Optical Characterization Methods Research Articles

4H-SiC epitaxial layer growth by trichlorosilane (TCS)

The growth rate of 4H-SiC epilayers has been increased up to 100 μm/h with the use of trichlorosilane instead of silane as the silicon precursor. The epitaxial layers grown with this process have been characterized by electrical, optical and structural characterization methods. Schottky diodes, manufactured on the epitaxial layer grown with trichlorosilane at 1600 °C, have higher yield and lower defect density in comparison to diodes realized on epilayers grown with the standard epitaxial process. Both very low (<10 13/cm 3) and very high (∼10 19/cm 3) doping levels have been reached with this process. The interface between very high and low doped regions is on the order of 30–50 nm. Very thick (>100 μm) epitaxial layer has been grown and the Schottky diodes realized on these layers with a good yield (>87%). This process gives the opportunity to realize very high-power devices with breakdown voltages in the range of 10 kV with a low cost SiC epitaxy process.

SiC-4H Epitaxial Layer Growth by Trichlorosilane (TCS) as Silicon Precursor at Very High Growth Rate

The growth rate of 4H-SiC epi layers has been increased up to 100 µm/h with the use of trichlorosilane instead of silane as silicon precursor. The epitaxial layers grown with this process have been characterized by electrical, optical and structural characterization methods. Schottky diodes, manufactured on the epitaxial layer grown with trichlorosilane at 1600 °C, have higher yield and lower defect density in comparison to diodes realized on epilayers grown with the standard epitaxial process.

Preparation of Fullerene (C60) Based SiO2 Sonogel Hybrid Composites: UV Laser Induced Photo-Polymerization, Morphological, and Optical Properties

Fullerene (C60) spheres were successfully embedded by diffusion into a catalyst-free SiO2 sonogel network. By this method, homogeneous and stable solid state hybrid samples suitable for optical characterization were produced. Due to the high porosity exhibited by the sonogel matrix on the nanometric length-scale, the preparation of several C60 doped composites with variable dopant concentrations was achieved. The obtained bulk hybrids were irradiated with a pulsed Kr-F laser system in order to induce photo-polymerization of the implanted C60 guest molecules at optimal experimental conditions established according to the pulsed laser photoacoustic technique (LPAT). The adequate purity level displayed by the sonogel route allowed us to perform optimal optical characterizations of these composites (reference hybrids and photo-polymerized samples) in order to evaluate their photo-physical properties for potential photonic applications. In this work the sonolysis process used to prepare amorphous catalyst-free SiO2:C60 sonogel hybrids and their linear and nonlinear optical (NLO) properties are extensively discussed. UV-VIS absorption-, Photo Luminescent (PL)-, and Raman-spectroscopies in conjunction with nonlinear optical limiting (OL) measurements, gave experimental evidence of the inclusion of C60 molecules within the sonogel environment, the laser induced photo-polymerization process and the induced changes of the optical and NLO-properties. Results are discussed in combination with atomic force microscopy (AFM) studies in order to explore the surface morphology of these samples.

Effect of compositional variations on the optical properties of Sb–Ge–Se thin films

Compositional dependences of the optical and physical properties of as-deposited amorphous SbxGe25−xSe75 films (x = 0, 5, 10, 15 and 20 at%), prepared by thermal evaporation, have been studied. The refractive index, n, and film thickness, d, have been determined from the upper and lower envelopes of the transmission spectra, measured at normal incidence, in the spectral range from 400 to 2500 nm. An optical characterization method for uniform films based on Swanepoel's ideas has been employed, and it has allowed us to determine the average thickness, , and the refractive index, n, of the films, with accuracies better than 1%. It has been found that the refractive index of the SbxGe25−xSe7 samples increases with increasing x, over the entire spectral range, which is related to the increased polarizability of the larger Sb atomic radius 1.38 Å compared with the Ge atomic radius 1.22 Å. The dispersion parameters such as E0 (single-oscillator energy) and Ed (dispersive energy) were discussed in terms of the single-oscillator Wemple–DiDomenico model. The absorption coefficient, α, and therefore the extinction coefficient, k, have been determined from the transmittance and reflectance spectra in the strong absorption region. The optical energy gap is derived from Tauc's extrapolation and a decrease in with increasing Se content. Finally, in terms of the chemical bond approach, cohesive energy has been applied to interpret the decrease in the glass optical gap with increasing Sb content in SbxGe25−xSe75.

