Spectroscopic Ellipsometry Investigation Research Articles

Spectroscopic ellipsometry investigation of a 6H–SiC single crystal plate for potential use in graphene optoelectronic devices

At room temperature, the spectroscopic ellipsometry SE parameters Epsi (ψ) and Delta (Δ) for 6H–SiC crystal substrate were measured in the wavelength range 350–1700 nm at four different angles of incidence, 60°, 65°, 70°, and 75°. An SE model was developed to extract optical consents of 6H–SiC crystal substrate from the experimentally measured SE parameters. In the wavelength range 350–1700 nm, the refractive index of the 6H–SiC crystal substrate was extracted and fitted to two terms of the Sellmeier dispersion function. In the wide spectral range (350–1700 nm), the Wemple-DiDomenico (WDD) dispersion model shows how the real refractive index changes with wavelength. The analysis of refractive index dispersion of the 6H–SiC crystal substrate using Sellmeier dispersion function and WDD optical model allows to extract oscillator strength Nfij, average oscillator energy Eo, dispersion energy of the single oscillator Ed, lattice oscillator strength El, Abbe dispersion number, energy gap Eg, density of valence electrons nv, refractive index at infinite wavelength n∞, lattice dielectric constant εL, the ratio of free carrier density to free carrier effective mass (N/m∗), and plasma frequency ωp. For the 6H–SiC crystal substrate, in the desired spectral range (240–1700 nm), it does not have any depolarization effect on the reflected polarized light.

Chemical state and atomic structure in stoichiovariants photochromic oxidized yttrium hydride thin films

Abstract We investigate the effective oxidation state and local environment of yttrium in photochromic YHO thin film structures produced by e-beam evaporation, along with their chemical structure and optical properties. Transmission electron microscopy images reveal the oxidized yttrium hydride thin film sample exhibiting a three-layered structure. X-ray photoelectron spectroscopy (XPS) measurements manifest that the oxidation state of yttrium is modified, dependent on the film’s composition/depth. Furthermore, Ion beam analysis confirms that this variability is associated with a composition gradient within the film. X-ray absorption spectroscopy at the Y K-edge reveals that the effective oxidation state of yttrium is approximately +2.5 in the transparent/bleached state of YHO. Spectroscopic ellipsometry investigations showed a complex non-linear optical depth profile of the related sample confirming the dominant phase of YHO and the presence of Y2O3 and Y towards the middle of the film. The first evidence of (n; k) dispersion curves for e-beam sputtered photochromic YHO thin films are reported for transparent and dark states.

In Operando Spectroscopic Ellipsometry Investigation of MOF Thin Films for the Selective Capture of Acetic Acid.

The emission of polar volatile organic compounds (VOCs) is a major worldwide concern of air quality and equally impacts the preservation of cultural heritage (CH). The challenge is to design highly efficient adsorbents able to selectively capture traces of VOCs such as acetic acid (AA) in the presence of relative humidity (RH) normally found at storage in museums (40-80%). Although the selective capture of VOCs over water is still challenging, metal-organic frameworks (MOFs) possess highly tunable features (Lewis, Bronsted, or redox metal sites, functional groups, hydrophobicity, etc.) suitable to selectively capture a large variety of VOCs. In this context, we have explored the adsorption efficiency of a series of MOFs thin films (ZIF-8(Zn), MIL-101(Cr), and UiO-66(Zr)-2CF3) for the selective capture of AA based on a UV/vis and FT-IR spectroscopic ellipsometry in operando study (2-6% of relative pressure of AA under 40% of RH), namely conditions close to the realistic environmental storage conditions of cultural artifacts. For that purpose, optical quality thin films of MOFs were prepared by dip-coating, and their AA adsorption capacity and selectivity were evaluated under humid conditions by measuring the variation of the refractive index as a function of the vapor pressures while the chemical nature of the coadsorbed analytes (water and AA) was identified by FT-IR ellipsometry. While thin films of ZIF-8(Zn) strongly degraded upon exposure to AA/water vapors, films of MIL-101(Cr) and UiO-66(Zr)-2CF3 present a high chemical stability under those conditions. It was shown that MIL-101(Cr) presents a high AA adsorption capacity due to its high pore volume but exhibits a poor AA adsorption selectivity under humid conditions. In contrast, UiO-66(Zr)-2CF3 was shown to overpass MIL-101(Cr) in terms of AA/H2O adsorption selectivity and AA adsorption/desorption cycling stability because of its high hydrophobic character, suitable pore size for adequate confinement, and specific interactions.

