Spectroscopic Ellipsometry Study Research Articles

Characterization of the Oxide Film Obtained by Wet Oxidation of Al-Rich AlGaAs

We investigate the characteristics of oxide film obtained by wet oxidation of AlxGa1-xAs with its Al concentration x between 0.55 and 0.99. 300-nm-thick AlxGa1-xAs grown on GaAs substrate by molecular beam epitaxy is wet-oxidized in a furnace at 410 °C for 120 min. Samples having x greater than 0.8 are confirmed to be oxidized from their observation, examined by X-ray diffraction showing the vanished crystallographic diffraction peaks. Scanning electron microscopy, X-ray reflectivity, and spectroscopic ellipsometry study determines the thickness, density and refractive index of the oxide films.

Spectroscopic Ellipsometry Analysis of a Thin Film Composite Membrane Consisting of Polysulfone on a Porous α-Alumina Support

Exposure of a thin polymer film to a fluid can affect properties of the film such as the density and thickness. In particular in membrane technology, these changes can have important implications for membrane performance. Spectroscopic ellipsometry is a convenient technique for in situ studies of thin films, because of its noninvasive character and very high precision. The applicability of spectroscopic ellipsometry is usually limited to samples with well-defined interfacial regions, whereas in typical composite membranes, often substantial and irregular intrusion of the thin film into the pores of a support exists. In this work, we provide a detailed characterization of a polished porous alumina membrane support, using variable-angle spectroscopic ellipsometry in combination with atomic force microscopy and mercury porosimetry. Two Spectroscopic ellipsometry optical models are presented that can adequately describe the surface roughness of the support. These models consider the surface roughness as a distinct layer in which the porosity gradually increases toward the outer ambient interface. The first model considers the porosity profile to be linear; the second model assumes an exponential profile. It is shown that the models can be extended to account for a composite membrane geometry, by deposition of a thin polysulfone film onto the support. The developed method facilitates practicability for in situ spectroscopic ellipsometry studies of nonequilibrium systems, i.e., membranes under actual permeation conditions.

Models and simulations in material science: two cases without error bars

We discuss two research projects in material science in which the results cannot be stated with an estimation of the error: a spectroscopic ellipsometry study aimed at determining the orientation of DNA molecules on diamond and a scanning tunneling microscopy study of platinum‐induced nanowires on germanium. To investigate the reliability of the results, we apply ideas from the philosophy of models in science. Even if the studies had reported an error value, the trustworthiness of the result would not depend on that value alone.

Optical, Structural, and Electrical Properties of Vanadium Dioxide Grown on Sapphire Substrates with Different Crystallographic Orientations

ABSTRACTThe phase transition of VO2 grown on sapphire having different crystallographic growth planes is examined experimentally. Measurements of electrical resistivity are compared with spectroscopic ellipsometry studies, to obtain complex index of refraction and plasma frequency, and transmission in the terahertz frequency range, each as a function of temperature.

Spectroscopic ellipsometry study of the Ag2O film deposited by radio-frequency reactive magnetron sputtering

Ag2O film has a potential application in high-density optical and magneto-optical disks. In this paper, a series of Ag2O films is deposited by radio-frequency reactive magnetron sputtering at different substrate temperatures, a deposition pressure of 0.2 Pa and an oxygen flow ratio of 2:3. The spectroscopic ellipsometry spectra of the films are fitted by using a general oscillator model (including one Tauc-Lorentz oscillator and two Lorentz oscillators). In an energy range between 1.5 eV and 3.5 eV, the refractive index and extinctive coefficient of the film are in ranges between 2.2 and 2.7, and between 0.3 and 0.9, respectively. The film indicates a clear abnormal dispersion in an energy range of 3.5 eV and 4.5 eV, meaning that the plasma oscillator frequency of the film is in this energy range . A redshift of the absorption edge of the film occurs with substrate temperature increasing, which can be attributed to the increased lattice strain. The optical constants of the film clearly show the dielectric properties.

Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2

We report the results of a comprehensive spectroscopic ellipsometry study of NaCu2O2, a compound composed of chains of edge-sharing Cu2+O4 plaquettes and planes of Cu1+ ions in a O-Cu1+-O dumbbell configuration, in the spectral range 0.75-6.5 eV at temperatures 7 -300 K. The spectra of the dielectric function for light polarized parallel to the Cu1+ planes reveal a strong in-plane anisotropy of the interband excitations. Strong and sharp absorption bands peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along (perpendicular) to the Cu2+O2 chains. They are superimposed on flat and featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on density-functional calculations, the anomalous absorption peaks can be assigned to transitions between bands formed by Cu1+ 3dxz(dyz) and Cu2+ 3dxy orbitals, strongly hybridized with O pstates. The major contribution to the background response comes from transitions between Cu1+ 3dz2 and 4px(py) bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the Cu2+O2 chains develops a weak two-peak structure centered at 2.1 and 2.65 eV upon cooling below 100 K, along with the appearance of spin correlations along the Cu2+O2 chains. These features bear a striking resemblance to those observed in the single-valent Cu2+O2 chain compound LiCuVO4, which were identified as an exciton doublet associated with transitions to the upper Hubbard band that emerges as a consequence of the long-range Coulomb interaction between electrons on neighboring Cu2+ sites along the chains. An analysis of the spectral weights of these features yields the parameters characterizing the on-site and long-range Coulomb interactions.

Magneto-optical Kerr-effect studies on copper oxide thin films produced by atomic layer deposition on SiO2

This work demonstrates the sensitivity of magneto-optical Kerr-effect (MOKE) spectroscopy to ultra-thin nonmagnetic films using the example of copper oxide. The films with an effective thickness between 0.6nm and 6nm are produced by atomic layer deposition (ALD) on silicon oxide substrates based on the Cu(I) β-diketonate precursor [(nBu3P)2Cu(acac)] (acac=acetylacetonate) at a process temperature of 120°C. The copper oxide films exhibit magneto-optical activity in the spectral ranges around 2.6eV and above 4eV. The evolution of the spectral features as a function of the number of ALD cycles is simulated numerically using the dielectric function and the Voigt constant of Cu2O as input parameters. The comparison between experimental and simulated MOKE spectra strengthens the conclusion drawn from spectroscopic ellipsometry studies that the thin film optical constants differ markedly from the bulk ones.

Annealing effects on microstructural and optical properties of Nanostructured-TiO 2 thin films prepared by sol–gel technique

In this paper we report on the effect of annealing on the microsctructural and optoelectronic properties of titanium dioxide (TiO 2) thin films prepared using sol–gel method onto silicon (Si) (100) and quartz substrates. The annealing temperatures range from 200 to 1000 °C. The Microstructural properties of annealed thin films were investigated by Thermal gravimetric analyses (TGA), X-ray diffraction (XRD) and Raman Spectroscopy. The surface morphology of the film was examined using Atomic Force Microscopy (AFM) method. The optical properties of TiO 2 thin films were characterized using UV–VIS and Spectroscopic ellipsometry. The results have shown that the TiO 2 thin films persist in the anatase phase even after annealing at 800 °C. The phase transformation from anatase to rutile occurred only when the films were annealed at 1000 °C. AFM studies revealed nanocrystalline structure where their shape and density depend strongly on the annealing temperatures. The elaborated nanostructured-TiO 2 thin films present a high transparency in the visible range. Spectroscopic ellipsometry (SE) study was used to determine the effect of annealing temperature on the thickness and on the optical constant of TiO 2 thin films. Spectroscopic ellipsometry and UV–VIS shows that the band gap of TiO 2 thin films was found to decrease when the annealing temperature increases. The Anatase phase was find to show higher photocatalytic activity than the rutile one.

Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500°C by reactive rf sputtering of GaAs target in argon–nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAsxN1−x (x≈0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.

Evolution of vertical phase separation in P3HT:PCBM thin films induced by thermal annealing

The achievement of the desirable morphology at the nanometer scale of bulk heterojunctions consisting of a conjugated polymer with fullerene derivatives is a prerequisite in order to optimize the power conversion efficiency of organic solar cells. The various experimental conditions such as the choice of solvent, drying rates and annealing have been found to significantly affect the blend morphology and the final performance of the photovoltaic device. In this work, we focus on the effects of post deposition thermal annealing at 140 °C on the blend morphology, the optical and structural properties of bulk heterojunctions that consist of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM). The post thermal annealing modifies the distribution of the P3HT and the PCBM inside the blend films, as it has been found by Spectroscopic Ellipsometry studies in the visible to far-ultraviolet spectral range. Phase separation was identified by AFM and GIXRD as a result of a slow drying process which took place after the spin coating process. The increase of the annealing time resulted to a significant increase of the P3HT crystallinity at the top regions of the blend films.

