Spectroscopic Ellipsometric Data Research Articles

Deconvolution of the biexciton structure of monolayer MoSe2 in spectroscopic ellipsometric data: a comparison of maximum-entropy methods

Deconvolution of the biexciton structure of monolayer MoSe2 in spectroscopic ellipsometric data: a comparison of maximum-entropy methods

Design of Liquid Crystal Materials Based on Palmitate, Oleate, and Linoleate Derivatives for Optoelectronic Applications

Herein, liquid crystalline derivatives based on palmitate, oleate, and linoleate moieties with azomethine cores were synthesized, and their physical, chemical, optical, and photophysical properties were investigated in detail. The mesomorphic activity of these materials was examined through polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The observed results revealed that the stability of the thermal mesophase depends on the terminal polar as well as on the fatty long-chain substituents. Purely smectogenic phases were detected in all three terminal side chains. A eutectic composition with a low melting temperature and a broad smectic A range was found by constructing a binary phase diagram and addressing it in terms of the mesomorphic temperature range. The energy bandgap of the palmitate-based derivative (Ia) was determined as 3.95 eV and slightly increased to 4.01 eV and 4.05 eV for the oleate (Ib) and linoleate (Ic) derivatives, respectively. The optical constants (n, κ, εr, and εi) were extracted from the fitting of measured spectroscopic ellipsometer data. The steady-state spectra of these samples exhibited a broad emission in the range 400-580 nm, which was found to be blue shifted to 462 nm for both Ib and Ic derivatives. The average fluorescence decay lifetime of the Ia derivative was found to be 598 ps, which became faster for the Ib and Ic derivatives and slower for the sample with a chloride end polar group.

Modulation of optical, photophysical and electrical properties of poly(3-hexylthiophene) via Gd:CdS nanoparticles

Herein, the impact of Gadolinium (Gd) doped cadmium sulphide (CdS) nanoparticles on the structural, optical, photophysical and electrical properties of poly(3-hexylthiophene) (P3HT) thin films have been investigated. The structural, morphological and vibrational properties of fabricated films were probed by measuring X-ray diffraction spectra, scanning electron microscope images and FT-Raman spectra, respectively. The absorption spectra of P3HT thin films was noted to be red shifted and energy bandgap slightly decreases with incorporation of Gd:CdS nanoparticles. The extinction coefficient (k), refractive index (n) and dielectric constants (εr, εi) of pure and hybrid films were extracted from spectroscopic ellipsometer data fitted by new amorphous dispersion model (for P3HT) and Adachi-New Forouhi dispersion model (for CdS) and noted that the optical constants decreases with increase of Gd:CdS nanoparticles concentrations. The photoluminescence intensity of P3HT/Gd:CdS films was quenched as compare to pristine P3HT, also an additional bands observed in hybrid films corresponds to emission from Gd:CdS nanoparticles. The average fluorescence life times of pristine P3HT thin films was evaluated to be 154 ps which drop to 109 ps for hybrid film, ascribes to formation of additional decay paths created due to Gd:CdS doping. The electrical resistivity (ρ) and mobility (μ) of P3HT films were measured from Hall Effect systems and noted to be decreases for hybrid films whereas charge carrier concentration (nC) and sheet resistance (RS) increases.

Influence of the indium on the structure and the optical properties of the ZnO thin film: Kramer kronig relation and the spectroscopic ellipsometry

The ZnO thin film doped indium has been fabricated by the sol–gel spin coater technique. The XRD confirmed the pristine ZnO and ZnO:In are polycrystalline phase. The optical constants (n, k) have been determined in terms of two analyses methods, the first was by Kramer kronig relation, and the second was by fitting a spectroscopic ellipsometric (SE) data (ψ, Δ) by means of the three-layer optical model. The calculated optical constants in terms of two analyses were comparable and affected by additional In at the expense of ZnO. The optical band gap reduced by the increase of the indium content at the expense of Zn from 3.1 to 2.7 eV. The atomic force microscope (AFM) was used to investigate the surface roughness, which was nearly approached with the surface roughness investigated by SE.

