Temperature-dependent Spectroscopic Ellipsometry Research Articles

Temperature Coefficient of Electronic Polarizability in Thin Polymer Films Deposited on Si and SiO2 Substrates Determined via Spectroscopic Ellipsometry

Ellipsometry is widely used to determine the thermo-optical properties of thin polymer films. However, if the thermo-optic coefficient (TOC) and the linear thermal expansion coefficient (LTEC) are to be used to determine the temperature coefficient of electronic polarizability (TCEP) in thin polymer films, their values must be determined with the greatest possible accuracy, as both have the opposite effect. In this article, we analyze changes in ellipsometric parameters resulting from changes in the thin film temperature in order to develop a data analysis method for temperature-dependent spectroscopic ellipsometry that will facilitate the accurate determination of thermo-optical parameters, including the TCEP, in polymer thin films. As practical application examples, we identified optimal spectral windows to accurately determine the thermo-optical parameters of 50 to 150 nm-thick PMMA thin films deposited on Si and SiO2 substrates. The influence of thin-film thickness on the accuracy of TOC and LTEC determination is discussed.

Thermal and optical properties of P3HT:PC70BM:ZnO nanoparticles composite films

The results of studies on the influence of zinc oxide nanoparticles (ZnO-NPs) on the structural, thermal and optical properties of thin films of mixtures of phenyl-C71-butyric acid methyl ester (PCBM) with poly[3-hexylthiophene] (P3HT) of various molecular weights are described in this article. The structural properties of the layers of: polymers, mixtures of polymers with fullerenes and their composites with ZnO-NPs were investigated using X-ray diffraction. Whereas their glass transition temperature and optical parameters have been determined by temperature-dependent spectroscopic ellipsometry. The presence of ZnO-NPs was also visible in the images of the surface of the composite layers obtained using scanning electron microscopy. These blends and composite films have also been used as the active layer in bulk heterojunction photovoltaic structures. The molecular weight of P3HT (Mw = 65.2; 54.2 and 34.1 kDa) and the addition of nanoparticles affected the power conversion efficiency (PCE) of the obtained solar cells. The determined PCE was the highest for the device prepared from the blend of P3HT:PCBM with the polymer of the lowest molecular weight. However, solar cells with ZnO-NPs present in their active layer had lower efficiency, although the open-circuit voltage and fill factor of almost all devices had the same values whether they contained ZnO-NPs or not. It is worth noting that thermal studies carried out using temperature-dependent ellipsometry showed a significant effect of the presence of ZnO-NPs on the value of the glass transition temperature, which was higher for composite films than for films made of a polymer-fullerene blend alone.

Dielectric functions evolution and electronic bandgap manipulation by silicon doping for Sb2Te3 phase change films: Temperature dependent spectroscopic ellipsometry study

The dielectric function evolution and electronic bandgap manipulation by silicon (Si) doping for Sb2Te3 phase change films have been investigated by temperature dependent spectroscopic ellipsometry measurements. During the phase change from amorphous to rhombohedral structures, the significant contrast of electronic band structure for pure Sb2Te3 and Si-doped Sb2Te3 (SST) films as functions of temperature (210–620 K) and Si concentration (0%–12%) has been systematically studied by analyzing the dielectric functions, Tauc gap energy, and partial spectral weight integral. The distinct differences can be mainly attributed to the increment of structure order degree, originated from the change of local bonding arrangement. Based upon the evolutions of Tauc gap energy and partial spectral weight integral with increasing temperature for all four samples, it can be concluded that Si doping can inhibit the crystallization of amorphous films and accelerate the phase change process by serving as nanoscale heaters, which is helpful in improving the thermal stability of amorphous films. The elevated crystallization temperature and phase change rate by Si doping contribute to the dependability and endurance for SST-based phase change memory. The present data provide an important direction on the physical mechanism investigation of Si doping Sb2Te3 by optical techniques.

