Variable Angle Spectroscopic Ellipsometry Measurements Research Articles

Optical Properties of Reactive RF Magnetron Sputtered Polycrystalline Cu3N Thin Films Determined by UV/Visible/NIR Spectroscopic Ellipsometry: An Eco-Friendly Solar Light Absorber

Copper nitride (Cu3N), a metastable poly-crystalline semiconductor material with reasonably high stability at room temperature, is receiving much attention as a very promising next-generation, earth-abundant, thin film solar light absorber. Its non-toxicity, on the other hand, makes it a very attractive eco-friendly (greener from an environmental standpoint) semiconducting material. In the present investigation, Cu3N thin films were successfully grown by employing reactive radio-frequency magnetron sputtering at room temperature with an RF-power of 50 W, total working gas pressure of 0.5Pa, and partial nitrogen pressures of 0.8 and 1.0, respectively, onto glass substrates. We investigated how argon affected the optical properties of the thin films of Cu3N, with the aim of achieving a low-cost solar light absorber material with the essential characteristics that are needed to replace the more common silicon that is currently in present solar cells. Variable angle spectroscopic ellipsometry measurements were taken at three different angles, 50∘, 60∘, and 70∘, to determine the two ellipsometric parameters psi, ψ, and delta, Δ. The bulk planar Cu3N layer was characterized by a one-dimensional graded index model together with the combination of a Tauc–Lorentz oscillator, while a Bruggeman effective medium approximation model with a 50% air void was adopted in order to account for the existing surface roughness layer. In addition, the optical properties, such as the energy band gap, refractive index, extinction coefficient, and absorption coefficient, were all accurately found to highlight the true potential of this particular material as a solar light absorber within a photovoltaic device. The direct and indirect band gap energies were precisely computed, and it was found that they fell within the useful energy ranges of 2.14–2.25 eV and 1.45–1.71 eV, respectively. The atomic structure, morphology, and chemical composition of the Cu3N thin films were analyzed using X-ray diffraction, atomic force microscopy, and energy-dispersive X-ray spectroscopy, respectively. The Cu3N thin layer thickness, profile texture, and surface topography of the Cu3N material were characterized using scanning electron microscopy.

Toward high efficiency for long-term stable Cesium doped hybrid perovskite solar cells via effective light management strategy

Power conversion efficiency (PCE) and device stability are the two most important concerns for next generation perovskite solar cells (PSCs). Herein, we report an investigation for long-term stable perovskite Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 solar cells toward high efficiency via the light management strategy. To that end, we first experimentally determine complex optical constants of this kind of perovskite thin film by using variable angle spectroscopic ellipsometry measurements. By constructing the proper model, the calculated photocurrent density (Jsc) of 23.2 mA/cm2 of the planar PSC model is only slightly higher than the measured 23.0 mA/cm2 of our champion planar cell with excellent consistency, where the PCE is up to 20.5% with active area of 0.16 cm2, demonstrating a long-term stability capable of retaining 91% of initial PEC for 40 days. It is further found that when the front texture of nanocone array design is applied to the planar cell as a surface antireflection, the Jsc value of 24.8 mA/cm2 with the resulting PCE of 22.1% is projected from the present real planar PSC device with active area of 0.16 cm2.

Determination of complex optical constants and photovoltaic device design of all-inorganic CsPbBr3 perovskite thin films

All-inorganic perovskites exhibit interesting properties and unprecedented stability compared to organic-inorganic hybrid lead halide perovskites. This work focuses on depositing and characterizing cesium lead bromide (CsPbBr3) thin films and determining their complex optical constants, which is a key requirement for photovoltaic device design. CsPbBr3 thin films are synthesized via the solution method followed by a hot-embossing step to reduce surface roughness. Variable angle spectroscopic ellipsometry measurements are then conducted at three angles (45°, 55°, and 65°) to obtain the ellipsometric parameters psi (Ψ) and delta (Δ). For the present model, bulk planar CsPbBr3 layer is described by a one-dimensional graded index model combined with the mixture of one Tauc-Lorentz oscillator and two Gaussian oscillators, while an effective medium approximation with 50% air void is adopted to describe surface roughness layer. The experimental complex optical constants are finally determined in the wavelength range of 300 to 1100 nm. Furthermore, as a design example demonstration, the simulations of single-junction CsPbBr3 solar cells are conducted via the finite-difference time-domain method to investigate the properties of light absorption and photocurrent density.

