Far-infrared Reflectivity Spectra Research Articles

Optical properties of amorphous Sb2Se3:Sn films

The measurements and analysis of optical transmission and far-infrared (IR) reflectivity spectra of thermally evaporated Sb2Se3:Sn films are reported. The refractive index and film thickness have been determined from the upper and lower envelopes of the transmission spectra (Swanepoel's standard envelope method), measured at normal incidence, in the spectral range from 800 to 2500 nm. Values of the refractive index fit well to Cauchy's dispersion relation. The optical gap decreases with an increase in the Sn content, while a maximum in the tailing parameter and Urbach's energy occurs with only a small amount (∼1 at %) of this additive. Characteristic vibrational bands for SbSe3 structural units are revealed in the far-IR spectrum with no additional ones arising from the Sn additive. The Kramers–Kronig analysis has been used to calculate the dielectric constants and hence the longitudinal optic and transverse optic splitting for various compositions. The inclusion of Sn as a charged entity along with the Coulomb interactions which serve to polarize the glass medium is found to be responsible for these results.

Far-infrared study of impurity local modes in Co-doped PbTe

Far-infrared reflection spectra in wide temperature range were used to investigate the vibrational properties of the PbTe single crystals doped with Co. In the analysis of the experimental results we used dielectric function that took into account the existence of plasmon-ionised impurity–phonon interaction. The results obtained from the experimental spectra as the best fit, are in very good agreement with the theoretical prediction. Together with the strong coupling we obtained three local modes of Co at about 125, 190 and 275 cm −1, which corresponded to the impurity atoms in different valence states. The positions of these modes depend on the impurity center charge, and their intensities depend on the temperature and Co concentration.

Structural analysis of Zn2TiO4 doped with MgO

The starting mixtures of ZnO, TiO 2 , and MgO (0, 1.25, and 2.5 wt.%) powders are mechanically activated in a high-energy planetary mill for 15 min and then sintered between 800 and 1100°C for 2 h. The influence of MgO addition on the structure of the samples obtained is analyzed with X-ray diffraction and infrared reflection spectroscopy. Room-temperature far-infrared reflectivity spectra of the samples doped with MgO (0, 1.25, and 2.5 wt.%) and sintered at 1100°C in the frequency range between 100 and 1200 cm -1 are measured and analyzed. Optical parameters are determined for seven oscillators belonging to the spinel structure using the four-parameter model of coupled oscillators. Born effective charges are calculated from the transversal/longitudinal splitting.

Temperature-dependent far-infrared properties of Bi12GeO20 single crystal

Far-infrared reflectivity spectra of bismuth germanium oxide (Bi12GeO20) single crystals are measured from room temperature down to 10 K. All the reflectivity spectra are fitted to a complex dielectric function ∊(ω) in the factorised form. Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrum weight below 150 K. The temperature-dependent static dielectric constants are obtained from the Lyddane–Sachs–Teller relation based on the obtained oscillator parameters. The dielectric constants are found to increase upon decreasing temperature, which is attributed to the charge transfer among the ions in the unit cell with the temperature varying.

Far-infrared spectroscopy of CdTe/ZnTe self-assembled quantum dots

In this paper we present far-infrared (FIR) reflection spectra and results of photoluminescence (PL) measurements of CdTe/ZnTe self-assembled quantum dots (SAQD). The electronic band structure was characterized by a photoluminescence spectrum. The PL spectrum displayed two main emission peaks, both connected with existing of QDs. One presents direct deexcitation to ground state and the other is optical phonon assisted deexcitation. The FIR reflectivity spectra were measured in the spectral range of 70–650 cm −1. Analyses of the spectra were made by fitting parameters in the complex dielectric function that depends on 3D distribution of constituents. CdTe buffer and ZnTe dielectric barrier layer act as a bulk semiconducting crystal and have usual values for phonon frequencies. The confined CdTe modes from SAQD could not be completely resolved in FIR reflectivity spectra because these modes are hidden by the strong peak of CdTe buffer layer, but it was necessary to include a CdTe mode confinement in the fitting procedure in order to obtain good agreement with the experimental data.

Plasmon–phonon interaction in Hg 1− xMn xSe alloys

Far-infrared reflection and Raman spectra of Hg 1− x Mn x Se ( x ≤ 0.3) alloys were measured at room temperature. The analysis of the Raman spectra was made by deconvolution technique. The analysis of the far-infrared spectra was made by a fitting procedure based on dielectric function which includes spacious distribution of free carrier influence on the plasmon–phonon interaction. In spite of strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed two-mode behaviour. The model of phonon behaviour was developed, and we obtain very good agreement with experimental results.

Combined FTIR and SEM‐EDS study of Bi2O3 doped ZnO–SnO2 ceramics

The effects of Bi(2)O(3) addition on the phase composition, microstructure and optical properties of ZnO-SnO(2) ceramics were investigated. Starting powders of ZnO and SnO(2) were mixed in the molar ratio 2:1. After adding Bi(2)O(3) (1.0 mol.%) this mixture was mechanically activated for 10 min in a planetary ball mill, uniaxially pressed and sintered at 1300 degrees C for 2 h. Far-infrared reflection spectra were measured (100-1000 cm(-1)). To investigate the occurred differences in FTIR spectra, the Bi(2)O(3)-doped sample was examined more carefully with a Perkin-Elmer FTIR spectrometer (Perkin Elmer, Waltham, MA, USA) connected with a Perkin-Elmer FTIR microscope and itemized points of interest were also studied with SEM-EDS.