Magnetic and Optical Study of Nematic Liquid Crystal E7 Mixed Fe3O4 Ferrofluid

The magnetic and optical properties of suspension of ferrofluid (FF)-based spherical Fe3O4 magnetic nanoparticles (MNPs) in E7 nematic liquid crystal (NLC) matrix have been investigated experimentally in an isotropic phase. The ferronematic liquid crystal (FNLC) mixture exhibits a uniform molecular interaction under an externally applied magnetic field as low as 2 kOe. While the pure NLC shows the expected positive birefringence behavior, the FNLC mixture shows an improvement in tunability with mainly a negative birefringence behavior. The magnetic measurements of samples, including the magnetization curve, hysteresis loop, and pertinent parameters, are carried out using a vibrating sample magnetometer (VSM). The optical properties of samples within the visible spectral range of 380–900 nm are investigated using spectroscopic ellipsometer (SE) device. The results and analysis of magnetic measurements using VSM device show initial magnetic susceptibility of $2.03\times 10^{-6}$ , $8.25\times 10^{-5}$ , $9.85\times 10^{-4}$ , and $3.38\times 10^{-2}$ for FLC0, FLC1, FLC2, and FLC3 samples, respectively. The analysis of SE data shows ordinary and extraordinary refractive indices $n_{\mathrm {o}}$ and $n_{\mathrm {e}}$ of FNLCs samples from 1.85 to 1.67 and from 1.73 to 1.40, respectively.

Study of the phase transition properties and electronic band structures of VO2 thin films grown on different substrates

Optical properties and electronic structures of VO2 films obtained by magnetron sputtering method on quartz and sapphire substrates were analyzed by transmission spectrum and spectroscopic ellipsometry (SE). The metal-to-insulator transition (MIT) characteristics of the two thin films were compared and discussed correlating with their structural details. By fitting spectroscopic ellipsometric data, the dielectric functions, electronic transitions, and optical bandgap of VO2 films can be tuned by substrate effects. On quartz substrate, the VO2 film exhibits a lower transition temperature and broader thermal hysteresis (ΔH). It is found that the lowest order transition (E1) on quartz substrate does not reduce to zero in the metallic phase as expected. There are two reasons: one is the slight decrease in the filled a1g bands compared to the bands due to a greater compressive stress in the quartz films; the other is associated with the donor level (ED) and acceptor level (EA).

Investigating the Electron–Phonon Coupling of Molecular Beam Epitaxy-Grown Hg1−xCdxSe Semiconductor Alloys

Using spectroscopic ellipsometry, the temperature-dependence of the dielectric functions of a series of Hg1−xCdxSe thin films deposited on both ZnTe/Si(112) and GaSb(112) substrates were investigated. Initially, for each sample, room-temperature ellipsometric spectra were obtained from 35 meV to 6 eV using two different ellipsometers. Subsequently, ellipsometry spectra were obtained from 10 K to 300 K by incorporating a cryostat to the ellipsometer. Using a standard inversion technique, the spectroscopic ellipsometric data were modeled in order to obtain the temperature-dependent dielectric functions of each of the Hg1−xCdxSe thin films. The results indicate that the E1 critical point blue-shifts as a function of Cd-alloy concentration. The temperature-dependence of E1 was fitted to a Bose–Einstein occupation distribution function, which consequently allowed us to determine the electron–phonon coupling of Hg1−xCdxSe alloys. From the fitting results, we obtain a value of 17 ± 2 meV for the strength of the electron–phonon coupling for Hg1−xCdxSe alloy system, which compares nominally with the binary systems, such as CdSe and CdTe, which have values around 38 meV and 16 meV, respectively. This implies that the addition of Hg into the CdSe binary system does not significantly alter its electron–phonon coupling strength. Raman spectroscopy measurements performed on all the samples show the HgSe-like transverse optic (TO) and longitudinal optic (LO) phonons (∼ 130 cm−1 and ∼ 160 cm−1, respectively) for all the samples. While there is a slight red-shift of the HgSe-like TO peak as a function of the Cd-concentration, HgSe-like LO peak does not significantly change with the alloy concentration.

