Modulation Ellipsometry Research Articles

Demulsification of Silica Stabilized Pickering Emulsions Using Surface Freezing Transition of CTAC Adsorbed Films at the Tetradecane-Water Interface.

The adsorbed film of cetyltrimethylammonium chloride (CTAC) at the tetradecane (C14) - water interface undergoes a first-order surface transition from two-dimensional liquid to solid states upon cooling. In this paper, we utilized this surface freezing transition to realize a spontaneous demulsification of Pickering emulsions stabilized by silica particles. In the temperature range above the surface freezing transition, the interfacial tension of silica laden oil-water interface was lower than CTAC adsorbed film, hence, stable Pickering emulsion was obtained by vortex mixing. However, the interfacial tension of CTAC adsorbed film decreased rapidly below the surface freezing temperature and became lower than the silica laden interface. The reversal of the interfacial tensions between silica laden and CTAC adsorbed films gave rise to Pickering emulsion demulsification by the desorption of silica particles from the oil-water interface. The exchange of silica particles and CTAC at the surface of emulsion droplets was also confirmed experimentally by using phase modulation ellipsometry at the oil-water interface.

Nucleation of Surfactant-Alkane Mixed Solid Monolayer and Bilayer Domains at the Air-Water Interface.

We investigated the wetting transitions of tetradecane and hexadecane droplets in dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), and hexadecyltrimethylammonium bromide (C16TAB) aqueous solutions. By varying the surfactant concentration, the formation of mixed monolayers of a surfactant and an alkane was observed at the air–water interface. Depending on the combination of surfactant and alkane, these wetting monolayers underwent another thermal phase transition upon cooling either to a frozen mixed monolayer (S1) or a bilayer structure composed of a solid monolayer of a pure alkane rested on a liquid-like mixed monolayer (S2). Based on the phase diagrams determined by phase modulation ellipsometry, the difference in the morphology of the nucleated S1 and S2 phase domains was also investigated using Brewster angle microscopy. Domains of the S1 phase were relatively small and highly branched, whereas those of the S2 phase were large and circular. The difference in domain morphology was explained by the competition of the domain line tension and electrostatic dipole interactions between surfactant molecules in the domains.

Linear electro-optic effect in ferroelectric HfO2-based epitaxial thin films

Electro-optic (EO) modulators for silicon photonics using CMOS-compatible materials and processes are in great demand. In this study, epitaxial (100)-undoped HfO2 and Y-doped HfO2 thin films were fabricated on Sn-doped In2O3/yttria-stabilized zirconia(100) substrates at room temperature via magnetron sputtering. EO measurement of the Y-HfO2 film using modulation ellipsometry showed that the phase was changed by 180° after application of positive and negative poling biases, and the modulation amplitude increased linearly with increasing AC electric field, indicating a linear EO effect based on ferroelectricity. The observed results indicate that ferroelectric HfO2-based films are viable candidates for CMOS-compatible EO devices.

Large impact of strain on the electro-optic effect in (Ba, Sr)TiO3 thin films: Experiment and theoretical comparison

(001)-epitaxial (Ba0.5Sr0.5)TiO3 (BST) thin films with different magnitudes of compressive strain were fabricated on SrRuO3/SrTiO3 substrates by pulsed laser deposition, and their electro-optic (EO) properties were characterized by modulation ellipsometry at different temperatures. All fabricated films showed an increased paraelectric-to-ferroelectric phase transition temperature upon compressive strain and revealed c-domain structures in the ferroelectric phase. We experimentally clarified that the EO properties of compressively strained BST thin films are enhanced toward the phase transition temperature modified by the strain. The experimental results were compared with the theoretical prediction based on a phenomenological thermodynamic model. Although the measured EO coefficient rc was less than that theoretically predicted, the experimentally observed strain effect on the EO properties is in good qualitative agreement.

Spurious electro-optic coefficients inferred from modulation ellipsometry measurements in the presence of an air cavity

This paper describes how thin air gaps in multilayer polymer thin film structures can lead to unexpectedly large signals in modulation ellipsometry experiments, which can then be misinterpreted as the electro-optic effect. The contributions from the electro-optic effect and polarisation on reflection from the air cavity are indistinguishable and the reflection contribution can be on the order of 100 times that of the electro-optic effect. Caution must thus be exercised in any attempt to measure electro-optic coefficients with modulation ellipsometry in the presence of air gaps, to avoid spuriously high results. Thin film multilayer structures containing air gaps may be suitable for some of the same applications as electro-optic reflectance modulators.

