Hans Arwin

Infrared dielectric functions and phonon modes of high-quality ZnO films

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Imaging ellipsometry revisited: Developments for visualization of thin transparent layers on silicon substrates

Imaging ellipsometry is presented as a technique for quantification and visualization of the lateral thickness distribution of thin (0–30 nm) transparent layers on solid substrates. The main advantage of imaging ellipsometry is that every point on a surface is measured at the same time with a high lateral resolution. The method is based on the use of combined null and off‐null ellipsometry at an incident angle close to the pseudo‐Brewster angle of a high index substrate such as silicon. In the present experimental setup, a xenon lamp, a collimator, and a wavelength‐selective filter provide an expanded collimated probe beam with a diameter of 25 mm. Other major components in the system are a polarizer, a compensator, and an analyzer. In this way, a 15×30 mm2 image of a sample surface can be focused onto a charge‐coupled‐device video camera and transferred to a computer for further evaluation by image processing. Thickness measurements are performed for calibration purposes with ordinary null ellipsometry. ...

Imaging surface plasmon resonance sensor based on multiple wavelengths: Sensitivity considerations

A new, multiple wavelength surface plasmon resonance apparatus for imaging applications is presented. It can be used for biosensing, e.g., for monitoring of chemical and biological reactions in real time with label-free molecules. A setup with a fixed incident angle in the Kretschmann configuration with gold as the supporting metal is described, both theoretically and experimentally. Simulations of the sensor response based on independently recorded optical (ellipsometric) data of gold show that the sensitivity for three-dimensional recognition layers (bulk) increases with increasing wavelength. For two-dimensional recognition layers (adlayer) maximum sensitivity is obtained within a limited wavelength range. In this situation, the rejection of bulk disturbances, e.g., emanating from temperature variations, decreases, with increasing wavelength. For imaging surface plasmon resonance the spatial resolution decreases with increasing wavelength. Hence, there is always a compromise between spatial resolution, bulk disturbance rejection, and sensitivity. Most importantly, by simultaneously using multiple wavelengths, it is possible to maintain a high sensitivity and accuracy over a large dynamic range. Furthermore, our simulations show that the sensitivity is independent of the refractive index of the prism

Total internal reflection ellipsometry: principles and applications

A concept for a measurement technique based on ellipsometry in conditions of total internal reflection is presented. When combined with surface plasmon resonance (SPR) effects, this technique becomes powerful for monitoring and analyzing adsorption and desorption on thin semitransparent metal films as well as for analyzing the semitransparent films themselves. We call this technique total internal reflection ellipsometry (TIRE). The theory of ellipsometry under total internal reflection combined with SPR is discussed for some simple cases. For more advanced cases and to prove the concept, simulations are performed with the Fresnel formalism. The use of TIRE is exemplified by applications in protein adsorption, corrosion monitoring, and adsorption from opaque liquids on metal surfaces. Simulations and experiments show greatly enhanced thin-film sensitivity compared with ordinary ellipsometry.

A spectroscopic ellipsometry study of cerium dioxide thin films grown on sapphire by rf magnetron sputtering

Variable angle spectroscopic ellipsometry (VASE) has been used in the photon energy range 1.25–5.0 eV to study the structure and optical properties of cerium dioxide (CeO2) films. Both amorphous and highly oriented crystalline films were grown on sapphire by rf magnetron sputtering. The crystallinity, chemical structure, and surface morphology of the films were studied by x‐ray diffraction, x‐ray photoelectron spectroscopy, and atomic force microscopy, respectively. The measured VASE spectra on a series of films with different thicknesses were analyzed by using multiple optical models. In this way, the complex refractive index N=n+ik of CeO2, the film thicknesses, and the surface roughness of the different films could be determined. The ellipsometrically deduced refractive index spectrum was observed to be strongly dependent on the film structure. Highly oriented crystalline CeO2 films exhibited a higher refractive index and a higher band gap energy than the amorphous film. The surface roughness of the cr...

Optical properties of MgH2 measured in situ by ellipsometry and spectrophotometry

The dielectric properties of alpha-MgH2 are investigated in the photon energy range between 1 and 6.5 eV. For this purpose, a sample configuration and experimental setup are developed that allow bo ...

