Angle-resolved X-ray Photoelectron Spectroscopy Measurements Research Articles

Formation of Al/Au Bimetallic Interface Studied by Angle-Resolved X-Ray Photoelectron Spectroscopy (ARXPS)

ARXPS (Angle-resolved X-ray photoelectron spectroscopy) measurements were performed on the thin aluminum films deposited on the gold polycrystalline substrate kept at the temperature of liquid nitrogen in order to characterize the interdiffusion process during early stages of Al/Au interface formation. The intermixing of Al and Au elements at Al/Au bimetallic interface occurred already at temperature of liquid nitrogen leading to the formation of the well-resolved component on the high binding energy side of Au(4f) core line corresponding to AlAu intermetallic phase. The formation of initial AlAu phase at Al/Au interface in our experimental conditions can be explained on the base of its thermodynamic stability. The sample annealing resulted in the growth of the amount of AlAu intermetallic compound formed at liquid nitrogen temperature with subsequent decomposition of the parent alloy phase and formation of AlAu2 intermetallics richer in Au atoms. Tikhonov regularization method was used to extract the aluminum in-depth concentration profile from the measured ARXPS intensities. L-curve analysis was applied to determine the regularization parameter.

Investigation of band alignment between AlN and atomic-layer-deposited ZrO2 by X-ray photoelectron spectroscopy

The band alignment between AlN and atomic-layer-deposited ZrO2 was investigated by X-ray photoelectron spectroscopy (XPS). The dependence of Al 2p core-level positions on the take-off angle θ indicated upward band bending at the AlN surface. On the basis of angle-resolved XPS measurements combined with numerical calculations, the valence band of ZrO2 was found to be 0.23 ± 0.2 eV below that of AlN by taking the AlN surface band bending into account. By taking the band gaps of ZrO2 and AlN as 5.6 and 6.2 eV, respectively, a type-II energy band alignment with the conduction band discontinuity ΔEC of 0.83 ± 0.2 eV was found between ZrO2 and AlN.

Observation of Substrate Silicon Incorporation into Thin Lanthanum Oxide Film during Rapid Thermal Annealing

Lanthanum oxide (La2O3) has been proposed as the promising gate dielectric material for future complementary metal-oxide-semiconductor (CMOS) technology. However, unlike the conventional homopolar materials such as silicon oxide or silicon nitride, La2O3 is more ionic and in particular at the La2O3/Si interface is less thermally stable. This work investigates the chemical and compositional variations of La2O3 thin film on the silicon substrate during rapid thermal annealing by using angle-resolved x-ray photoelectron spectroscopy (ARXPS) measurements. Results show that thermal annealing at temperatures above 500 °C would result in the incorporation of substrate Si atoms deep into the bulk of the La2O3 film and forming silicate phases both at the interface and in the bulk. These effects would result in the characteristic degradation of CMOS devices.

Photopolymerized Network Polysiloxane Films with Dangling Hydrophilic/Hydrophobic Chains for the Biofouling Release of Invasive Marine Serpulid Ficopomatus enigmaticus.

Novel photopolymerized network films based on a polysiloxane matrix containing varied amounts of polyoxyethylene (P3) or perfluorohexylethyl (F) dangling side chains were investigated. For films containing less than 10 wt % P3 and F, the wettability and elastic modulus were similar to those of the photopolymerized network matrix. However, angle-resolved X-ray photoelectron spectroscopy measurements proved that the surface of films with F dangling chains was highly enriched in fluorine depending on both the amount of P3 and F and their relative ratio in the films. The biological performance of the films was evaluated against a new widespread and invasive marine biofoulant, the serpulid Ficopomatus enigmaticus. The diatom Navicula salinicola was also assayed as a conventional model organism for comparison. Films richer in P3 better resisted the settlement and promoted the release of calcified tubeworms of F. enigmaticus.