Real time spectroscopic ellipsometry of CuInSe2: Growth dynamics, dielectric function, and its dependence on temperature

AbstractThis Letter reports real time spectroscopic ellipsometry studies of copper indium diselenide (CuInSe2; CIS). A Volmer‐Weber nucleation process was identified from the measured thickness dynamics, and accurate dielectric functions were obtained in‐situ, avoiding oxidation while correcting for surface roughness. The energy and broadening parameters of the critical points in the dielectric functions obtained versus measurement temperature (including three previously unreported ones) yield a database that is valuable for on‐line materials analysis. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopic Ellipsometry Studies of CdS:O Layers for Solar Cells

This work reports the results of spectroscopic ellipsometry study of CdS thin films, deposited on glass substrates in the presence of oxygen at room temperature by rf sputtering. The data acquisition, together with the subsequent linear regression analysis to restore dielectric function has been performed over the photon energy range 1.5–5.0 eV. The obtained oxygen-free CdS films and reference bulk CdS have been found to be very much the same regarding their dielectric function spectra. On the other hand, blue-shift of the main structures in dielectric function has been observed for CdS:O films deposited at 5% value of O/Ar ratio. In spite of the broadening taking place on a par with the shift, the last turns out to be so large that provides transparency of CdS:O thin films in a spectral range above the energy gap of CdS. The obtained results allow to considering CdS:O as an improved window-layer material for solar cell application.

Vacuum ultra-violet spectroscopic ellipsometry study of sputtered BeZnO thin films

This paper reports on a systematic investigation of the optical properties of BeZnO thin films fabricated by radio frequency reactive magnetron sputtering technique using vacuum ultraviolet spectroscopic ellipsometry (VUV-SE). The thicknesses and optical constants of the thin films were determined in the wavelength range 138–1650 nm, using VUV-SE through the Tauc–Lorentz and Gaussian models. Refractive indices and extinction coefficients of the thin films were determined to be in the range n = 1.58–1.99 and κ = 1.0 × 10 −27–0.37, respectively. The absorption coefficient and the optical bandgap energy were then calculated. Measurement of the polarized optical properties reveals a high transmissivity (>90%) and very low absorptivity (<4%) for BeZnO films in the visible and near infrared regions at different angles of incidence. From the angle dependence of the p-polarized reflectivity we deduced a Brewster angle of about 58.5°.

Superconductivity-induced optical anomaly in an iron arsenide.

One of the central tenets of conventional theories of superconductivity, including most models proposed for the recently discovered iron-pnictide superconductors, is the notion that only electronic excitations with energies comparable to the superconducting energy gap are affected by the transition. Here, we report the results of a comprehensive spectroscopic ellipsometry study of a high-quality crystal of superconducting Ba0.68K0.32Fe2As2 that challenges this notion. We observe a superconductivity-induced suppression of an absorption band at an energy of 2.5 eV, two orders of magnitude above the superconducting gap energy 2Δ≈20 meV. On the basis of density functional calculations, this band can be assigned to transitions from As-p to Fe-d orbitals crossing the Fermi level. We identify a related effect at the spin-density wave transition in parent compounds of the 122 family. This suggests that As-p states deep below the Fermi level contribute to the formation of the superconducting and spin-density wave states in the iron arsenides.

Optical properties and switching of a Rose Bengal derivative: A spectroscopic ellipsometry study

Optical properties in terms of the complex-valued dielectric function were determined for spin-coated films of a Rose Bengal derivative using variable angle of incidence spectroscopic ellipsometry in the visible and infrared wavelength regions. In addition, the thickness and roughness of the films were determined and related to the solution concentration of Rose Bengal. Switching between two different oxidation states of the Rose Bengal derivative was investigated. The two states were chemically induced by exposure to vapors of hydrochloric acid and ammonia, respectively. A substantial and reversible change of the optical properties of the films was observed.

Spectroscopic ellipsometry study of the effect of illumination and thermal annealing on the optical constants of thin Ge–As–S films

The effects of illumination and post-illumination thermal annealing on the optical properties of chalcogenide thin (∼150 nm) films were studied by spectroscopic ellipsometry. The films were thermally evaporated from Ge30.8As5.7S63.5 and Ge32As5S63 glasses. They were exposed to illumination with an HBO 500 lamp and to subsequent thermal annealing at a temperature of 350 °C. Ellipsometric measurements in the spectral range 300–820 nm were carried out after each technological step. From the ellipsometric data analysis the optical constants (n, k, ε), optical band gap energy Eog and film thickness have been determined, while the oscillator energies E0 and Ed have been estimated applying the single-oscillator approximation theory. In the ε2 spectra three peaks, denoted by E1, E2 and E3, have appeared, which are attributed to interband transitions. By illumination, the values of the complex refractive index (), dielectric function () and dispersion energy Ed decrease, whereas the band gap energy (Eog) and the oscillator energy (E0) values increase. All these are accompanied by a ∼12–13% enhancement of film thickness. Thermal annealing leads to a further increase of the energetic parameters values, but causes a ∼14–15% decrease of film thickness in comparison to that of illuminated films. The E1 and E2 peaks diminish on illumination and post-illumination annealing, whereas the magnitude of the E3 peak decreases on illumination and increases by annealing, approaching its initial value for the as-deposited state.

