Spectroscopic Phase-modulated Ellipsometry Research Articles

Enhanced optical absorption of graphene-based heat mirror with tunable spectral selectivity

Graphene based heat mirror (i.e., graphene/Cu/graphene multilayer coating) was designed and developed to achieve enhanced optical absorption with tunable spectral selectivity. The concept of a heat mirror and a high/low/high refractive index stack was used to develop the graphene/Cu/graphene absorber and the transition from high transmittance to high absorptance was realized on quartz substrates. The thickness of the Cu layer has an important role in creating the destructive interference, for achieving high absorptance and low emittance. The refractive index of the multilayer graphene transferred on Si substrate was studied using phase-modulated spectroscopic ellipsometry. An absorptance of 0.91 and emittance of 0.22 was achieved for the SiO2/graphene/Cu/graphene multilayer coating. For the first time, the feasibility of graphene/Cu/graphene multilayer coating for photothermal applications have been demonstrated.

Optical properties of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO tandem absorber coatings by phase-modulated spectroscopic ellipsometry

TiAlC, TiAlCN, TiAlSiCN, TiAlSiCO, and TiAlSiO layers of thicknesses ~2.2 μm, 755, 491, 393, and 431 nm, respectively, were deposited on stainless steel, silicon, and glass substrates to study their refractive indices and extinction coefficients using the phase-modulated spectroscopic ellipsometry in the wavelength range of 300–1200 nm. Absorption coefficient of each layer was calculated from the extinction coefficient of the layer. The results indicate that the first three layers (i.e., TiAlC, TiAlCN, and TiAlSiCN) are absorbing in nature, while TiAlSiCO and TiAlSiO act as intermediate and antireflection layers. Subsequently, a tandem absorber of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO with layer thicknesses of 62, 20, 18, 16, and 27 nm, respectively, was deposited on stainless steel substrates to fabricate a spectrally selective coating with absorptance of 0.961 and emittance of 0.15 at 82 °C. The obtained refractive indices and extinction coefficients of the tandem absorber were used to simulate the reflectance of the deposited tandem absorber using SCOUT software. Simulated reflectance data of the tandem absorber showed a good agreement with the experimental data measured by UV–Vis–NIR and FTIR spectrophotometry. The angular dependence of the selective properties of the tandem absorber was studied by measuring the reflectance spectra of the tandem absorber at different incident angles.

Microstructure and Optical Properties of SS/Mo/Al2O3 Spectrally Selective Solar Absorber Coating

Surface-textured Mo thin film is fabricated by magnetron sputtering through the adjustment of deposition parameters, which exhibits a high absorptance of 0.80 and a low emittance of 0.09. The single-layer Mo deposited on stainless steel (SS) is characterized by x-ray diffraction (XRD), ultra-high resolution scanning electron microscope, atomic force microscope and optical measurement. The controlled surface roughness combined with larger aspect ratio contributes much to the high absorptance and low emittance. Based on the SS/Mo coating, a spectrally selective coating (SS/Mo/Al2O3) is designed and fabricated. The coating shows an amorphous structure and exhibits an absorptance of 0.90 and an emittance of 0.08. Tauc-Lorentz and Drude free-electron models are used to modeling the optical properties of Al2O3 and Mo layers by phase-modulated spectroscopic ellipsometry.

Spectroscopic ellipsometry and UV–vis studies at room temperature of the novel organic–inorganic hybrid of salt Bis (4-acetylanilinium) tetrachlorocadmiate

The optical properties of Bis (4-acetylanilinium) tetrachlorocadmiate compound were studied using phase modulated spectroscopic ellipsometry (PMSE) and ultraviolet–visible (UV–Vis) spectroscopy in the range 200–800nm. The optical absorbance were measured in order to deduce the absorption coefficient α and optical band gap Eg, thus the Tauc model was used to determine the optical gap energy of the synthesized (C8H10NO)2CdCl4 compound. The analysis of the data revealed the existence of optical allowed direct transition mechanisms with the band gap energy equal to 3.17eV. On the other hand the single-effective-oscillator model was used to fit the calculated data to the experimental ellipsometric spectra. Thus the values of the dispersion energy and single-oscillator strength are determined. Also, the extinction coefficient, refractive index, dispersion parameter and both the real εr and imaginary parts εi of the dielectric permittivity of Bis (4-acetylanilinium) tetrachlorocadmiate compound were calculated and the results are discussed.