Annealing-enhanced birefringence and aggregation in MEH-PPV: A spectroscopic ellipsometry study

We have used absorption, photoluminescence (PL), and variable angle spectroscopic ellipsometry (VASE) measurements to investigate the structural changes that take place upon high temperature annealing in spin-coated films of the prototypical conjugated polymer (CP) poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV). Absorption and VASE measurements reveal that the birefringence of the films increases by approximately a factor of two upon heating, which indicates a significant increase in the alignment of the conjugated polymer (CP) strands within the film plane. Absorption and PL spectra indicate the formation in annealed films of interchain species having lower energy transitions. But these measurements alone do not reveal the type of interchain species formed, such as excimers or aggregates. VASE measurements were used to investigate this feature and clearly reveal a new, low energy, feature with a shoulder at 650 nm in the dispersion relations of the extraordinary (out-of-plane) extinction and absorption coefficients of the annealed films, which we assign to aggregate absorption. Thus, our work shows that VASE is sufficiently sensitive to measure aggregate absorption in CP films. In the case of the ordinary (in-plane) extinction and absorption coefficients, there is an increased amplitude of the lower energy peak upon heating, owing to increased uniaxial anisotropy, along with a broadening and a longer red tail, but the well-resolved red-shifted absorption band seen for the extraordinary absorption coefficient is not observed. Therefore, we conclude that while in-plane and out-of-plane aggregation occurs in annealed spin-coated films of MEH-PPV, aggregate absorption is only clearly observed when the aggregate electronic transition dipole is oriented preferentially in a direction perpendicular to the film plane. This conclusion is consistent with the usual observation that aggregate absorption in MEH-PPV films is not easily observed using absorption spectra alone, which are typically measured at normal incidence.

Direct retrieval method of the effective permittivity and permeability of bulk semi-infinite metamaterials by variable-angle spectroscopic ellipsometry

In this work, we present a simple method for the direct retrieval of the effective permittivity and permeability of a bulk semi-infinite metamaterial from variable-angle spectroscopic ellipsometry measurements. Starting from the well-known Fresnel equations, we derive an analytical expression in which unknown coefficients are fitted to the experimental data using a linear regression model. The effective permittivity and permeability are then determined by solving a simple system and the correct solution is selected based on physical criteria. As an example, the method is applied to the case of a self-assembled metamaterial exhibiting strong isotropic optical magnetism.

The effect of thermal annealing on Fe/TiO2 coatings deposited with the help of RF PECVD method. Part II. Optical and photocatalytic properties

In this work, morphology, optical properties, photowetting effect, and bactericidal behavior of titanium dioxide coatings, both plain and iron doped within the range of 0.52 and 4.76at%, are presented. The coatings were synthesized with the help of radio frequency plasma enhanced chemical vapor deposition technique followed by thermal annealing at 500°C under normal atmospheric conditions. Atomic force microscopy examination of the film morphology reveal roughness differences depending on the concentration of iron. Measurements of optical properties, carried out with the UV–VIS absorption spectrometry, show high transmittance in the visible range. Optical gap values, determined with the help of the Tauc equation, exhibit a decreasing tendency with an increasing iron content, but only up to the concentration of 2.5at%, with thermally annealed coatings characterized by higher Eg values than those of the non-annealed materials. Results of variable angle spectroscopic ellipsometry measurements indicate that both iron doping and thermal annealing have the effect of increasing refractive index of the films. An analysis of the coatings surface wettability, performed under conditions of an alternative exposure either to daylight or to the UV-B radiation, show the most important parameter to be the time of water contact angle return to its initial value under darkroom conditions. Finally, bactericidity studies of the UV-B irradiated samples, performed with the Escherichia coli DH5α bacteria, reveal the most extensive bactericidal effect observed for low iron concentrations, equally for both non-annealed and thermally annealed materials.