Amphiphilic pentablock copolymers and their blends with PDMS for antibiofouling coatings

Well-defined amphiphilic pentablock copolymers Siy-(EGx-FAz)2 composed of polysiloxane (Si), polyethylene glycol (EG), and perfluorohexylethyl polyacrylate (FA) blocks are synthesized by ATRP of FA monomer starting from a difunctional bromo-terminated macroinitiator. Diblock copolymers EGx-FAz are also synthesized as model systems. The block copolymers are used, either alone or blended with a PDMS matrix in varied loadings, to prepare antibiofouling coatings. Angle-resolved XPS and contact angle measurements show that the coating surface is highly enriched in fluorine content but undergoes reconstruction after contact with water. Protein adsorption experiments with human serum albumin and calf serum highlight that diblock copolymers resist protein adhesion better than do pentablock copolymers. Blending of the pentablock copolymer with PDMS results in increased protein adsorption. By contrast, the PDMS-matrix coatings show high removal percentages of sporelings of the green fouling alga Ulva linza. © 2015 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 2015, 53, 1213–1225

Thermal annealing, interface reaction, and lanthanum-based sub-nanometer EOT gate dielectrics

In future nanoscale complementary metal-oxide-semiconductor (CMOS) devices, the high dielectric constant (high-k) gate dielectric film will be shrunk down to a couple of nanometers or down to the sub-nanometer range in the sense of oxide equivalent thickness (EOT). However, as high-k materials, including La2O3 which has been considered to be one of the promising next generation gate dielectric materials, are only marginally stable on the silicon substrate, some nominal temperature processes used for the device fabrication may still give rise to interface reactions and result in a low-k interface layer which will be a critical constraint for achieving the ultrathin EOT gate dielectric film. In this work, some issues related to the material interaction at the lanthanum oxide/Si and lanthanum oxide/metal interfaces will be discussed with the supports of interface bonding structures as revealed by using angle-resolved x-ray photoelectron spectroscopy (ARXPS) measurements. We show that thermal annealing at temperature above 500 °C would result in the migration of Si atoms deep into the bulk of the La2O3 film and formation of silicate phases both at the interface and in the bulk. These effects would significantly lower the dielectric constant and thus increases the effective thickness of the dielectric film from the equivalent silicon oxide thickness (EOT) point of view.

Cross-linking and ultrathin grafted gradation of fluorinated polymers synthesized via initiated chemical vapor deposition to prevent surface reconstruction.

Poly(fluoroalkyl acrylate)s with long perfluorooctyl pendant groups have been found to lead to the release of biopersistent perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). Those with no more than six perfluorinated carbons in pedant groups do not cause such problems. They, however, give poor dynamic water repellency due to extensive reorganization of surface fluorinated groups when exposed to the water interface. In this work, thin films exhibiting improved dynamic water repellency, as evidenced by water contact angle (WCA) measurements, were synthesized via substrate-independent initiated chemical vapor deposition (iCVD) from 1H,1H,2H,2H-perfluorooctyl acrylate (C6PFA) and divinylbenzene (DVB) using two methods: copolymerization and ultrathin grafted gradation. The copolymerization between C6PFA and the cross-linker, DVB, was confirmed by Fourier transform infrared (FTIR) spectroscopy. The cross-linking is concluded to hinder the reorganization of surface fluorinated groups. The grafted gradation, consisting of an ultrathin pC6PFA top layer and a pDVB base layer, was characterized by angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurements, which indicated that the top layer of pC6PFA is <2.5 nm thick to achieve the best dynamic water repellency. The outmost surface of this structure is fully covered by fluorinated groups, giving hydrophobicity. Concurrently, thanks to the interlayer grafting and the ultrathinness of the top layer, the fluorinated groups' tendency to migrate away from water interface is sterically blocked by the highly cross-linked pDVB base layer. The proposed approaches effectively reduced WCA hysteresis of C6PFA-based thin film to as low as 26.9° while maintaining sufficient hydrophobicity (advanced WCA of 119.6°). Due to the conformal and substrate-independent nature of iCVD technique, the films could be used to coat textured surfaces to generate superhydrophobicity.

Band alignment of HfO2/In0.18Al0.82N determined by angle-resolved x-ray photoelectron spectroscopy

The band-alignment of atomic layer deposited (ALD)-HfO2/In0.18Al0.82N was studied by high resolution angle-resolved X-ray photoelectron spectroscopy measurements. The band bending near the HfO2/In0.18Al0.82N interface was investigated, and the potential variation across the interface was taken into account in the band alignment calculation. It is observed that the binding energies for N 1s and Al 2p in In0.18Al0.82N decreases and the corresponding extracted valence band offsets increases with increasing θ (i.e., closer to the HfO2/In0.18Al0.82N interface), as a result of an upward energy band bending towards the HfO2/In0.18Al0.82N interface. The resultant valence band offset and the conduction band offset for the ALD-HfO2/In0.18Al0.82N interface calculated was found to be 0.69 eV and 1.01 eV, respectively.

