Showing papers by "Solid State Physics Laboratory published in 1993"

Influence of the crystalline phase on the molecular mobility of PVDF

Abstract: Dielectric relaxation and pyrocurrent of PVDF were studied by thermostimulated current spectroscopy. The transition spectrum of the material was investigated by DSC. Two well-resolved relaxation peaks have been observed in the temperature range [-100-100°C]. The molecular mechanisms of these phenomena have been discussed, based on a comparative study of α-PVDF and β-PVDF. The β relaxation mode is located at -41°C in α-PVDF and is slightly shifted toward higher temperatures in the stretched material. This mode has been ascribed to the dielectric manifestation of the glass transition (T g ) of PVDF. It is comprised of two components corresponding to the free and constrained amorphous phases, respectively, in the order of increasing temperatures

Structural and Electronic Properties of Carbon-Doped β-Rhombohedral Boron

Abstract: β-rhombohedral boron doped with carbon up to about 1 at% is investigated by X-ray diffraction, by optical transmission spectroscopy in the spectral range of the absorption edge and its low energy tail, by I R reflection spectroscopy in the lattice vibration range, and by Raman scattering. The carbon atoms substitute for boron sites in the icosahedra shrinking as a consequence of the increased intraicosahedral bonding force. Hence the lattice constants decrease with increasing carbon content. Within the accuracy of measurement the interband transition energies are not influenced by the carbon content, and the p-type conductivity remains qualitatively unchanged. Additionally to the series of six ionization energies of B12 electron trapping levels, which are equidistantly arranged below the conduction band in pure boron, a second series of B11C trapping levels develops. The traps are esscntially generated by the interaction between one electron each and up to six pairs (probably triples) of defined phonons. These phonons are quenched as a consequence of the occupation of these traps. In correlation with the new optical results ESR investigations of other authors can now be interpreted. Important details of the electronic structure and its modification by carbon doping are determined. The relevance for other boron-rich solids with icosahedral structure is discussed. β-rhomboedrisches Bor, dotiert mit Kohlenstoff bis zu etwa 1 At%, wird rnit Rontgenbeugung, mit optischer Transmissionsspektroskopie im Spektralbereich der Absorptionskante und ihres Auslaufers, mit Reflexionsspektroskopie im Gitterschwingungsbereich und mit Ramanstreuung untersucht. Die Kohlcnstoffatome substituieren Borplatze in den Ikosaedern, die infolge der wachsenen Bindungsstarke schrumpfen. Dies fuhrt zu einer Verkleinerung der Gitterkonstanten. Die Interbandubergange werden durch den Kohlenstoff im Rahmen der Mesgenauigkeit nicht verandert, und die p-Leitung bleibt qualitativ erhalten. Zusatzlich zu der Serie von sechs B12-Elektronenhaftniveaus, die in reinem Bor aquidistant unter dem Leitungsband angeordnet sind, tritt eine zweite Serie von B11 C-Haftstellen auf. Die Haftstellen entstehen durch Wechselwirkung zwischen je einem Elektron und bis zu sechs Paaren (moglicherweise Tripeln) definierter Phononen. Diese werden bei Besetzung der Haftstellen gequencht. In Zusammenhang mit den neuen optischen Ergebnissen konnen nun ESR-Untersuchungen anderer Autoren interpretiert werden. Wichtige Details der elektronischen Struktur und ihre Veranderung bei Kohlenstoffdotierung werden bestimmt. Die Bedeutung der Ergebnisse fur andere borreiche Festkorper wird diskutiert.

