UV-visible Spectroscopic Ellipsometry Research Articles

Oxides Film Formed on Fe- and Ni-Based Alloys: An Ellipsometry Insight

UV-visible spectroscopic ellipsometry was used to study the thickness and composition of the oxidized zone in Fe- and Ni-based alloys as a function of oxygen partial pressure. In the case of AISI 304 stainless steel, the weathered thickness increases with oxygen partial pressure, whereas in the case of Inconel 600, it appears to be independent of oxygen pressure. This trend is confirmed by the AFM measurements. For both materials studied, the oxygen-modified zone consists of two layers as confirmed by glow discharge optical emission spectrometry (GDOES) measurements. The thicknesses of these two layers vary differently on either side of an oxygen partial pressure of 0.1 Torr. In the case of AISI 304 stainless steel, the thickness of the Fe-rich outer layer decreases in favor of the Cr-rich inner layer. In the case of Inconel 600, the trend is reversed. The Ni-rich outer layer increases significantly above this critical pressure, while the Cr-rich inner layer decreases slightly. The composition of each layer in the oxidized zone is discussed in terms of its dielectric function in relation to reference material. The use of UV-visible ellipsometry as a non-destructive tool to study the structure and composition of the oxide bilayer of absorbing systems such as the alloys under investigation is a first.

Role of H3 + ions in deposition of silicon thin films from SiH4/H2 discharges: modeling and experiments

A 1D fluid model including a detailed chemistry for silane-hydrogen discharges as well as surface reactions to account for deposition and etching processes has been implemented to study the effects of gas pressure (1 to 3.5 Torr) and silane concentration (2 to 10%) on the deposition rate of silicon thin films in a standard RF-PECVD reactor. The thickness of the films and their deposition rate as functions of the process conditions were determined from optical modelling of UV-visible spectroscopic ellipsometry measurements. The experimental values of the deposition rate were compared with results from the 1D fluid model. SiH3 radicals are found to be the main contributor to the computed deposition rates, while H3 + ions play the main role in the etching process. The study reveals that etching by hydrogen ions must be taken into account to reproduce properly the experimental deposition rate. In particular etching by H3 + ions must be taken into account to achieve a good agreement between the experimental and modelled values of the deposition rate as a function of the total gas pressure and the silane fraction in the discharge.

Spectro-ellipsometric modeling and optimization of two-dimensional Ge layer and three-dimensional Ge dot/island structures on SiO2 substrates

Morphological structures of two-dimensional (2D) Ge thin films and three-dimensional (3D) Ge dots/islands grown on SiO2 substrates were analyzed with UV–visible spectroscopic ellipsometry. The pseudo-dielectric functions (〈ε〉 = 〈ε 1〉 + i〈ε 2〉) were calculated under the Bruggeman effective medium approximation. The 〈ε〉 spectra of 2D films were well fitted with a simulation assuming a single-layer or two-layer model. The single-layer model has a mixture of c-Ge, a-Ge, and void components. The two-layer model has a base-layer consisting of 100% c-Ge and a surface-roughness layer consisting of a mixture of c-Ge, a-Ge, and voids. In the case of 3D films, the overall shape of the 〈ε〉 spectra could only be reproduced by using a two-layer model: a base-layer consisting of mixtures of c-Ge and a-Ge with a surface-roughness layer consisting of c-Ge, a-Ge, and voids. The degree of disagreement correlated with the root mean square surface roughness measured by atomic force microscopy.

Improvement of corrosion resistance of Zn-Ni alloy coatings by anodizing in selected alcoholic solutions

The subject of this investigation is the anodic treatment of galvanic Zn-Ni alloy coatings in alkaline solutions of selected alcohols and their characterization. The process was carried out galvanostatically at an anodic current density in the range 0.06–6.25 A/dm2 for 2–60 minutes. The characterization of the formed coatings was based on the morphological studies (OM), determination of the chemical and phase composition (XPS and GIXD), as well as film thickness (UV–vis spectroscopic ellipsometry) and corrosion investigations (EIS). It was found that the coatings have a mixed oxide-alkoxide passive character with the alkoxide-rich layer on the top. The anodic oxidation led to the coloration of the alloy; various colors (beige, blue, and violet) could be obtained depending on the process parameters. The colors were strongly related to the thickness of the films, which is similar to the passive layers found on valve metals (e.g. titanium). The thickest of the conversion coatings had ca. 250 nm. The EIS investigations showed the improvement of corrosion resistance of the substrate after the anodic treatment.