Spectroscopic ellipsometry and reflectometry of statistically rough surfaces exhibiting wide intervals of spatial frequencies

AbstractTwo optical methods for the optical characterization of the statistically rough surfaces exhibiting wide intervals of spatial frequencies are presented. These methods employ the combination of variable angle spectroscopic ellipsometry and near‐normal spectroscopic reflectometry. The first method is based on combining the scalar diffraction theory and effective medium theory while the second method combines the scalar diffraction theory with Rayleigh‐Rice theory.Both the methods are applied to the optical characterization of statistically rough GaAs surfaces prepared by thermal oxidation. It is shown that both the methods can be utilized for characterization of these surfaces in a reasonable way, however, the latter is more suitable for this purpose. The results of the optical characterization of the selected rough GaAs surface are supported by those obtained using atomic force microscopy. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hollow-core waveguide characterization by optically induced particle transport

We introduce a method for optical characterization of hollow-core optical waveguides. Radiation pressure exerted by the waveguide modes on dielectric microspheres is used to analyze salient properties such as propagation loss and waveguide mode profiles. These quantities were measured for quasi-single-mode and multimode propagation in on-chip liquid-filled hollow-core antiresonant reflecting optical waveguides. Excellent agreement with analytical and numerical models is found, demonstrating that optically induced particle transport provides a simple, inexpensive, and nondestructive alternative to other characterization methods.

A novel ATR‐FTIR method for functionalised surface characterisation

AbstractWe demonstrate a novel method to analyse ex situ prepared chips by attenuated total reflection Fourier transform infrared spectroscopy (ATR‐FTIR), which circumvents tedious functionalisation steps of internal reflection elements (IREs), and simultaneously allows for complementary measurements by other analytical techniques. This concept is proved by utilising immobilised metal affinity capture (IMAC) chips containing about 10 µm thick films of copolymers coated with nitrilotriacetic acid (NTA) groups. With this so‐called ‘upside‐down’ ATR‐FTIR technique, each chemical modification step can be followed and optimised with respect to concentration, buffer, pH, ionic strength, and so on, and there are no limitations in variations or numbers of functionalised surfaces that can be generated. We have demonstrated the feasibility of this approach to determine the molecular structure of ligand bonded to immobilised polypeptide, directly observed in the raw ATR‐FTIR spectrum. Peptide adsorption in a thick NTA–copolymer matrix yields a high peptide concentration as determined by the analysis of the Langmuir adsorption isotherm. Combined with the ‘upside‐down’ ATR‐FTIR approach which samples the outermost region of the exposed NTA–copolymer film, this generates well‐resolved amide I and II absorption bands that reduce the necessity of using D2O based buffers, which otherwise is common in mid‐IR spectroscopy of proteins. We believe that this new optical surface characterisation method has a great potential as a stand‐alone or complementary analytical tool. We emphasise further that with this approach no chemical treatment of IREs is needed; the chips can be regenerated and reused, and analysed by complementary analytical techniques such as mass spectrometry. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Development of measurement techniques to characterize the optical properties of transparent films with application in in-mould decoration

This paper details the development of appropriate techniques for quantifying and comparing the optical properties of transparent polymer films used in in-mould decoration (IMD) and the effects of heat and strain on these properties. Currently there are no approved methods for optical characterization of transparent films, with most techniques being based on reflectance measurement of printed materials for the graphic arts industry. Original experimentation was therefore required in order to characterize the optical properties of transparent polymer films used in IMD or similar applications and to predict the changes in appearance during the forming process. A range of measurement techniques was utilized to characterize the films and their interaction with light in terms of light scattering and transmission. As a result of the investigation, the optical properties of these films can now be quantified and the changes due to the different mechanisms of softening and crystallization can be separately distinguished.

Very High Growth Rate Epitaxy Processes with Chlorine Addition

The growth rate of 4H-SiC epi layers has been increased by a factor 19 (up to 112 μm/h) with respect to the standard process with the introduction of HCl in the deposition chamber. The epitaxial layers grown with the addition of HCl have been characterized by electrical, optical and structural characterization methods. An optimized process without the addition of HCl is reported for comparison. The Schottky diodes, manufactured on the epitaxial layer grown with the addition of HCl at 1600 °C, have electrical characteristics comparable with the standard epitaxial process with the advantage of an epitaxial growth rate three times higher.