Interband and Gap State Related Transitions in β‐Rhombohedral Boron

Abstract: The anisotropy of the electronic interband transitions of β-rhombohedral boron is determined by optical absorption measurements. The band gaps (indirect allowed transitions) are 1.32(1) and 1.50(1) eV for E ∥ c, respectively, 1.29(1) and 1.46(1) eV for E ⟂ c (extrapolated to T = 0 K). At low temperatures these gaps decrease, while the thermal equilibrium develops. The temperature dependence of the gap is attributed to the thermal disorder caused by phonons of 121.9 (E ⟂ c) and 94.1 meV uE ⟂ i.c) representing the most prominent intraicosahedral vibrations. Six intrinsic trapping levels for electrons in distances between 0.17 and 1.15 eV from the conduction band edge are determined, the closer ones in agreement with different photo effects. The trapping levels are attributed to distortion states of the icosahedra due to a dynamical Jahn-Teller effect. This conception allows the interpretation of other physical properties like internal friction, discontinuities of thermal expansion, and electrical conductivity, of the temperature dependence of the ESR linewidth, and of the density of paramagnetic centers (spin density and magnetic susceptibility). Die Anisotropy der elektronischen Interbandubergange des β-rhombomboedrischen Bors wurde aus der oprischen Absorption bestimmt. Die Bandabstande (indirekt erlaubte Ubergange) betragen, extrapoliert auf T = O K, 1,32(1) und 1,50(1) eV fur E ∥ c bzw. 1,29(1) und 1,46(1) eV E ⟂ c. Bei tiefen Tenperaturen nehmen die Bandabstande im Verlauf der Einstellung des thermischen Gleichgewichtes ab. Der Temperaturgang der Bandabstande wird der temperaturbedingten Fehlordnung des Gitters durch die thermische Anregung der starksten intraikosaedrischen Phononen bei 121.9 (E ∥ c) und 94,1 meV (E ⟂ c) zugeordnet. Sechs intrinsische Haftniveaus fur Elektronen in Abstanden zwischen 0,17 und 1,15 eV vom Leitungsband werden nachgewiesen die bandnahen sind in Ubereinstimmung mit verschiedenen Photoeffekten. Die Haftniveaus werden Verzerrungszustanden der Ikosaeder infolge eines dynamischen Jahn-Teller-Effektes zugeschrieben. Diese Konzeption erlaubt die Erklarung anderer Physikalischer Eigenschaften wie der inneren Reibung, der Diskontinuitaten der thermischen Dilatation und der elektrischen Leitfahigkeit, der Temperaturabhangigkeit der ESR-Linienbreite sowie der Lonzentration lokalisierter paramagnetischer Zentren (Spindichte und magnetische Suszeptibilitat).

Improved Raman Effect Studies on Boron Carbide (B4.3C)

Abstract: With a FT Raman spectrometer highly resolved and reliable Raman spectra of B4.3C were obtained largely refuting previous results of other authors. The symmetry selection rules of IR and Raman active phonons are proved to hold. The most prominent Raman doublet at 270/320 and that at 869/928 cm−1 are due to pentagonal pyramids of the icosahedra symmetrically vibrating relative to the end atoms of the three-atomic chain. The broad band at 1065 cm−1 is attributed to corresponding vibrations of pairs of pentagonal pyramids in adjacent icosahedra. The remaining Raman-active phonons, mainly attributed to intraicosahedral vibrations, are comparably weak. At higher photon energies the luminescence radiation due to electron transitions is essential.

Effect of heat treatment on the structural and optical properties of CuInTe2 thin films

Abstract: The CuInTe2 thin films were prepared by thermal vacuum evaporation of the bulk compound. The structural and optical properties in the temperature range 300–47 K of thin films grown on glass substrates and annealed in vacuum were studied. The films were investigated by X-ray diffraction and electron microscope techniques. The calculated lattice constants for CuInTe2 powder were found to bea=0.619 nm andc −1,234 nm. From the reflection and transmission data, the optical constants, refractive indexn, absorption index,k, and the absorption coefficient, α, werw computed. The optical energy gap was determined for CuInTe2 thin films heat treated at different temparatures for different periods of time. It was found thatE g increases with both increasing temperature and time of annealing.

Stretched Exponential Relaxation of Persistent Photoconductivity Due to the Si-Related DX Centre in AlxGa1−xAs

Abstract: We have shown that the decay of the photoexcited carrier after termination of disequilibrating illumination follows the stretched-exponential-relaxation law n(t) = n(0)exp[−(t/τ)β]. It is found that the stretching parameter β and relaxation time constant τ depend on temperature with β ranging from 0.2 to 0.4. The stretched exponential relaxation is attributed to the capture-enhanced motion of an Si donor atom from the substitutional to an interstitial site in an AlxGa1−xAs lattice during the capture of photoexcited electrons from the conduction band. Temperature-dependent β is associated with the distribution of activation energies due to local atomic alloy disorder in AlxGa1−xAs. The distribution of activation energies is also verified with the novel spectroscopic technique based on deep-level transient spectroscopy.

Some remarks on the reverse monte-carlo method

Abstract: An attempt is made to specify the role of the reverse Monte Carlo method, the experimental input and possible models in the evaluation of a diffraction experiment.