Spectroscopic ellipsometry study of gold nanostructures for LSPR bio-sensing applications

This main aim of this work is the development of optical biosensors based on the LSPR phenomenon in nano-structured gold films suitable for detection of low molecular weight analytes such as mycotoxins. A simple technology of annealing thin gold films was utilized for the formation of gold nano-islands exhibiting the LSPR effect. The morphology of gold nano-structures produced was studied with SEM and AFM, and their optical properties were analysed with UV-visible absorption spectroscopy and spectroscopic ellipsometry (SE). The position of LSPR band appeared to depend on the dimensions of gold nano-islands. The dependence of the LSPR band spectral shift on the refractive index of a medium was studied with both UV-vis absorption and SE, and the refractive index sensitivity (RIS) was evaluated. The method of SE gave from two to three times higher values of RIS as compared to those obtained by absorption spectroscopy. LSPR bio-sensing tests were attempted using total internal reflection ellipsometry (TIRE); a noticeable spectral shift was recorded on course of immune binding of Aflatoxin B1 to specific antibodies immobilized on the surface of gold.

Etching of a-Si:H thin films by hydrogen plasma: a view from in situ spectroscopic ellipsometry.

When atomic hydrogen interacts with hydrogenated amorphous silicon (a-Si:H), the induced modifications are of crucial importance during a-Si:H based devices manufacturing or processing. In the case of hydrogen plasma, the depth of the modified zone depends not only on the plasma processing parameters but also on the material. In this work, we exposed a-Si:H thin films to H2 plasma just after their deposition. In situ UV-visible spectroscopic ellipsometry measurements were performed to track the H-induced changes in the material. The competition between hydrogen insertion and silicon etching leads to first order kinetics in the time-evolution of the thickness of the H-modified zone. We analyzed the correlation between the steady state structural parameters of the H-modified layer and the main levers that control the plasma-surface interaction. In comparison with a simple doped layer, exposure of a-Si:H based junctions to the same plasma treatment leads to a thinner H-rich subsurface layer, suggesting a possible charged state of hydrogen diffusing.

Effects of germane flow rate in electrical properties of a-SiGe:H films for ambipolar thin-film transistors

In this work, the study of germane flow rate in electrical properties of a-SiGe:H films is presented. The a-SiGe:H films deposited by low frequency plasma-enhanced chemical vapor deposition at 300°C were characterized by Fourier transform infrared spectroscopy, measurements of temperature dependence of conductivity and UV–visible spectroscopic ellipsometry. After finding the optimum germane flow rate conditions, a-SiGe:H films were deposited at 200°C and analyzed. The use of a-SiGe:H films at 200°C as active layer of low-temperature ambipolar thin-film transistors (TFTs) was demonstrated. The inverted staggered a-SiGe:H TFTs with Spin-On Glass as gate insulator were fabricated. These results suggest that there is an optimal Ge content in the a-SiGe:H films that improves its electrical properties.

Spectroscopic ellipsometry studies of sol-gel-derived Cu-doped ZnO thin films

Undoped and Cu-doped ZnO (Zn1−xCuxO: x=0.00, 0.03, 0.06, 0.10) films were prepared on Si(100) substrates by the sol-gel method, and the effects of Cu content on the structural, and optical properties of these films were investigated by X-ray diffraction (XRD) and UV–visible spectroscopic ellipsometry (SE). XRD patterns revealed that all the films were monophasic and had wurtzite structure with (002) preferential orientation along c-axis, indicating there were not any secondary phases. The measured UV–visible SE spectra were analyzed with an appropriate procedure to accurately determine the thickness and the optical constants. We found that the refractive index and the extinction coefficient increase with Cu content increasing. The optical band gap (Eg) has been calculated by classical Tauc formula, and the Eg values decrease as Cu content increasing. The variation in Eg with different Cu content revealed that the Eg value was affected by doping Cu. These mean that we can control the optical properties of the ZnO films by varying Cu content, which might be important in view of designing integrated optic devices.

