Spectromicroscopy and internal photoemission spectroscopy of semiconductor interfaces

For decades, techniques based on synchrotron light sources have played a central role in solid interface research. This role has been recently enhanced by two factors: the commissioning of the third generation of sources, characterized by unprecedented levels of brightness, and the first utilization cases of another class of photon sources related to synchrotron facilities, the free electron lasers (FEL's). This review will first present some relevant examples of how the new facilities are changing the scene of interface research, most notably in the domain of spectromicroscopy. We will specifically illustrate how the crucial problem of the lateral fluctuations of interface properties is being attacked with both synchrotron-light and FEL techniques. Then, we will argue that the present applications are only marginally exploiting the amazing capabilities of the new sources. The main case to illustrate this point is coherence-sharpened x-ray imaging, a very promising and spectacular technique developed for medical radiology, which could find extremely interesting applications in interface research.     

In situ infrared spectroscopy at electrochemical interfaces

An insight into the in situ FTIR spectroscopy method as applied in Electrochemistry is given. The particular aspects inherent to the electrochemical method are described in a concise form. Selected examples cover the results of about the last 8 years, on a variety of systems including carbon monoxide, small organic molecules and double-layer components (hydrogen, anions and water). The experimental data refer mostly to adsorption on well-defined single-crystal surfaces. Analogies and differences with data from the metal/gas interface are discussed.     

Injection spectroscopy of localized states in thin insulating layers on semiconductor surfaces

An overview of charge injection techniques for studies of localized electron states in insulators is presented. These states control most important electrical properties of the materials, especially their thin layers and the multilayered structures containing them. Electronic properties of thin insulating layers are usually different from those of bulk material because in vicinity of an interface the thin film is subjected to action of structural strain, spatial confinement, extra energy release during growth or deposition, e.c. All these factors substantially affect the spectrum of localized electron states (intrinsic defects, dopants, impurities) and influence their spatial distribution. The interfaces often exhibit an unusual behaviour relative the defect generation and an impurity transport and segregation comparing with a bulk of insulator. The characterization of these states in thin-layered systems suffers from low sensitivity of both electron and optical spectroscopies to shallow electron states those have to be detected on the background of the signal from substrate. Nevertheless, the problem may be resolved by means of charge carrier injection into the insulating layer followed by monitoring of the charge becomes trapped. The localized states are detected by this way as a potential wells for the injected a mobile charge carriers. In thin layers the mean path of injected electron (hole) with respect to interaction with such a trap may greatly exceed thickness of the layer. In offers possibility to use linear modes for the trapping and detrapping analysis. We will show that under these conditions the charge-injection methods provide the most important phenomenological parameters of a localized state: the density per unit area, in-depth location, cross section for capture of carriers or recombination, energy position of the trapped carrier level in the insulator bandgap, e.c. On the base of these possibilities a non-destructive methods for characterization of extrinsic and intrinsic defects states are developed. Various charge injection and charge detection methods are compared. It shows unambiguously the advantages of using a semiconducting substrate for the insulator trapping studies. The limits of the linear models applicability are discussed in detail and the effects arising from the non-linear correlation phenomena are analyzed.     

Exploring catalytic solid/liquid interfaces by in situ attenuated total reflection infrared spectroscopy

In situ attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy has gained considerable attention as a powerful tool for exploring processes occurring at solid/liquid and solid/liquid/gas interfaces as encountered in heterogeneous catalysis and electrochemistry. Understanding of the molecular interactions occurring at the surface of a catalyst is not only of fundamental interest but constitutes the basis for a rational design of heterogeneous catalytic systems. Infrared spectroscopy has the exceptional advantage to provide information about structure and environment of molecules. In the last decade, in situ ATR-FT-IR has been developed rapidly and successfully applied for unraveling processes occurring at solid/liquid interfaces. Additionally, the kinetics of complex reactions can be followed by quantifying the concentration of products and reactants simultaneously in a non-destructive way. In this tutorial review we discuss some key aspects which have to be taken into account for successful application of in situ ATR-FT-IR to examine solid/liquid catalytic interfaces, including different experimental aspects concerned with the internal reflection element, catalyst deposition, cell design, and advanced experimental methods and spectrum analysis. Some of these aspects are illustrated using recent examples from our research. Finally, the potential and some limitations of ATR will be elucidated.     

Tight binding models for non ideal semiconductor interfaces

The electronic structure of the (100) Ge-GaAs is studied for a non ideal model which assumes a stoichiometrically disordered interface. By using an ETB model with a 1 s 3p basis, which suffices to describe reasonably the valence bands, and by employing a method of calculation based on the Surface Green Function Matching formalism a fairly simple calculation can be set up which explains adequately the experimentally established semiconductor nature of this interface.     

