Thermally stimulated currents in disordered solids at fractional heating

Thermally stimulated currents (TSCs) in disordered solids, registered at fractional sample heating, are investigated theoretically. The Gobrecht–Hofmann method of determining trap distribution in energy gap is extended to the case of significant carrier retrapping. Two specific cases, when the TSCs are limited either by carrier recombination or by carrier neutralization on collecting electrode, are considered. The accuracy of proposed method is estimated from the analysis of TSC curves, calculated numerically for exponential and Gaussian trap distributions.     

Thermally stimulated currents in amorphous solids at mixed carrier recombination,Part II: Quasi-equilibrium approximation

Thermally stimulated carriers (TSCs) in amorphous materials are investigated theoretically for the case of ‘mixed carrier recombination’, when the recombination rate is the sum of terms corresponding to monomolecular and bimolecular one. The basic approximations are quasi-equilibrium carrier distribution in the traps and low trap occupancy. The accuracy of the obtained formulas is verified for the case of exponential trap distribution by comparison of TSC curves calculated analytically and numerically. Methods of determining the trap distribution and the carrier recombination kinetics from experimental data are discussed.     

Thermally stimulated currents in amorphous solids at mixed carrier recombination,Part I: Non-equilibrium approximation

Thermally stimulated currents (TSCs) in amorphous solids are studied theoretically for the case of ‘mixed carrier recombination’, when two sets of traps exist in the energy gap, one of them relatively deep and initially filled. The main simplifying assumptions are a strongly non-equilibrium carrier distribution in the active traps and their low occupancy. Simple formulas for TSCs are derived and their accuracy is verified by comparison with the numerical results for exponential trap distribution. Methods of determining the trap distribution and the carrier recombination kinetics from experimental data are considered.     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films

The study, by differential thermal analysis (DTA) of the glass transition of thermally evaporated amorphous selenium, allows the effect of ageing on structural properties to be observed. An enthalpic diagram is used to explain the shift of the glass transition region, with heating rate, and also with ageing. These shifts are directly dependent on structural relaxation in amorphous selenium. The model used, to express the change of the structural time relaxation with temperature, and with the departure of thermodynamic equilibrium leads to a slightly different expression for the dependence of T_g on heating rate than that previously published (by Moynihan et al.).The activation enthalpies of the studied phenomena compared with other results, seem to show that structural relaxation in a-Se principally involves bond breaking and reformation of chains.     

Thermo-mechanical behavior of uniaxially drawn and crystallized poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films

Thermo-mechanical properties of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films in the amorphous state, uniaxially stretched and thermally crystallized were compared and show different microstructure–property relationships. The glass transition temperature shifts to higher temperatures when films are stretched above T_g at 160 °C, in relation with a crystallinity increase and subsequent confinement effects of the amorphous phase, but films stretched below T_g at 100 °C show an opposite trend while drawing. This is discussed in relation with the highly mobile structural state arising from plastic deformation of the amorphous phase. The confinement effects were particularly assessed by the estimation of a ‘rigid amorphous fraction’ (RAF). Crystallinities and RAF were estimated from DSC data for the uniaxially drawn and thermally crystallized films respectively. It was observed that the RAF was higher in thermally crystallized films than in uniaxially drawn samples in relation to a lamellar semi-crystalline morphology. In addition to this detailed thermo-mechanical characterizations, low stress creep measurements were carried out during heating scans. They can be related to the microstructure and thermal transitions of the films. In particular, for the uniaxially stretched films, the measured shrinkage decreases when the draw ratio increases because the crystalline entities developed during the draw process are locking the chain motions.     

Atomistic examinations of the solid-phase epitaxial growth of silicon

The low-temperature vapor deposition of silicon thin films and the ion implantation of silicon can result in the formation of amorphous silicon layers on a crystalline silicon substrate. These amorphous layers can be crystallized by a thermally activated solid-phase epitaxial (SPE) growth process. The transformations are rapid and initiate at the buried amorphous to crystalline interface within the film. The initial stages of the transformation are investigated here using a molecular dynamics simulation approach based upon a recently proposed bond order potential for silicon. The method is used first to predict an amorphous structure for a rapidly cooled silicon melt. The radial distribution function of this structure is shown to be similar to that observed experimentally. Molecular dynamics simulations of its subsequent crystallization indicate that the early stage, rate limiting mechanism appears to be removal of tetrahedrally coordinated interstitial defects in the nominally crystalline region just behind the advancing amorphous to crystalline transition front. The activation barriers for this interstitials migration within the bulk crystal lattice are calculated and are found to be comparable to the activation energy of the overall solid-phase epitaxial growth process simulated here.     

