Showing papers by "Solid State Physics Laboratory published in 1994"

Study of cooperative relaxation modes in complex systems by thermally stimulated current spectroscopy

Abstract: Cooperative relaxation modes associated with transitions of various systems have been experimentally resolved into elementary processes characterized by activation enthalpies and entropies following a compensation law. Two transitions with no kinetic effect-solid-solid transition and Curie transition have been studied. In both cases, the corresponding dielectric relaxations are characterized by a compensation phenomenon with transition temperature as the compensation temperature. Dielectric relaxations liberated at the glass transition, T g , have also been studied. They are also associated with a compensation law but, in that case, the compensation temperatures T g + 10/30°. This lag has been attributed to the kinetic character of T g . Note that it is lower in amorphous polymers than in semicrystalline polymers. The distribution function of activation parameters is broader in amorphous polymers than in semicrystalline ones.

Optical Evidence of Anderson-Mott Localization in FeSi

Abstract: We have investigated the electrodynamic response of single-crystalline FeSi over a frequency range from d.c. to 105 cm-1. At low frequencies, we found evidence for an Anderson-Mott localization behaviour of the low-temperature optical conductivity, while at high frequencies the excitation spectrum resembles that of a conventional semiconductor. The missing spectral weight below the gap energy is redistributed around the gap edge, in disagreement with previous claims based on optical measurements.

Radiation damage in NaCl. I. Optical-absorption experiments on heavily irradiated samples.

Abstract: Results of optical-absorption experiments on heavily irradiated NaCl single crystals are presented. The dose rates were between 4 and 250 Mrad/h; the doses between 1 and 7 Grad. The irradiation temperatures were in the range of 20--150 \ifmmode^\circ\else\textdegree\fi{}C. Because of the intense optical bands, the thickness of the samples had to be reduced to values between 2 and 100 \ensuremath{\mu}m using a thinning device. With this device we are able to investigate samples containing about 1 mol % Na and ${\mathrm{Cl}}_{2}$. The optical spectra of the samples, irradiated at low temperatures, show intense F, M, R, and N bands. The positions of the optical bands of the electron excess centers are in agreement with the literature. With increasing irradiation temperature, the contribution of these centers to the total amount of radiation damage decreases due to coagulation to colloidal Na particles. The colloid band, however, is much wider than the literature values; in addition, it is located at longer wavelengths. These observations can be explained by assuming that we are dealing with extremely small colloids, showing appreciable quantum size effects.

Precipitation in CdTe crystals studied through Mie scattering

Abstract: Below gap optical losses in as-grown n-type CdTe crystals were analyzed in terms of free carrier absorption and Mie extinction due to Te precipitates. Experimental absorption spectra measured between 2 to 20 μm exhibited the well-known free carrier absorption behavior αFCA∼λx with x=3 due to scattering by polar optical phonons. In shorter wavelength regions below 6 μm, however, additional contributions to the light loss due to absorption and scattering by precipitates were also observed. Assuming a log-normal size distribution, the precipitate extinction spectra were calculated according to Mie theory within the electric and magnetic dipole and electric quadrupole approximation. A comparison with the experimental spectra identifies the precipitates and enables estimation of their sizes and total number density. In this investigation, both undoped and In-doped CdTe crystals grown from stoichiometric melts by vertical asymmetric Bridgman method were used. It was found that In doping, in general, suppresses Te precipitation. At high doping level (melt containing∼1019 In atoms cm−3), the formation of In2Te2 is also indicated. It is demonstrated that the Mie extinction analysis offers an, expedient method to rapidly analyze the precipitates in CdTe and in similar other wide gap materials in a nondestructive manner.