Ellipsometry on thin organic layers of biological interest : characterization and applications

The thickness resolution and in situ advantage of ellipsometry make this optical technique particularly suitable for studies of thin organic layers of biological interest. Early ellipsometric studies in this area mainly provided thickness quantification, often expressed in terms of surface mass. However, today it is possible to perform monolayer spectroscopy, e.g. of a protein layer at a solid/liquid interface, and also to resolve details in the kinetics of layer formation. Furthermore, complicated microstructures, like porous silicon layers, can be modeled and protein adsorption can be monitored in such layers providing information about pore filling and penetration depths of protein molecules of different size and type. Quantification of adsorption and microstructural parameters of thin organic layers on planar surfaces and in porous layers is of high interest, especially in areas like biomaterials and surface-based biointeraction. Furthermore, by combining ellipsometric readout and biospecificity, possibilities to develop biosensor concepts are emerging. In this report we review the use of ellipsometry in various forms for studies of organic layers with special emphasis on biologically-related issues including in situ monitoring of protein adsorption on planar surfaces and in porous layers, protein monolayer spectroscopy and ellipsometric imaging for determination of thickness distributions. Included is also a discussion about recent developments of biosensor systems and possibilities for in situ monitoring of engineering of multilayer systems based on macromolecules.

Spectroscopic ellipsometry studies of the optical properties of doped poly(3,4-ethylenedioxythiophene): an anisotropic metal

Abstract Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer with high electrical conductivity in the doped state, good thermal and chemical stability and fast electrochemical switching. The material is used as an antistatic layer, and has potential for use as a (transparent) electrode material and for electrochromic applications. Thin films ( μ m) of doped PEDOT have been studied with variable angle spectroscopic ellipsometry. Reflectance and transmission measurements have also been performed. From ellipsometry and transmission data, the optical properties and thickness of the PEDOT layers were extracted. By simultaneously fitting optical functions for multiple samples using a recently discussed 4×4 matrix formalism, the analysis reveals that the material is optically anisotropic and of uniaxial character with the optic axis normal to the film surface. The uniaxial anisotropy of PEDOT films reveals an entirely different appearance for the ordinary and extraordinary indices of refraction with the former showing metallic character. The metallic state behavior agrees well with the high electrical conductivity measured for this material at different doping levels. From the measured optical properties and from supplementary optical and structural analysis, a preferential structural orientation of the polymer chains can be deduced.

Optical optimization of polyfluorene-fullerene blend photodiodes

Blends of polyfluorene-fullerenes are promising materials for polymer-based photovoltaic devices (PPVD). Using spectroscopic ellipsometry we deduce the dielectric function for the blend of the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and the alternating polyfluorene copolymer, poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] DiO-PFDTBT (4:1 by weight), for the wavelength interval 250–1300nm. n reaches above 2 and saturates to 1.9 for high wavelengths. Absorption starts at 720nm (1.72eV) and reaches a crest around 550nm (2.25eV). The spin coating introduces anisotropy in the blend, manifested in birefringence as well as in dichroism. The dielectric function for the blend versus its constituents is not additive. There are indications that the constituents lost their dielectric identity, as screening cannot explain the experimental data. Simulations of optical absorption inside a PPVD are performed for both monochromatic and polychromatic ...

Spectroscopic ellipsometry and biology : recent developments and challenges

Abstract In surface biology there are numerous studies carried out using single wavelength ellipsometry, especially in the area of macromolecular adsorption on solid surfaces. The results obtained contribute significantly to the understanding of the basic mechanisms of adsorption and surface dynamics of organic molecules, especially of proteins. An example of an area of great importance is biomaterials, where ellipsometry is used as a tool in the process of acquiring knowledge about the biological acceptance of new as well as currently used implant materials. In the area of affinity biosensors, ellipsometry has been suggested as a potential readout principle. Ellipsometry is also a tool in emerging technologies, such as surface molecular engineering with the aim to construct molecular superstructures with predesigned biological functions and to interface biology with electronics. However, in most cases when ellipsometry is applied in biology, it has been used for surface mass determination. The potential in using spectroscopic data for resolving microstructural and dynamic information has not been exploited fully. From the above perspective, this report reviews the use of spectroscopic ellipsometry for studies in surface biology and highlights the advantages it offers. Two main themes are developed. The first is spectroscopy on monolayers of macromolecules with emphasis on determination of their dielectric functions and microstructure. A specific example discussed is ferritin adsorption on gold. The results, including dynamics of both the surface mass and layer microstructure, indicate an adsorption model based on a two-state adsorption mechanism. The second theme is ellipsometrically based biosensor systems. The discussion covers aspects of what imaging ellipsometry can provide in this context and is exemplified by results from affinity biosensor and gas sensor systems.