Subtle vertical structures of porous anodic alumina films for use as waveguides

Since porous anodic alumina (PAA) is a frequently-used optical waveguide material, accurate characterization of its structure parameters and optical properties is in urgent need. To characterize PAA, spectroscopic ellipsometry is preferred due to its undamaged detection, no sample pretreatment, and having a coverage area relatively larger than that of scanning electron microscopy. For spectroscopic ellipsometric data fitting, previous studies usually adopted a four-layer model, which displays a large bias from the raw data. Here, a modified six-layer model is built in consideration of the more elaborate porous layer that is the dominating contributor for the optical property of a PAA film. By using this six-layer model, PAA films with different thicknesses and under different oxidation voltages were analyzed, and the disperse curves of the porous layer were provided. This study will be helpful for learning the subtle structure of PAA and widen its applications for optical purposes.

Characterization of non-vacuum CuInxGa1 − xSe2 thin films prepared by low-cost sequential electrodeposition technique

Copper indium gallium diselenide (CuInxGa1−xSe2) thin films were grown on indium tin oxide coated glass substrate by a sequential electrodeposition technique. The deposition bath consisted of an aqueous solution of 10mM CuCl2, 10mM InCl3, 20mM GaCl3, 20mM H2SeO3 as precursors, and 200mM LiCl. The pH of the solution was adjusted to 1.7 by adding HCl. Cu, In, Cu, Ga, Cu, Se components were deposited sequentially. Cu/In/Cu/Ga/Cu/Se stacked layers were annealed at 250, 350, 450 and 550°C for 30 min. X-ray diffraction (XRD) studies showed the produced CIGS thin films had polycrystalline structure. From scanning electron microscopy studies, it was found that CIGS thin films exhibited different surface appearances depending on annealing temperature. Energy dispersive x-rays analysis showed that elemental ratio of Se decreased from 46.99 to 14.84 (%) with increasing of annealing temperature. Optical band gap of the CIGS films varied between 1.41 and 2.19 eV. Thicknesses and refractive indices of the produced CIGS thin films were calculated by fitting spectroscopic ellipsometric data (ψ and ∆) by using Cauchy model. Deposited CIGS thin films were p-type semiconductor with carrier concentration of ~1016 cm−3.

Investigation of localized surface plasmon resonance of TiN nanoparticles in TiN_xO_y thin films

Titanium oxynitride (TiNxOy) thin films are deposited by RF sputtering with various N2 flow rates. Two distinct absorption bands, one in the photon energies of ~0.5 – 2.5 eV and the other in the energies of ~3.5 – 5 eV, which correspond to the localized surface plasmon resonance (LSPR) of the TiN nanoparticles in the thin films and the interband transitions in TiNxOy, respectively, are observed in the absorption measurement. A Drude-Lorentz model, including the contributions of the free electrons, LSPR and interband transitions, is able to well fit the spectroscopic ellipsometric (SE) data. The resonance energy and strength of the LSPR oscillator are accurately determined from the SE analysis. The resonance energy is in the range of ~1 – 1.3 eV and blue-shifts with increasing N2 flow rate; and the strength decreases significantly with increasing N2 flow rate. The plasma energy yielded from the SE analysis shows a correlation with the conduction electron concentration obtained from the Hall effect measurement. It is shown that the LSPR plays a significant role in the complex dielectric function of the TiNxOy grains at the low photon energies (~0.5 – 1.5 eV).