Structure of liquid caesium-lead alloys

Abstract: Neutron diffraction measurements have been made on liquid Cs-Pb alloys at the Intense Pulsed Neutron Source. Equiatomic CsPb has been shown in previous work to be a Zintl alloy with well-defined Cs 4 Pb 4 structural units, explaining the anomalously high electrical resistivity and specific heat observed in the molten state. These studies have been extended to the Cs 80 Pb 20 and Cs 25 Pb 75 compositions and also repeated with the equiatomic composition as a reference. The structure of liquid Cs 25 Pb 75 resembles that of solid KPb 2 . Liquid Cs 80 Pb 20 exhibits large composition fluctuations

The structure of liquid LiSi alloys

Abstract: The reverse Monte Carlo method is used to analyse neutron diffraction data on liquid Li 65 Si 35 . A well defined SiSi partial pair correlation function is obtained with strong indications for covalent SiSi bonds. It is also clear that most of the Si 4 stars and Si 5 rings occurring in solid Li 12 Si 7 have disappeared in the liquid state. Si clusters are present in various forms, ranging from single Si atoms to large clusters.

Bright red electroluminescence in diffused porous siliconp-n junction

Abstract: We report the first operation of light emittingp-n junction diode in porous silicon fabricated by diffusion.

Compound formation in liquid alkali indium alloys

Abstract: Resistivity measurement on liquid RbIn and CsIn alloys provide evidence for compound formation at a composition of 50% In. The discussion is based on existing electronic structure calculations for solid alloys of Li and Na with Ga, In and Tl. Several possible models for describing liquid equiatomic RbIn and CsIn are discussed. In particular the NaTl-type and CsCl-type local structures are compared.

Resistivity of liquid Sr-Bi and Sr-Sb alloys

Abstract: Recently, measurements of the electrical resistivity, ϱ, of the liquid SrBi and SrSb systems have been performed as a function of temperature for the entire concentration range. A very high and sharp resistivity maximum occurs at 60 at.% Sr for both systems, indicating that the mixture is semiconducting at this composition. Comparison with the MgBi system is made and an attempt is made to give an explanation in terms of ionic ordering.

Electrical conductivity and luminescence of 9-chloro-10,10′-bis-(dichloromethyleno)-(9′H)-10,10′-dihydro-9,9′-bianthryl (CDDB)

Abstract: The absorption and fluorescenc of 9-chloro-10,10′-bis-(dichloromethyleno)-(9′H)-10,10′-dihydro-9,9′-bianthryl (CDDB) has been studied in polar and nonpolar solvents and also in microcrystal. In polar solvents CDDB emits from two molecular forms, the normal charge transfer form (locally excited, LE form) and the solvent-induced twisted intramolecular charge transfer (TICT) form. Electrically, CDDB possesses semiconducting property with conductivity approximately 10−9 S cm−1 and this conductivity further increases to 10−7 S cm−1 on photoexcitation. Intramolecular charge transfer by hopping mechanism is assumed to be the main process for controlling activation energy and electronic conduction.

Identification of the crystallographic polarity of the {111} CdTe surfaces

Abstract: A simple, non-destructive chemical method has been described for the determination of the crystallographic polarity of the {111} CdTe surfaces. The chemicals used are dilute HCl and zinc. The hydrogen gas evolved during the reaction preferentially gathers only on the Cd surface (as per Fewster's convention) of 〈111〉 CdTe, and can be used to identify the crystallographic polarity of 〈111〉 CdTe.

The electrical resistivities of liquid K-Sn and Cs-Sn alloys

Abstract: The electrical resistivity, ϱ, of liquid KSn and CsSn alloys has been measured as a function of composition and temperature. The resistivities of both systems exhibit very sharp peaks close to the equiatomic composition. At the same composition, the temperature dependence of the resistivity has a been minimum. The maximum resistivities are 3100 and 14200 μΩ cm respectively. Comparison with the LiSn and NaSn alloys and with the alkali-lead systemsstrongly suggests that Zintl ions are formed in the liquid.

Laser and thermal processing for Ti:LiNbO3 waveguide fabrication

Abstract: Ti diffusion into LiNbO3 single crystal was studied as a result of Ruby laser irradiation of a metallic Ti thin film covering the crystal surface and subsequent thermal processing in a furnace. Different regimes of irradiation and heating were used. SIMS analysis was employed for investigation of the treatment. The combined method used proved to be able to form waveguides of 3–4 μm thickness.