In situ study of electrodeposited thin layers by Mössbauer spectroscopy

New experimental method was developed to study the in situ electrochemical deposition process. The formation of ultra thin and submonolayers were observed and the atomic interaction in the layers and interlayers were determined. ⁵⁷Co deposited on Ag cathode in the thickness range of 0.2–15 ML showed different atomic interactions depending on the layer thicknesses. No oxide formation on the deposited layers and on submonolayers appeared. The Mössbauer method was first applied for in situ study of the electrolytic deposition of ⁵⁷Co. Further applications to other Mössbauer active isotopes are possible.     

Chromatographic interfaces for supersonic jet spectroscopy

Until recently, applications of supersonic jet spectroscopy to chemical analysis have been plagued by relatively poor detection limits and the lack of suitable interfaces to standard chromatographic techniques. Supersonic jet nozzles based upon new technologies have been developed which permit low picogram detection limits to be obtained and provide convenient interfaces for capillary gas chromatography. Extension of these methods to liquid and supercritical fluid chromatography may also be possible.     

In situ measurement of conformational changes in proteins at liquid interfaces by circular dichroism spectroscopy

A new circular dichroism (CD) spectroscopy technique for studying conformational changes in proteins in situ at the air-water interface is described. By using this technique, conformations of four proteins, viz., beta-casein, bovine serum albumin (BSA), lysozyme, and fibrinogen in the adsorbed state at the air-water interface have been studied. beta-Casein, which is predominantly in a disordered state in solution, assumes a beta-sheet conformation at the air-water interface. On the other hand, lysozyme and fibrinogen, which are alpha+beta-type proteins in solution, become beta-type proteins by completely transforming their alpha-helix structure into beta-sheets. Bovine serum albumin, which is an alpha-type protein in solution, loses its alpha-helix and becomes a disordered protein at the air-water interface. The results indicated that during unfolding and film formation at the interface, structural changes in proteins, regardless of their initial native state, follow the course of increasing beta-sheet and disordered structure and decreasing alpha-helix content. Although this seems to be the general trend, the exceptional case of BSA suggests, however, that this is not universal.     

Surface analysis of functional layers for semiconductor production

Summary Graphite components used as supports during the production of epi-layers need to be sealed by CVD-SiC coating. In connection with the scaling up of the SiC-coating process the requested quality, i.e. the chemical purity and the microstructure has to be guaranteed. Very large discrepancies have been observed between the various analytical methods (NAA, OES-ICP, SNMS, EPMA) on the determination of the impurities of those coatings in a concentration of some ppm and lower. The chemical analysis of such materials is not conventional and is the aim of future investigations. Using microstructural characterization methods a high quality of this coating material has been established for the mentioned application with a unidirectional crystal growth on the <111> direction of the cubic structure. This quality could also be produced in industrial scale using a large CVD-reactor.     

Modulated electron emission for structural characterization of buried layers and interfaces

Principles and performances of Primary-beam Diffraction Modulated Electron Emission are briefly reviewed. This approach exploits the spatial modulation of the beam electrons waves intensity in ordered atomic arrays, which in turn results in a marked dependence of the electron emission on the incidence angle. Detection of Auger electrons or inelastically backscattered electrons (ionization losses) provides chemical specificity. Surface sensitivity is related to the inelastic mean free path of the detected electrons. Examples are given of the capability of this technique for structural characterization of buried layer and interfaces. They include: i) epitaxial growth of Co oxide on the (001) surface of Co bct film, ii) subsurface behaviour of Co epitaxial layers during the bct→hcp phase transition.     

Structural control of surface layer proteins at electrified interfaces investigated by in situ Fourier transform infrared spectroscopy

In situ Fourier Transform Infrared (FTIR) Spectroscopy complemented by Electrochemical Quartz Microbalance (EQMB) investigations allowed a detailed insight into the influence of the electrode potential on competing adsorption processes and bonding mechanisms of buffer ions and S-layer protein molecules of Lysinibacillus sphaericus CCM2177 at an electrified liquid/gold interface. The S-layer proteins adsorb on gold polarized positively of the point of zero charge by displacing perchlorate anions in the Helmholtz plane by their carboxylate groups. This is indicated by an increase of the peptide and carboxylate infrared absorption signals accompanied by a decrease of the perchlorate signal. S-layers interlinked laterally with Ca(2+) ions, positive of the point of zero charge, resulted in the formation of a crystalline layer participating in the Helmholtz layer. In contrast to the absence of the Ca(2+)-linkers, S-layers remain structurally intact also in the negative polarization domain where the Helmholtz layer is solely sustained by mainly solvated cations without participation of the negatively charged protein carboxylate functions.     