Band alignment between GaN and ZrO2 formed by atomic layer deposition

The band alignment between Ga-face GaN and atomic-layer-deposited ZrO2 was investigated using X-ray photoelectron spectroscopy (XPS). The dependence of Ga 3d and Zr 3d core-level positions on the take-off angles indicated upward band bending at GaN surface and potential gradient in ZrO2 layer. Based on angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔEV of 1 ± 0.2 eV and conduction band discontinuity ΔEC of 1.2 ± 0.2 eV at ZrO2/GaN interface were determined by taking GaN surface band bending and potential gradient in ZrO2 layer into account.

Combined UHV/high-pressure catalysis setup for depth-resolved near-surface spectroscopic characterization and catalytic testing of model catalysts

An ultra-high vacuum (UHV) setup for "real" and "inverse" model catalyst preparation, depth-resolved near-surface spectroscopic characterization, and quantification of catalytic activity and selectivity under technologically relevant conditions is described. Due to the all-quartz reactor attached directly to the UHV-chamber, transfer of the catalyst for in situ testing without intermediate contact to the ambient is possible. The design of the UHV-compatible re-circulating batch reactor setup allows the study of reaction kinetics under close to technically relevant catalytic conditions up to 1273 K without contact to metallic surfaces except those of the catalyst itself. With the attached differentially pumped exchangeable evaporators and the quartz-microbalance thickness monitoring equipment, a reproducible, versatile, and standardised sample preparation is possible. For three-dimensional near-surface sample characterization, the system is equipped with a hemispherical analyser for X-ray photoelectron spectroscopy (XPS), electron-beam or X-ray-excited Auger-electron spectroscopy, and low-energy ion scattering measurements. Due the dedicated geometry of the X-ray gun (54.7°, "magic angle") and the rotatable sample holder, depth analysis by angle-resolved XPS measurements can be performed. Thus, by the combination of characterisation methods with different information depths, a detailed three-dimensional picture of the electronic and geometric structure of the model catalyst can be obtained. To demonstrate the capability of the described system, comparative results for depth-resolved sample characterization and catalytic testing in methanol steam reforming on PdGa and PdZn near-surface intermetallic phases are shown.

Silicon Surface Passivation Technology for Germanium-Tin P-Channel MOSFETs: Suppression of Germanium and Tin Segregation for Mobility Enhancement

We report a two-step silicon (Si) surface passivation technology for germanium-tin (Ge1-xSnx) p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs), featuring an initial low temperature atomic-layer-epitaxy or passivation step to suppress the segregation of subsurface germanium (Ge) and tin (Sn) atoms into the first monolayer of the ultrathin Si passivation layer. Evidence of reduced Ge and Sn segregation into the ultrathin Si passivation layer is provided by angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurement. As compared to a Si passivation process performed at a higher temperature (370°C), the two-step Si passivation process gives ∼13% higher drive current in Ge0.94Sn0.06 pMOSFETs. It also achieved a high effective hole mobility μeff of 472 cm2/V·s at inversion carrier density Ninv of 1 × 1013 cm−2. The two-step Si surface passivation technology enables the formation of high-quality gate stack on Ge1-xSnx surface channel for high performance logic applications.

Advanced surface characterization of silver nanocluster segregation in Ag–TiCN bioactive coatings by RBS, GDOES, and ARXPS

Surface modification by means of wear protective and antibacterial coatings represents, nowadays, a crucial challenge in the biomaterials field in order to enhance the lifetime of bio-devices. It is possible to tailor the properties of the material by using an appropriate combination of high wear resistance (e.g., nitride or carbide coatings) and biocide agents (e.g., noble metals as silver) to fulfill its final application. This behavior is controlled at last by the outmost surface of the coating. Therefore, the analytical characterization of these new materials requires high-resolution analytical techniques able to provide information about surface and depth composition down to the nanometric level. Among these techniques are Rutherford backscattering spectrometry (RBS), glow discharge optical emission spectroscopy (GDOES), and angle resolved X-ray photoelectron spectroscopy (ARXPS). In this work, we present a comparative RBS-GDOES-ARXPS study of the surface characterization of Ag-TiCN coatings with Ag/Ti atomic ratios varying from 0 to 1.49, deposited at room temperature and 200 °C. RBS analysis allowed a precise quantification of the silver content along the coating with a non-uniform Ag depth distribution for the samples with higher Ag content. GDOES surface profiling revealed that the samples with higher Ag content as well as the samples deposited at 200 °C showed an ultrathin (1-10 nm) Ag-rich layer on the coating surface followed by a silver depletion zone (20-30 nm), being the thickness of both layers enhanced with Ag content and deposition temperature. ARXPS analysis confirmed these observations after applying general algorithm involving regularization in addition to singular value decomposition techniques to obtain the concentration depth profiles. Finally, ARXPS measurements were used to provide further information on the surface morphology of the samples obtaining an excellent agreement with SEM observations when a growth model of silver islands with a height d = 1.5 nm and coverage θ = 0.20 was applied to the sample with Ag/Ti = 1.49 and deposited at room temperature.