Temperature-dependent phonon mode and interband electronic transition evolutions of ε-InSe films derived by pulsed laser deposition

We report the temperature-dependent phonon modes and interband electronic transitions of InSe films on SiO2/Si substrates prepared by pulsed laser deposition. The microstructure results proved the ε phase structure and monochalcogenide phase composition with well-defined hexagonal InSe sheets. The temperature effects on lattice vibrations were discovered by Raman spectra from 123 K to 423 K. The frequency and full width at half maximum of the A2g1(LO) mode show a strong nonlinearity with the temperature. The energy band structure and electron–phonon interaction were studied by temperature-dependent spectroscopic ellipsometry with the aid of the Tauc–Lorentz model. It was found that five electronic transitions around 1.33, 1.61, 2.53, 3.73, and 4.64 eV generally show a redshift trend with the temperature. The present results can provide a valuable reference for future optoelectronic applications of InSe films.

Temperature-Dependent Spectroscopic Ellipsometry of Thin Polymer Films.

Thin polymer films have found many important applications in organic electronics, such as active layers, protective layers, or antistatic layers. Among the various experimental methods suitable for studying the thermo-optical properties of thin polymer films, temperature-dependent spectroscopic ellipsometry plays a special role as a nondestructive and very sensitive optical technique. In this Review Article, issues related to the physical origin of the dependence of ellipsometric angles on temperature are surveyed. In addition, the Review Article discusses the use of temperature-dependent spectroscopic ellipsometry for studying phase transitions in thin polymer films. The benefits of studying thermal transitions using different cooling/heating speeds are also discussed. Furthermore, it is shown how the analysis and modeling of raw ellipsometric data can be used to determine the thermal properties of thin polymer films.

Characterization of optical properties and thermo-optic effect for non-polar AlGaN thin films using spectroscopic ellipsometry

The non-polar a-plane AlGaN thin films with various Al compositions were grown successfully on the r-plane semi-polar sapphire substrates using the continuous growth, two-way pulsed-flow, and three-way pulsed-flow growth methods, respectively with metal-organic chemical vapor deposition. The optical properties and thermo-optic effect of these films were studied extensively for the first time with angle- and temperature-dependent spectroscopic ellipsometry (SE) under both isotropic and anisotropic fitting modes. The SE fitting results for the energy band-gap, the layer thickness, and the surface roughness of the non-polar GaN, AlGaN, and AlN thin films were found to be comparable or even consistent quite well with the characterization results of high-resolution x-ray diffraction, ultraviolet-visible absorption spectroscopy, and atomic force microscopy. It was revealed that the non-polar AlGaN has higher refractive index than its polar counterpart with the same Al composition, and the non-polar AlGaN with the lowest surface roughness could be achieved with the three-way pulsed-flow growth method. Moreover, it was demonstrated that the anisotropy for the non-polar AlGaN thin film increased with increasing the Al composition. These characterization results should be useful for the fabrication of the non-polar AlGaN-based high-temperature power and ultraviolet-polarized optoelectronic devices utilizing thermo-optical effect and optical anisotropy.

Exploration of a Ca1−x(NaCe)x/2Bi4Ti3.98(WNb)0.01O15 ceramic intermediate phase by temperature-dependent spectroscopic ellipsometry and Raman scattering

Different doping concentrations of Ca1−x(NaCe)x/2Bi4Ti3.98(WNb)0.01O15 (x=0, 3%, 4%, 5%, and 6%) were prepared by a traditional solid state reaction method. The effects of doping on the crystal structure, dielectric constant, electronic transitions, and lattice dynamics of doped CaBi4Ti4O15 were investigated by x-ray diffraction, temperature-dependent spectroscopic ellipsometry, and Raman scattering. The results showed that, when x<4%, the substitution of Na [rNa=0.95 Å with a coordination number (CN) = 6] is dominant, and the crystal structure symmetry is weakened. When x>4%, the substitution of Ce (rCe=1.01 Å, CN=6) is dominant, and the crystal structure symmetry begins to increase. As the doping concentration increases, the optical bandgap initially increases and then decreases, which is consistent with the evolution of the structural symmetry. Finally, evidence of the existence of an intermediate phase transformation of CaBi4Ti4O15-based ceramics is extracted by analyzing the thermal evolution of the electronic transitions and Raman modes. Therefore, spectroscopic ellipsometry and Raman spectroscopy can be used to probe intermediate phase transformations in ferroelectrics.