Electrochemical behaviour of conducting polymers obtained into clay-catalyst layers. An in situ Raman spectroscopy study

In this paper we present the study of the electrochemical properties of the following conducting polymers: poly(o-anisidine), polyaniline and copolymers of aniline and o-anisidine obtained by a new synthetic method. The polymers are synthesized in free-of-acid conditions, using an activated montmorillonitic clay catalyst, known as Maghnite-H⁺ (Mag-H) as proton source. The electrochemical behaviour of poly(o-anisidine) created using Mag-H (PoANI-MagH) and their copolymers with aniline is quite different of those polymers created in HCl solution. In situ Raman data suggest that the structure of PoANI-MagH is a mixture of conducting (polyaniline-type) and redox (phenoxazine or phenazine-type) segments.     

Structure of mixed beta-lactoglobulin/pectin adsorbed layers at air/water interfaces; a spectroscopy study

Based on earlier reported surface rheological behaviour two factors appeared to be important for the functional behaviour of mixed protein/polysaccharide adsorbed layers at air/water interfaces: (1) protein/polysaccharide mixing ratio and (2) formation history of the layers. In this study complexes of beta-lactoglobulin (positively charged at pH 4.5) and low methoxyl pectin (negatively charged) were formed at two mixing ratios, resulting in negatively charged and nearly neutral complexes. Neutron reflection showed that adsorption of negative complexes leads to more diffuse layers at the air/water interface than adsorption of neutral complexes. Besides (simultaneous) adsorption of protein/polysaccharide complexes, a mixed layer can also be formed by adsorption of (protein/)polysaccharide (complexes) to a pre-formed protein layer (sequential adsorption). Despite similar bulk concentrations, adsorbed layer density profiles of simultaneously and sequentially formed layers were persistently different, as illustrated by neutron reflection analysis. Time resolved fluorescence anisotropy showed that the mobility of protein molecules at an air/water interface is hampered by the presence of pectin. This hampered mobility of protein through a complex layer could account for differences observed in density profiles of simultaneously and sequentially formed layers. These insights substantiated the previously proposed organisations of the different adsorbed layers based on surface rheological data.     

X-ray photoelectron spectroscopy for nondestructive analysis of buried interfaces

The possibility to use high energy X-ray photoelectron spectroscopy as a nondestructive method of in-depth chemical phase analysis of solids is demonstrated. Theoretical background and optimal experimental conditions of in-depth analysis are considered. Interfaces in 5 nm thick HfO₂ films synthesized by molecular layering and deposition of organometallic compounds from the gas phase were studied. It was shown that thickness of the layers in multilayer structures can be determined by measuring the intensity of photoelectron peaks.     

Relaxation of surfactants adsorption layers at liquid interfaces

The relaxation behaviour of surfactant layers provides a deep insight into the composition and structure of adsorbed layers at liquid interfaces. The development of professional experimental tools created a helpful basis for an increasing interest in these studies. In addition, the theoretical basis has been improved in many aspects such that for several surfactant systems a quantitative understanding is already possible. In particular the consideration of the changes in molar area of adsorbed molecules, introduced into the thermodynamics of adsorbed layers first by Fainerman in 1995, due to changes in the surface coverage allowed a considerably better, in many cases even quantitative understanding of the surface relaxation. Another important additional property, introduced into the thermodynamics and consequently also into the mechanisms of relaxation processes in interfacial layers, is the two-dimensional compressibility, important for the response to deformations of rather packed interfacial layers. The experimentally observed negative dilational viscosity is discussed only briefly and considered essentially in terms of experimental and theoretical shortcomings. The relaxation behaviour of nano- and microparticles, in literature often addressed is compounds able to act “instead of surfactants” are also addressed and some peculiarities discussed, while the obvious interrelation between the dilational rheology and stability of foams and emulsions is not analysed in detail.     

FTIR spectroscopy of buried interfaces in molecular junctions

We demonstrate that ATR-FTIR spectroscopy can be used to record high-quality vibrational spectra of molecules at buried interfaces in metal-molecule-silicon and metal-molecule-metal junctions. This provides quantitative information on the structure and conformation of molecules at buried interfaces, an issue of critical importance to molecular electronics. In the model systems of Au on octadecyltrichlorosilane self-assembled monolayer on Si or mecaptohexadecanoic acid multilayers on Au-covered Si, ATR-FTIR suggests that metal deposition leads to not only conformational disorder within the film but also the direct interaction of metal atoms/clusters with alkyl backbones.     

Optical characterisation of sol-gel optical composites

Sol-gel optical composites doped with optically active materials prepared by the post-doping method have been characterised in relation to their optical and structural properties. The dopants of interest have been found to be homogeneously distributed throughout the gel matrix. The in-situ impregnation of an index-matching polymer such as PMMA within the porous structure improves both the optical and mechanical properties of the composites significantly. The as-prepared materials have a non-porous feature and show a low optical loss.