Influence of impurities,irradiation and temperature on the mechanical and electrical properties of LiF crystals

On the basis of study of the temperature dependence of mechanical and electrical properties of LiF crystals with different impurities an evident interaction between radiation defects and impurities is revealed. As a result of trapping of radiation defects (F- and H-aggregate colour centres) by impurity centres the reconstruction of impurity centres takes place, causing a change of mechanical and electrical properties.     

Growth and characterization of GaP and GaAs1−xPx

The minority carrier lifetime at low and high excitation densities was determined in VPE layers of GaP and GaAs_?0.2 P_?0.8. The lifetime at high excitation densities, having a value up to ∽350 ns, is one to two orders of magnitude larger than the lifetime at low excitation densities. It is shown that impurities are involved in some saturable killer centres dominating at low excitation densities. In the case of the largest values of the minority carrier lifetime and at a dislocation density of > 10⁵ cm^?2, the non-radiative recombination at high excitation densities is shown to occur at dislocations; at lower values of the minority carrier lifetime the killer action may be due to microprecipitates. These findings also hold for LPE layers of GaP. It is shown that by measuring the minority carrier lifetime as a function of temperature a discrimination is possible between killer action due to diffusion of minorities towards sinks like dislocations or microprecipitates and due to capture by a normal point-defect type recombination centre.     

Effect of the magnetic flux trapping upon the temperature dependence of the flow stress change at the superconducting transition

The effect of the superconducting transition on the flow stress in single and polycrystals of Pb-In and Pb-Sn alloys has been studied in the temperature range 1.8 K to T_e. Unlike earlier efforts, these experiments involved investigation of the magnetic flux trapping. Two types of temperature dependences of the magnetic flux trapping, B₀(T), were revealed. In Pb-1.9 at.% In and Pb-6 at.% Sn alloys (subjected to various thermal treatment), B₀ monotonically rises with lowering temperature. In Pb-1.8, 3 at.% Sn single crystals the flux trapping arises at high strains (> 50%) and low temperatures (<2.5 K). In this case B₀ magnitude depends on the strain rate. The difference in the B₀(T) behaviour is associated with dissimilar flux pinnig by pinning centres. The date on B₀(T) obtained give an insight into the two types of the Δσ(T) dependences; taking into account of the flux trapping allowed the Δσ values to be corrected, so that as a result, a single Δσ_SN(T) curve was obtained for all the alloys and structures studied.     

Capture of adatoms by clusters in thin film deposition

The adatom diffusion and capture by clusters are considered as a random-walk problem. The method of solution is developed. Green's function for a square substrate lattice is obtained. This permits one to give a general solution for a steady-state adatom distribution around clusters and for Halpern's arrival probability. This solution is applicable for calculating capture numbers of some small capture centres. An interpolation formula for the capture number, as dependent on the perimeter length of the capture centre, is proposed for λ a. The adatoms fluxes to the 10 and 11 steps on the surface are calculated. The fluxes are anistropic but the anisotropy disappears with λ/a → ∞. The applicability of the steady-state approach is discussed. It is shown that this approximation is valid for low adatom density. The adatom fluxes to monatomic and diatomic immobile clusters are calculated under conditions when these clusters cannot be treated as capture centres. The conservation equations for these clusters are formulated whereby two different configurations of three-atomic clusters are taken into account.     

Quantum-theoretical calculation of activation parameters for mass transfer and mass transport processes on ionic crystals (V)

A report is given on the calculation of activation parameters for mass transfer and mass transport processes for ionic crystals on a quantum-theoretical basis. Einstein's model for calculating the internal energy of ionic crystals and the specific heat was extended and applied to the problems of mass transfer and transport, the frequency of the optical phonons and the characteristic oscillation frequencies of the crystals (Einstein frequency, Debye-frequency) being employed for the calculation. The experimentally determined activation energies of the mass transfer and transport processes were compared with the values calculated on a quantum theoretical basis which resulted in good agreement.     

On the Quantum Theoretical Calculation of Activation Energies for the Self-Diffusion of Single Atoms and the Diffusion of Adatoms on Metal Surfaces (I)

By application and modification of the equation given by A. Einstein for calculating the internal oscillation energy of crystals, the activation energies for the self-diffusion of single atoms on the metal surfaces of Rh, Pt, and for the surfaces diffusion of adatoms on metal surfaces were calculated on the basis of quantum theory from the surface Debye frequency and the bulk Debye frequency, neglecting the frequency distribution. A comparison of the activation energies calculated by quantum theory with the experimental values of the literature obtained by the field-ion microscope shows a relatively good agreement for the single crystal surfaces and temperature ranges. The calculation showed a strong increase of the activation energies for the self-diffusion at higher temperatures and a relatively great dependence on the respective crystal surface of the metal. Analogous results were also obtained for the surface diffusion of adatoms on the metals.     