Pulsed laser deposited GeTe-rich GeTe-Sb2Te3 thin films.

Pulsed laser deposition technique was used for the fabrication of Ge-Te rich GeTe-Sb2Te3 (Ge6Sb2Te9, Ge8Sb2Te11, Ge10Sb2Te13, and Ge12Sb2Te15) amorphous thin films. To evaluate the influence of GeTe content in the deposited films on physico-chemical properties of the GST materials, scanning electron microscopy with energy-dispersive X-ray analysis, X-ray diffraction and reflectometry, atomic force microscopy, Raman scattering spectroscopy, optical reflectivity, and sheet resistance temperature dependences as well as variable angle spectroscopic ellipsometry measurements were used to characterize as-deposited (amorphous) and annealed (crystalline) layers. Upon crystallization, optical functions and electrical resistance of the films change drastically, leading to large optical and electrical contrast between amorphous and crystalline phases. Large changes of optical/electrical properties are accompanied by the variations of thickness, density, and roughness of the films due to crystallization. Reflectivity contrast as high as ~0.21 at 405 nm was calculated for Ge8Sb2Te11, Ge10Sb2Te13, and Ge12Sb2Te15 layers.

Effect of electron correlations on theFe3Siandα−FeSi2band structure and optical properties

We use the Vienna ab initio simulation package (vasp) for evaluation of the quasiparticle spectra and their spectral weights within Hedin's $\text{GW}$ approximation (GWA) for ${\text{Fe}}_{3}\text{Si}$ and $\ensuremath{\alpha}{\text{-FeSi}}_{2}$ within the non-self-consistent one-shot approximation ${\text{G}}_{0}{\text{W}}_{0}$ and self-consistent $\text{scGWA}$ with the vertex corrections in the particle-hole channel, taken in the form of two-point kernel. As input for ${\text{G}}_{0}{\text{W}}_{0}$, the band structure and wave functions evaluated within the generalized gradient corrected local-density approximation to density functional theory (GGA) have been used. The spectral weights of quasiparticles in these compounds deviate from unity everywhere and show nonmonotonic behavior in those parts of bands where the delocalized states contribute to their formation. The ${\text{G}}_{0}{\text{W}}_{0}$ and scGWA spectral weights are the same within 2%\char21{}5%. The scGWA shows a general tendency to return ${\text{G}}_{0}{\text{W}}_{0}$ bands to their GGA positions for the delocalized states, while in the flat bands it flattens even more. Variable angle spectroscopic ellipsometry measurements at $T=296$ K on grown single-crystalline $\ensuremath{\sim}50$-nm-thick films of ${\text{Fe}}_{3}\text{Si}$ on $n$-Si(111) wafer have been performed in the interval of energies $\ensuremath{\omega}\ensuremath{\sim}(1.3\char21{}5)$ eV. The comparison of ${\text{G}}_{0}{\text{W}}_{0}$ and $\text{scGW}$ theory with experimental real and imaginary parts of permittivity, refractive index, extinction and absorption coefficients, reflectivity, and electron energy loss function shows that both ${\text{G}}_{0}{\text{W}}_{0}$ and $\text{scGW}$ qualitatively describe experiment correctly, the position of the low-energy peaks is described better by the $\text{scGW}$ theory, however, its detailed structure is not observed in the experimental curves. We suggest that the angle-resolved photoemission spectroscopy experiments, which can reveal the fine details of the quasiparticle band structure and spectral weights, could help to understand (i) if the scGWA with this type of vertex correction is sufficiently good for description of these iron silicides and, possibly, (ii) why some features of calculated permittivity are not seen in optical experiments.

