Reflectivity Spectra Of Thin Films Research Articles

In2Se3, In2Te3, and In2(Se,Te)3 Alloys as Photovoltaic Materials.

In its lowest-energy three-dimensional (3D) hexagonal crystal structure (γ phase), In2Se3 has a direct band gap of ∼1.8 eV and displays high absorption coefficient, making it a promising semiconductor material for optoelectronics. Incorporation of Te allows for tuning the band gap, adding flexibility to device design and extending the application range. Here we report results of hybrid density functional theory calculations to assess the electronic and optical properties of γ-In2Se3, γ-In2Te3, and γ-In2(Se1-xTex)3 alloys, and initial experiments on the growth and characterization of γ-In2Se3 thin films. The predicted band gap of 1.84 eV for γ-In2Se3 is in good agreement with the absorption onset derived from transmission and reflection spectra of thin films. We show that incorporation of Te gives γ-In2(Se1-xTex)3 alloys with a band gap ranging from 1.84 eV down to 1.23 eV, thus covering the optimal band gap range for single-junction solar cells. In addition, the γ-In2Se3/γ-In2(Se1-xTex)3 bilayer could be employed in tandem solar-cell architectures absorbing at Eg ≈ 1.8 eV and at Eg ≤ 1.4 eV, toward overcoming the ∼33% efficiency set by the Shockley-Queisser limit for single junction solar cells. We also discuss band gap bowing and mixing enthalpies, aiming at adding γ-In2Se3, γ-In2Te3, and γ-In2(Se1-xTex)3 alloys to the available toolbox of materials for solar cells and other optoelectronic applications.

IR absorptance of thin carbon multi-walled nanotubes layers

The IR transmittance and reflectance spectra of thin films which consist of multi-walled carbon nanotubes (MWCNT) are measured in the range from 2 to 17 μm. The MWCNT films of various thicknesses not exceeding 1.2 μm were deposited on silicon substrates with Si3N4/Al2O3 dielectric layers. It is shown that the deposition of an Al2O3 layer with a thickness of 2 nm allows to significantly improve the homogeneity and continuity of the MWCNT films. A technique is proposed for measuring the average thickness of rough MWCNT films by means of atomic force microscopy after laser engraving. Suggested method of spectra processing allows to estimate the spectral dependence of the IR radiation absorptance in MWCNT films, which are diffusely scattering objects and have a large absorption coefficient.

Effect of B2O3 additive on the properties of ZnS-Ge system and the resulting thin-film coatings

The effect of B2O3 additive to ZnS and ZnS-Ge system on their properties and parameters of thin-film coatings obtained from them has been studied. The essence of the additive is the binding of ZnO in Zinc sulfide, which has a negative effect due to the interaction with the evaporator material. According to X-ray diffraction, the introduction of B2O3 additive to ZnS results in the disappearance of the wurtzite phases (β-ZnS and ZnO). There is also a change in the IR spectra, in which reveal the bands at ≈300 cm−1 and 415 cm−1, which correspond to the valence vibrations of the Zn-S and Zn-O bonds, respectively. It is established that the addition of B2O3 noticeable improves the technological mode of evaporation in vacuum of materials due to the increase of their volatility. The reflection spectra of thin films of undoped and doped ZnS indicate a certain increase in the refractive index of the latter, while in the ZnS-Ge system such a difference is not observed. The mechanical durability of coatings increases especially significantly - more than 10 times for ZnS and more than 2 times - for the ZnS-Ge system. Therefore, the use of B2O3 as an additive has proved to be an effective method for minimizing the effects of oxide impurities in chalcogenide materials on the properties of thin-film coatings.

Optical and mechanical properties of Ag doped thermally evaporated SeTe thin films for optoelectronic applications

Se–Te materials have shown promise in optoelectronic applications. A combination of enhanced optical and mechanical properties represents a good candidate for optical limiting devices. Here, we report the influence of Ag additive on the optical and mechanical properties of (Se70Te30)100−xAgx(0.0≤x≤8.0at.%) chalcogenide thin films. The reflectance spectra of amorphous thin films have been analyzed in the spectral range 0.3 μm–2.5 μm. The optical parameters are obtained using reflectance spectra of the thin films. The refractive index increases with the addition of Ag, while the optical band gap decreases from 1.40 eV to 1.11 eV. The reduction in the optical band gap is correlated with the raise in the concentration of defect states and the disorder. The nonlinear refractive index and nonlinear susceptibility have been evaluated and found to show an augmented response with Ag content. The value of (χ(3)) increases from 3.44 × 10−11 esu to 7.66 × 10−11 esu and n2 from 3.98 × 10−10esu to 8.08 × 10−10esu. The mechanical properties have also been analyzed theoretically and ultrasonically with Ag incorporation to SeTe thin films.