Ellipsometry of TiO_x coatings deposited in a magnetron apparatus with an unbalanced magnetic system: effect of the oxygen concentration and the magnetron–substrate distance

The method of spectral modulation ellipsometry is used to investigate the structure and optical properties of thin TiOx coatings deposited by magnetron with various oxygen concentrations in the plasma, a working pressure of 0.28 Pa, and a discharge current of 5.6 A. It is established that, depending on the fraction of O2, three types of coatings are formed in the plasma: semitransparent three-layer coatings, transparent two-layer coatings, and transparent three-layer coatings. The last coatings have a dense contact layer up to 50 nm thick, whose optical characteristics correspond to crystalline anatase and/or rutile. A correlation is established between the roughness, the band gap, and the impurity concentration in the plasma. It is pointed out that, with 19 vol. % O2, coatings with a low refractive index (2.347), a small band gap (3.31 eV), and high roughness (16.8 nm) are deposited in the plasma, and this must increase the photocatalytic activity of the coatings.

The effect of bulky groups on the electro-optic coefficient [formula omitted] of a pyridine-donor nonlinear optical chromophore

The effect of bulky groups attached to pyridine-donor chromophores on the electro-optic coefficient r33 of host-guest polymers was measured by modulation ellipsometry. Overall, the experimental results indicated no significant beneficial effect of the bulky groups on r33. The maximum r33 achieved was just under 6pm/V for the plain chromophore, compared with the expected 20pm/V based on the molecular properties. Decomposition of the chromophore absorption band revealed significant aggregation, with the monomer fraction estimated to be marginally less than 1/3 for all chromophores studied. We attribute the lower achieved r33 values to increased molecular aggregation.

In situ calibration technique for photoelastic modulator in ellipsometry

The modulation amplitude of a phase modulator is calibrated under a fixed incident angle in photoelastic modulation (PEM) ellipsometry. In addition to its modulation amplitude (Δ o), we also calibrated its static phase retardation at 632.8 nm. The ellipsometric parameters of a sample were measured by a multiple harmonic intensity ratio technique and proved that ellipsometric parameters can be obtained under various modulation amplitudes. Since the physical size of PEM is constant, we calibrated the modulation amplitude at multiple wavelengths by setting the Δ o value at 0.383 for 568.2 nm. These calibrations provided enough information for establishing an in situ/real time spectroscopic ellipsometry.

Digital Modulation Ellipsometry in Monitoring the Growth of Thermooxide Layers

A unified method of upgrading the polarization–optical reflectometric schemes for continuously monitoring the thickness of surface oxide layers is developed on the basis of digital modulation ellipsometry. A phase-modulation attachment for universal automation and computerization of a wide class of reflectometers and hand-operated ellipsometers, is constructed. An automated modulation spectral ellipsometer with a direct digital detection of a signal from a photoreceiver based on the hand-operated Gartner L-119XUV ellipsometer was used in contactlessly monitoring the growth of thin nonuniform layers on the iron surface at an oxygen pressure of 10–4 to 1 mmHg and a low temperature (300°C). The accuracy of the device is 0.05°, and the spectral range is from 0.35 to 0.8 μm. The speed of response is mainly determined by the spectral analyzer, analog–digital transformer, and computer used.

Azimuth Alignment in Photoelastic Modulation Ellipsometry at a Fixed Incident Angle

A high intensity ratio technique is proposed to align all the azimuthal angles of a photoelastic modulation ellipsometric system in a fixed incident angle. This technique is used to determine the azimuthal orientations of the polarizer, the photoelastic modulator and the analyzer to the incident plane. Two pre-designed testing samples and two probing wavelengths for a thin film are introduced to locate the incident plane by the intensity ratio technique in a polarizer-sample-analyzer ellipsometric setup. Then, the azimuth of the photoelastic modulator is directly obtained from two DC radiances separated by 45°. In addition to azimuth deviation, the ellipsometric parameter Ψ can also be determined from the same measurement for determining the incident angle on-line.

Extension of Phase Modulation Ellipsometry to Measure Refractive Indices of Liquid Crystals

We present an extension of phase modulation ellipsometry to measure refractive indices in the visible and near infrared spectrum of multilayer and anisotropic structures, in particular liquid crystal cells, with arbitrary orientation of optical axis. An original software code was derived to process ellipsometer measurements, based on Jones formalism to express sinusoidal components of detected optical intensity reflected by a sample for different angles of the polariser and analyser respect to the sample orientation. In this way it is possible to obtain independent equations whose unknowns are the reflection matrix coefficients. Since it is possible only to obtain values of detected intensity and reflection coefficients given the refractive indices of the sample by using Berreman formalism, an algorithm has been developed to search best refractive index values which fit intensity measurements at each wavelength. Dispersion curves of refractive indices of 5CB have been obtained with accuracy of about 1%.