Thermally-stimulated currents controlled by fluctuating localized states

Thermally stimulated currents (TSC) are examined in a material with randomly fluctuating energies of the localized states. It is shown that, these conditions, distributions of the localized state density recovered from TSC data, represent distributions of effective activation energies (DEAE) rather than the true density-of-state (DOS) function. The difference between DEAE and DOS increases with decreasing heating rate used to obtain the TSC transients, as well as with decreasing starting temperature.     

Determination of Kinetic Parameters of Bi2Se3 Thin Films by Computation

Bi₂Se₃ thin films are prepared by thermal evaporation method onto well‐cleaned glass substrates. From the XRD analysis, the film is found to be amorphous in nature. Thermal analysis of Bi₂Se₃ sample scraped off from the glass substrate is performed using the technique differential scanning calorimetry (DSC) at different heating rates. Kissinger method is used to analysis these constant heating rate spectra and the activation energy 'E_a′ corresponding to the phase transformation is calculated through the examination of the DSC peak temperature shift induced by variation of the heating rate. An analytical equation with three kinetic parameters (activation energy (E_a), transformation order (n) and the frequency factor (v)) is derived from the Johnson‐Mehl‐Avrami (JMA) theory to describe the constant heating rate DSC spectra. Using the activation energy from Kissinger plot, theoretical constant heating rate DSC traces are generated for arbitrary values of 'v' and 'n' using the analytical equation. Finally, theoretically generated DSC spectra is compared with that of the experimental one and the unknown values 'v' and 'n' are determined.     

An anomalous X-ray structural study of an amorphous La55Al25Ni20 alloy with a wide supercooled liquid region

The atomic structures of amorphous La₅₅Al₂₅Ni₂₀ alloys which have a wide supercooled liquid region and a high reduced glass transition temperature have been studied using anomalous X-ray scattering (AXS) at the Ni K-absorption edge as well as the ordinary X-ray diffraction with Mo K radiation. The interference functions and the radial distribution functions were determined for the amorphous alloys as-quenched and after annealing at various temperatures and also for a fully crystallized sample. These systematic structural studies revealed a drastic change in Ni environment upon crystallization. The need for such atomic rearrangements around the Ni atoms during crystallization may be the reason why the amorphous phase is thermally stable.     

Study on the effectiveness of thermally treated rice husks for petroleum adsorption

Rice husks, an agricultural waste, were thermally treated and evaluated as an adsorbent for petroleum with the aim of being used as a remediation strategy for petroleum spills. The petroleum sorption capacity was examined for thermally treated rice husks, which mainly composed of amorphous SiO₂. Results showed that the petroleum sorption capacity of this material prepared at 700 °C was 15 g/g for heavy crude petroleum. The effects of heating temperature, contact time and petroleum density on the petroleum sorption capacity of thermally treated rice husks were further studied on the basis of phase composition, microstructure and morphology using X-ray diffraction analysis, FTIR spectrometry and scanning electron microscopy (SEM).     

Study of dielectric relaxations in cellulose by combined DDS and TSC

In this work, thermally stimulated currents (TSC) analyses combined with dynamic dielectric spectroscopy (DDS) have been applied to the investigation of molecular mobility of cellulose. The correlation between results obtained by both methods allows us to attribute the low temperature DDS relaxation mode to the γ-mode resolved in TSC. The values of its activation parameters point out that the chain mobility remains localized. At high temperature, the various dielectric relaxation phenomena are separated by applying a recent analytical protocol. The comparison between the activation enthalpy values obtained by DDS and TSC leads to the assignment of the so-called α-mode to cooperative movements of polymeric sequences. The Arrhenius behavior of α-relaxation time is explained using the strong/fragile pattern. The influence of water content on secondary and primary relaxation modes was examined as well.     

Conduction mechanism in amorphous Si films

dc conductivity as a function of temperature has been measured for as-evaporated and annealed films of amorphous Si, grown by the vacuum evaporation technique. The experimental data suggest that conduction in the higher temperature range (～175–300 K) is by the thermally activated holes in the localized states near the valence band edge while conduction in the lower temperature range (～77–175 K) is found to be thermally assisted tunnelling in the localized states near the Fermi level. The activation energy for both the processes is found to increase with an increase in the annealing temperature. The average hopping distance, calculated for conduction near the Fermi level, is also found to increase with an increase in the annealing temperature.     

Thermally stimulated currents in thin silicon films arising from atmospheric effects without light exposure

A study of the effects of atmospheric adsorption on conductivity of thin silicon films upon heating is presented. A phenomenon is observed without illumination of the sample, that is similar to light-induced thermally stimulated current except no exciting light is applied. The effect is greatly reduced if the sample is annealed prior to cooling. A model involving electrostatic effects coupled with desorption is presented to account for this.     