Hole scattering mechanisms in Hg 1−x Cd x Te

Abstract: In this paper, we analyze and discuss the roles of nine different scattering mechanisms—ionized impurity, polar and nonpolar optical, acoustic, dislocation, strain field, alloy disorder, neutral impurity, and piezoelectric—in limiting the hole mobilities in p-type Hg1−xCdxTe crystals. The analysis is based on obtaining a good fit between theory and experiment for the light and heavy hole drift mobilities by optimizing certain unknown (or at the most vaguely known) material parameters such as the heavy hole mobility effective mass, degree of compensation, and the dislocation and strain field scattering strengths. For theoretical calculations, we have adopted the relaxation time approach, keeping in view its inadequacy for the polar scattering. The energy dispersive hole relaxation times have been drawn from the published literature that take into account the p-symmetry of valence band wave functions. The temperature dependencies of multiple charge states of impurities and of Debye screening length have been taken into account through a numerical calculation for the Fermi energy. Mobility data for the present analysis have been selected from the HgCdTe literature to represent a wide range of material characteristics (x=0.2–0.4, p=3×1015–1×1017 cm−3 at 77K, μpeak≅200-1000cm2V−1s−1). While analyzing the light hole mobility, the acoustic deformation and neutral impurity potentials were also treated as adjustable. We conclude that • the heavy hole mobility is largely governed by the ionized impurity scattering, unless the strain field or dislocation scattering below 50K, or the polar scattering above 200K, become dominant; • the light hole mobility is mainly governed by the acoustic phonon scattering, except at temperatures below 30K where the neutral impurity, strain field and dislocation scattering also become significant; • the intervalence scattering transitions make negligible impact on the heavy hole mobility, but virtually limit the light hole mobility; • the alloy disorder scattering does not dominate in any temperature region, although it exercises some influence at intermediate temperatures; • the heavy hole mobility effective mass ratio mhh/mo∼-0.28–0.33 for crystals with x<0.4; and • the light hole band deformation potential constant is ∼12 eV.

Mercury-enhanced high Tc in the Bi (Pb)SrCaCuO system

Abstract: This short communication reports an onset Tc as high as 135 K and a Tc0 (zero resistivity) of 114 K in a Bi1.7Pb0.3Sr2−xHgxCa2Cu3Oy composition. Mercury does not seem to be incorporated in the lattice. It appears probable that mercury enhances the oxidation state of copper in the CuO2 layers and thus raises the Tc. Very interestingly, the XRD spectra of these specimens show the presence of only one phase, that is, the 2212 phase of the Bi-system. Highest value of Tc0 (114 K) was consistently found for Hg=0.3, even when the entire series of specimens was prepared four times under identical conditions. The studies hint at the possibilities of obtaining Tc values higher than the standard assigned Tc values to various oxide superconductors.

Radiation damage in NaCl. III. Melting phenomena of sodium colloids

Abstract: We report measurements on the melting behavior of colloids produced in irradiated NaCl. With differential scanning calorimetry (DSC) several latent-heat peaks near the melting temperature of pure bulk sodium metal have been detected. It appears that the total latent heat in these peaks is an accurate measure of the amount of damage in the crystal. Peak temperature and shape provide more detailed information about the properties of the colloids. The different melting temperatures can be explained by differences in the typical sizes of the colloid, based on the theory for melting of small particles. This DSC technique provides a method to evaluate the production of radiation damage in detail without changing the properties of the damaged crystal.

Radiation damage in NaCl. II. The early stage of F-center aggregation.

Abstract: We study the early stage of aggregation of F centers into colloids in pure NaCl under irradiation. The crystals have been electron irradiated with a dose rate of 2 Mrad/h up to doses of 1500 Mrad and measured by optical-absorption spectroscopy. The major bands, the F, M, and the colloid band, are analyzed qualitatively as well as quantitatively. We have observed a relationship between the concentrations of the F and the M centers, which changes from quadratic to linear. The colloid band appears to peak at two distinct wavelengths, indicating that two types of colloids are formed during the nucleation stage. The defect concentrations are determined as a function of the dose and the irradiation temperature and are discussed in terms of models which describe the kinetics of defect formation.