Spectroscopic ellipsometry study of N+ ion-implanted ethylene–norbornene films

The optical properties of 150keV N+ implanted ethylene–norbornene (TOPAS 6017S-04) copolymer were investigated using phase modulated spectroscopic ellipsometry (PMSE) and ultraviolet–visible (UV–Vis) spectroscopy in the ranges of 0.6–6.5eV and of 1.5–6.2eV, respectively. The single-effective-oscillator model was used to fit the calculated data to the experimental ellipsometric spectra. The results show that the oscillator and dispersion energies decrease with increasing ion fluence up to 1015cm−2, and then these parameters increase with further fluence increasing. Analysis of the UV–Vis absorption spectra revealed the presence of indirect electronic transitions with the band gap energy in the range of 1.3 to 2.8eV. It was found that both the band gap energy and the energy width of the distribution of localized band tail states decrease, while the values of Tauc coefficient increase with increasing the ion fluence. From the ellipsometric data we found that the real part of the dielectric function increased about 7% after irradiation with 1015cm−2, and decreased about 10% in samples modified with 1016cm−2.

Dependence of Optical Anisotropy of ZnO Thin Filmson the Structural Properties

The structural properties and optical anisotropy of strained ZnO films prepared by rf magnetron sputtering were investigated. Anisotropic complex refractive index spectra of the films were determined by using the Adachi-New Forouhi dispersion model with phase-modulated spectroscopic ellipsometry in the range of 1.0 4.0 eV. A negative strain was induced by the mismatch between the ZnO film and the Al2O3 substrate and decreased with improving crystallinity of the film. The size of the grains formed in the films showed a linear dependence on the strain. The optical anisotropy of the ZnO films was enhanced with improving crystallinity of the films. A shifting of the critical points of the anisotropic complex refractive index spectra was observed due to variations in the strain. The anisotropic complex refractive index spectra showed a strong dependence on structural changes in the ZnO films.

Temperature-dependent and pump-probe ellipsometric studies of TlInSe 2

Temperature-dependent phase-modulated spectroscopic ellipsometry over the photon energies 1.5–5.5 eV and temperatures 300–400 K, as well as pump-probe single wavelength rotating-analyzer laser ellipsometry at photon energy 1.61 eV and pumping power-density up to 160 W/cm 2 has been carried out on TlInSe 2 that has a quasi one-dimensional crystalline structure and apparently undergoes an incommensurate phase transition with temperature. Refractive indices and extinction coefficients, as well as their variation with temperature and pumping power-density have been determined. The change of the surface profile under pumping has been disclosed. The reversibility of the effect has been confirmed by the dynamic reflectance examination. Obtained results are discussed in terms of thermal effects and incommensurate phase transition in TlInSe 2. It is proposed that a giant thermo-elastic effect and a soliton wave formation are behind the observed reversible change of the surface profile.

Characterization of non-uniform diamond-like carbon films by spectroscopic ellipsometry

Optical characterization of diamond-like carbon (DLC) films non-uniform in thickness is performed using spectroscopic phase-modulated ellipsometry. This characterization is based on new formulas for the associated ellipsometric parameters of thin films exhibiting a wedge-shaped thickness non-uniformity. These formulas express the associated ellipsometric parameters by means of the density of distribution of local film thickness. The spectral dependences of the optical constants of these non-uniform DLC films are expressed using the dispersion model based on parametrization of density of electronic states. It is shown that this model of the thickness non-uniformity provides a relatively good agreement between the experimental and theoretical data, indicating that the results of the optical characterization of the non-uniform DLC films are close to the correct results. Moreover, it is shown that the model of uniform thin films is unsuitable for the optical characterization of the non-uniform DLC films studied.