Dynamic optical response of SU-8 upon UV treatment.

We report the optical properties of SU-8 in the mid-infrared (mid-IR) region before and after UV treatment. Samples consisted of SU-8 films of thickness ranging from 10 um to 157 um deposited on gold coated silicon substrates and were prepared using spin coating. Mid-IR diffuse reflectance measurements were conducted using Fourier transform infrared spectroscopy. Spectra measurements imply a change in optical properties of SU-8 upon exposure to UV and heat treatment. A gradual change in optical properties is seen after each step of UV treatment and the baking process. Reflectance spectra of thin-films were also observed to be thickness dependent. We calculate the dielectric function of SU-8 in the range 2 um to 15 um using the reflectance spectra of the samples.

Photoinduced changes in optical properties of Ga–Sb–Ge–Se glasses

The kinetics of photoinduced effects on Ga 5Sb 10Ge 25Se 60 thin film exposed to continuous wave laser radiations are studied as a function of exposure time and laser intensity. The transmission and reflection spectra of thin films before and after exposure are investigated. The optical band gap and the refractive index are derived from the above spectra. Generalized Miller's rule and linear refractive index are used to find the nonlinear susceptibility and nonlinear refractive index of the thin films. The studies show a red shift in the band gap with increase in exposure time and laser power which is attributed to the photoinduced darkening in the films.

Crystallization kinetics and composition dependence of some physical properties of Sn–Sb–Bi–Se chalcogenide glasses

Abstract Chalcogenide alloys of Sn 10 Sb 20− x Bi x Se 70 (0⩽ x ⩽8) system were prepared by the melt quenching technique. Thin films were prepared on well-cleaned glass substrates by the thermal evaporation technique. The X-ray diffractogram for x =0 reveals the amorphous nature as no sharp peak is observed but the sample with x =4 shows sharp peaks. The glass transition, crystallization, melting temperatures and glass-forming tendency of the amorphous samples were determined from differential scanning calorimetric measurements. The glass transition activation energies and the crystallization activation energies were determined using the Kissinger method. Optical transmission and reflection spectra of thin films were obtained in the range 400–2500 nm. The conductivity activation energy and optical gap initially increases with increasing Bi concentration (for x =2) and then decreases sharply for higher Bi content. The values of the band tailing parameter and the pre-exponential factor are also reported and discussed.

Closed equation for the normal incidence reflectance of thin films on absorbing substrates

The optical constants of thin films can be obtained from inversion of spectrophotometric measurements by using minimization gradient methods. The computational approach of these minimization methods requires closed compact formulas for reflectance and/or transmittance. For normal incidence closed compact formulations for the direct transmittance, both for thin films on transparent or absorbing substrates, and for the reflectance of thin films on transparent substrates, are available in the literature. We report here a closed compact formula to evaluate reflectance spectra of thin films on absorbing substrates, and it is shown that for vanishing substrate absorption this new, to the best of our knowledge, approach gives the same results obtained from the formulation corresponding to thin films supported by transparent substrates.

Effects of matrix variations on pH and Cu2+sensing properties of sol–gel entrapped Eriochrome Cyanine R

Eriochrome Cyanine R has been entrapped in a range of acid- and base-catalysed silica sol–gel matrices using various values of the water ∶ alkoxide ratio (R) and total precursor volume and incorporating varying amounts of the organic modifier methyltriethoxysilane (MTES). Diffuse reflectance spectra of thin films of polyisobutylene-bound finely-ground samples of these carefully aged materials have been measured as a function of time in solutions of various pH and Cu2+ concentration. Effects of pore size distribution and organic modification on these sensing properties have been interpreted in terms of data from nitrogen adsorption porosimetry and EXAFS. Optimum sensitivity to low Cu2+concentrations is achieved for gels incorporating 10% MTES, and the use of the polymer-bound ground samples increases response speed by a factor of 300 relative to bulk monolith sol–gel samples with elimination of cracking effects.

The energy-dispersive reflectometer at BESSY II: a challenge for thin film analysis

Installed in 1999 the energy-dispersive reflectometer at the 13.2 bending magnet employs the exponentially decaying white X-ray emission spectrum of the 1.7 GeV storage ring of BESSY II outside the vacuum. Using an energy-dispersive detector specular and longitudinal-diffuse reflectivity spectra of thin films can be recorded simultaneously between 0.2 Å −1< q z <1.2 Å −1 within a few seconds. The capability of the experiment is demonstrated probing the correlation length of Cd-arachidate and -stearate films at room temperature and its change during annealing. At T=70°C we observe an instantaneous decay of specular Bragg peaks accompanied with an increase of the diffuse scattering. This indicates the onset of the melting of 2D-ordered acid domains. The vertical diffusion coefficient is estimated to be about 2×10 −24 m 2/s.

