Spectral Ellipsometry Measurements Research Articles

AgGaTe2 – The thermoelectric and solar cell material: Structure, electronic, optical, elastic and vibrational features

Silver gallium telluride AgGaTe2 is a promising and one of few multifunctional energy convertor crystal for processing solar energy or waste heat into electricity (by the thermoelectric, photovoltaic and recently intensively studied thermophotovoltaic systems). Its electronic structure, optical, elastic and vibrational properties were consistently and systematically studied in the framework of the first principles calculations. It should be emphasized that the results of our investigations were verified by comparison with the collected available experimental and vestigial theoretical data. The main spectral features of the electronic band structure and origin of the electronic levels affecting the physical and chemical properties are analyzed in detail. Spectral dependencies of both real ε1 and imaginary ε2 components of dielectric function ε(ω) have been calculated showing a good agreement with spectral ellipsometry measurements. The anisotropy of optical and elastic properties, which is important for possible applications, has been highlighted by means of the 2D and 3D graphs. Phonon dispersion curves and corresponding density of states are studied in the frame of linear response approach based on the density functional perturbation theory. Unified approach focused on the analysis of a wide-range physical properties of anisotropic crystals not only results in good agreement with the experimental data, but also allows to explain the observed optical effects at the microscopic level. Therefore, the presented results should be very useful for future possible improvements in the desired properties of the base material both by changing the chemical composition and/or introducing stresses by epitaxial growth of thin films.

Plasmon modes characterization in hybrid noble/ ferromagnetic nanodisks using an effective medium approximation

Magneto-plasmonic systems composed by noble/ferromagnetic metals structures can be used to control light polarization. The extraordinary optical properties arising from combining strong local enhancements of electromagnetic fields in surface plasmon excitations with the magneto-optical activity inherent to ferromagnetic materials, can be controlled by external magnetic fields that have demonstrated the possibility to control and amplify the magneto-optical properties via plasmonic excitations. In this work, the anisotropic optical and ellipsometric sensitivity to dielectric environment of multi-layered hybrid gold/cobalt magnetoplasmonic nanodisks are studied in the framework of the effective-medium approximation, where the components of the dielectric tensor of the system are modelled using Lorentz-like oscillators. Furthermore, full electromagnetic simulations were performed using the standard Finite Element Method, which allow us to characterize the modes and explore the role of the dielectric environment of the nanodisks in the optical properties of the nanostructures via the variation of a coating silicon dioxide (SiO2) layer thickness. Our model and numerical results show a very good agreement with spectral ellipsometry measurements, where two orthogonal plasmonic modes one in-plane and one out-of-plane- are well characterized. These results lead us to conclude that the effective-medium approximation is good enough to accurately describe the plasmonic behavior of multi-layered confined nanostructures.

Ellipsometric and Spectrometric Studies of (Ga0.2In0.8)2Se3 Thin Film

Thermal evaporation technique is used to deposite (Ga0,2In0,8)2Se3 thin films. The refractive index and extinction coefficient dispersions are obtained from the spectral ellipsometry measurements. The dispersion of the refractive index is described in the framework of the Wemple–Di Domenico model. The optical transmission spectra of a (Ga0,2In0,8)2Se3 thin film are studied in the temperature range 77–300 K. The temperature behavior of the Urbach absorption edge, as well as the temperature dependences of the energy pseudogap and Urbach energy, are investigated. The influence of different types of disordering on the optical absorption edge of a (Ga0,2In0,8)2Se3 thin film is discussed. Optical parameters of a (Ga0,2In0,8)2Se3 thin film and a single crystal are compared.

Optical dielectric constants of single crystalline silver films in the long wavelength range

Optical dielectric constants are critical to modeling the electronic and optical properties of materials. Silver, as a noble metal with low loss, has been extensively investigated. The recently developed epitaxial growths of single crystalline Ag on dielectric substrates have prompted efforts to characterize their intrinsic optical dielectric function. In this paper, we report spectral ellipsometry measurements and analysis of a thick, epitaxially-grown, single-crystalline Ag film. We focus on the range of 0.18 – 1.0 eV or 1.24 – 7 µm, an energy and wavelength range that has not been examined previously using epitaxial films. We compare the extracted dielectric constants and the predicted optical performances with previous measurements. The loss is appreciably lower than the widely quoted Palik’s optical constants (i.e., up to a factor of 2) in the infrared frequency range. The improved knowledge of fundamental optical properties of the high-quality epitaxial Ag film will have a broad impact on simulations and practical applications based on Ag in the long wavelength range.