Observation of Optical Properties of Neodymium Oxide with Spectroscopic Ellipsometry

The annealing dependence of the optical properties of Nd2O3 films deposited by atomic layer deposition has been investigated by variable-angle spectroscopic ellipsometry (VASE). Based on VASE measurements, it is found that the refractive index and high-frequency dielectric constant decrease with increasing annealing temperature. The optical bandgap energy extracted using the Cody method varies from 4.13 eV for the as-grown state to 4.78 eV for the film annealed at 900°C. Moreover, three proposed physical models with corresponding optical models are applied to fit the experimental data to understand the possible structural change of the films with annealing temperature. Fitting results indicate that the interfacial interaction between the substrate and the film bulk occurs during the postdeposition annealing procedure and the surface properties of the Nd2O3 films are improved after annealing at 900°C. It is shown that the extracted VASE parameters well represented the characteristics of Nd2O3/SiO2/Si stacks with different annealing temperatures.

Iron doped thin TiO2 films synthesized with the RF PECVD method

Iron doped titanium dioxide coatings were synthesized with the help of RF plasma enhanced CVD technique. As a source of titanium, titanium chloride (IV) TiCl4 was used while iron pentacarbonyl (0) Fe(CO)5 served as iron supply. The coatings were diagnosed using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Their elemental and chemical composition was studied with the help of X-ray energy dispersive spectroscopy (EDS) and Fourier transform infrared (FTIR) spectroscopy, respectively. For the determination of their optical properties, variable angle spectroscopic ellipsometry (VASE) and ultraviolet-visible (UV-Vis) spectroscopy techniques were used. Iron content in the range of 0.07–11.5 at% was found in the films. Apart from oxygen, titanium and iron, a presence of trace amounts of chlorine, very likely originating from the titanium precursor, was recorded. FTIR studies showed that iron was built-in in the structure of TiO2 matrix. Surface roughness, assessed using SEM and AFM techniques, increases with an increasing content of this element. VASE measurements revealed an increase of the coatings refractive index with a growing iron concentration, with the extinction coefficient remaining low and independent of that parameter. Trace amounts of iron resulted in a lowering of an absorption threshold of the films as well as their optical gap, but the tendency was reversed for high concentrations of that element.

Optical characterization of epitaxial single crystal CdTe thin films on Al2O3 (0001) substrates

The optoelectronic properties of single crystal CdTe thin films were investigated by photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51eV and was consistent between spectroscopic measurements and previously reported values. Breadth of bandgap emission was consistent with high quality material. Low temperature photoluminescence spectra indicated a dominant emission consistent with bound excitons. Emissions corresponding to self-compensation defects, doping and contaminants were not found. Variable angle spectroscopic ellipsometry measurements over the near-UV to infrared range demonstrated sharp resonance peaks. All spectroscopic measurements indicate high quality thin film material of comparable or better quality than bulk CdTe.

Refractive Index and Effective Thickness Measurement System for the RGB Color Filter Coatings With Absorption and Scattering Properties

During the manufacturing processes of the thin-film transistor liquid-crystal display (TFT-LCD) panel, red, green, and blue (RGB) color filter coatings undergo the layer-adding process which causes the rough surfaces between the layers. The rough surfaces make acquiring an accurate measurement of the optical properties and thickness (n, k, d) much more difficult because the scattering effects occur. The effective layer-included model is considered in determining the (n, k, d) by including effective layers to reside between and above the multilayer (ML). To show the feasibility of the effective layer-included model, we examined the model by fitting the (n, k, d) for different virtual systems which contain different kinds of scatters reside between and above it. Our findings show that the fitted (n, k, d) can be closer to the assumed (n, k, d) by using the effective layer-included model rather than the standard model. Also, the tolerance of initial assigned (n, k, d) regions to obtain the accurate results are investigated. Further, both models are used to determine the (n, k, d) of the fabricated RGB color filter samples. In the experimental measurements, all reflection and transmission signals are measured by utilizing the in-house variable angle spectroscopic ellipsometry (VASE) system. Consequently, the thicknesses determined from effective layer-included model are closer to the thicknesses measured from profilometry (Alpha-step 100). Also, the transmissions under 0 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , 15 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , and 30 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> illuminations calculated from the fitted (n, k, d) through the effective layer-included model are closer to the VASE measurements rather than the standard model for each sample. We conclude that the effective layer-included model can be used to determine the accurate (n, k, d) of RGB color filter coatings with rough surface.