Silane injection in a high-density low-pressure plasma system and its influence on the deposition kinetics and material properties of SiO2

High-rate, low temperature deposition is an essential requirement for industrial fabrication technology to be suitable for the deposition of optical and protective coatings. High-density, low-pressure plasmas have received significant attention for such applications due to their ability to create large and controllable ion fluxes onto the substrate. In this study, the high-rate deposition of silica films from a silane and oxygen gas mixture in a high-density plasma system based on a matrix distributed electron cyclotron resonance (MDECR) plasma source is investigated using directional jet injection of undiluted silane. The influence of process parameters such as the microwave power, radio frequency biasing of the substrate holder, and gas flows on the OH content of the oxide films is studied using phase-modulated spectroscopic ellipsometry (SE), Fourier transform infrared (FTIR) spectroscopy, and transmission measurements. The results of the measurements, taken at various points across the wafer, show a decrease in the thickness-normalized OH concentration in the areas of higher deposition rates. The corresponding gas phase composition is investigated using optical emission spectroscopy and compared to the FTIR, transmission and SE measurement results in order to validate our findings and ultimately optimize the deposition process. It is found that the primary silane flux onto the surface, which depends on the positioning of the jet injection point and gas flow rate, plays an important role not only on the deposition rate but also on the OH content of the films. The authors conclude that high-density plasma deposition systems such as the MDECR plasma enhanced chemical vapor deposition system cannot be considered as well mixed for gases with dissociation products that have high sticking coefficients, contrary to the accepted paradigm.

Temperature‐dependent spectro‐ellipsometric studies of optical transitions near absorption edge of TlInS2

AbstractThe temperature dependence of the absorption edge of the layered TlInS2 was studied in a region of the incommensurate phase by spectroscopic phase modulated ellipsometry (SPME) on (001) surfaces in incoherent reflection mode that was sensitive to both E⊥C* and E//C* components of the reflected light (C*: the direction bisecting the angle between two optic axes). The energy position of the absorption edge, which was obtained from the measured Ic signal, was found to exhibit an irregular behavior with temperature in a similar way with the obtained earlier temperature dependence of excitons. It is proposed that this behaviour reflects the changes in electronic spectrum due to the temperature phase transition while the peaking anomaly of the measured in‐layer‐plane optical anisotropy is mostly related with the spatial structure of the incommensurate phase. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectrally selective NbAlN/NbAlON/Si 3N 4 tandem absorber for high-temperature solar applications

A new spectrally selective NbAlN/NbAlON/Si 3N 4 tandem absorber was deposited on copper substrates using a reactive direct current magnetron sputtering system. A high solar absorptance (0.956) and a low emittance (0.07) were achieved by gradually decreasing the refractive index from the substrate to the surface. The tandem absorber was characterized using solar spectrum reflectometer and emissometer, X-ray photoelectron spectroscopy, phase-modulated spectroscopic ellipsometry, atomic force microscopy and micro-Raman spectroscopy techniques. In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) at different durations and temperatures. The tandem absorber deposited on copper substrate exhibited high solar selectivity in the order of 13–15 even after heat treatment in air up to 500 °C for 2 h. These tandem absorbers also exhibited high thermal stability (450 °C) in air for longer durations (116 h). The onset of oxidation for the tandem absorber deposited on silicon substrates was 650 °C, indicating a high oxidation resistance. The results of the present study indicate the importance of NbAlN/NbAlON/Si 3N 4 tandem absorber for high-temperature solar selective applications.

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.

Spectroscopic phase modulated ellipsometry in application to some novel single and polycrystalline ternary compounds

AbstractThe results of the application of the spectroscopic phase modulated ellipsometry (SPME) to a series of novel ternary compounds from anisotropic to polycrystalline are presented to show the high effectiveness of this optical technique with regard to the dielectric function of the new materials. Besides, SPME in specular reflection mode to study the polarization degree of the light reflected at specular angle is shown to be very informative in determination of the best conditions for optical measurements on rough surfaces such as, for example, surfaces of the polycrystalline ternary thiogallate compounds studied in this work. An incoherent approach relating the measured depolarization with the surface variance and optical constants of a material or thin film is proposed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical constants at and below energy gap of layered ternary Tl compounds