Radiation damage in NaCl. IV. Raman scattering

Abstract: Raman-scattering experiments on heavily irradiated pure and doped NaCl crystals are described. The experiments have been performed at room temperature and at approximately 25 K. The crystals had been irradiated up to a maximum dose of 95 Grad by means of electrons from a Van de Graaff accelerator. The Raman spectra show the set of phonon peaks corresponding to the NaCl modes. At low frequencies the reduced Raman intensity shows a clear power-law dependence on the frequency, ${\mathit{I}}_{\mathrm{red}}$(\ensuremath{\omega})\ensuremath{\propto}${\mathrm{\ensuremath{\omega}}}^{\ensuremath{ u}}$, with an exponent \ensuremath{ u}\ensuremath{\simeq}1.4, indicating that the colloids in heavily irradiated samples exhibit a fractal structure. The change in the melting peak pattern reveals that the initial form of the sodium particles is strongly affected by the performance of a Raman experiment at room temperature. At low measuring temperatures two unknown phonon peaks at 330 and 560 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are observed. These peaks are located well above the phonon cutoff frequency of NaCl at 260 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and are associated with local modes due to the ultrafine structure of the colloids.

C-axis infrared response of tl2ba2ca2cu3o10 studied by oblique-incidence polarized-reflectivity measurements

Abstract: We show that from measurements of the reflectivity of a uniaxial medium taken at a finite incidence angle with s- and p-polarized light it is possible to determine the dielectric function both parallel and perpendicular to the optical axis. When applied to layered compounds with its surface parallel to the layers, this technique allows for an accurate determination of the loss function perpendicular to the layers. This is demonstrated for the example of c-axis oriented thin films of the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$, on which we carried out polarized reflectivity measurements at 45\ifmmode^\circ\else\textdegree\fi{} incidence angle above and below ${\mathit{T}}_{\mathit{c}}$.

Optical and electrical properties of transparent conducting ZnO films prepared by spray pyrolysis

Abstract: Transparent conducting zinc oxide (unintentionally doped) films were prepared by spray pyrolysis using air as the carrier gas The optical and electrical properties of the films are presented and discussed as a function of the substrate temperature The optical properties were studied in the ultraviolet (uv) visible and near infrared (ir) regions The transmittance data were used to determine the optical constants, the refractive index, n, the extinction coefficient, k, and the absorption coefficient, α The reflectivity of the films was calculated and found to be very small in the wavelengths investigated, showing a tendency to increase in the ir range in order to act as a host reflector

Temperature dependence of the pyroelectric coefficient in polyvinylidene fluoride

Abstract: The pyroelectric properties of bioriented β-PVDF were studied in the temperature range [- 100-70°C]. The temperature dependence of the pyroelectric coefficient was obtained, and its variation was compared with the DSC thermogram and the TSC complex spectrum. Three transitions have been observed in the investigated temperature range. They have been associated with the glass transition of the free and constrained amorphous phases (Tgl = -40°C and Tgu = 15°C) and to conformational reorganizations at the crystallites surface (Tac = 47°C). A correlation has been established between these pyroelectric transitions and volumetric variations. Dimensional effects have been found to reasonably describe the increase of the pyroelectric coefficient through the glass transition range. These changes in the thickness of the film correspond to secondary pyroelectricity and explain nearly 50% of the room temperature coefficient. In the glassy state, primary pyroelectricity is dominant.

Charge transport in germanium-substituted magnesium ferrites

Abstract: Electrical conductivity (σ) and thermopower measurements have been carried out on some Mg1+xGexFe2−2xO4 (0 < x < 0.4) ferrite samples over a temperature range 300–700 K. Using the experimental values of Seebeck coefficient at various temperatures, the values of charge carrier concentration have been determined. On the basis of the temperature variation of charge carrier mobility, the conduction mechanism in these ferrites has been discussed.

Microwave Lithium Ferrites

Abstract: Among the various magnetic oxide materials used in devices operating at microwave frequency ranges lithium ferrite is presently occupying a prominent position. Although its potential as rectangular loop material was being utilised for long in memory devices the exploitation of its attractive feature as a microwave material was much delayed due to excessive losses observed in early samples. Many researchers working in the field put up a good deal of effort in the early 70s to obtain lithium ferrites with improved dielectric properties. Preparation of low loss lithium ferrite was first reported in 1971 by Collins and Brown [1]. Considerable interest has persisted in lithium ferrites since then. With the establishment of reproducible procedures for the preparation of lithium ferrites, there has been no looking back and at this juncture lithium ferrites are among the most wanted materials for the microwave frequency ranges. The family of lithium ferrites has expanded to such an extent that it now covers a very wide spectrum of properties for utilisation in microwave devices. The frequency range of its use extends even to millimeter wave bands.