Compatibility of plasma-deposited linalyl acetate thin films with organic electronic device fabrication techniques

Contact angle and temperature-dependent variable angle spectroscopic ellipsometry measurements have been performed on plasma-deposited linalyl acetate thin films in order to provide insight into the compatibility of the material with current organic electronic fabrication techniques. XPS data on several substrates confirmed that the chemical properties of the thin films were substrate independent. The plasma-deposited layers were found to be insoluble in many solvents commonly used in the deposition of organic semiconducting layers, including chloroform and dichlorobenzene, and the wetting envelope for the surfaces presented. Thermal degradation was found to begin at ~200 °C, and up until this temperature the material’s thickness, refractive index and transparency in the visible region were constant. The exhibited properties show plasma-deposited linalyl acetate thin films to be compatible with state of the art organic electronic processing techniques.

Characterization of Interfacial Layer of DUV Coatings by Spectroscopic Ellipsometry

The Interfacial layer has an important influence on the optical performance of the deep ultraviolet (DUV) coatings. The variable angle Spectroscopic ellipsometry measurements of three kinds of DUV coating samples were performed in the wavelength range 150-500nm using a purged variable angle Spectroscopic ellipsometer. The samples include the single layer sample, double layers sample, and three layers sample. The thickness and optical index of the MgF2 layer and LaF3 layer as well as those of the interfacial layer between them were obtained by successful regression of the SE measurements. For the single layer, the agreement of the results between the SE and the spectrophotometic techniques was very good. The obtained thickness of the two kinds of interfacial layer was in consistent with the RMS results of the single layer obtained by AFM, indicating the obtained results were reliable.

Optical and microstructural characterisation of Au–Sn and Cu–Sn diffusive layers

Phase composition, crystallinity, optical and electrical properties were determined for Au–Sn and Cu–Sn ultra-thin films produced by sequential evaporating and co-depositing of metals on glass plates in a vacuum. Thickness of Sn films grown on top of Au(Cu) nanolayers (dAu(Cu)=20nm) was varied to obtain different atomic concentration ratios of Au(Cu)-rich diffusive samples up to 1:1. The samples were characterised using the XRD, SEM, spectroscopic ellipsometry and transmittance measurements. The XRD patterns indicated creation of AuSn and AuSn2 intermetallic phases at room temperature in both types of Au–Sn samples, formation of Cu6Sn5 compound in bilayer Sn/Cu samples and Cu10Sn3 intermetallic in the co-deposited Sn–Cu film. There was observed a substantial influence of morphology and phase composition on the effective complex dielectric functions and optical conductivity of the multiphase films, determined using the transmittance and variable angle spectroscopic ellipsometry measurements in the photon energy range of 0.6–6.5eV. Adopting the Drude–Lorentz parameterisation approach to optical spectra enabled to extract contributions related to the free-carriers, interband transitions and plasmonic effects. The optical resistivity agreed reasonably with the dc-resistivity results, which changed approximately from 17.5μΩcm to 26μΩcm and from 24μΩcm to 96μΩcm for investigated Au–Sn and Cu–Sn systems, respectively.

The role of alkane dithiols in controlling polymer crystallization in small band gap polymer:Fullerene solar cells

AbstractThe effect of the addition of 1,8‐octanedithiol (ODT) during processing on the microstructure of blend films of poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7(2,1,3‐benzothiadiazole)] (PCPDTBT) and [6,6]‐phenyl‐C71 butyric acid methyl ester ([70]PCBM) is studied. Grazing incidence X‐ray diffraction and absorption spectroscopy show that the crystalline order of PCPDTBT increases when ODT is introduced in the solution phase either to neat polymer systems or to blends with [70]PCBM. The increased crystalline order is accompanied by less dispersive hole transport in the polymer, and leads to a more efficient formation of a percolating fullerene network within the blend. This contributes to an increase in photocurrent generation. However, the bimolecular recombination rate as determined from photovoltage transients increases upon addition of ODT, limiting the power conversion efficiency to values well below those expected from the energy levels of PCPDTBT. We propose some explanations for this increase in bimolecular recombination, based also on variable angle spectroscopic ellipsometry measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Optical characteristics of pulsed laser deposited Ge–Sb–Te thin films studied by spectroscopic ellipsometry