Incoherent reflection mode of spectroscopic phase modulated ellipsometry (SPME) was firstly applied to quasi-two-dimensional TlInS2, TlGaS2 and TlGaSe2 to obtain uniaxial optical anisotropy of these materials at room temperature. Simulation of the measured SPME signal was performed within uniaxial incoherent reflection model, with a result of perfect match to experimental data for all three compounds. Birefringence in a photon energy range from below the energy gap of each material and down to 0.76eV was reliably determined. Obtained birefringence agrees well with optical anisotropy reported earlier from light figure studies of the above materials in the visible spectral range. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth-dependent refractive index nonlinearity and mean microstructural properties of codeposited composite gadolinia silica films

Codeposited gadolinia silica composite films have been probed for their growth-dependent optical and microstructural properties using phase-modulated spectroscopic ellipsometry and scanning probe microscopy. The mean refractive indices were computed using an effective ellipsometric multilayer modeling approach. Most of the composite films have shown growth-induced nonlinear refractive indices to some extent. However, the mean optical properties have depicted interesting trends in the microstructural evolutions. Gadolinia silica composite films in the composition ratio ranging from 90:10 to 70:30 have depicted superior optical as well as morphological properties. Unlike conventional oxide films, these composite films displayed microstructural, spectral refractive index, and bandgap supremacy over the pure films. Such an observation cannot be explained by the empirical Moss law. Atomic force microscopy also revealed a superior morphology in the composite films. The autocorrelation and height-height correlation functional analysis have distinctly supported such superior microstructural features in the composite films, which justifies the supremacy of the optical properties. Such an observation has opened up possibilities to utilize such composite films toward deep-and extreme-ultraviolet spectral regions of the electromagnetic spectrum.

Spectroscopic ellipsometry study of Ir(ppy)3 organic light emitting diode

Abstract We studied the optical properties of fac tris(2-phenylpyridine) iridium (Ir(ppy) 3 ) emitting layer in single-layer organic light emitting diode (OLED) device and in thin film grown on quartz plate using a phase modulated spectroscopic ellipsometry (PMSE). Accurate spectra of refractive index n and extinction coefficient k of Ir(ppy) 3 were obtained in the wavelength range of 330–830 nm in the device and film. Difference was observed in the n and k spectra between the single layer thin film evaporated on quartz plate and OLED. Additionally difference was found between the layer thickness estimated by quartz oscillator and thickness estimated from the PMSE measurement.

Role of H in hot-wire deposited a-Si:H films revisited: optical characterization and modeling

Device quality hydrogenated amorphous silicon (a-Si:H) thin films were grown by hot wire chemical vapor deposition (HWCVD) on glass (Corning 7059) using silane under high hydrogen dilution as a function of substrate temperature ( T S) ranging 50–515 °C. As a consequence of variation of T S, the hydrogen concentration [ C H] varied from 20.0% to 0.2%. They are optically examined ex situ using Raman spectroscopy (RS) and spectroscopic phase modulated ellipsometry (SPME) from near IR to near UV (1.5–5.0 eV) obtaining their vibration frequencies and pseudo-dielectric function, respectively for analyzing network disorder. The ellipsometry raw data (〈 ε r ( E)〉, 〈 ε i ( E)〉) were modeled using Bruggeman effective medium theory (BEMT) and the dispersion relations for the amorphous semiconductors comprising a two-layer model consisting of a top surface roughness layer ( d S) containing an effective medium mix of 50% a-Si:H and 50% voids and a single “bulk” layer ( d B) of a-Si:H to simulate the data reasonably well. We performed these simulations by non-linear least-square regression analysis based on Marquardt–Levenberg algorithm and it was possible to estimate the true dielectric function of a-Si:H thin films and the energy band gap ( E g), besides film thickness ( d SE), bulk void fraction, surface roughness layer ( d S), and the confidence limits ( χ 2). Moreover, it is shown that the Tauc–Lorentz (TL) model fits the ellipsometry data much better than Forouhi and Bloomer (FB) and help elucidate the layered structure of a-Si:H thin films. We also compared the optical band gap ( E g) determined using ellipsometry modeling and the Tauc gap ( E T) using conventional approach. We discuss the possible physical meaning and the variation of the deduced parameters in terms of role of T S ( T-role) or of hydrogen ( H-role) in the dispersion model abovementioned. The bandgap was found to decrease systematically with increasing T S, reconfirming the role of hydrogen as alloy in conjunction with network relaxation, which is in agreement with excitation dependent Raman spectroscopy (i.e. strongly versus weakly absorbing) results interpreted in terms of the variation of three “order parameters” relating the short- and medium-range order with respect to substrate temperature. This is since, previous Raman scattering studies in various hydrogenated amorphous silicon (a-Si:H) materials resulted in contradicting conclusions as to the role of hydrogen on the atomic-network order. Hereby, we elucidated these contradictions using the abovementioned optical probes by establishing that the surface and the bulk of HW a-Si:H films behave differently due to their different hydrogen concentrations. Micro-Raman spectroscopy (RS) and atomic force microscopy (AFM) were used to validate the simulations also. Further consequences of these findings are also discussed in terms of other optical and structural properties. These analyses led to a correlation between the films’ microstructure (or network disorder) and their electronic properties for several technological applications, in general, and for solar cells applications, in particular.