Atomic-scale flattening mechanism of 4H-SiC (0 0 0 1) in plasma assisted polishing

Plasma assisted polishing (PAP), in which the irradiation of atmospheric pressure water vapor plasma and ceria (CeO2) abrasive polishing are combined, is a novel finishing technique for single-crystal silicon carbide (4H-SiC). An atomically flat 4H-SiC surface (rms about 0.2nm) with a well-ordered step/terrace structure was obtained by PAP. Cross-sectional transmission electron microscopy (XTEM) observation revealed that plasma oxidation atomically flattened the interface of SiO2/SiC. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurement results showed the existence of a thin silicon oxycarbide layer, which is corrosion-resistant to hydrofluoric acid, at the interface. The combination of water vapor plasma oxidation and the mechanical removal of silicon oxide as well as silicon oxycarbide layers by a soft abrasive is effective in obtaining an atomically flat surface of 4H-SiC (0001) without introducing crystallographic subsurface damage.

Ultrathin Antifouling Coatings with Stable Surface Zwitterionic Functionality by Initiated Chemical Vapor Deposition (iCVD)

Antifouling thin films of poly[N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)-co-2-(dimethylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] (PDDE) were synthesized via a substrate-independent and all-dry-initiated chemical vapor deposition (iCVD) technique followed by a diffusion-limited vapor-phase reaction with 1,3-propane sultone. Coated surfaces exhibited very low absorption of various foulants including bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA), as measured with the quartz crystal microbalance with dissipation monitoring (QCM-D). The fouling by humic acid was dependent on the presence of divalent cations such as Ca(2+). Both depth profiling and angle-resolved X-ray photoelectron spectroscopy (XPS) measurements indicated that the zwitterionic groups were highly concentrated in the top ~3 nm of the film. The contact angle measurements revealed a limited degree of surface chain reorganization upon contacting water. The dynamic contact angles remained unchanged after 100 days of storage in air, indicating the stability of the interface. The coating was substrate-independent, and the film was conformal on surface nanostructures including trenches, reverse osmosis membranes, and electrospun nanofiber mats.

A Study of Chemical Modifications of a Nafion Membrane by Incorporation of Different Room Temperature Ionic Liquids

AbstractSurface and bulk chemical changes in a Nafion membrane as a result of room temperature ionic liquids (RTILs) incorporation were determined by X‐ray photoelectron spectroscopy (XPS) and elemental analysis, respectively. RTILs with different physicochemical properties were selected. Two imidazolium based RTIL‐cations (1‐octyl‐3‐methylimidazolium and 1‐butyl‐3‐methylimidazolium) were used to detect the effect of cation size on membrane modification, while the effect of the RTIL hydrophilic/hydrophobic character was also considered by choosing different anions. Angle resolved XPS measurements (ARXPS) were carried out varying the angle of analysis between 15° and 75° to get elemental information on the Nafion/RTIL‐modified membranes interactions for a deepness of around 10 nm. Moreover, changes in the RTIL‐modified membranes associated to thermal effect were also considered by analyzing the samples after their heating at 120 °C for 24 h. Agreement between both chemical techniques, bulk and destructive elemental analysis and surface and non‐destructive XPS, were obtained.

Understanding the role of Si doping on surface charge and optical properties: Photoluminescence study of intrinsic and Si-doped InN nanowires

In the present work, the photoluminescence (PL) characteristics of intrinsic and Si-doped InN nanowires are studied in detail. For intrinsic InN nanowires, the emission is due to band-to-band carrier recombination with the peak energy at $\ensuremath{\sim}$0.64 eV (at 300 K) and may involve free-exciton emission at low temperatures. The PL spectra exhibit a strong dependence on optical excitation power and temperature, which can be well characterized by the presence of very low residual electron density and the absence or a negligible level of surface electron accumulation. In comparison, the emission of Si-doped InN nanowires is characterized by the presence of two distinct peaks located at $\ensuremath{\sim}$0.65 and $\ensuremath{\sim}$0.73--0.75 eV (at 300 K). Detailed studies further suggest that these low-energy and high-energy peaks can be ascribed to band-to-band carrier recombination in the relatively low-doped nanowire bulk region and Mahan exciton emission in the high-doped nanowire near-surface region, respectively; this is a natural consequence of dopant surface segregation. The resulting surface electron accumulation and Fermi-level pinning, due to the enhanced surface doping, are confirmed by angle-resolved x-ray photoelectron spectroscopy measurements on Si-doped InN nanowires, which is in direct contrast to the absence or a negligible level of surface electron accumulation in intrinsic InN nanowires. This work elucidates the role of charge-carrier concentration and distribution on the optical properties of InN nanowires.