Pulsed laser deposition technique was used for the fabrication of (GeTe)1−x(Sb2Te3)x (x = 0, 0.33, 0.50, 0.66, and 1) amorphous thin films. Scanning electron microscopy with energy-dispersive x-ray analysis, x-ray diffraction, optical reflectivity, and sheet resistance temperature dependences as well as variable angle spectroscopic ellipsometry measurements were used to characterize as-deposited (amorphous) and annealed (rocksaltlike) layers. In order to extract optical functions of the films, the Cody–Lorentz model was applied for the analysis of ellipsometric data. Fitted sets of Cody–Lorentz model parameters are discussed in relation with chemical composition and the structure of the layers. The GeTe component content was found to be responsible for the huge optical functions and thickness changes upon amorphous-to-fcc phase transition.

Thickness dependent absorption spectra in conjugated polymers: Morphology or interference?

The thickness dependence of the absorption spectrum of spin-coated films of poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] has been studied using reflectivity and variable angle spectroscopic ellipsometry measurements. It is found that, for films with thicknesses in the range of 18–178 nm, a single set of optical constants is sufficient to simulate accurately all the experimental data used, including the absorption spectra, independently of the film thickness or the processing conditions. Thus, the observed changes in the absorption spectrum with thickness can be fully accounted for by reflectivity and interference effects alone without the need to invoke morphology differences between films.

Electrophoretic Deposition of Au Nanocrystals inside Perpendicular Mesochannels of TiO2

Au nanocrystals stabilized by dodecanethiol were deposited into 100−150 nm thick TiO2 films with evenly spaced perpendicular nanopillars and mesochannels on the order of 10 nm supported on conducting ITO/glass electrodes. Electrophoretic deposition was used to enhance nanocrystal deposition within the mesoporous TiO2 film. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive X-ray (EDX), UV−vis spectroscopy, variable-angle spectroscopic ellipsometry (VASE), and scanning surface potential microscopy (SSPM) were used to characterize the resulting Au nanocrystal/TiO2 composites. Au nanocrystal loadings reached 21 wt % and were not kinetically limited at 10 min, relative to depositions performed for 20 h. Both VASE measurements of the anisotropy of the imaginary refractive index, k, and X-ray photoelectron spectroscopy (XPS) depth profiling studies indicate that Au nanocrystals are dispersed within the vertically aligned mesopores and distributed throughout the film. The mean penetration depth of a single nanocrystal penetrating inside the film is described with a model in terms of the electric field and a local deposition rate constant, which is influenced by ligand binding and architecture on the nanocrystal surface.

UV‐induced in‐plane anisotropy in layers of mixture of the azo‐dyes SD‐1/SDA‐2 characterized by spectroscopic ellipsometry

AbstractThe optical properties of the azo‐dyes SD‐1/SDA‐2, which are used for photoaligning of liquid crystals (LCs), are investigated with Variable Angle Spectroscopic Ellipsometry (VASE). Films of mixture of SD‐1/SDA‐2 are deposited by spin coating on silicon wafers. The estimated thickness is approximately 10 nm. To achieve photo‐induced anisotropy, one of the samples is illuminated during 15 minutes with linearly polarized UV light followed by thermal stabilization during 1 hour at 150 °C. VASE measurements are performed in the wavelength range 200–1350 nm at several angles of incidence and at different sample orientations. Dielectric functions of azo‐dye films without/with polarized UV light illumination were modelled using an ensemble of Lorentz oscillators. The results confirm the diffusion model proposed recently for explanation of the formation of the photo‐induced order in azo‐dye films under the action of polarized light. Refractive indices, their wavelength dispersion and thicknesses of films of SD‐1/SDA‐2 are reported here. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)