Optical characterization of polycrystalline ZnSe1−xTex thin films using variable angle spectroscopic ellipsometry and spectrophotmetery techniques

Polycrystalline ZnSe1−xTex (0.0≤x≤1.0) thin films were deposited by the electron beam deposition technique on corning glass substrate. Two different techniques, Variable Angle Spectroscopic Ellipsometry (VASE) and spectrophotometry, have been applied and compared for characterization of the optical properties of ZnSe1−xTex films. The film thickness and optical constants (refractive index (n) and extinction coefficient (k)) of polycrystalline ZnSe1−xTex films were obtained by fitting the spectroscopic ellipsometric data (ψ, ∆) using a three-layer model system in the wavelength range from 400 to 1100nm. Nevertheless, the optical band gap Egopt determined from k values indicating a direct allowed transition. The optical studies of the polycrystalline ZnSe1−xTex films showed that the refractive index increases and the Egopt decreases. It is worth noting that the obtained values of the optical band gap of the different Te doped films remain in between the reported optical energy gap values of the two ends of the solid solution,ZnSe and ZnTe thin films,. Furthermore, the transmittances spectra of the ZnSe1−xTex films are obtained experimentally from spectrophotometry measurements and theoretically calculated using Murmann's exact equation. Comparing the results yielding a fully agreement between experimental and fitted transmittance data.

Spectroscopic ellipsometry of Zn1−xCuxO thin films based on a modified sol–gel dip-coating technique

Nanocrystalline Zn1−xCuxO thin films (x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by sol–gel dip-coating technique on a quartz substrate. These films were annealed at 350°C for 2h. The X-ray diffraction showed a hexagonal crystal structure with high intensity peak for the (002) reflection plane indicating preferential growth along the c-axis of the crystal lattice. The peak position related to the (002) peak was shifted as a result of the copper ion incorporation, confirming the interstitial substitution of the zinc ions by the copper ions. This interstitial substitution leads to a decrease of an average crystallite size and lattice constants and an increase of the micro-strain up to 2at.% of the copper amount. The surface morphology was explored by scanning electron microscopy which confirmed the homogenous distribution of nanoparticles in the deposited films along the quartz substrates. The energy dispersion X-ray spectroscopy revealed absence of impurities in the as-deposited films. The high resolution electron microscopy and selected area electron diffraction depicted that the films have polycrystalline nature. The film thickness and optical constants of the Zn1−xCuxO thin films were estimated by fitting the spectroscopic ellipsometric data (ψ and Δ) using three different models. The refractive index was fitted using harmonic oscillator model from which the oscillator and the dispersive energies were found. The dielectric constant, dielectric loss, energy loss functions were also determined.

Spectroscopic ellipsometry and magneto-transport investigations of Mn-doped ZnO nanocrystalline films deposited by a non-vacuum sol–gel spin-coating method

Abstract Nanocrystalline Zn 1− x Mn x O films ( x = 0, 0.05, 0.1, 0.15, and 0.2) were deposited onto corning glass substrates by a non-vacuum sol–gel spin coating method. All of the films were annealed at 450 °C for 2 h. The structural, optical and magneto-transport properties were investigated by X-ray diffraction, spectroscopic ellipsometry and a system for the measurement of the physical properties. X-ray diffraction analysis of the films reveals that the Mn-doped ZnO films crystallize in the form of a hexagonal wurtzite-type structure with a crystallite size decreases with an increase of the Mn concentration. It was also found that the microstrain increases with the increase of the Mn content. Evidence of nanocrystalline nature of the films was observed from the investigation of surface morphology using transmission, scanning electron microscopy and atomic force microscopy. The optical constants and film thicknesses of nanocrystalline Zn 1− x Mn x O films were obtained by fitting the spectroscopic ellipsometric data ( ψ and Δ ) using a three-layer model system in the wavelength range from 300 to 1200 nm. The refractive index was observed to increase with increasing Mn concentration. This increase in the refractive index with increasing Mn content may be attributed to the increase in the polarizability due to the large ionic radius of Mn 2+ compared to the ionic radius of Zn 2+ . The optical band gap of the nanocrystalline Mn–ZnO films was determined by an analysis of the absorption coefficient. The direct transition of the series of films was observed to have energies increasing linearly from 3.17 eV ( x = 0) to 3.55 eV ( x = 0.2). Magnetoresistance (MR) was measured from 5 K to 300 K in a magnetic field of up to 6 T. Low-field positive MR and high-field negative MR were detected in Mn-doped ZnO at 5 K. Only negative MR was observed for temperatures above 200 K. The positive MR in Mn-doped ZnO films was observed to decrease drastically when the temperature increased from 5 K to 100 K. The isothermal MR of Zn 1− x Mn x O films with different Mn concentrations at 5 K reveals that the increase of the Mn content induces a giant positive MR above x = 0.05 and reaches up to 55% at an applied field of 30 kOe for x = 0.2.