A Direct Determination Technique for Azimuth Alignment in Photoelastic Modulation Ellipsometry

Instead of the nulling method, a three-intensity-measurement technique is proposed to determine the azimuth deviation of the polarizer (P), photoelastic modulator (PEM) and analyzer (A) with respect to the specimen surface for ellipsometric measurements. After the initial alignment in a straight-through setup, we adjusted the azimuth of P at 45° to the strain axis of the PEM. Arranging a Polarizer-PEM-Sample-Analyzer ellipsometer by subjecting a specimen at the required incident angle, we measured a set of three DC radiances at the zero point of the zero-order Bessel function. In addition to the azimuth deviation, the ellipsometric parameter can also be determined from the same measurements.

Control of the bias tilt angles in nematic liquid crystals

The pretilt angle controlled by electric field was studied by the modulation ellipsometry technique. The easy direction of compensated nematic liquid crystals was controlled by surface flexoelectric torque created by the linear coupling of the director deformation and electric field. The weak anchoring energy necessary for the occurrence of flexoelectric distortion was produced by unidirectional rubbing of the clean indium–tin–oxide covered glasses with a cotton cloth. The pretilt angle was measured as a function of electric field. Long relaxation times of the optical response (hundreds of seconds) were observed. The rubbed thin polyvinyl alcohol and polyimide aligning layers were seen to promote strong anchoring energy (>0.5 erg/cm2) preventing any deviation of pretilt angle and, consequently, to suppress the optical response. The probable applications of the obtained results are discussed.

1-10a 変調エリプソメトリー法によるネマチック液晶セルのプレチルト評価

1-10a 変調エリプソメトリー法によるネマチック液晶セルのプレチルト評価

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ0 of the director and the dynamic deviation δ0 of the tilt bias angle. The angles θ0 and δ0 measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ 0=5.1°, δθ=0.5° and θ0=89°, δθ=0.06°, respectively.

Modulation ellipsometry measurements and density matrix modeling of a p-i-n-i superlattice optoelectronic modulator

Differential reflection measurements are performed on a n-i-p-i modulator with superlattice in the active layer. We use a modulation ellipsometer with pulsed laser excitation of the sample, avoiding electrical contacting of the device. We present for the first time a density matrix model which can be used for calculating the dielectric function of both superlattices as well as multiple quantum wells. The model includes the off-diagonal Coulomb coupling potentials, which are crucial for superlattice structure modeling, and the applied electric field which is required in a modulator device. An optical transfer-matrix model is used to obtain the differential reflection spectra of the device, using the dielectric function of the superlattice calculated with the density matrix model. Thus a direct and quantitative comparison with the experimental spectrum is shown.

Optical functions of chemical vapor deposited thin-film silicon determined by spectroscopic ellipsometry

The optical functions of several forms of thin-film silicon (amorphous Si, fine-grain polycrystalline Si, and large-grain polycrystalline Si) grown on oxidized Si have been determined using 2-channel spectroscopic polarization modulation ellipsometry from 240 to 840 nm (∼1.5–5.2 eV). It is shown that the standard technique for simulating the optical functions of polycrystalline silicon (an effective medium consisting of crystalline Si, amorphous Si, and voids) does not fit the ellipsometry data.

Optical functions of silicon-germanium alloys determined using spectroscopic ellipsometry

Abstract The optical functions of silicon-germanium alloys have been determined using two-channel spectroscopic polarization modulation ellipsometry. The optical thickness of the overlayer is determined using the experimental ellipsometry parameters at long wavelengths, thereby increasing the accuracy of the optical functions determined from the data. Extensive tables of the refractive index and extinction coefficient are presented for wavelengths between 240 and 840 nm (5.16 to 1.48 eV), and the results are compared with other results in the literature.

Optical functions of GaAs, GaP, and Ge determined by two-channel polarization modulation ellipsometry

The optical functions of (100) and (111) GaAs, (100) GaP, and (100) Ge havebeen determined from 234 to 840 nm (5.30 to 1.48 eV) at room temperature using two-channel spectroscopic polarization modulation ellipsometry. The results are corrected for oxide overlayers, where the layer thickness is measured using nulling ellipsometry at 1152 nm. The results are tabulated in terms of the refractive index n and extinction coefficient k, including the propagated errors, and are compared with some recent measurements.

Sample depolarization effects from thin films of ZnS on GaAs as measured by spectroscopic ellipsometry

Thin films of ZnS grown on GaAs by laser ablation are examined using spectroscopic two-channel polarization modulation ellipsometry (2-C PME). It is found that variations in the film thickness over the illumination spot result in the quasidepolarization of the incident light, which can be measured directly using 2-C PME. Quantitative fits of the ellipsometry data using a distribution-of-thicknesses model agree with independent reflectivity measurements of the thickness gradient, and allow for the accurate determination of the optical functions of the ZnS film.