Spectroscopic ellipsometry investigation of free liquid-liquid and liquid-air interfaces

As ellipsometry capabilities gain wider publicity there is a remarkable interest in using the technique for unusual material characterizations. In the past decades, the possibility of non-destructive, contactless investigations regarding the structural and optical properties of free liquid surfaces has gained importance for both chemical and biological applications. To obtain high-accuracy ellipsometric measurements on liquid surfaces certain technical obstacles must be eliminated.The first obvious difficulty stems from the presence of environmental vibrations which make the liquid surface oscillate in a way that the resulting surface waves scatter out the light upon reflection. The second problem is that the liquid surface is always horizontal (vid. spirit leveled), thus the incident plane must be aligned according to it.In this study we present a solution for both above-mentioned problems by utilizing active pneumatic isolators. The capability of the method is demonstrated on bulk liquid samples of high purity distilled water, glycerol, and Polydimethylsiloxane (PDMS) and on liquid layer on liquid substrates (PDMS on glycerol).

Growth of Multiple Island Layers during Iron Oxide Atomic Layer Deposition: An Electron Microscopy and Spectroscopic Ellipsometry Investigation

Growth of Multiple Island Layers during Iron Oxide Atomic Layer Deposition: An Electron Microscopy and Spectroscopic Ellipsometry Investigation

Samarium doped cerium oxide thin films deposited by pulsed laser deposition

Samarium doped ceria thin films are an important material for the electrochemical devices. Fluorite structured Samarium Doped Cerium Oxide (SDC) thin films were deposited on Si (100) at 500 °C and 600 °C substrate temperatures by PLD (Pulsed Laser Deposition) from Sm0,24 Ce0,13 O0.63 target. X-ray Diffraction (XRD) proved the existence of SDC crystalline compound in both samples, deposited at 500 and 600 °C but not in the target. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) investigations revealed a high quality of the films, with lower roughness, of under 5 nm in the case of the sample deposited at 600 °C. The beginning of sintering process was evidenced by SEM in the case of samples deposited at 600 °C. X-ray Photoelectron Spectroscopy (XPS) indicated the increasing proportion of Ce3+ for PLD films comparatively to the target, in both cases, of substrate temperatures of 500 and of 600 °C, higher in the first case. Spectroscopic Ellipsometry (SE) investigation permitted the calculation of a refractive index under 2.2 in the visible (Vis)-near infrared (near-IR) domain for both samples. Nanoindentation tribometric measurements concluded that both elasticity modulus and hardness decreases with substrate temperature increasing. The obtained films are candidates for potential uses as electrolyte material for a new generation of electrochemical devices.

Evolution of Nanocrystalline Graphite’s Physical Properties during Film Formation

Nanocrystalline graphite (NCG) layers represent a good alternative to graphene for the development of various applications, using large area, complementary metal-oxide semiconductor (CMOS) compatible technologies. A comprehensive analysis of the physical properties of NCG layers—grown for different time periods via plasma-enhanced chemical vapour deposition (PECVD)—was conducted. The correlation between measured properties (thickness, optical constants, Raman response, electrical performance, and surface morphology) and growth time was established to further develop various functional structures. All thin films show an increased grain size and improved crystalline structure, with better electrical properties, as the plasma growth time is increased. Moreover, the spectroscopic ellipsometry investigations of their thickness and optical constants, together with the surface roughness extracted from the atomic force microscopy examinations and the electrical properties resulting from Hall measurements, point out the transition from nucleation to three-dimensional growth in the PECVD process around the five-minute mark.

Spectroscopic Ellipsometry Investigation of a Sensing Functional Interface: DNA SAMs Hybridization

AbstractHere, a comprehensive study of a label‐free detection platform for the recognition of oligonucleotide sequences based on hybridization of thiol‐tethered DNA strands self‐assembled on flat gold films is presented. The study exploits in‐buffer spectroscopic ellipsometry (SE) measurements, a noninvasive method sensitive to monolayer films, supported by surface mass density change measurements (Quartz Crystal Microbalance with Dissipation, QCM‐D) obtained under comparable experimental conditions. SE and QCM‐D allow monitoring deposition of molecular precursors and DNA chain hybridization. Combining SE measurements with QCM‐D data paves the way for quantification of the assay through the possible calibration of SE data. Optical measurements also demonstrate the selectivity and recovery properties of the sensing platform. Broadband SE measurements are interpreted by means of an effective optical model. The model, complemented by information on film thickness (scanning probe nanolithography), and surface composition (monochromatic X‐ray Photoemission Spectroscopy, XPS), enables a clear spectral identification of UV DNA resonances and the formation of the thiolate interface with gold. Spectroscopic validation of the hybridization is complemented by employing labeled target strands. The influence of hybridization on UV resonances and optical thickness of the DNA film is discussed in the light of hypochromism, through comparison with QCM‐D data.