Photovoltage spectroscopy of direct and indirect bandgaps of strained Ge1-xSnx thin films on a Ge/Si(001) substrate

The near-bandgap optical properties of Ge1-xSnx alloys were characterized by photovoltage spectroscopy and spectral ellipsometry measurements. Contributions of Urbach tailing as well as direct and indirect optical transitions were observed. The compositional dependence of direct bandgaps of strained GeSn films grown on a Ge buffered Si substrate was studied for up to 15% Sn content. The contribution to the photovoltage spectra of Ge1-xSnx alloys (x < 6%) from indirect optical transitions was observed at lower energies than from direct bandgaps. Using bowing parameters of bGeSnΓ = 3.16–0.5x and bGeSnL = 1.93 eV, a correlation was detected between calculated and measured indirect and direct bandgaps at 82 K. As the Sn content was increased, the difference between the energies of the indirect and direct bandgaps decreased, resulting in a smaller contribution of the indirect transitions due to competition with direct transitions and Urbach tails. Two sublayers with different Sn content, strain values and bandgaps were observed for samples with x ∼12%. The results indicated that strain relaxation in films with thicknesses exceeding a critical value occurs via formation of a Sn-rich top layer with higher direct bandgap. These findings have important implications when designing IR photodetectors or solar cells.

Optical characteristics of charge carrier transfer across interfaces between YBa2Cu3O6+δ and La0.7Ca0.3MnO3

Transition metal oxides undergo many different phase transitions when doped with charge carriers. Thanks to recent advances in the synthesis of metal-oxide heterostructures, doping can now be achieved through charge transfer across interfaces. This paper reports spectral ellipsometry measurements on multilayers of superconducting cuprates and ferromagnetic manganates and demonstrates that the optical spectra reveal fingerprints of interfacial charge transfer. Optical spectroscopy can thus serve as a nondestructive probe of the doping level at buried metal-oxide interfaces.

Optical properties of zinc phthalocyanine thin films prepared by pulsed laser deposition

ZnPc thin films were prepared by pulsed laser deposition (KrF laser, λ = 248 nm, τ = 5 ns, f = 50 Hz) on suprasil substrates in vacuum. Optical properties in UV–Vis spectral region were analyzed as functions of laser fluence from 40 to 100 mJ/cm2 by spectrophotometric and spectral ellipsometry measurements. The spectral ellipsometry data were treated using a three-layer model (substrate, film, roughness). The best results of data fitting were obtained when Q band was characterized by two Lorentz oscillators, while two Gaussian oscillators were used for B and C band fitting. We derived the band gap using Tauc plot considering ZnPc a direct band gap semiconductor. The band gap values were found decreasing from 3.13 to 3.09 eV with increasing laser fluence, which might be related with formation of trapping sites at higher fluence.

Characterization of Ultra-thin μc-Si:H Films for Silicon Heterojunction Solar Cells

The present work demonstrates the suitability of spectral ellipsometry (SE) to properly characterize ultra-thin (< 15nm) hydrogenated microcrystalline silicon (μc-Si:H) and its possible advantages over alternative methods, e.g. Raman spectroscopy. For very thin μc-Si the undesirable amorphous and void-rich initial incubation layer can make up a significant fraction of the actual film, thereby adversely affecting the effective electrical and optical film properties. Based on SE measurements and a suitable model to fit the experimental data, the strong material inhomogeneities induced by the incubation layer can be analyzed and accounted for. This allows for (i) the non-invasive determination of the μc-Si:H film thickness and crystalline volume fraction and (ii) the evaluation of the material density. Using this approach, the growth behavior of μc-Si:H on different surfaces, the influence of doping precursor gases and different deposition parameters has been determined. A strong dependence between the film thickness and its density has been observed as well as its correlation with the electrical and optical properties in silicon heterojunction (SHJ) structures. Additionally, a good monocrystalline silicon (c-Si) surface passivation and high selectivity for the extraction of charge carriers from the absorber into the Transparent Conductive Oxide (TCO) electrode using n-type μc-Si:H as front layer is being demonstrated by high open-circuit voltages of up to 730mV and good contacts to the TCO (both ITO and AZO). The approach described and demonstrated in this work provides a suitable tool to engineer the partially crystallized doped layers of SHJ cells with respect to their electrical and optical properties at a device relevant level.

Study of hydrogenated AlN as an anti‐reflective coating and for the effective surface passivation of silicon

AbstractStacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual‐layer stack. Deposited on 1 Ω cm p‐type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately –1 × 1012 cm–2 and a very low interface defect density below 5 × 1010 eV–1 cm–2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti‐reflective coatings. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Dynamics of moisture interaction with polyelectrolyte multilayers containing nanofibrillated cellulose

Abstract Recent findings have shown that it is possible to use the Layer-by-Layer technique to create nanofibrillated cellulose / polyethyleneimine interference films whose colour change with relative humidity. This study uses different optical models to describe spectral ellipsometry measurements data of interference films and how the film properties alter in dry and humid environments. The results indicate that water condensation initially is filling the surface pores within seconds whereas relaxation of the film to adjust to the added water is a slower process that reaches a steady state after »20 min. The maximum swelling ratio of the LbL films is almost independent of the number of layers within the film, but decreases considerably by crosslinking via heat treatment. The films show a distinct birefringence with optical axis perpendicular to the surface. Analysis of the moisture response with different optical models indicates that the films swell uniformly in the thickness direction with no separate water film on top. The results provide important understanding for the design of NFC based LbL films for visual moisture sensors and interactive security paper.