Yeast Cytochrome c Monolayer on Flat and Nanostructured Gold Films Studied by UV–Vis Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) has been used to investigate the adsorption of yeast cytochrome c (YCC) on gold films. YCC is a prototypical molecular system to evaluate the sensitivity and reliability of optical methods in probing conformational changes upon adsorption at surfaces. Measurements have been performed in situ in aqueous solutions at room temperature. We have applied to monolayer-thick YCC films the same scheme of analysis of SE spectra used in our recent works on thiolate self-assembled monolayers on gold, exploiting the so-called δΨ and δΔ difference spectra. A comparison between δΨ and δΔ spectra obtained from YCC adsorption on flat and cluster-assembled nanostructured gold films, obtained by supersonic cluster beam deposition is proposed. For both kinds of substrate δΨ and δΔ, difference spectra show well-defined features related to molecular optical absorptions typical of the so-called heme group, in particular the Soret band at about 410 nm. These features appear in the same position found for molecules in solution, suggesting that YCC native conformation is maintained upon adsorption also in the case of a substrate with typical roughness at the nanoscale.

Study of the effects of different fractions of large grains of μc-Si:H:F films on the infrared absorption on thin film solar cells

We have studied microcrystalline silicon films (μc-Si:H:F) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) at low temperature (∼200°C) from SiF4, H2 and Ar gas mixtures. We found that the H2/SiF4 ratio plays an important role on the crystalline fraction of the films, specifically on the films grain size, deduced from UV–vis spectroscopic ellipsometry. We observed that higher crystallinity (and larger fraction of large-grains) in the films provides a higher absorption on the infrared (IR) region, which is of particular interest for tandem solar cells. PIN solar cells were fabricated with intrinsic μc-Si:H:F films of different crystallinity (and different fraction of large grains) in order to observe their effect on the solar cells IR absorption and on the cells performance characteristics.

Investigation of annealing-treatment on structural and optical properties of sol-gel-derived zinc oxide thin films

The transparent ZnO thin films were prepared on Si(100) substrates by the sol-gel method. The structural and optical properties of ZnO thin films, submitted to an annealing treatment in the 400–700°C ranges are studied by X-ray diffraction (XRD) and UV-visible spectroscopic ellipsometry (SE). XRD measurements show that all the films are crystallized in the hexagonal wurtzite phase and present a random orientation. Three prominent peaks, corresponding to the (100) phase (2θ ≈ 31.8°), (002) phase (2θ ≈ 34.5°), and (110) phase (2θ ≈ 36.3°) appear on the diffractograms. The crystallite size increases with increasing annealing temperature. These modifications influence the optical properties. The optical constants and thickness of the films have been determined by analysing the SE spectra. The optical bandgap has been determined from the extinction coefficient. We found that the refractive index and the extinction coefficient increase with increasing annealing temperature. The optical bandgap energy decreases with increasing annealing temperature. These mean that the optical quality of ZnO films is improved by annealing.

Electronic properties of ZnO field-effect transistors fabricated by spray pyrolysis in ambient air

We report the application of spray pyrolysis (SP) for the deposition of high quality zinc oxide (ZnO) films and the fabrication of thin-film transistors. The chemical, structural, optical, and electronic properties of as-deposited ZnO films are studied using infrared spectroscopy, atomic force microscopy, UV-visible spectroscopic ellipsometry, and field-effect measurements. SP ZnO films are found to be uniform and polycrystalline with a band gap of 3.32 eV. ZnO transistors exhibit n-channel characteristics with electron mobility in the range 10–22 cm2/Vs. Device performance is found to depend on the work function of source/drain metal electrodes and on the device architecture employed.