The effects of Al doping on the optical constants of ZnO thin films prepared by spray pyrolysis method

Aluminum doped ZnO (AZO) thin films doped with different aluminum concentrations have been prepared by spray pyrolysis method onto glass substrates. The optical and structural properties of the films have been investigated by X-ray diffraction and optical characterization methods. The X-ray diffraction spectra showed that all of the thin films are of polycrystalline nature. The thin films have (002) as the preferred orientation. The optical band gaps of the films were calculated. The Eg values decrease with increasing Al doping concentration. The refractive index, the extinction coefficient, and the real and imaginary components of dielectric constant are calculated. The obtained results show that all optical parameters keep constant in the visible region, whereas in the ultraviolet region, doping concentration strongly affects the optical parameters of AZO thin films. Optical constants tend to decrease with increasing doping concentration.

Electrical conductivity and single oscillator model properties of amorphous CuSe semiconductor thin film

The structural, optical dispersion and electrical conductivity properties of the CuSe thin film have been investigated using X-ray diffraction, electrical and optical characterization methods. X-ray diffraction results indicate that CuSe thin film has an amorphous structure. The electrical conductivity of the CuSe film increases with increasing temperature. The activation energy and room temperature conductivity values of the film were found to be 1.32meV and 3.89×10−3S/cm, respectively. The refractive index dispersion of the thin film obeys the single oscillator model and single oscillator parameters were determined. The Eo, n∞, and So values of the CuSe thin film were found to be 5.08eV, 3.55 and 1.92×1014m−2, respectively. The obtained results suggest that CuSe film is an amorphous semiconductor.

Linear and non-linear optical properties of 2,5-disubstituted pyrroles supported by a catalyst-free SiO 2 sonogel network

The preparation and optical characterization of pyrrole based sol–gel hybrid materials generated by ultrasonic irradiation (Sonogel composites) are presented in this work. Pyrrole compounds were recently synthesized in our group by a modification of the Schulte–Reisch reaction; these molecular systems were dissolved at different concentrations in tetrahydrofuran (THF) and optimally embedded into a catalyst-free SiO 2 sonogel network. For this purpose, we exploited the novel catalyst-free (CF) sonolysis route to produce highly pure sol–gel glasses, generated via sonochemical reactions. This approach has been recently developed in our research group and has been successfully implemented to develop several hybrid composites for optical applications. By this method, homogeneous and stable solid-state hybrid samples suitable for optical characterization can be produced. The high porosity exhibited by the sonogel matrix allowed us to prepare several pyrrole doped composites with variable dopant concentration. The linear and nonlinear optical (NLO) properties of these amorphous hybrid structures were determined by absorption- and photoluminescent (PL)-spectroscopies, and by the optical third harmonic generation (THG) techniques, respectively. The implemented catalyst-free sonolysis route produced SiO 2-host networks of high chemical and optical purity, suitable for optical and photonic applications.

Reflectance methods for determining the optical constants of highly absorbing films: comparative analysis of the accuracy

The accuracies of the most widely used methods for optical characterization of absorbing films with thicknesses greater than the penetration depth of light are compared quantitatively through estimation of the errors Δn and Δk in refractive index, n, and extinction coefficient, k. Among the great variety of possible combinations of reflectance, R, measured in both planes of polarization, Rp and Rs, reflectance ratios, Rp/Rs, and pseudo-Brewster angle, β, only four are selected as the most suitable methods: (R,Rp70), (Rpβ/R,β), (Rpβ,β) and (R,β). It is shown that the methods using β are more accurate than those with reflectance measurements only. Among the three methods with β, the least accurate is (Rpβ/R,β), while (R,β) and (Rpβ,β) have similar accuracies but slightly different application ranges. It is found that only samples with n, k≤3 can be characterized with methods including β if the largest incident angle, which can be measured, is 70°. Application of the error analysis predictions is illustrated through determination of the optical constants of 150 nm thick Au film.

Epitaxial Layers Grown with HCl Addition: A Comparison with the Standard Process

The growth rate of 4H-SiC epi layers has been increased by a factor 3 (up to 18μm/h) with respect to the standard process with the introduction of HCl in the deposition chamber. The epitaxial layers grown with the addition of HCl have been characterized by electrical, optical and structural characterization methods. An optimized process without the addition of HCl is reported for comparison. The Schottky diodes, manufactured on the epitaxial layer grown with the addition of HCl at 1600 °C, have electrical characteristics comparable with the standard epitaxial process with the advantage of an epitaxial growth rate three times higher.