Interplay between electronic and structural transitions in VO2 revealed by ellipsometry

Temperature-dependent spectroscopic ellipsometry measurements, combined with AFM investigations, have been performed to reveal the interplay between the structural and electronic phase transitions during the insulator-to-metal transition in VO2 thin films with different thicknesses. A comprehensive analysis of the macroscopic optical response in the framework of an anisotropic Bruggeman effective medium approximation yields the hysteretic shape evolution of the metallic inclusions and the changes in film roughness due to the structural transitions during a temperature cycle. The authors show that the structural modifications in the material across the transition affect the metallic cluster shape in the vicinity of the percolation threshold, leading to an altered absorption by the localized plasmon resonances. The structural changes are supported by AFM measurements and can be explained by the particular strain present in different films.

Investigation of the Optical Properties of InSb Thin Films Grown on GaAs by Temperature-Dependent Spectroscopic Ellipsometry

InSb thin films were grown on GaAs substrates by metal organic chemical vapor deposition (MOCVD) and investigated by temperature-dependent spectroscopic ellipsometry (TD-SE). The refractive index, extinction coefficient, and dielectric function of the InSb films were extracted. The variation of critical point energies (E1, E1+Δ1, E2, $$ {E}_1^{\hbox{'}} $$ ) related to the excited state transitions of InSb and the second energy derivatives of the dielectric function at different temperatures showed that the InSb thin film had high electrical and optical stability at the evaluated temperatures. TD-SE analysis revealed a temperature range suitable for the use of InSb/GaAs-based devices. Beyond 250°C, InSb was heavily oxidized to form a thin In-O layer, causing a pronounced change in the optical constants. The results indicated that optimized InSb thin films grown on GaAs by MOCVD possess good optical and structural properties.

Dielectric functions and phase transitions of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ceramics revealed by spectroscopic ellipsometry

The dielectric functions and phase transitions of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZTN-x wt% MgO, x = 0, 0.1, 0.2, 0.5) ceramics have been investigated using temperature-dependent spectroscopic ellipsometry. With the aid of standard critical-point (SCP) model, four typical interband transitions can be observed from the second derivative of dielectric functions. Three transition process are observed and can be linked to antiferroelectric (AFE)-ferroelectric (FE) (T1 ∼ 250 K), low temperature ferroelectric phase [FER(LT)]to high temperature one [FER(HT)] (T2 ∼ 300 K), and ferroelectric-paraelectric phase (TC ∼ 500 K).

Synchrotron Radiation X-Ray Absorption Spectroscopy and Spectroscopic Ellipsometry Studies of InSb Thin Films on GaAs Grown by Metalorganic Chemical Vapor Deposition

A series of ultrathin InSb films grown on GaAs by low-pressure metalorganic chemical vapor deposition with different V/III ratios were investigated thoroughly using spectroscopic ellipsometry (SE), X-ray diffraction, and synchrotron radiation X-ray absorption spectroscopy. The results predicted that InSb films on GaAs grown under too high or too low V/III ratios are with poor quality, while those grown with proper V/III ratios of 4.20–4.78 possess the high crystalline quality. The temperature-dependent SE (20–300°C) and simulation showed smooth variations of SE spectra, optical constants (n, k, e1, and ε2), and critical energy points (E1, E1+Δ1, E′0, E2, and E′1) for InSb film when temperature increased from 20°C to 250°C, while at 300°C, large changes appeared. Our study revealed the oxidation of about two atomic layers and the formation of an indium-oxide (InO) layer of ∼5.4 nm. This indicates the high temperature limitation for the use of InSb/GaAs materials, up to 250°C.

Phase transition analysis of thermochromic VO2 thin films by temperature-dependent Raman scattering and ellipsometry

Vanadium dioxide (VO2) has a fantastic phase transition behavior from insulator to metal which could be used for numerous areas. Research on the optical constant and fine structure of VO2 at various temperatures is favorable to investigate the mechanism of phase transition, and benefits the design and fabrication of vanadium dioxide integrated devices or applications. So the micro structure variation and evolution process during the transformation of VO2 thin film which deposited via DC reactive magnetron sputtering method was characterized by temperature-dependent spectroscopic ellipsometry and Raman spectroscopy, along with X-ray diffraction. It could be observed that the Raman scattering peaks decreased corresponded with temperature, as well as the blue shift and red shift occurred in V–V vibration (@192 cm−1) and V–O vibration (@613 cm−1), respectively. The result revealed the M1 phase to R phase transform process of VO2 film. The refraction index determined by ellipsometry keep almost stable in ultraviolet and visible wavelength at various temperatures, but show abrupt decline at MIT temperature in near-infrared wave band. Moreover, the tendency of temperature-dependent refraction index at 990 nm match well with the sheet resistance which could be described by Boltzmann model.