Quantenmechanische Gesetzmäßigkeiten bei Kristallisationsprozessen an Phasengrenzflächen von Ionenkristallen (III)

The activation energies for the phase boundary processes on AgCl and AgBr precipitates were measured by the method of labelled atoms and compared with the values obtained by quantum mechanical calculations. A good agreement of the measured and the calculated values was found. – Quantum mechanical laws in mass transfer processes at the phase boundaries of AgCl and AgBr precipitates are discussed. A direct proportionality exists between the activation energy for the mass transfer in crystallization processes and the interfacial energy, respectively, and the frequency of the longitudinal optic phonones, the proportionality constant being the Planck's constant.     

The effect of local centers on conduction and luminescence of boron nitride

Energetic and kinetic characteristics of charge carrier trapping and recombination centres in graphite-like boron nitride produced by gas-phase deposition are discussed. Local levels in X-ray excited PBN are studied using the method of thermally activated spectroscopy of luminescence and conduction, and their position in the band gap is determined from the temperature dependences of the diffusive current and depolarization current. Acceptor and donor level models for different PBN are developed. Their influence on conduction and on luminescence is determined. The scheme of hole-electron transitions by luminescence is suggested. The carbon influence on the nature of the defects creating local levels is discussed.     

AMPS-1D simulation studies of electronic transport in n+-μc-Si:H thin films

To study the electronic transport in highly n-doped microcrystalline silicon (n⁺-μc-Si:H) thin films, grain-boundary trapping model is implemented in AMPS (analysis of microelectronic and photonic structure)-1D. This approach is based on the traditional thermionic-emission model and considering the electronic transport parallel to the substrate. In spite of its simplicity, the model leads to the simulated values of activation energy, free carrier concentration, interface trap charge density and mobility which are in good agreement with the referred Hall effect measurement results for electron cyclotron resonance-chemical vapor deposited (ECR-CVD) highly n-doped μc-Si:H thin films.     

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO₂ doped with rare-earth ions Er³⁺ and Eu³⁺ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er³⁺ and Eu³⁺ ions are expected to act as acceptors in SnO_2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO₂ lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO₂ and SnO₂:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO₂ than in Er-doped SnO₂, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu³⁺ centers.     

The nature of the defect states above midgap and their role in the light-induced metastability of a-Si:H

Defect states of a-Si:H above midgap, with which the electrons interact through trapping and thermal release and make an important contribution to the imaginary (Y) term of the modulated photocurrent, are determined. Analysis of the ‘transition’ region between the low- and high-frequency regimes enables the densities and capture coefficients of defect states to be probed. A reasonable fit to the data is obtained by assuming three defect states, and it is suggested that the defects with the higher capture probability are hydrogen-related centers that are removed during the initial stage of light degradation.     

Investigation of the interaction of quartz with H2 under supply of mechanical energy (I)

The gas molecules absorbed during mechanical activation of quartz in a H₂ atmosphere are situated as well as the formed E′ centres in the amorphous ranges. The concentrations of the E′ centres are about one order of magnitude less after activation in H₂ than after an activation in Ar. Hence and because of the existence of a proton doublet (splitting of about 73 G) in the EPR spectra a chemical interaction of a part of the absorbed H₂ molecules with quartz is concluded.     

Impedance and Electric Modulus Analysis of Cd0.6Zn0.4Te Thin Films

Cd_0.6Zn_0.4Te thin films were deposited by single source vacuum evaporation technique. The impedance of polycrystalline Cd_0.6Zn_0.4Te thin films has been measured as a function of frequency. The experimental data were measured in the temperature range of 300 – 445K and have been analyzed in the complex plane formalism and suitable equivalent circuits have been proposed in different regions. The values of resistance and capacitance of bulk and grain boundary contributions have also independently calculated from the peak of spectroscopic plots. The role of bulk and grain boundary in the overall conduction process has been discussed with realistic justification. The frequency analysis of ac conduction properties showed distribution of relaxation times due to the presence of multiple grain boundaries in the films. The activation energy calculated from the complex impedance analyses was found to be 0.29eV. The values of activation energies decrease with increase in frequency and are in agreement with that calculated from the impedance plot.     