Transparent Dielectric TiO2/SiO2 Coatings for Thermal Shielding and Self-Cleaning Applications

The economical fabrication of one-dimensional dielectric reflectors using the hybrid sol-gel spin coating process is significant compared to sophisticated chemical and physical vapor deposition techniques. In this work, we opted for alternate layers of TiO2 and SiO2 films to fabricate near-infrared dielectric reflectors owing to their high refractive index contrast and easy tunability of the reflection window in the desired spectral range. X-ray diffraction (XRD) studies of the monolayer TiO2 and SiO2 confirmed the existence of anatase-TiO2 and amorphous-SiO2 phases, respectively. Spectroscopic ellipsometry investigation of TiO2 and SiO2 films revealed the refractive indices of 2.6 and 1.5, respectively. The field-emission scanning electron microscopy (FESEM) analyses evidenced the fabrication of 2.5 stacks/bilayers of TiO2/SiO2 (TiO2/SiO2)2.5S. The reflectance measurement demonstrated 100% reflection in the near-infrared region with its center wavelength of 833 nm. In addition, we have examined the water contact angle of various samples using the sessile drop technique, and 2.5 stacks-based reflector showed its lowest contact angle of 29.3o, which suggests its anti-fogging and self-cleaning applications.

Spectroscopic Ellipsometry Investigation of Au‐Assisted Exfoliated Large‐Area Single‐Crystalline Monolayer MoS2

The recent Au‐assisted mechanical exfoliation technique has enabled scalable fabrication of millimeter‐sized, high‐quality monolayer 2D crystals. Herein, spectroscopic ellipsometry characterization of as‐exfoliated single‐crystalline monolayer MoS2 is reported. By developing layer‐by‐layer ellipsometry measurements and analysis, the complex dielectric function of monolayer MoS2 is extracted. Compared with bulk MoS2, the A and B exciton peaks in the dielectric function spectrum nearly disappear in the 1L MoS2–Au hybrid system. The charge transfer at the 1L MoS2–Au interface is responsible for this observation and the conclusion is also supported by Raman, photoluminescence, and differential reflectance measurements. These results reveal the impact of the Au substrate on the dielectric response of 2D monolayers with strong excitonic effect and will deepen our understanding of the 1L MoS2–Au interaction.

Spectroscopic ellipsometry investigation to study the microstructure evolution in boron-doped amorphous silicon films as a result of hydrogen dilution

Spectroscopic ellipsometry (SE) is a sophisticated technique to find the optical constants, bandgap and microstructure of thin layer. SE is used to study the microstructure evolution in boron-doped amorphous silicon films for different hydrogen flow rates (HFR). Spectral dependance of the real and imaginary parts of pseudo-dielectric constant is obtained at a fix angle of incidence (70°). Tauc–Lorentz (T–L) optical model is used to estimate the thickness, bandgap, optical constant and thickness of the top rough layer of the films, whereas Bruggeman effective medium approximation (BEMA) is applied to find the volume fractions of amorphous, crystalline and void phases. A shift in peak position from 3.65 to 4.1 eV in dielectric constant is observed as the hydrogen flow rate is increased from 30 to 70 SCCM. This is accompanied by the emergence of a peak near 3.4 eV, which belongs to the direct bandgap of c-Si. These observations suggest an improvement in microstructure of the films deposited at higher HFR. It is also supported by the observation that films deposited at higher HFR have higher magnitude of amplitude parameter and less broadening. Fitting of experimental data using BEMA also suggests that crystalline fraction increases and amorphous fraction decreases at higher HFR. The bandgap and thickness of top rough layer estimated from SE data are matched well with those obtained using transmission data and atomic force microscopy.

