Charged Dislocations Research Articles

Measurement of Vibrational Current by Charged Dislocation in NaCl Crystal

To study the structure of the core of an edge dislocation in ionic crystal, we measured the amplitude dependence of internal friction by automated Marx's composite oscillator method and the vibrational current caused by charged dislocations. The phase angle between vibrational current and strain suddenly changed by 180 degrees at a strain amplitude of 10-6. From this fact we discuss the structure of the dislocation core.

On the mechanism of seismo-electromagnetic phenomena and their possible role in the electromagnetic radiation during periods of earthquakes, foreshocks and aftershocks

Abstract We describe generation of low- and radio-frequency electromagnetic radiation (EMR) by seismic waves of pressure applied to a neutral nonpiezoelectrical medium. The EMR is induced during the process of ionic crystal fracture and formation of a mosaic of fluctuation charges on the opposite sides of the cracks, the formation and oscillation of charged dislocations and the vibrations of very small conducting or magnetized particles in dielectrics with fluctuating charges proportional to their radius and temperature. The most interesting source of EMR emission is the process of formation and oscillation of double electric layers (DEL). The DEL can be modelled as a system of capacitors or electric dipoles. The variation of their parameters by external pressure, by changes of water content, etc., leads directly to the EMR emission. As DELs are widespread in rocks, they can make an essential contribution to the EMR. DEL oscillation induced by tectonic processes is probably the main contribution.

Amplitude dependence of the internal friction of alkali-halide crystals in an electric field

The influence of an electrical field on the internal friction of an alkali-halide crystal (AHC) was investigated for relative deformation amplitudes corresponding to separation of dislocations from fastening centers, and for higher amplitudes when plastic deformation of the specimen occurs in an ultrasonic wave field. The influence of the electrical field in the deformation domain under consideration reduces to direct action on charged dislocations, re-orientation of dipole fastening centers, and activation of sources in block boundaries.

Electrical conductivity of ZnS crystals: ZnS crystals as quasi-one-dimensional conductors.

Due to their special morphology many vapor-grown ZnS crystals behave as quasi-one-dimensional conductors. These faulted single crystals display a highly anisotropic dc conductivity, the conductivity at 90\ifmmode^\circ\else\textdegree\fi{} to the c axis being in some cases up to ${10}^{7}$ times larger than its counterpart along the c axis. In the latter direction there exist intrinsic builtin electrical fields of different polarities and field strengths. These internal fields are due to charged dislocation layers appearing between different structures and polytypes which have different basal-plane densities. The influence of these phenomena on the ac conductivity has been investigated for frequencies between 5 kHz and 150 MHz which was found to be both direction and frequency dependent. The peculiar properties of these crystals made it possible to distinguish between effects of the electrodes, surface conductivity, dislocations, and bulk conductivity proper. Even at microwave frequencies up to 3 GHz there is a small anisotropy in the electrical conductivity. A theoretical approach developed for the excitation dynamics in random one-dimensional systems which was previously applied to a random barrier model in the case of the anomalous low-frequency behavior of the ionic conductivity in a quasi-one-dimensional superionic conductor, has been found to be appropriate for a quantitative evaluation of the described experiments.

Charged dislocation induced optical absorption in GaAs

We propose a model of optical absorption induced by electric fields resulting from charged dislocations. It is shown to account satisfactorily for the deformation induced absorption in GaAs, which exhibits the shift of the band-to-band transitions often observed in semiconductors. This model considers that dislocations introduce energy levels in GaAs and its implications should be rather general for most semiconductors in which dislocations introduce energy levels in the band gap.

Theoretische und experimentelle Untersuchungen über die Stromverteilung in elektrochemischen Zellen mit Gasverzehrelektroden

AbstractThis paper deals with charge dislocation problems occuring during overcharge in sealed nickel‐cadmium‐accumulators. In the case of catalyst particle forming gas consumption electrodes, the model “point electrode over flat electrode” was used for the determination of the current distribution. It was solved for linear polarized electrodes. The solution integral is analyzed numerically. A test apparatus for experimental testing was designed and built. This enlarged model enables a measurement of the current distribution. The theoretically predicted results are confirmed experimentally. Additionally the current distribution in the model “point electrode between two ideal unpolarized electrodes” is calculated.

Charged dislocations and properties of alkali halide crystals

Theoretical and experimental investigations of charged dislocations in alkali halide crystals are reviewed. A description is given of the fine structure of an edge dislocation with defects on the dislocation line. A systematic account is given of theories of a sessile charged dislocation ranging from its representation by an infinitely long charged filament to an allowance for the fine structure of the dislocation line. Phenomenological models dealing with a charge on a moving dislocation are discussed. Forces acting on a dislocation are considered and the characteristics of its motion under the influence of an external mechanical load and of an electric field are dealt with. The direct and converse dislocation piezoelectric effects are described for a crystal containing mobile charged dislocations. In presentation of experimental results special attention is given to studies providing quantitative estimates of the linear charge density on a dislocation.