Damping mechanism of the strongly renormalized c-axis plasma frequency in high-Tc cuprates

Abstract: We study the charge dynamics of high-Tc superconductors with the electric field perpendicular to the planes, using polarized oblique-incidence reflectometry for thin films of Tl2Ba2Ca2Cu3O10 and normal incidence reflectometry for single crystals of La1.85Sr0.15CuO4. In Tl2Ba2Ca2Cu3O10 we observe no c-axis optical plasmon either above or below Tc. For E//c in La2−xSrxCuO4, no plasmon is observed in the normal state, but as soon as T drops below Tc, a plasma edge in the reflectivity occurs, which moves up to about 2kBTc for T→0. The electronic contribution to σ(ω)≈7ω−1 cm−1 is independent of frequency up to several hundred cm−1, which implies that the optical scattering rate (τc−1) is large compared to the c-axis plasma frequency (ωpc). We prove experimentally that h /τ c h ω pc ⪢ 3.5k E T c , hence the c-axis response is in a universality class different from the dirty (Mattis-Bardeen) limit, and the absence of a plasma edge in the normal state is due to overdamping. Recent experimental data of various groups show that this is a generic feature of high-Tc cuprates. There is no need to assume a large many-body reduction of the ab initio LDA-RPA plasma frequency along the c-axis. Yet the overdamping of the plasmon could be a many-body effect. The ‘confinement’ to the planes as was proposed by Anderson is therefore due to a strong scattering in the c-direction. In the dirty limit only the relatively small amount of oscillator strength in the below-gap region is transferred to the pair-response (the δ-function at zero frequency). As a result the plasma edge in the superconducting state occurs at about 3.4(k B T c h /τ) 0.5 ω pc ⪡3.5k B T c ⪡ h ω pc .

Influence of deviation from stoichiometry on the photoluminescence in CdTe doped with indium

Abstract: Low temperature photoluminescence of vacuum and cadmium annealed CdTe:In is reported here. A new peak at similar to 1.14 eV related to transitions from the conduction band to an acceptor involving a tellurium vacancy has been observed.

Isotope substitution in B2O3 glass: a molecular dynamics study

Abstract: The results of molecular dynamics simulations of boron oxide glasses with different isotopic compositions ( 11 B, 10 B, 16 O, 18 O) are presented. The influence of boron and oxygen mass on the vibrational spectra is considered and compared with experimental data from the literature. The position of the dominant Raman line at 805 cm −1 appears to depend only on the oxygen mass while the position of the infrared absorbtion band at 1260 cm −1 is influenced by the boron as well as the oxygen mass. These results are in agreement with the experimental data.

Radiation-damage in nacl

Abstract: In this contribution we will consider some of the characteristic effects associated with the formation of radiation damage in NaCl. This work is directly related to the problem of the disposal of high level nuclear waste. For this reason, we will discuss in particular die production of radiation damage at moderate temperatures (between 30 and 150°C). We were forced to use relatively high dose rates (between 4 and 250 Mrad/hr) in order to reach the high doses, which are accumulated under disposal conditions. The basic Jain-Lidiard model, which has been modified in our laboratory, plays an important role in the analysis of the experimental results, which have been obtained to date. With this model we are able to “translate” our experimental results to predictions for the practical situation of disposal of radioactive waste in rock salt.

Some new results in porous silicon

Abstract: Efforts have been made to see the effect of some standard microelectronic processing steps on porous silicon. Our diffusion experiments for making p-n junctions confirm that this material can withstand high temperatures of the order of 800°C to 1000°C. A new technique for photolithography has been suggested to obtain porous silicon in selected areas. Etch stop method to control the thickness of the porous layer and an organic protective layer for porous silicon have also been suggested. Models proposed by other workers to explain luminescence in porous silicon are not sufficient to explain many experimental observations. A hybrid model is suggested.