Infrared and Raman analysis of plasma CVD boron nitride thin films

We present an optical investigation, by means of polarized IR reflectivity and Raman scattering, of the quality and stability of PECVD grown BN films obtained on the anode and on the cathode of a parallel-plate radio-frequency reactor by glow discharge decomposition of two different gas mixtures, B2H6-H2-NH3 and B2H6-N2. The addition of Ar to B2H6-N2 has also been investigated. It is shown that polarized IR reflection spectra of thin films have a complex optical behaviour, which is a function of the oblique angle of incidence and the substrate material used in the deposit. However, these spectra are predictable when optical theory is applied. The results obtained show the capabilities of polarized IR reflection and Raman scattering to perform detailed investigations on the microstructure and aging of BN thin films, independently of the substrate material.

Low-temperature refractive indices of Cd1−xMnxTe and Cd1−yMgyTe

The refractive indices of Cd1−xMnxTe and Cd1−yMgyTe (0&amp;lt;x&amp;lt;0.28 and 0&amp;lt;y&amp;lt;0.70) were determined by analyzing interference fringes in reflectivity spectra of thin films grown by molecular beam epitaxy. It is shown that their dispersions in the transparent region at 4 K are well described by a Sellmeier relation. These results have been applied to grow CdMnTe/CdMgTe Bragg mirrors incorporated in a II–VI microcavity operating in the strong exciton-photon coupling regime.

Infrared conductivity of YBCO from transmission and reflection spectra of thin films

We measured far infrared transmittance and reflectance spectra of thin Y 1Ba 2Cu 3O 7− x films grown on MgO. At 295 K reflectance measurements were extended up to 20 000 cm −1. We model the dielectric function of Y 1Ba 2Cu 3O 7− x by using free-carrier and mid-infrared terms. Our data are consistent with a huge mid-infrared component, in which low frequency contributions are very important and appear to be temperature dependent. The free-carrier term is described by the usual Drude model in the normal phase and by a two-fluid model in the superconducting phase.

In Situ Studies of Coatings on Metal Wires by FT-IR External Reflectance Spectroscopy

Diffuse reflectance FT-IR spectroscopy was originally developed for particulate samples dispersed in KBr powders. Now, by using ellipsoid mirrors for scattered light collection, we have taken advantage of the collection efficiency of diffuse reflectance optics and have extended their use to an in situ study of coatings on metal wires and of the broken surface of a reinforcement wire as well. The result is that spectra of coatings on wire show good overall agreement with external reflection spectra of thin films. The technique we have developed has an advantage over infrared attenuated total reflection (ATR), in that there is no optical contact problem between the wire sample and ATR crystal.

The influence of preparation techniques on the fundamental reflectivity spectra of thin ZnTe films

The dependence of the reflectivity coefficients of ZnTe textured thin films evaporated under various conditions in the energy range 3–6 eV was investigated. The analysed reflectivity spectra of thin films were compared with the reflection spectra of ideal monocrystals. Careful examination of these results suggests the influence of internal randomly distributed stresses introduced into the samples during the evaporation process and the cooling of the thin film-substrate system.

Thin Film Reflection Spectroscopy. I. Determination of Optical Constants of Amorphous Selenium and As2Se3

The reflectivity spectra of thin films of amorphous As2Se3 and Se have been obtained. The films were deposited on aluminum substrates and had a range of thickness from 2 to 50 μ. The frequency range covered by these spectra was from 650 to 40 wavenumbers (15 to 250 μ). Two methods have been used to determine the optical constants, N and K, from the reflectivity spectra. The first employs a single equation nonlinear regression analysis on the radiant reflectance equation. The second method uses a two equation maximum likelihood analysis on the real and imaginary parts of the amplitude reflectance equation coupled with a Kramers-Kronig analysis of the phase angle. The real and imaginary parts of the complex dielectric constant, the absorption coefficient, the energy loss function and the transverse and longitudinal vibrational frequencies are then determined from the optical constants.

Longitudinal Optical Phonons in CaF2, SrF2, and BaF2

Off-normal reflection spectra of thin films of CaF2, SrF2, and BaF2 deposited on a silver substrate have been obtained. They exhibit decreased reflectivity at the long wave limit of the longitudinal optical branch. The values so obtained are in agreement with the predictions by the Lyddane—Sachs—Teller relation. They are also in agreement with vibrational levels obtained from vibronic data for SrF2:Sm2+ and BaF2:Sm2+. The observed longitudinal optical phonons are at 466 cm—1 in CaF2, 366 cm—1 in SrF2, and 319 cm—1 in BaF2. The close agreement between vibronic data and direct observation of these vibrations is considered further evidence that a good model for these vibronic transitions must consider vibrations of a relatively large segment of crystal and not merely nearest neighbors.