Preparation and characterization of nano-SiO2/fluorinated polyacrylate composite latex via nano-SiO2/acrylate dispersion

The organic nano-SiO2/acrylate dispersion in which methyl methacrylate (MMA) and butyl acrylate (BA) in the micelles act as dispersing media of nano-SiO2 was prepared by sol–gel method with tetraethyl orthosilicate (TEOS) as precursor, hydrochloric acid as catalyst and methacryloylpropyl trimethoxysilane (MPS) as modifier. Subsequently, the nano-SiO2/fluorinated polyacrylate composite latex was synthesized by two-stage semi-continuous starved emulsion polymerization based on the organic nano-SiO2/acrylate dispersion. The SiO2/fluorinated polyacrylate composite latex and the resultant films were characterized by Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), Zeta potential (ζ), angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and contact angle measurements (CA). The nano-SiO2/fluorinated polyacrylate composite latex possessed very narrow particle size distributions and good latex stability. Fluorinated groups had a strong tendency to migrate to film surface and the surface hydrophobicity of the resultant films was remarkably improved. The mass ratio of methyl methacrylate to butyl acrylate (MMA/BA) has an important influence on the hydrophobicity of the composite films. When the content of dodecafluoroheptyl methacrylate (DFHMA) was 6wt% and the mass ratio of MMA/BA was 1/3, the water contact angle could reach 121°. The possible mechanism of migration and enrichment of fluorinated groups to the film surface under different MMA/BA mass ratio was proposed.

SERS, XPS, and DFT Study of Adenine Adsorption on Silver and Gold Surfaces.

The adsorption of adenine on silver and gold surfaces has been investigated combining density functional theory calculations with surface-enhanced Raman scattering and angle-resolved X-ray photoelectron spectroscopy measurements, obtaining useful insight into the orientation and interaction of the nucleobase with the metal surfaces.

Behavior of N atoms after thermal nitridation of Si1 − xGex surface

Behavior of N atoms after thermal nitridation of Si1−xGex (100) surface in NH3 atmosphere at 400°C was investigated. X-ray photoelectron spectroscopy (XPS) results show that N atomic amount after nitridation tends to increase with increasing Ge fraction, and amount of N atoms bonded with Ge atoms decreases by heat treatment in H2 at 400°C. For nitrided Si0.3Ge0.7(100), the bonding between N and Si atoms forms Si3N4 structure whose amount is larger than that for nitrided Si(100). Angle-resolved XPS measurements show that there are N atoms not only at the outermost surface but also beneath surface especially in a deeper region around a few atomic layers for the nitrided Si(100), Si0.3Ge0.7(100) and Ge(100). From these results, it is suggested that penetration of N atoms through around a few atomic layers for Si, Si0.3Ge0.7 and Ge occurs during nitridation, and the N atoms for the nitrided Si0.3Ge0.7(100) dominantly form a Si3N4 structure which stably remains even during heat treatment in H2 at 400°C.

XPS and UPS study on band alignment at Pt–Zn-terminated ZnO(0 0 0 1) interface

The interface between Pt and Zn-terminated ZnO(0001) was investigated by X-ray and ultra violet photoelectron spectroscopy in order to examine electronic band alignment. An angle-resolved X-ray photoelectron spectroscopy measurement of the clean ZnO(0001) surface has revealed a downward band bending by 0.25eV. The results of the valence band analysis show that the work function and the valence band maximum of clean ZnO(0001) were 4.49eV and 2.79eV, respectively. Platinum was then deposited in several deposition steps onto a clean ZnO(0001) surface up to 0.6nm in thickness. After the deposition, the binding energy of Zn 2p doublet peak was shifted towards lower value by 0.77eV, and the measured work function changed to 5.51eV. As a result, the Schottky barrier height of Pt/ZnO(0001) interface was 1.11eV.