx interface layer. These results were contributed by factors such as the charge-trapping characteristics in the interface layer and the some damage repairing mechanisms. In summary, this research proved that the high-k film's interface layer and bulk properties could be were improved with the doping process as well as the insertion of an ultra-thin TaNx interface film.

Formation and Electric Properties of Disordered Yb Layers on Si(111)7×7 Surface

AbstractInterface formation in Yb/Si(111) system has been investigated by AES and EELS spectroscopy and in situ Hall measurements at room temperature. It was found that interface formation process may be divided into five stages: 1) two-dimensional growth of Yb, 2) intermixing and formation of two-dimensional Yb silicide, 3) formation of 3D silicide islands, 4) growth of Yb on 3D silicide islands, 5) coalescence of 3D Yb – Yb silicide islands and formation of continuos Yb film. We attribute the observed character of conductivity in Yb/Si(111) system to the evolution of morphological and electrical properties of the growing Yb layer (2D Yb, silicide, metal) rather than to the changes within the space charge layer under the surface. Some amplitude oscillations have been observed in sheet conductivity, hole mobility and surface hole concentration within the coverage range below 6 ML where formation of a continuos Yb silicide film completes. Conductivity oscillations are explained by transition from semiconductor-type conductivity at the first growth stages (two-dimensional Yb growth) to metal-like conductivity of 2D and 3D Yb silicide films.

A combinatorial approach in oxide/semiconductor interface research for future electronic devices

A combinatorial methodology was employed to investigate oxide/semiconductor interfaces for future devices using a combination of temperature gradient pulsed laser deposition and transmission electron microscopy. In growing oxide materials on Si substrate, a proper termination was found to be inevitable for avoiding surface oxidation. For this purpose, arsenic was used to obtain a durable surface of Si in oxygen atmosphere. On this surface, CeO 2 and SrTiO 3 were grown to study interface stability and phenomena. CeO 2 and SrTiO 3 were found to have abrupt structures at 200 °C. At a higher temperature at 550 °C, an amorphous interfacial layer was formed for the SrTiO 3/Si interface. From the results, surface termination and the growth temperature were identified as factors governing the process of abrupt interface formation.

Identification of crystal nucleation and growth in an amorphous FeZr2 alloy by means of electrical resistance measurements

A polymorphous crystallization process in an amorphous FeZr2 alloy has been investigated by means of accurate electrical resistance measurements (ERMs) at elevated temperatures. It was found that, upon crystallization of the amorphous alloy, the electrical resistance increased with increasing temperature, exhibiting three distinct stages. Quantitative microscopy observations revealed that the three stages originated from crystal nucleation, from subsequent growth of crystal nuclei and from coarsening of the crystallites respectively. The activation energies for the crystal nucleation and growth determined from the ERM data agree satisfactorily with the data in the literature. The success in identification of the crystal nucleation and growth processes by means of ERMs may originate from differences in electrical resistance changes due to the crystal formation and the crystalline-amorphous interface formation processes from the amorphous phase.

Observation of intermixing at the buried CdS/Cu(In, Ga)Se2 thin film solar cell heterojunction

A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.

Formation of the Zn/CdTe(100) interface: Interdiffusion, segregation, and Cd-Zn exchange studied by photoemission

The interface formation of Zn/CdTe(100) has been investigated using synchrotron and Mg $K\ensuremath{\alpha}$ x-ray photoelectron spectroscopy. We identify five distinct phases of the interface formation process, including the passivation of surface defects by small amounts of adsorbed Zn $(l~$ 0.1 \AA{}), diffusion of Zn into Cd vacancies and lattice-site defects, and a Cd-Zn exchange, thus forming the ternary compound ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{Te}$ in a near-surface region. In the final stage of the indiffusion, we find significant Cd segregation and also, to a smaller extent, Te segregation. The formation of a metallic Zn overlayer for high Zn coverages is associated with a surface photovoltage effect at room temperature. The results derived from the investigation of Zn-induced band bending, surface core-level shifts, and peak area evaluations are discussed, and a model based on the variation of the photoemission information depth is given. We also present a simple method to determine the onset of Cd segregation in order to identify the ternary ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{Te}$ surface-alloy with maximum Zn content.