Polarization behavior of zinc oxide thin films studied by temperature dependent spectroscopic ellipsometry

We report on the influence of temperature on the polarization behavior of highly oriented ZnO thin film. First, the investigation of crystal structure change is studied, providing supporting information on the macroscopic-scale polarization of the ZnO thin film. Here, the lattice distortion is investigated by using X-ray diffraction. Furthermore, the role of temperature on the polarization behavior of the ZnO thin film is comprehensively studied by using temperature dependent spectroscopic ellipsometry. Here, the temperature dependent dielectric function analysis and electronic excitation models are used to understand the mechanism of polarization. We found an interesting temperature dependence of electronic transition, where the red-shift absorption and exciton-phonon interaction are observed on the system. This interaction is responsible for the increase of polarization response, which is confirmed by dielectric susceptibility spectra. These results provide important understanding for the control of the polarization dependence on the working temperature of ZnO thin film, which is the essential key in the fabrication of switchable optical devices.

Determination of the glass transition temperature in thin polymeric films used for microelectronic packaging by temperature-dependent spectroscopic ellipsometry

We characterized the glass transition temperature Tg of thin polyimide films by temperature-dependent spectroscopic ellipsometry and compared the results to DSC measurements of the bulk polymer. The effect of the curing temperature on Tg and the thermal expansion α(T) was analyzed. An improved ellipsometric data evaluation was used to get most precise and reliable Tg data. Tg increased with increasing curing temperature, while the bulk Tg was considerably lower than the thin film Tg. Both observations are attributed to the temperature sensitive release of the imidization by-product 2-hydroxyethyl methacrylate (HEMA) and crosslinker components as well as decomposition products from the material. Variation in the curing temperatures of 230–380 °C led to an increase in the Tg of 34 °C.

Spectroscopic ellipsometry studies on ZnCdO thin films with different Cd concentrations grown by pulsed laser deposition

A set of Zn1-xCdxO thin films with different Cd concentrations was deposited on quartz substrates by Pulsed Laser Deposition (PLD). The properties of these films were investigated by variable angle and temperature dependent spectroscopic ellipsometry (SE). The experimental Zn1-xCdxO thin films showed a red shift in the absorption edge with increasing Cd contents at room temperature. For ZnCdO films with the similar Cd concentration, it has been found that the film thickness has important effects on the optical constants (n, k). The variations of optical constants (n, k) and the band gap, E0, with temperature (T) in 25°C–600°C for a typical Zn0.95Cd0.05O sample were obtained. The E0 vs T relationship is described by a T- quadratic equation.

Temperature dependent spectroscopic ellipsometry and Raman scattering of PbTiO_3-based relaxor ferroelectric single crystals around MPB region

To discover intrinsic relationship between optical bandgap and structural transformations in relaxor ferroelectric single crystals, electronic band structures and dielectric functions of xPb(In1/2Nb1/2)O3-(1 − x − y)Pb(Mg1/3Nb2/3)O3-yPbTiO3 single crystals (x∼0.27–0.28, y∼0.29–0.35) around morphotropic phase boundary have been investigated by variable-temperature (200–750 K) spectroscopic ellipsometry. It was found that the discontinuous evolution from the second derivative of dielectric functions corresponds to structural transformation patterns. Using the SCP (standard critical point) model, four typical interband transitions (Ea∼2.8 eV, Eb∼3.6 eV, Ec∼4.6 eV, and Ed∼5.4 eV) can be uniquely assigned. These interband transitions are mainly attributed to the contributions from B-O bonds and multiphase coexistence. Furthermore, a modified phase diagram based on interband transition variations with the temperature and PT composition for PIMN-PT crystals was provided. In order to verify the accuracy of phase transition temperature, temperature-dependent low-wavenumber Raman scattering was used as a support. The present results provide important supports for the theoretical model, which establish a quantitative relationship between the electronic transition and structural transformation for ferroelectric oxides.