Film-shaped sol–gel Li4Ti5O12 electrode for lithium-ion microbatteries

Homogeneous and transparent film-shaped spinel Li4Ti5O12 electrodes on gold coated quartz were synthesized by sol–gel and a dipping procedure after thermal treatments at 700°C. The influence of the lithium precursor content on the phase composition, structure and electrochemical behavior of this material was studied. Spectral ellipsometry measurements and SEM-EDX analysis indicate an average thickness between 385 and 560nm. Rutile TiO2 is detected using grazing incidence X-ray analysis and Micro-Raman in compositions with a lower content of lithium. Good galvanostatic cycling and very flat discharge–charge profiles confirm the high purity of Li4Ti5O12 electrodes obtained with the higher lithium content composition.

Dependence of optical properties on structural and compositional parameters in CuGaSe2

The dielectric constant of CuGaSe2 as a function of stoichiometric deviations has been obtained from photomodulated spectral ellipsometry measurements. Optical gaps have been computed by fitting experimental and differentiated data of the imaginary part of the dielectric constant to function ε2 of the Lorentz model and to the second derivative of ε2. In each sample, three transition energy values have been found in the 1.65–1.72, 1.83–1.95, and 2.97–3.14 eV ranges. The presence of point defects reduces the energy gap values. For nonstoichiometric samples, changes in the first energy gap values have been analyzed as a function of the displacement of the position of the anion in the unit cell. The shifts in the valence band structure have been analyzed and it is concluded that the difference between the first and second transition energies, (Eg2−Eg1), is also affected by stoichiometric deviations, so that the Γ5v(2) level in samples with point defects is closer to Γ4v(2) level than in stoichiometric ones. Changes in (Eg3−Eg2) and (Eg3−Eg1) are related to Se position.

Determination of optical parameters of GeTe semiconductor films after thermal treatment

The optical parameters of GeTe semiconductor films after various thermal treatments have been measured using a novel method. A comparative study using a spectrum ellipsometer is presented. The optical parameters of the films were extracted precisely by data analysis and corrections have been made to previous calculations. Calculations based on the spectral ellipsometry measurements are presented finally, and the complex refractive index curves of the samples in the spectral range from 250 to 830 nm have been obtained.

Modeling the Optical Constants of Diamond- and Zincblende-Type Semiconductors: Discrete and Continuum Exciton Effects at E0 and E1

We present a comprehensive model dielectric function ε(E) [= ε1 + iε2] for diamond- and zincblende-type semiconductors based on the energy-band structure near critical points (CPs) plus discrete as well as continuum excitonic effects at the E0, E0 + Δ0,, E1, and E1 + Δ1 CPs. In addition to the energies of these band-to-band CPs, our analysis also yields information about the binding energies of not only the 3D exciton associated with E0 (R0), when resolved, but also the 2D exciton related to the E1, E1 + Δ1 CPs (R1). This model has been applied to spectral ellipsometry measurements of ε1, ε2 (0.3 eV < E < 5.5 eV) of ZnCdSe/InP, CdTe1—xSx, In0.66Ga0.34As, and GaSb and a surface photovoltage spectroscopy determination of the absorption coefficient of GaAs near E0. This work shows conclusively that even if the exciton at E0 is not resolved the lineshape is continuum exciton. The obtained values of R1 exhibit a trend which is in good agreement with effective mass/k · p theory. Our analysis will be compared with the modeling of Adachi and the University of Illinois-Chicago group, both of whom neglect exciton continuum effects and hence have not evaluated R1. Our results, particularly for exciton continuum effects at E1, have considerable implications for recent first-principles band structure calculations which include exciton effects.

Characterization and Dielectric Properties of Fluorinated Amorphous Carbon Measured by Capacitance-Voltage Response and Spectroscopic Ellipsometry

This paper describes a comparison between capacitance-voltage (CV) and spectral ellipsometry (SE) measurements of the dielectric constant of fluorinated amorphous carbon films. Whereas CV measurements require the construction of metal-insulatorsemiconductor (MIS) structures, SE measurements can be made on as-deposited films, and are much easier to implement. An examination of the results indicate that under certain conditions, the SE measurements give values that are in reasonable agreement with the more direct measurements based on the electrical (CV) response of the dielectric layer. The films reported on here had dielectric constants ranging from 3 – 5, and, in some cases, show promising characteristics for use as low-k dielectrics in semiconductor devices.