Charged dislocations in semiconductors

A modification of the theory of charged dislocations is proposed which enables one to allow for several discrete electron levels in a band gap of semiconductor. The equilibrium characteristics of a semiconductor with charged dislocations are studied for the case when a spectrum of electrons captured by a dislocation has several levels. A set of kinetic equations is written describing the dynamics of filling charged dislocations under conditions close to the real ones, i.e. with allowance for a complex electron spectrum, the deformation potential, a finite conductivity of the Read cylinder etc. Results of these calculations are compared with the experimental data on capacitance spectroscopy of plastically deformed silicon. [Russian Text Ignored].

Dye‐Laser 4f–4f‐Line Probing of Crystal‐Field Fluctuations in ZnS:Sm3+ Polycrystalline Films

AbstractUsing the dye‐laser site‐selective excitation technique, a study is made of 4G5/2 → 6H9/2, 6H7/2 inter‐Stark level transitions for a special type of Sm3+ centers in ZnS polycrystalline films. It is found that the emission lines become narrowed compared with the case of “nonselective” excitation, with the peak positions and intensities depending on the location of the dye‐laser energy within the 4f–4f excitation line profiles. The results can be quantitatively described by a simple model which involves small centre‐to‐centre fluctuations of crystal‐field parameters. A comparison with the (much sharper) lines measured for the same type of Sm centers in ZnS bulk crystals shows that these fluctuations have a Gaussian distribution with maximum corresponding to the bulk crystal situation. An explanation can be given on the basis of random strain originating from high dislocation densities. A discussion is made of whether the linewidths are affected by interactions between the Sm centres and by electric fields due to dislocation charge.

Thermally stimulated depolarization currents in OH−‐doped NaCl and KCl crystals

AbstractThe main features of the high‐temperature ITC band are analyzed on the basis of experimental results obtained for NaCl and KCl crystals containing the hydroxyl ions. The unusual behaviour of this band has been compared with that described previously for alkali halides, AH, (LiF, NaCl, KCl) doped with some divalent cations, Me++, (Be, Eu, Ni, Pb).It has been shown that — similar to the case of the Me++‐doping‐most of the results obtained can be explained in frames of the Maxwell‐Wagner‐Sillars model of interfacial polarization induced by the impurities forming the charge clouds to the dislocation lines. An ITC band can be detected due to the fact that these regions exhibit electric conductivity and dielectric constants different from those characteristic of the bulk material. The role of the charge of dislocations in the induced polarization phenomenon is shortly discussed.

Zaryazhennye dislokatsii i svoistva shchelochnogaloidnykh kristallov

Zaryazhennye dislokatsii i svoistva shchelochnogaloidnykh kristallov

Dislocations and electrical characteristics of HgCdTe

The electrical characteristics of HgCdTe samples have been experimentally examined as a function of dislocation density. MIS measurements on samples with a graded dislocation density ranging from about 2×105 to 2×106 cm2 showed the net doping concentration is independent of local dislocation density, but that MIS dark current varies linearly with dislocation density. No evidence for impurity segregation to dislocations was observed between the low and high dislocation density region. Hall measurements on n-type HgCdTe samples both before and after 2% plastic deformation showed that the net donor density increased slightly by (1−2)×1014cm-3 possibly from residual point defects generated by deformation. Conversely, the electron mobility at low temperatures decreased by about an order of magnitude due to increased scattering from charged dislocations whose density increased by a factor of more than 100. Deformed HgCdTe shows a larger decrease in the electron mobility after low temperature plasma hydrogenation than do undeformed samples. Dislocation density may influence the hydrogen solubility in these samples. Collectively, both the MIS and Hall results suggest dislocations themselves do not contribute to the carrier concentration in HgCdTe.

Influence of the Electric Charge of Dislocations on the Hardening of Alkali Halides

It is verified that the electrostatic interaction between dislocations and divalent cation–cation vacancy dipoles gives the same kind of dependence on temperature, effective stress, and dislocation velocity as those deduced from the elastic interaction between dislocations and tetragonal defects. It is observed that if the temperature remains constant when the stresses are low the relationship between stress and dislocation velocity can be approximated by a simple exponential relation and when the stresses are higher it follows a potential expression, as deduced empirically by Gilman and Johnston. Il a été vérifié que l'interaction électrostatique entre les dislocations et les dipoles cation divalent—vacante de cation donne le měme type de dépendance entre la température et la vitesse des dislocations que celles déduites de l'interaction entre dislocations et défauts tétragonaux. On a observé que si la température demeure constante quand les efforts sont faibles, la relation entre l'effort et la vitesse des dislocations peut etre approchée par une simple relation exponentielle et quand les efforts sont plus importants il existe une expression de puissance comme celle qui a été déduite empiriquement par Gilman et Johnston.