Minimization of suboxide transition regions at Si–SiO2 interfaces by 900 °C rapid thermal annealing

Transitions regions at Si–SiO2 interfaces contain excess suboxide bonding arrangements which contribute to interface roughness and also can give rise to electronically active defects. This article provides insights into the origin and temperature stability of these suboxide bonding arrangements by studying different interface formation processes, e.g., rapid thermal oxidation and plasma-assisted oxidation, and then subjecting these interfaces to rapid thermal annealing (RTA). The interfacial bonding chemistry has been studied before and after the RTA by Auger electron spectroscopy and it has been demonstrated that interfacial transition regions with suboxide bonding are a direct result of thermal and plasma-assisted oxidation at temperatures up to at least 800 °C, and that the excess suboxide bonding in interfacial transition regions is significantly reduced following a 30 s, 900 °C RTA. The kinetics of this interfacial annealing process are essentially the same as observed for the RTA-induced separation of homogeneous suboxide thin films (SiOx, x<2) into silicon nanocrystals and stoichiometric SiO2.

Cr 3d Surface and Bulk States in Sn1-xCrxTe/Cr Crystals

We report a new approach to investigate metal-semiconductor interface formation. Photoemission spectroscopy was applied in order to investigate the clean surface of a Sn0.97 Cr0.03Te crystal and to observe its changes under sequential deposition of small amounts of Cr atoms. In order to analyse the Cr 3d contribution to the valence band, the Fano-type resonance tuned to the Cr 3p-3d transition was used. The experiment was designed to follow the Sno.97Cr0.o3Te/Cr interface formation process. At the clean Sn0.97Cro.o3Te surface, the Cr 3d states contribution to the valence band was found to be positioned 0.8 eV below the Fermi level. After the Cr deposition processes the contribution shifted to a higher binding energy and another contribution 5.8 eV below the Fermi level was also observed. PACS numbers: 79.60.-i, 68.35.Fx

Moving contact lines in liquid/liquid/solid systems

A general mathematical model which describes the motion of an interface between immiscible viscous fluids along a smooth homogeneous solid surface is examined in the case of small capillary and Reynolds numbers. The model stems from a conclusion that the Young equation, σ1 cos θ = σ2 − σ3, which expresses the balance of tangential projection of the forces acting on the three-phase contact line in terms of the surface tensions σi and the contact angle θ, together with the well-established experimental fact that the dynamic contact angle deviates from the static one, imply that the surface tensions of contacting interfaces in the immediate vicinity of the contact line deviate from their equilibrium values when the contact line is moving. The same conclusion also follows from the experimentally observed kinematics of the flow, which indicates that liquid particles belonging to interfaces traverse the three-phase interaction zone (i.e. the ‘contact line’) in a finite time and become elements of another interface – hence their surface properties have to relax to new equilibrium values giving rise to the surface tension gradients in the neighbourhood of the moving contact line. The kinematic picture of the flow also suggests that the contact-line motion is only a particular case of a more general phenomenon – the process of interface formation or disappearance – and the corresponding mathematical model should be derived from first principles for this general process and then applied to wetting as well as to other relevant flows. In the present paper, the simplest theory which uses this approach is formulated and applied to the moving contact-line problem. The model describes the true kinematics of the flow so that it allows for the ‘splitting’ of the free surface at the contact line, the appearance of the surface tension gradients near the contact line and their influence upon the contact angle and the flow field. An analytical expression for the dependence of the dynamic contact angle on the contact-line speed and parameters characterizing properties of contacting media is derived and examined. The role of a ‘thin’ microscopic residual film formed by adsorbed molecules of the receding fluid is considered. The flow field in the vicinity of the contact line is analysed. The results are compared with experimental data obtained for different fluid/liquid/solid systems.

Photoelectron spectromicroscopy as a microchemical probe of high temperature superconductors

Abstract Synchrotron-radiation photoemission spectroscopy at high lateral resolution is a fairly new branch of surface experimental science. Nevertheless, it has already emerged as one of the most powerful probes of local surface and interface chemical properties. We show that this approach can play a very important role in the research on high temperature superconductors (HTSCs). Experiments on these materials are becoming quite sophisticated and require extremely high-quality single crystals and films. The success of such experiments depends critically on the local chemical properties of the specimens, notably on their chemical homogeneity. We used synchrotron imaging spectromicroscopy to explore this issue for high-quality single crystals and films, and found that even these specimens sporadically exhibit inhomogeneities. Such inhomogeneities are quite critical, as they could simulate spurious effects and jeopardize the reliability of experiments, notably those on bandgap spectroscopy and on interface formation processes.