Phase-modulated spectroscopic ellipsometry and polarized transmission intensity studies of wide-gap biaxial CaGa 2S 4

Differently prepared natural (100)-oriented cleavage planes of the orthorhombic wide-gap ( E o∼4.1 eV) single crystalline CaGa 2S 4 were examined in two symmetric positions at room temperature over the photon energies 0.8–6.5 eV by spectroscopic ellipsometry. Complimentary, polarized transmission intensity technique was applied. The data obtained by both techniques were further treated within standard biaxial approach and the major refraction indices were restored for the practically important range below energy gap. The structure and polarization peculiarities of the obtained pseudo-dielectric function were related with apparently four critical points of interband electronic transitions in ZZ ( E 1=4.49 eV, E 3=5.34 eV) and YY ( E 2=5.00 eV, E 4=6.21 eV) configurations, respectively.

Optical properties of silicon carbide polytypes below and around bandgap

The optical properties of SiC polytypes are discussed with relation to the structure of SiC. The main part of the paper review the detailed measurements of the uniaxial dielectric function below and around the minimum band gap, using phase-modulated spectroscopic ellipsometry, polarised light transmission measurements or crossed polariser variable angle of incidence interferometry. In particular, the oscillations in the recorded ellipsometric intensities, due to interference between the ordinary and extraordinary modes, specific to thick uniaxial wafers, are used to accurately measure the below band gap dielectric difference. It is focussed on the differences in optical properties among the polytypes, in order to investigate SE and closely related techniques as a candidate for bulk-material wafer inspection and characterisation.

Optical and electrical properties of amorphous carbon films deposited using filtered cathodic vacuum arc with pulse biasing

Passive devices using metal containing amorphous (a-C) films have been successfully fabricated. However, difficulties in the etching of these films as well as their inferior inertness compared to pure a-C films led us to study the electrical and optical properties of pure a-C films. The films were deposited using a filtered cathodic vacuum arc system (FCVA) in conjunction with high substrate pulse biasing. It is possible to control the sp 2 content and hence the properties, by varying the substrate pulse bias voltage. In this study, the a-C films were prepared by varying the high substrate bias between 3 and 11 kV using a Plasma immersion ion implantation (PI 3) system. Characterization of these samples gives us an indication about the suitability of the films for integrated passive devices and other applications. Four-point probe measurement has been carried out to study the resistivity of the films deposited on quartz and SiO 2. The resistivity decreases with increasing pulse bias voltage, which is likely attributed to the sp 2 fraction in the film as well as the substrates’ resistivity. The sp 2 content in the films is estimated using XPS and Raman spectroscopy. Optical properties of the films are characterized using spectroscopic phase-modulated ellipsometry. The band gap decreases from 2.3 to 1.49 with increasing bias voltage.