Dielectric response of AlP by in-situ ellipsometry

We present and analyze pseudodielectric function data < ε> = < ε 1> + i< ε 2> of AlP from 0.75 to 5.05 eV. The sample is a 1.0 μm thick AlP film grown on (001) GaAs by molecular beam epitaxy (MBE). Spectroscopic ellipsometric data were obtained before removing the sample from the MBE chamber to avoid oxidation and related artifacts. Analysis of interference oscillations and corrections for overlayer effects with a multilayer parametric model yield the closest representation to date of the intrinsic bulk dielectric response ε of AlP. From this analysis we obtain the energies of the E 0′ and E 1 critical points of AlP.

Ellipsometric studies on TiO 2 thin films synthesized by spray pyrolysis technique

TiO 2 thin films were synthesized on quartz substrates at substrate temperatures of 350 °C and 450 °C by thermal spray pyrolysis technique using titanium oxy-acetyl acetonate as a precursor. The optical properties of the thin films were characterized by a Spectroscopic Ellipsometer (SE). The surface morphology of the thin films was studied using Atomic Force Microscopy (AFM). The surface roughness values obtained using AFM and SE was compared. The refractive indices of the films were computed using a point by point ellipsometric data extraction procedure. The porosity of the films were modeled from the optical data by effective medium approximation and corroborated from empirical relations. Using Forouhi–Bloomer optical dispersion model, further treatment of SE data was carried out. The experimental investigations and modeling of the data were directed towards optical benchmarking of spray pyrolyzed titania thin films.

Synthesis and Characterization of Mo Films Deposited by RF Sputtering at Various Oxygen Ratios

We report here the results of deposition and physicochemical characterization of molybdenum thin films deposited by RF magnetron sputtering. The physical and the chemical properties of these films were examined with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The obtained film at 0% oxygen ratio showed a crystalline cubic Mo(110) phase. After the oxygen ratio had been increased to more than 5% in the sputtering gas, the molybdenum films were formed as an amorphous phase. The thickness of the Mo thin film was drastically decreased from 1000 to ca. 70 nm after introducing oxygen in the sputtering gas, which was confirmed by spectroscopic ellipsometer (SE) and scanning electron microscopy (SEM). The calculated band gap of the film deduced from the SE data increased from 3.17 to 3.63 eV by adding oxygen to the sputtering gas. The roughness of the Mo film was examined with atomic force microscopy (AFM), and it was dramatically decreased by introducing oxygen during sputtering. XPS results revealed that the ratio of metallic Mo species in the film decreased, as the ratio of oxygen increased in the sputtering gas but the contents of the Mo(VI) species increased. In addition, Mo species with low oxidation states (IV and V) were fully oxidized to Mo(VI) at low oxygen content in the sputtering gas.

Spectroscopic Ellipsometry Characterization of Optical Properties for Ti-Doped SiO2 Mesoporous Films

Ti-doped SiO2 Mesoporous films were synthesized by sol-gel process using surfactant cetyltrimethyl ammonium bromide as the template. Spectroscopic ellipsometer and UV-VIS spectrophotometer were used to characterize the optical properties. The Cauchy model is presented well in fitting spectroscopic ellipsometric data for the optical constants. The relation between miscrostructure, optical constants and optical character are discussed. The results show that Ti+4 cations have no effect on the dispersion relation of the films while they have obvious effect on the value of the optical constants. The refractive index is inversely proportional to the porosity of the films.