Spectroscopic ellipsometry study on the dielectric function of bulk Ti2AlN, Ti2AlC, Nb2AlC, (Ti0.5,Nb0.5)2AlC, and Ti3GeC2 MAX-phases

The averaged complex dielectric function ε=(2ε⊥+ε∥)/3 of polycrystalline Ti2AlN, Ti2AlC, Nb2AlC, (Ti0.5,Nb0.5)2AlC, and Ti3GeC2 was determined by spectroscopic ellipsometry covering the mid infrared to the ultraviolet spectral range. The dielectric functions ε⊥ and ε∥ correspond to the perpendicular and parallel dielectric tensor components relative to the crystallographic c-axis of these hexagonal compounds. The optical response is represented by a dispersion model with Drude–Lorentz and critical point contributions. In the low energy range the electrical resistivity is obtained from the Drude term and ranges from 0.48 μΩ m for Ti3GeC2 to 1.59 μΩ m for (Ti0.5,Nb0.5)2AlC. Furthermore, several compositional dependent interband electronic transitions can be identified. For the most important ones, Im(ε) shows maxima at: 0.78, 1.23, 2.04, 2.48, and 3.78 eV for Ti2AlN; 0.38, 1.8, 2.6, and 3.64 eV for Ti2AlC; 0.3, 0.92, and 2.8 eV in Nb2AlC; 0.45, 0.98, and 2.58 eV in (Ti0.5,Nb0.5)2AlC; and 0.8, 1.85, 2.25, and 3.02 eV in Ti3GeC2.

Spectroscopic ellipsometry studies on hydrogenated amorphous silicon thin films deposited using DC saddle field plasma enhanced chemical vapor deposition system

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, ε(E)=ε1(E)+iε2(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5–4.5eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.

Extension of Far UV spectroscopic ellipsometry studies of High-κ dielectric films to 130 nm

Next generation CMOS devices use a high-κ dielectric layer (HfO2, HfSiO, HfSiON and La2O3) grown on thin interfacial silicon dioxide as the gate dielectric. The higher dielectric constant of the Hf oxide based film stack allows a decrease in equivalent oxide thickness (EOT). Because the high-κ film stack has a greater physical thickness than an electrically equivalent SiO2 film, the tunneling current decreases. It is a critical metrology requirement to measure the thickness of silicon dioxide and high-κ film stacks. Spectroscopic ellipsometry (SE) in the far UV wavelength region can be used to differentiate the high-κ films from silicon dioxide. This is due to the non-zero nature of the imaginary part of the dielectric function (beyond 6eV) in the far UV region for high-κ films. There has been some conjecture that optical studies should be extended beyond 150nm further into the VUV. This study addresses these concerns through determination of the dielectric function down to 130nm. We show the fitted dielectric function of hafnium silicates and lanthanum oxide down to 130nm. X-ray reflectivity (XRR) measurements were also performed on the high-κ films to complement the thickness measurements performed with SE.

Gravitational waves detector mirrors: Spectroscopic ellipsometry study of Ta2O5 films on SiO2 substrates

A stack of Ta2O5/SiO2 layers is presently used as coating layer of mirrors in interferometric detectors for gravitational waves. The sensitivity of these detectors is limited in the 50–300Hz frequency range by the mirror thermal noise, and it was suggested that mechanical losses in the Ta2O5 are the dominant source of noise. We focus here on Spectroscopic Ellipsometry (SE) results (in the 0.75÷5eV spectral range) obtained on high quality Ta2O5 films deposited on SiO2 substrates by Double Ion Beam Sputtering at the Laboratoire des Matériaux Avancés (Lyon, France). The films are extremely flat as indicated by the 0.2nm RMS roughness determined by Atomic Force Microscopy (AFM) on (20×20)μm2 areas. The comparison of the optical properties determined by SE with literature data, corroborated by X-ray Photoelectron Spectroscopy (XPS) data, suggests that the films present a non-ideal bulk stoichiometry and/or some degree of nanoporosity. The possible influence of an interface layer is also discussed.