Effects of In, Al and Sn dopants on the structural and optical properties of ZnO thin films

Effect of In, Al and Sn dopants on the optical and structural properties of ZnO thin films have been investigated by X-ray diffraction technique and optical characterization method. X-ray diffraction patterns confirm that the films have polycrystalline nature. The thin films have (0 0 2) as the preferred orientation. This (0 0 2) preferred orientation is due to the minimal surface energy which the hexagonal structure, c-plane to the ZnO crystallites, corresponds to the densest packed plane. The grain size values of the films are found to be 29.0, 35.2 and 39.5 nm for In, Al and Sn doped ZnO thin films, respectively. The optical band gaps of the films were calculated. The absorption edge shifts to the lower wavelengths with In, Al and Sn dopants. The inclusion of dopant into films expands also width of localized states as E UIn > E UAl > E USn. The refractive index dispersion curves obey the single oscillator model. The dispersion parameters and optical constants of the films were determined. These parameters changed with In, Al and Sn dopants.

Experimental investigation of R-134a and R-245fa two-phase flow in microchannels for different flow conditions

An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. The tests were run in 0.5 mm and 0.8 mm diameter glass channels using R-134a and R-245fa at 26, 30 and 35 °C saturation temperatures. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were detected. The two-phase flow pattern transitions observed with R-134a did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. No significant influence of the inlet subcooling (2, 3, 5 °C) nor the saturation pressure has been observed on the flow pattern transitions. Furthermore, changing the heated length from 70 to 30 mm before the glass tube did not influence the location of the transition lines in the flow pattern map. The 0.8 mm diameter also did not show any significant difference to the 0.5 mm channel, although bubbly/slug flow was present over a wider range of mass flux. The transitions between flow regimes are less influenced by the mass flux for R-245fa than for R-134a; thus for low mass fluxes the difference is significant and the flow regimes appear earlier in terms of vapor quality.

4H SiC Epitaxial Growth with Chlorine Addition

AbstractThe growth rate of a 4H‐SiC epitaxial layer has been increased by a factor of 19 (up to 112 μm h–1) with respect to the standard process, with the introduction of HCl in the deposition chamber. The epitaxial layers grown with the addition of HCl has been characterized by electrical, optical, and structural characterization methods. The effects of various deposition parameters on the epitaxial growth process have been described, and an explanation of this behavior in terms of the diffusion coefficient on the surface, Ds, and the ratio between the characteristic times, τD:τG, has been provided. The diodes, manufactured on the epitaxial layer grown with the addition of HCl at 1600 °C, have electrical characteristics comparable with the standard epitaxial process. This process is very promising for high‐power devices with a breakdown voltage of 10 kV.

Characterization of diabatic two-phase flows in microchannels: Flow parameter results for R-134a in a 0.5 mm channel

An optical measurement method for two-phase flow pattern characterization in microtubes has been utilized to determine the frequency of bubbles generated in a microevaporator, the coalescence rates of these bubbles and their length distribution as well as their mean velocity. The tests were run in a 0.5 mm glass channel using saturated R-134a at 30 °C (7.7 bar). The optical technique uses two laser diodes and photodiodes to measure these parameters and to also identify the flow regimes and their transitions. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions were detected and observed also by high speed video. It was also possible to characterize bubble coalescence rates, which were observed here to be an important phenomena controlling the flow pattern transition in microchannels. Two types of coalescence occurred depending on the presence of small bubbles or not. The two-phase flow pattern transitions observed did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. Time averaged cross-sectional void fractions were also calculated indirectly from the mean two-phase vapor velocities and compared reasonably well to homogeneous values.

High Growth Rate Process in a SiC Horizontal Reactor with HCl Addition: Structural and Electrical Characterization

AbstractThe growth rate of 4H-SiC epitaxial layer has been increased by a factor 19 (up to 112 μm/h) with respect to the standard process with the introduction of HCl in the deposition chamber. The epitaxial layers grown with the addition of HCl have been characterized by electrical, optical and structural characterization methods. The effects of different deposition parameters on the epitaxial growth process have been described in detail. This process can be very promising for high power devices with a breakdown voltage of 10 kV.

Theoretical realization of holey fiber with flat chromatic dispersion and large mode area: an intriguing defected approach

We present a novel design approach for realizing holey fibers (HFs) with flat dispersion characteristics and large mode area based on the existence of an artificially defected air-hole ring in the cladding, and on the inclusion of additional defected air holes in the core of the fiber. This unique type of HF can be used for achieving remarkable flat dispersion characteristics as well as a large mode area, which are particularly useful for high-speed data transmission. The validation of the proposed design is done by adopting an efficient full-vectorial finite element method for optical characterization of HFs. The proposed fiber can be employed in reconfigurable optical transmission systems for performing wavelength division multiplexing operation. Typical characteristics of the proposed HF are a flattened dispersion of 6.3 +/- 0.5 ps/km/nm from 1.45 to 1.65 microm and an effective mode area as large as 100 microm2 in the same frequency range.