The localization of electrons in amorphous semiconductors: A twenty-first century perspective

This review covers those consequences of the localization of electronic states that appear to be universal features embracing all amorphous semiconducting materials where the electron–lattice interaction can be neglected, such as hydrogenated amorphous silicon. Several experimental measurements of these features are described. The role of strong electron–lattice interactions in some amorphous semiconducting systems, such as many chalcogenide glasses, is also discussed. In these systems, the electron–lattice interaction is so strong that it more than offsets the coulomb repulsion needed to put two electrons in the same energy state. Some experimental consequences of these so-called negative-U_eff systems are described. In addition, some universal features of metastable excitations for systems with both weak and strong electron–lattice interactions are discussed in the light of some recent experimental results.     

Theoretical Study of Recrystallization Process of Amorphous Ge Layer Subjected to Pulsed Excimer (XeCl) Laser Radiation

Transient temperature profiles have been studied theoretically in amorphous germanium thin films on the crystal substrate during heating by a single laser pulse. Computer calculations have been based on the Fourier equation of the heat flow. The calculated threshold value of the energy density for the melting the whole of amorphous layer have been reported for excimer laser radiation.     

Influence of homocysteine on the physical structure and molecular mobility of elastin network in cultured arteries

The thermal and dielectric properties of the elastin network were investigated in arteries cultured with physiological and pathological concentrations of homocysteine, an aminoacid responsible of histological impairments in human arteries. The glass transition of this amorphous protein was investigated by Differential Scanning Calorimetry (DSC). To explore the molecular dynamics of the elastin network in the nanometer range, we used Thermally Stimulated Currents (TSC), a dielectric technique running at low frequency and measuring the dipolar reorientations in proteins subjected to a static electrical field. Combining TSC and DSC experiments with determination of the activation parameters of relaxation times reveals the molecular mobility of the proteins. The major differences in the relaxation behavior of elastin between arteries cultured with physiological and pathological concentrations of homocysteine are discussed.     

Evolution of pressure-amorphized zirconium tungstate upon annealing

Zirconium tungstate Zr(WO₄)₂ exhibits irreversible amorphization at high pressure. Upon heating, the pressure-amorphized phase transforms into different phase/phases depending on the pressure applied during heating. Transformation of pressure-amorphized samples to metastable cubic phase during isochronal annealing at ambient pressure is investigated using X-ray diffraction and Raman spectroscopy. Though the X-ray diffraction and Raman spectra show only monotonic changes up to 850 K in the amorphous state, the sample exhibits dramatic changes in the color from gray to black to white. The relaxation of the amorphous phase during annealing suggests gradual irreversible volume increase by about 6%, whereas the tungstate tetrahedra are found to shrink. Crystallization to cubic phase at 900 K is accompanied by a large increase in the sample volume. The specific volumes of the amorphous phases obtained from cycling the samples to different pressures suggest the possibility of polyamorphism in this system.     

Magnetothermal investigation of crystallization of Ni–P amorphous alloys

A new magnetic phase analysis method is presented and its application to a certain class of amorphous alloys is thoroughly discussed. Special attention is paid to the size of clusters with strong magnetic properties that form the separating phase. This magnetothermal (MT) method reveals the formation or disappearance of a phase with strong magnetic properties, and in certain cases it can give valuable information, even in the temperature range where the sample is paramagnetic. The accuracy and applicability of the MT method are discussed. Combining this method with differential scanning calorimetry (DSC), the transformations in melt-quenched (MQ) and electrolessly deposited (ED) amorphous Ni–P alloys with hypoeutectic and close to eutectic composition are investigated under the condition of a constant heating rate. Up to four stages of Ni precipitation in the alloys are revealed, depending on the phosphorous content and the method of sample preparation.     

Structure,Crystallization and Electrochemical Corrosion Behaviour of Amorphous Cu66Ti34 Alloy

The effect of thermally induced structural changes on electrochemical corrosion behaviour of amorphous Cu₆₆Ti₃₄ alloy is studied using electrochemical potentiodynamic polarization technique in conjunction with XRD, TEM and DTA. The heat treatment has been carried out at two temperatures: 300 °C — where the process of structural relaxation is only possible and 500 °C — where crystallization of the alloy occurs. As model corrosive media 1N H₂SO₄ and 1N HNO₃ solutions are used. The short range order of the amorphous samples is studied with the total pair correlation function. Some changes of the interatomic distances and a tendency to increasing of the density of the amorphous structure after low temperature heating are established. The crystallization leads to formation of Cu₃Ti₂, β-Cu₃Ti and Cu₂Ti crystalline phases. It has been found that structural relaxation may have a beneficial effect on the susceptibility of the amorphous alloy to passivation, while crystallization lowers considerably its corrosion resistance.