Optical constants of graphene measured by spectroscopic ellipsometry

A mechanically exfoliated graphene flake (∼150×380 μm2) on a silicon wafer with 98 nm silicon dioxide on top was scanned with a spectroscopic ellipsometer with a focused spot (∼100×55 μm2) at an angle of 55°. The spectroscopic ellipsometric data were analyzed with an optical model in which the optical constants were parameterized by B-splines. This parameterization is the key for the simultaneous accurate determination of the optical constants in the wavelength range 210–1000 nm and the thickness of graphene, which was found to be 3.4 Å.

Structure, optical properties and thermal stability of pulsed sputter deposited high temperature HfO x/Mo/HfO 2 solar selective absorbers

Solar selective coatings of HfO x /Mo/HfO 2 were deposited on copper (Cu) and stainless steel (SS) substrates using a magnetron sputtering system. The HfO x and HfO 2 layers were deposited from the sputtering of Hf target in Ar+O 2 plasma using an asymmetric bipolar-pulsed direct current generator, whereas the Mo layer was deposited from the sputtering of Mo target in the Ar plasma. The optimized HfO x /Mo/HfO 2 multilayer absorber on Cu substrate exhibited high solar absorptance ( α=0.905–0.923) and low thermal emittance ( ε 82 °C =0.07–0.09). Similarly, on SS substrates the optimized coatings exhibited α and ε 82 °C in the ranges of 0.902–0.917 and 0.15–0.17, respectively. The X-ray diffraction data showed that the HfO x /Mo/HfO 2 coating consists of tetragonal and monoclinic phases of HfO 2, which was confirmed by micro-Raman spectroscopy data. The bonding structure of the HfO x and the HfO 2 layers were confirmed using X-ray photoelectron spectroscopy data. The optical constants ( n and k), measured using spectroscopic ellipsometry, showed that the top HfO 2 layer acts as an antireflection coating and the bottom two layers (HfO x and Mo) are the main absorber layers. The analysis of the spectroscopic ellipsometric data indicated that the band gap of HfO x and HfO 2 layers were 3.90 and 5.82 eV, respectively, indicating non-stoichiometric nature of HfO x . In order to study the thermal stability of the HfO x /Mo/HfO 2 coatings, they were subjected to heat treatment in air and vacuum at different temperatures ( T A). The HfO x /Mo/HfO 2 coatings deposited on Cu substrates were thermally stable up to 400 °C for 2 h in air. Addition of a thin Mo interlayer (40 nm) in the HfO x /Mo/HfO 2 coating (i.e., Mo/HfO x /Mo/HfO 2) deposited on Cu substrates exhibited high solar selectivity ( α/ ε) of 0.872/0.09 even after heat-treatment in air up to 500 °C for 2 h. At T A=525 °C, the solar selectivity decreased drastically ( α/ ε=0.761/0.35) due to the formation of MoO 2, MoO 3 and HfMo 2O 8 phases. The Mo/HfO x /Mo/HfO 2 coatings deposited on SS substrates showed no significant changes in α and ε values after annealing at 500 °C in air and at 800 °C in vacuum. These results were confirmed by micro-Raman spectroscopy measurements, which showed the compositional stability of these coatings up to 500 °C in air and 800 °C in vacuum.

A study of the interaction of gallium arsenide with wet chemical formulations using thermodynamic calculations and spectroscopic ellipsometry

This paper investigates the effectiveness of different wet chemical treatments and their ability to produce/regrow a thin, stable surface oxide layer on GaAs. Results from thermodynamic considerations indicate that a stable surface oxide layer, free of excess arsenic and arsenic oxides, can be achieved by properly choosing aqueous solutions targeted within the GaAs solubility range (pH < 3 or pH > 11) followed by deionized (DI) water rinsing. This is further corroborated by spectroscopic ellipsometric data that can qualitatively, but correctly, identify the thickness of the surface oxide layer after different wet chemical treatments. Specifically, samples treated with acidic solutions based on HCl, HF, and H 3PO 4 and diluted ammonium hydroxide solution produce a more stable surface layer that is thinner than the native oxide layer on GaAs. The results and subsequent discussion are presented in the context of an attempt at achieving a well passivated GaAs surface, free of excessive surface state defects responsible for Fermi-level pinning.