Spectroscopic ellipsometry investigation and morphological characterization of electrodeposited Cu2O thin films: annealing effect

ABSTRACT The present paper demonstrates the annealing temperature effect on the structural, optical and morphological properties of electrodeposited copper (I) oxide (Cu2O) thin films on ITO-glass substrates. The optical properties were itemized by using spectroscopic ellipsometry (SE) and an adequate model was simulated using WinElli II software. The finite difference time domain simulation, X-ray diffraction and atomic force microscopy (AFM) data show good conformity with the SE measurements. For the surface roughness, good matching between the results of SE and AFM measurements was observed. The refractive index and extinction coefficient had decreased with increasing annealing temperature. On the contrary, the optical gap (Eg) had increased as the annealing temperature increases. Two optical band gaps observed in the sample annealed at 350°C indicate the presence of CuO and Cu2O phases. These results were confirmed by X-ray diffraction measurements.

Promising Shadow Masking Technique for the Deposition of High-Efficiency Amorphous Silicon Solar Cells Using Plasma-Enhanced Chemical Vapor Deposition

In this work, a detailed description of the various steps involved in the fabrication of high-efficiency hydrogenated amorphous-silicon cells using plasma-enhanced chemical vapor deposition, and a novel shadow masking technique is presented. The influence of the different masking methods on the cell parameters was experimentally investigated. Particularly, the short-circuit current density (Jsc), the fill factor, the open circuit voltage (Voc), and the resistive losses indicated by the shunt (Rsh) and series (Rs) resistances were measured in order to assess the performance of the cells as a function of the masks used during the cell fabrication process. The results indicate that the use of a masking technique where the p-i-n structure was first deposited over the whole surface of a 20 cm2 × 20 cm2 substrate, followed by the deposition, deposits the back contact through a metal mask, and by the ultrasonic soldering of indium to access the front contact is a good alternative to laser scribing in the laboratory scale. Indeed, a record efficiency of 8.8%, with a short-circuit current density (Jsc) of 15.6 mA/cm2, an open-circuit voltage (Voc) of 0.8 V, and a fill factor of 66.07% and low resistive losses were obtained by this technique. Furthermore, a spectroscopic ellipsometry investigation of the uniformity of the film properties (thickness, band gap, and refractive index) on large-area substrates, which is crucial to mini-module fabrication on a single substrate and for heterojunction development, was performed using the optimal cell deposition recipes. It was found that the relative variations of the band gap, thickness, and refractive index n are less than 1% suggesting that the samples are uniform over the 20 cm2 × 20 cm2 substrate area used in this work.

Spectroscopic ellipsometry investigation of electronic states and optical properties of thin films from Ge30AsxSe70-x system

This paper deals with the investigation of the optical properties of thin films from the Ge30AsxSe70-x system. The complex permittivity, ε^=ε1+iε2, and the optical band gap, Egopt, were determined by spectroscopic ellipsometry measurements. The spectra of ε2 in the ultraviolet spectral range were analysed on the base of the existing literature data for the valence band spectra obtained by X-ray photoelectron spectroscopy. It was found that the absorption in the spectral range 2.0–6.5 eV is related to the bonding and anti-bonding p-orbitals of Ge, Se and As atoms. The temperature coefficients of linear expansion, refractive index and the band gaps were determined. The evaluated values for the non-linear refractive index, γ, and the two-photon absorption coefficient, β, showed that the thin films exhibit a highly non-linear refractive index at the telecommunication wavelength.

Antireflective coatings with high damage threshold prepared by laser ablation

Latest developments in the field of high power ultra-short pulse lasers have led to intensive studies dedicated to the fabrication possibility of new antireflective coatings which exhibit high damage threshold. Therefore, this study is focused on the deposition and characterization of metal oxide heterostructures followed by laser-induced damage threshold tests which evidence their application in high power laser optics. Al2O3, SiO2, and HfO2 layers are combined to obtain different heterostructures, i.e. HfO2/Al2O3/HfO2/Al2O3/HfO2 and HfO2/SiO2/HfO2/SiO2/HfO2. The metal oxide heterostructures are deposited in a controllable oxygen atmosphere, either at room temperature or high temperatures (600 °C) by pulsed laser deposition (PLD). The morphological, structural and optical properties of the as-deposited heterostructures are first investigated. Atomic force microscopy and spectroscopic ellipsometry investigations reveal a lower roughness of the heterostructures based on HfO2/Al2O3 layers grown at 600 °C as compared to those grown at room temperature. Furthermore, following the laser-induced damage threshold (LIDT) tests carried out with a Ti–Sapphire laser, higher LIDT values are obtained for the HfO2/Al2O3-based heterostructures than for the HfO2/SiO2-based heterostructures. The ability to control the morphological and structural properties of the antireflective coatings by modifying the deposition parameters of the metal oxide heterostructures demonstrates that PLD is a suitable technique for the manufacturing of antireflective coatings for high power ultra-short laser systems.