Dislocation charge in potassium chloride temperature and divalent-impurity dependence

Abstract Using the ultrasonic composite oscillator technique at high strain amplitudes (8 × 10−6-2 × 10−4) we have measured the equilibrium dislocation charge in bent potassium chloride single crystals doped with Ca2+ (0·02–14 p.p.m.) over the temperature range 400–730°C. The large strain amplitude ensures that dislocation displacement exceeds the Debye-Hückel screening radius so that the resulting polarization is unscreened. In the amplitude-dependent region the isoelectric temperature at which the mean charge vanished was found to decrease slowly with strain amplitude. The temperature and impurity dependences of the charge are well characterized by a straightforward model of vacancy formation at core jogs.

The Electric Charge of Dislocations in Alkali Halides

The linear density of the electric charge of edge dislocations in Mg2+ doped LiF single crystals is measured. Assuming that mechanical and electrical forces exert the same effect on the dislocations at the arms of an indentation rosette, a theoretical model is developed which predicts a linear dependence between the applied stress and lg (dmax/dmin), where dmaxand dmin are the maximum and minimum lengths, respectively, of the arms of the rosette formed by edge dislocations around the indentation. On a mesure la densite lineaire de la charge electrique des dislocations en arěte dans des mono-cristaux de LiF:Mg2+. Le procede utilise suppose que les forces mecaniques et electriques ont un měme effet sur les dislocations des bras d'une rosette d'indentation. Le modele theorique developpe prevoit une dependance lineaire entre 1'effort de compression applique et le logarithme du quotient des longitudes maximale and minimale des bras de la rosette d'indentation formes par des disloca-tions en arete.

A dislocation model relating apparent charge density to lattice mismatch in epitaxial InGaAs/InP

Using a new model relating dislocation charge to Fermi level, we have simulated the differential capacitance resulting from the inclined dislocations accompanying the misfit dislocations in epitaxial InGaAs/InP samples. The result is a quadratic dependence of charge upon mismatch which is in quantitative agreement with experiments of Macrander et al. for a misfit dislocation segment length equal to about nine times the dislocation spacing.

Voltage-dependent plasticity in HgCdTe

The effect of an applied, time-varying voltage on the plastic behavior of (Hg,Cd)Te has been investigated. We observe that both positive and negative voltages suppress plasticity, with the negative voltage being more effective. The influence of this voltage polarity is strain dependent and provides information on the charge of the mobile dislocations.

Dislocation polarization and space-charge relaxation in solid solutions Ba1-xLaxF2+x.

Concentrated solid solutions ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$, x\ensuremath{\le}0.492, are studied by means of thermally stimulated depolarization [ionic thermal current (ITC)] or polarization [thermally stimulated polarization current (TSPC)] techniques. At high temperature two peaks are reported whose maxima shift to lower temperatures as x increases. The energies calculated for these relaxation processes are compared to the enthalpies deduced from conductivity measurements. The first high-temperature peak, D, has been already reported in a more restricted concentration range and has been attributed to either a space-charge accumulation near the electrodes or the polarization of the dislocations present in the crystal.We show here that it is the highest-temperature peak reported here for the first time and labeled as peak E which is best related to the displacement of free interstitial fluorines through the bulk of the crystal; that is, to the building up of a space charge near the electrodes. At low concentrations the behavior of peaks D and E is understood if the Debye-H\uckel interaction energy among unassociated defects is taken into account. For the more concentrated region (x\ensuremath{\gtrsim}0.05), the enhanced ionic motion model proposed to explain the variation of the ionic conductivity slopes as a function of x also applies here; the parameters present in the model are calculated and are found to be in agreement with those previously reported. The energy difference observed between peaks D and E is almost constant for x\ensuremath{\ge}0.05. This energy difference equal to 0.08 eV is attributed to the binding energy of the interstitial fluorines forming the dislocation charge cloud to the dislocation line. TSPC experiments are carried out on the same crystals and they show that peak D is present in all the TSPC spectra, thus confirming the localized displacement of the charges, while the highest-temperature peak E is replaced by a steep increase of the ionic current. Therefore, the origin of peak E observed in ITC must be attributed to the accumulation of charges near the electrodes; that is, the build up of the space-charge layer.

The influence of dislocation charge on the cleavage surface charge of alkali halide crystals

The influence of dislocation charge on the cleavage surface charge of alkali halide crystals

The sweep-up model of charged dislocations in ionic crystals

Abstract Edge dislocations moving in ionic crystals acquire a charge as a result of the sweeping-up of cation vacancies. The nature of this dynamic equilibrium process is analysed for non-irradiated extrinsic alkali halides at temperatures where diffusion can be ignored.