Structural and optical properties of epitaxial CaxBa1−xNb2O6 thin films grown on MgO by pulsed laser deposition

In the present work, we investigate in detail the effect of both the deposition temperature and the cooling rate on the growth of calcium barium niobate (CBN) thin films on a magnesium oxide (MgO) single crystal substrate. As revealed by x-ray diffraction measurements, epitaxy occurs for films deposited at 800°C with a slow in situ cooling rate. We have also investigated the chemical composition of the films by way of a Rutherford backscattering spectroscopy technique and of an elastic recoil detection analysis, which reveal a good stoichiometry for our samples. In addition, optical analysis (UV-visible spectroscopy and spectroscopic ellipsometry) is performed on both polycrystalline and epitaxial films. A high transmittance (>85%) and a refractive index close to that observed for bulk crystal are obtained for epitaxial CBN films deposited at a low oxygen pressure. X-ray reflectivity measurements have enabled us to correlate the density of the films with their refractive indices.

VUV processing of polymers: Surface modification and deposition of organic thin films

N-rich organic surfaces have been obtained by means of vacuum ultraviolet (VUV) processing, via the functionalization of low-density polyethylene (LDPE) surfaces and also by polymerization of hydrocarbons (CH 4 and C 2H 4) mixed with ammonia (NH 3). The fundamental mechanisms involved in grafting reactions, as well as in the polymerization of these binary mixtures are discussed as a function of wavelength, λ, of the VUV sources. The surface compositions and deposition rates are characterized by X-ray photoelectron spectroscopy (XPS) and by UV–vis spectroscopic ellipsometry, respectively. We show that photochemical reactions can be optimized by judiciously selecting λ of the VUV source, so that maximum absorption and photo-dissociation can be achieved.

Deposition of dielectrics using a matrix distributed electron cyclotron resonance plasma enhanced chemical vapor deposition system

We have studied the deposition of silicon oxide, oxynitrides and silicon nitride in a novel, scalable, low-pressure high-density plasma system based on the ECR effect and the matrix arrangement of microwave antennas. Silane, nitrogen and oxygen gases were used as precursors. Films were deposited onto different heated and unheated substrates, including silicon, glass and polymer. The influence of the magnetic field configuration, gas flows, reactor pressure, microwave power and radio frequency bias on the uniformity and properties of the films was studied. Ex-situ and in-situ UV-visible spectroscopic phase-modulated ellipsometry and FTIR spectroscopy were used to study the properties of materials and the uniformity of the depositions, along with electrical measurements to evaluate the dielectric properties of the films. Optimizing the magnetic field configuration and applying an RF bias, we were able to achieve an uniformity better than 3.3% across the 200 mm wafers. Stoichiometric silicon oxide films with index values matching that of thermal silica were grown on unheated substrates with rates exceeding 2 nm/s.

Structure of diamond-like carbon films containing transition metals deposited by reactive magnetron sputtering

The addition of transition metal (Mo, Nb, Ti, W) atoms within the matrix of diamond-like carbon (DLC) films leads to the improving of their mechanical properties. In this work, we study the effects of the different metal induced structures on the electrical, optical and mechanical properties of DLC thin films. The samples were prepared at room temperature by pulsed-DC reactive magnetron sputtering, as a suitable way to vary their composition. X-ray photoelectron spectroscopy (XPS) evidenced a smooth variation of the composition with relative methane flow. Metal-DLC (Me-DLC) films are generally uniform in composition, as confirmed by secondary ion mass spectrometry (SIMS). Cross-section micrographs of the films, observed by transmission electron microscopy (TEM), showed relevant structural differences, which led to strong variations of physical properties. Among these variations, we can point to a better control of roughness, measured by atomic force microscopy (AFM), and stress. Indentation essays on W-DLC films have been performed in order to obtain the dependence of hardness with W concentration. Optical properties determined by UV–VIS spectroscopic ellipsometry evidenced the influence of methane dilution. The electrical measurements show a thermally activated conductivity, whose range of values and activation energy depend on the metal abundance. Me-DLC films suppose an advance in hard and wear-resistant coatings development.

Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements

The dielectric functions of amorphous and polymorphous silicon films prepared under various plasma conditions have been deduced from UV-visible spectroscopic ellipsometry measurements. The measured spectra have been firstsimulated by the use of the Tauc-Lorentz dispersion model and then the compositions of the films have been obtained by the use of the tetrahedron model combined with the Bruggeman effective medium approximation. This approach allows us to determine the hydrogen content, the crystalline fraction, and the void fraction of the films. This is particularly important in the case of polymorphous films in which the low crystalline fraction (below 10%) can only be detected when an accurate description of the effects of hydrogen on the dielectric function through the tetrahedron model is considered. The hydrogen content and film porosity deduced from the analysis of the spectroscopic ellipsometry measurements are in excellent agreement with the hydrogen content and film density deduced from combined elastic recoil detection analysis and Rutherford backscattering spectroscopy measurements. Moreover, despite their high hydrogen content (∼15%-20%) with respect to hydrogenated amorphous silicon films deposited at the same temperature (8%), polymorphous silicon films have a high density, which is related to their very low void fraction.

From Hybrid Films to Mesoorganized Multi-metal-oxide Nanocrystalline Films (M3NF), preparation and characterization.

ABSTRACTHigh quality and reproducible thin silica and non-silica mesoporous films were obtained with mono-oriented organised mesoporosity, fitting various symmetry groups (i.e. p6m, Pm3n, Im3m, …). The Evaporation Induced Self Assembly (EISA) mechanism of structuration was studied through in-situ time-resolved SAXS, interferometry and spectroscopic ellipsometry investigations during dip-coating. A Modulable Steady State (MSS) was found during which the system is in quasi-equilibrium with its environment and during which the final structure is formed. The corresponding Self-Assembly mechanism was found to be governed by a competition between evaporation, micellization and condensation that depend on various critical chemical and processing parameters the influence of which have been summarized into directly usable phase diagrams. Calcined SiO2 and TiO2 mesoporous films structure (pore size and anisotropy, porous volume and surface) and mechanical properties (young modulus) were investigated by UV-Visible spectroscopic ellipsometry. Finally, thanks to a specially designed block copolymer, one further step was accomplished by achieving the dip coating and controlled nano-crystallisation of various metallic and multi-metal-oxides films (M3NF) of composition CoxTi(1-x)O(2-x) with ilmenite or doped Anatase structure, and SrTiO3 and MgTa2O6 with perovskite structure. Attending the well known magnetic, dielectric and catalytic properties of these structures, M3NF are highly compatible with high technology applications in microelectronic, energy transfer devices, spintronic, nano-mechanical adjustments, data storage, oxide fuel cells …

Ellipsometric characterization of expanding thermal plasma deposited SiO 2-like films

The hexamethyldisiloxane/oxygen-fed expanding thermal plasma deposition technique was used for a step-by-step tailoring of the film chemistry from C-rich polymeric films (so called silicone-like, SiC x H y O z ) to SiO 2-like films. Information related to the chemical composition of the film was obtained by means of infrared spectroscopy. In situ ellipsometry (632.8 nm) monitored the film growth in real time, providing the deposition rate and refractive index. On the basis of ex situ UV–Vis spectroscopic ellipsometry, an optical model is proposed in order to determine film porosity and to explain the presence of a small absorption in SiO 2-like films, attributed to residual carbon presence.

Experimental study of surface electronic properties of ECR-PECVD deposited a-C 1− xN x:H films using UPS, XPS and spectroscopic ellipsometry

Electronic properties of a-C 1− x N x :H (0< x<0.1) films deposited using ECR-plasma decomposition of C 2H 2/N 2 precursor gases have been investigated by combining photoelectron spectroscopy (PES) and UV–Vis spectroscopic ellipsometry (SE). Estimates of band positions with respect to the Fermi-level have been obtained. Ultraviolet photoemission spectroscopy (UPS) supplies the work function and the valence band maximum of the a-C 1− x N x :H surface, for as entered, heated and sputter cleaned samples. The extraction of the conduction band minimum from the combination of Tauc/ E 04 gap from SE analysis and the UPS valence band maximum is investigated. Furthermore, the most promising results from the current investigation, is the positioning of π states from data analysis of the UPS spectra, and π ∗ states from the combination of UPS and SE. The results show that a-C:H is p-type, lightly N doped a-C 1− x N x :H is n-type, while in the higher N doped a-C 1− x N x :H the Fermi-level seems to be pinned near the midgap.