Temperature dependent spectroscopic ellipsometry of Ag2Se and Ag2S with phase transitions from ionic to superionic conductivity state

AbstractTemperature dependent infrared (IR) and optical spectroscopic ellipsometry methods are used to investigate peculiarities of the transitions from the ionic conductivity to the superionic conductivity states in Ag2S and Ag2Se compounds. The structural phase transitions (SPTs), known as β→α transition in these semiconductors, are investigated also with spectroscopic ellipsometry methods. The electronic band structure of Ag2S at the SPT changes sharply from the monoclinic to the bcc phase, which occurs at T = 453 K in the super ionic conductivity state, where all the interband transitions in optic region, corresponding to the monoclinic structure of β‐phase, disappear. The α‐phase of Ag2S is characterised with new lines in the dielectric functions ε1 and ε2, which arise at 5‐6.5 eV. The interband transitions do not change in the SPT from the orthorhombic to the bcc in Ag2Se, which occurs at T > 406 K. In IR ellipsometry for the degenerated Ag2Se, the free carriers' plasma energy is displaced abruptly from ħω=0.075 eV at T=300 K to ħω=0.125 eV at T=338 K. The same effect is seen in the IR ellipsometry for Ag2S, where negative ε1 appears at T=434‐440 K. These facts indicate a drastic increase of the free carriers concentrations in both of the semiconductors at temperatures before the β→α SPT. In Ag2S the second abrupt shift of the free carriers plasma energy from 0.035eV to 0.1 eV occurs at T=453 K, corresponding to β→ α SPT. The second shift is due to the electronic band structure change of Ag2S. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase diagram and incommensurate antiferroelectric structure in (Pb1−1.5xLax)(Zr0.42Sn0.40Ti0.18)O3 ceramics discovered by band-to-band optical transitions

Optical properties and phase transitions of (Pb1−1.5xLax)(Zr0.42Sn0.40Ti0.18)O3 (PLZST 100x/42/40/18) ceramics with different compositions have been investigated by temperature dependent spectroscopic ellipsometry. Two interband critical points (Ecp1 and Ecp2) located at about 3.9 and 5.1 eV can be obtained by fitting standard line shapes to the second derivatives of the complex dielectric functions. Based on the band-to-band transitions, the phase diagram of PLZST ceramics can be well presented. Moreover, a peculiar incommensurate antiferroelectric state has been found to exist above the temperature of the normal commensurate antiferroelectric tetragonal structure. It can be stable below Curie temperature, evolving slowly with decreasing temperature towards the commensurate structure, which is due to strong pinning of incommensurate domain walls. The phenomena can result from a competition between ferroelectric ordering and antiferroelectric ordering caused by the lanthanum modification.

Phase transformation in multiferroic Bi5Ti3FeO15 ceramics by temperature-dependent ellipsometric and Raman spectra: An interband electronic transition evidence

Thermal evolution and an intermediate phase between ferroelectric orthorhombic and paraelectric tetragonal phase of multiferroic Bi5Ti3FeO15 ceramic have been investigated by temperature-dependent spectroscopic ellipsometry and Raman scattering. Dielectric functions and interband transitions extracted from the standard critical-point model show two dramatic anomalies in the temperature range of 200–873 K. It was found that the anomalous temperature dependence of electronic transition energies and Raman mode frequencies around 800 K can be ascribed to intermediate phase transformation. Moreover, the disappearance of electronic transition around 3 eV at 590 K is associated with the conductive property.

Intrinsic relationship between electronic structures and phase transition of SrBi2−xNdxNb2O9 ceramics from ultraviolet ellipsometry at elevated temperatures

The ferroelectric orthorhombic to paraelectric tetragonal phase transition of SrBi2−xNdxNb2O9 (x = 0, 0.05, 0.1, and 0.2) layer-structured ceramics has been investigated by temperature-dependent spectroscopic ellipsometry. Based on the analysis of dielectric functions from 0 to 500 °C with double Tauc-Lorentz dispersion model, the interband transitions located at ultraviolet region have shown an abrupt variation near the Curie temperature. The changes of dielectric functions are mainly due to the thermal-optical and/or photoelastic effect. Moreover, the characteristic alteration in interband transitions can be ascribed to distortion of NbO6 octahedron and variation of hybridization between Bi 6s and O 2p states during the structure transformation.