Kinetics and Trapping Parameters of Thermoluminecence of Sr1−xEuxF2+x Mixed Crystrals

The kinetics and trapping parameters of thermoluminescence of Sr_1−xEu_xF_2+x mixed crystals have been calculated by using different methods. The trap depths, frequency factors, and order of kinetics of glow curve pattern in several compositions of the system revealed variations with composition.     

Raman spectra of Ag- and Cu-photo-doped As40S60−xSex films

Raman spectra of the chalcogenide vitreous layers (As₄₀S₆₀, As₄₀S₄₀Se₂₀, As₄₀S₂₀Se₄₀, As₄₀Se₆₀) non-doped and photo-doped by Ag, Cu were measured. The spectra were analyzed in terms of a molecular model. It was ascertained that, for the spectra of photo-doped As₄₀S_60−xSe_x layers, the shift of the main bands to the high frequency side and the appearance of the additional scattering bands in the low frequency spectral range are characteristic features. Variations in the Raman spectra with photodoping by Ag or Cu are consistent with the supposition concerning normal covalent and coordinative bond formation between metal additives and chalcogen atoms, which results in the different activation energy values for the silver photo-stimulated diffusion into chalcogenide layers.     

Acoustic study of low-energy activation processes in amorphous Se,Ge and SeGe compounds

The study of the elastic properties of amorphous selenium over a wide range of frequency (30 kHz−100 MHz) reveal a single internal friction peak below the glass transition temperature.This one involves low activation energies ranging down to very small values and is reduced when the chains are crosslinked. In amorphous germanium, there is no evidence for the existence of activation processes exhibiting very small values. We suggest that the underlying microscopic mechanisms responsible for the peak in a-Se are closely related to the polymeric structure and so should exist in any amorphous polymers. We also suggest that the numerous other peaks existing in most polymers are not intrinsically related to structure in chains.     

Zur quantentheoretischen Berechnung von Aktivierungsenergien für den Stoffübergang an Phasengrenzflächen von Ionenkristallen (IV). Zur radiochemischen Bestimmung und quantentheoretischen Berechnung von Aktivierungsenergien für den Stoffübergang an den Kristallflächen des NaCl

It is shown that a direct proportionality exists between the activation energy for the mass transfer at the respective crystal faces of ionic crystals and the frequency of the phonones (longitudinal-optical), Planck's constant being found once more as a proportionality constant. Thus it could be demonstrated that the different activation energies measured at different time intervals for the mass transfer processes at phase boundaries of ionic crystals can be attributed to the specific growth of the crystal faces. Thus, NaCl crystal fractions which were mechanically stressed (pulverized and sifted) and consequently contained a great amount of {111}- and {110}-faces, respectively, experimentally yielded an activation energy wich agrees with the values determined by quantum theory when the frequency of propagation of the phonons is inserted into a derived equation. This relation was also confirmed by NaCl crystal fractions predominantly containing cubic faces. This was indicates that in mass transfer processes on phase boundaries of ionic crystals quantum mechanical laws are of importance.     

Gross dissolution of natural rhombohedral calcite crystal in nitric acid in dynamic etching condition

Gross dissolution of natural rhombohedral calcite crystal in concentrated nitric acid in dynamic etching condition is studied and interesting results are obtained. The gross dissolution is maximum at 6 molar concentration of nitric acid in dynamic etching condition which is similar to static etching condition. The activation energy and pre-exponential factor is calculated for different concentrations (range 3.0 to 7.5 molar) of nitric acid. The values of activation energy and pre-exponential factors increase as concentration increases in dynamic etching, and it is more than static etching. The higher values of gross dissolution, activation energy, and pre-exponential factor in dynamic etching condition is explained.     

On the Kinetics of Interface Processes on Silver Halide Crystals

In the present paper the activation energies of the transport of Ag⁺ ions in AgCl and AgBr crystals are calculated with the help of a quantum theoretical equation involving the frequency of the optical phonons of the crystal lattice, the mean coordination number for the diffusing ion, and the vibrational quantum number. Comparison of the calculated activation energies with values available from the literature, which were obtained by measurements of electrical conductivity, dielectric loss, diffusion and mass transfer processes, shows good agreement.     

Stability of Electronic Colour Centres in NaF:Li Single Crystals

The influence of the lithium impurity on the formation and the stability of electronic colour centres is studied in NaF crystals. It is found that F- and M-absorption bands are somewhat displaced to the long wavelengthes when doping with lithium owing to the overlapping with F_A- and M_A-absorption bands. It is shown that the lithium impurity is stabilising one for the charged M-centres though their accumulation in the doped crystals occurs slower. The activation energy is determined for the thermal destruction of M⁺- centres. Thermal destruction of M⁺-centres assumed to be resulting from the anion vacancy removal.