Effect of heating treatment and mixture on optical properties of coating materials used in gravitational-wave detectors

The interferometer mirrors of gravitational-wave detectors (GWDs) are Bragg reflectors made of alternate amorphous silica (SiO2) and titania-doped tantala (TiO2:Ta2O5) layers as low- and high-refractive index materials, respectively. A thermal treatment is usually performed to reduce both mechanical losses and near-infrared (NIR) optical absorptions of the coatings. The authors present a spectroscopic ellipsometry (SE) investigation of the effect of annealing and Ti:Ta mixing on Ta2O5 coatings deposited under conditions similar to those adopted for building up mirrors of GWDs. The broadband analysis covers both the NIR and the fundamental absorption threshold region. The data show an evident annealing-induced reduction of the fundamental optical absorption broadening. Modeling the data through the Cody-Lorentz formula confirms that NIR absorptions are below the SE sensitivity and shows a notable annealing-induced reduction of so-called Urbach tails. Titania-doping of tantala slightly reduces the Urbach energy. After the heating treatment, the resulting Urbach energy of the doped material is lower than that of annealed pure tantala. The observed reduction of Urbach tails is important because it parallels the reduction of the so-called internal friction observed in mechanical measurements so that SE emerges as a convenient tool for an agile diagnostic of both the optical and the mechanical quality of amorphous oxide coatings.

Spectroscopic Ellipsometry and White-Light Interferometry Investigation into Time-Dependent Oxidation Rates of Uranium in Pure Oxygen

Spectroscopic Ellipsometry and White-Light Interferometry Investigation into Time-Dependent Oxidation Rates of Uranium in Pure Oxygen

Optical constants of restored and etched reduced graphene oxide: a spectroscopic ellipsometry study

Structural and defective information in graphene are usually detected by Raman spectroscopy and electrical techniques; however, less attention has been paid to a spectroscopic ellipsometry investigation on graphene. Here, optical responses of graphene grown by chemical vapor deposition (G), reduced graphene oxide (RGO), restored RGO, and etched RGO are characterized by spectroscopic ellipsometry. The Lorentz oscillator model is used to fit the ellipsometric parameters. The resulting optical constants, refractive index and extinction coefficient are higher in less defective graphene, i.e. G and restored RGO. The results can be correlated with the Raman spectroscopic observation – fewer defects leading to higher density and more optical absorption. Our investigation opens a new angle of view to probe the structural information in the defective graphene and thus favors the production of high-quality graphene.

Microscopic nature of the asymmetric hysteresis in the insulator-metal transition of VO2 revealed by spectroscopic ellipsometry

Systematic spectroscopic ellipsometry investigations have been performed in order to elucidate the asymmetric insulator-to-metal transition in thin VO2 films. The comprehensive analysis of the obtained macroscopic optical response yields a hysteretic behavior, and in particular its asymmetry, when performed in the framework of an anisotropic effective medium approximation taking into account the volume fraction of the metal inclusions as well as their shape. We reveal the microscopic details of the percolation transition, namely that the shape of the metal inclusions goes through several plateaus, as seen in the evolution of the shape factor on both sides of the transition region and resulting in different critical volume fractions at the transition for the heating and cooling cycles.

Variable Angle Spectroscopic Ellipsometry investigation of CVD-grown monolayer graphene

Despite intensive investigations in the UV (ultraviolet) and visible range, the research on the optical properties of graphene in the extended near and mid infrared range by means of Spectroscopic Ellipsometry (SE) remains limited yet. Herein, the optical properties of a Chemical Vapor Deposition (CVD)-grown monolayer graphene, transferred from a copper substrate onto SiO2/Si, were studied in the broad energy range (0.38–6.2 eV) using Variable Angle Spectroscopic Ellipsometry (VASE). The morphological and the structural properties of the samples were investigated by Micro-Raman Spectroscopy, Wavelength Dispersive X-ray (WDX) analysis, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The Lorentz oscillator model proposed for the optical response of graphene fits very well the experimental data. An unintentional doping, revealed by Micro-Raman Spectroscopy and WDX, is reported.