Polarized infrared reflectivity study in charge density wave conductor Tl 0.3MoO 3

Polarized infrared reflectivity measurements between 300 and 10 K have been carried out on charge density waves (CDW) conductor blue bronze Tl 0.3MoO 3. Three important features are observed: (i) A bump at 1155 cm −1 in the reflectivity spectra of Tl 0.3MoO 3 at 300 K is a precursor of the Peierls gap due to optical excitations across a pseudogap, and this kind of Peierls-like gap opens gradually with decreasing temperature from 180 to 160 K. (ii) The three sharp modes as “triplet” of infrared reflectivity between 800 and 1000 cm −1 of Tl 0.3MoO 3 along [1 0 2] axis show red shift compared to K 0.3MoO 3 and Rb 0.3MoO 3, which is assigned to the increase of the distance of Mo–O bond with the substitution of thallium ions. (iii) Two peaks at about 514 and 644 cm −1 in the far-infrared reflectivity spectra of Tl 0.3MoO 3 along [1 0 2] direction are suggested to be the electronic transitions from the valence band to the midgap state and from occupied midgap state to the conduction band, respectively.

Probing the Superconducting Energy Gap from Infrared Spectroscopy on aBa0.6K0.4Fe2As2Single Crystal withTc=37 K

We performed optical spectroscopy measurement on a superconducting Ba0.6K0.4Fe2As2 single crystal with T{c}=37 K. Formation of the superconducting energy gaps in the far-infrared reflectance spectra below T{c} is clearly observed. A flat and close to unity reflectance is observed roughly below 150 cm;{-1} for T<<T{c}, following an s-wave pairing line shape. A more rapid decrease occurs near 200 cm;{-1}, leading to a peak in the ratio of the reflectance at T<<T{c} over that for T>or=T{c}. We determined the absolute value of the penetration depth at 10 K as lambda approximately 2000+/-80 A. A spectral weight analysis shows that the Ferrell-Glover-Tinkham sum rule is satisfied at low energy scale, less than 6Delta.

Infrared reflectivity spectra of η-Na1.3V2O5 in the charge disordered and ordered phase

We have measured the far-infrared reflectivity spectra of the sodium vanadium oxide η-Na1.3V2O5 polycrystals in the wide temperature (80–300 K) and frequency (150–1500 cm−1) range. Appearance of new phonon oscillators, phonon oscillator mode splitting and step-like shift of TO and LO frequencies at low temperatures are correlated with the charge-ordering phase transition, which takes place at about 120 K in this vanadium oxide.

Superconducting Properties of the Fe-Based Layered SuperconductorLaFeAsO0.9F0.1−δ

We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of over 50 T. A clear signature of superconducting gap opening below T(c) was observed in the far-infrared reflectance spectra, with 2Delta/kT(c) approximately 3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carriers with a rather low-carrier density.

Far-infrared reflectivity as a probe of point defects in Zn- and Cd-doped HgTe

We present a nondestructive method for quantitative determination of the vacancy concentration in the lattice of Hg-based semiconductor alloys with tetrahedral structure. The method is based on the identification of additional vibrational modes (AVMs) induced by lattice deformations in the far-infrared (FIR) reflectivity spectrum. Although the method is restricted by sensitivity limitations, recent FIR experimental data carried out on HgZnTe and HgCdTe samples containing Hg vacancies confirmed the presence of AVMs induced by Hg vacancies in as-grown crystals.

Far-infrared and galvanomagnetic study of gallium doped PbTe

The far-infrared reflectivity spectra of PbTe(Ga) ( N Ga = 0.1, 0.2, and 0.4 at.%) alloys were measured in the 30–500 cm −1 spectral range at different temperatures and analyzed using a numerical fitting procedure based on the plasmon–phonon interaction model. The three local Ga modes at about 120, 150 and 220 cm −1 together with the strong plasmon–phonon coupling are observed. Using results of galvanomagnetic measurements carrier concentrations and the Fermi level temperature dependence are determined for illuminated and unilluminated samples. Experimental results are interpreted assuming the mixed valence of Ga in lead telluride and the formation of centers with negative correlation energy. For both, p-type ( N Ga = 0.1 and 0.2 at.%) and n-type samples ( N Ga = 0.4 at.%) the Fermi level is in the forbiden band, but for n-type illuminated sample the Fermi level is in the conduction band at low temperature. Positions and concentrations of different Ga impurity states are obtained using the Gibbs distribution.

Optical properties of Nb-doped SrTiO3 single crystal

Far-infrared reflectivity spectra of (100) SrTi1−xNbxO3 single crystal have been measured under different temperatures in the frequency region between 100 and 6000cm−1. The obtained spectra are analyzed based on the double damping extended Drude model. Extra absorption near the highest frequency longitudinal phonon mode shows that the polaron coupling must be taken into account. The splitting of the last high reflectivity band, the temperature dependence of the 615cm−1 structure and the appearance of a new weak peak-like defect mode provide some evidence for the existence of small polarons.

Far-infrared study of impurity local modes in Ni-doped PbTe

In this paper, we present far-infrared reflection spectra of Ni doped PbTe single crystal samples. Crystals of PbTe(Ni) were grown by the Bridgman method. These spectra were analyzed using a fitting procedure based on the modified plasmon–phonon interaction model. Together with the strong plasmon–phonon coupling we obtained three local modes of Ni at about 130, 165 and 190 cm −1 which correspond to the impurity atoms in different valence states. The positions of these modes depend on impurity center charge, and their intensities depend on temperature and Ni concentration.

Plasmon–phonon and plasmon–two different phonon interaction in Pb 1− xMn xTe mixed crystals

Far-infrared reflection spectra in wide temperature range was used to investigate the vibrational properties of Pb 1− x Mn x Te ( x = 0.0002, 0.002, 0.02 and 0.1) mixed crystals. To analyse the experimental results we use dielectric function that takes into account the existence of plasmon–phonon as well as the plasmon–two different phonon interaction. The best fit method revealed two frequencies of plasmon–phonon coupled modes and three frequencies of plasmon–two different phonon coupled modes. Further, the values for two different LO modes and plasma frequency ( ω P) are calculated. Results obtained from experimental spectra as the best fit, are in very good agreement with the theoretical prediction. The model of phonon behaviour based on Genzel's model was developed. It was found that the long wavelength optical phonon modes of these mixed crystals, exhibit an intermediate and two mode behaviour, coincidentally.

Reflectivity and phase shift of semiconductor far-infrared mirrors

We have carried out an in-depth investigation on the reflectivity R and phase shift φ of a novel semiconductor mirror, which can be applied in the far-infrared (FIR) spectral range. By fitting FIR reflection spectra of the GaAs material with different doping concentrations, empirical formulas are obtained for the doping concentration-dependent carrier relaxation time τ of n-GaAs and p-GaAs. The effects of structure and material parameters on the reflectivity and the phase shift of the mirror are analytically studied on the basis of the deduced formulas. All the related parameters of the mirror are further optimized, and the corresponding wavelength selectivity is calculated according to the highest absorption in the detector cavity. The experimental FIR reflection result confirms the theoretical simulation, revealing the unique properties of this mirror, which establishes the basis of optimum designing of various FIR mirrors.

Infrared and Dielectrical Properties of SrTiO&lt;sub&gt;3&lt;/sub&gt;:Nd

Far-infrared reflection spectra and results of dielectric measurements of SrTiO3 single crystal doped by neodymium are presented in the paper. Dielectric investigations were conducted at temperature range from 25 K to room temperature for the test frequency 1MHz. The numerical analysis of the far-infrared reflection spectra made using a fitting procedure based on factorized form of the dielectric function. The results for SrTiO3: Nd are compared with those for pure crystal. The lowest-frequency phonon mode is found to be strongly influenced by presence of neodymium ions in SrTiO3 single crystal.

Phonon and vibrational spectra of hydrogenated CdTe

This work presents far-infrared reflectivity spectra collected with synchrotron radiation on specially prepared CdTe monocrystals in the temperature region of 30–300K. The investigated samples were of three different types characterized by the three different levels of hydrogenation—strong, middle, and low ones. In order to interpret the experimental data, the imaginary part of the dielectric function was evaluated by means of Kramers-Kronig transformation. To determine quantitatively the influence of hydrogen atoms on the phonon and vibrational spectra of hydrogenated CdTe crystals we used the special statistical model proposed in this paper. This model takes into account the tetrahedron as the basic structural unit and distinguishes the contribution of the hydrogen-bearing tetrahedra from the contribution of the hydrogen-free ones to the crystal phonon spectra.

First interpretation of phonon spectra of quaternary solid solutions using fine structure far-IR reflectivity by synchrotron radiation

In this work, we present and discuss far-infrared (FIR) reflectivity spectra obtained using infrared synchrotron radiation in different Zn x Cd y Hg 1− x− y Te (ZMCT) quaternary alloys. Reflectivity spectra were measured at the DAFNE-light facility of the INFN-LNF with samples placed under vacuum inside a BRUKER Equinox 55 interferometer. Samples were cooled in a helium cryostat in a temperature range from about 20 K to 300 K. The obtained spectra were standard Kramers–Kronig transformed to obtain the imaginary part of the dielectric functions, then unfolded into a set of Lorentzian oscillators. The fitting procedure yields a total of 21 lines in the Zn x Cd y Hg 1− x− y Te reflection spectra of these quaternary alloys. Sixteen lines have been correlated to ternary spectral contributions of the different components: CdHgTe, ZnHgTe and CdZnTe (with a precision of ±1.5 cm −1). In the wave number range between 108 cm −1 and 190 cm −1 the oscillator frequencies have been attributed to the spectral lines of these quaternary alloys.