Photoplastic Effect Research Articles

Influence of slip direction on the photoplastic effect in cadmium sulfide

Studies of the photoplastic behavior of CdS single crystals show that when slip occurs on the basal plane, a large increase in flow stress is observed in the presence of incident light. However, when the crystal is oriented for slip on the prismatic planes, a very small photoplastic effect is observed. Measurements of the current associated with dislocation motion on the active slip systems indicate that the photoplastic effect is proportional to the charge on the dislocations in the respective slip planes.

The photoplastic effect in II–VI compounds. Mercury cadmium telluride

Experiments demonstrating the photoplastic response in mercury cadmium telluride crystals are presented. These experiments show the presence of a series of time delays prior to the change in stress when (Hg1–xCdx)Te crystals are irradiated with light. The change in stress is observed to be positive or negative. In addition the application of a voltage – constant in magnitude or varying temporally – affects the plastic response of the deforming crystal. All of these results are explained by a model in which charged point defects interact coulombically with charged dislocations. The electrons and or holes introduced by the light diffuse to either point defects or dislocations, leading to changes in the interaction between point defects and dislocations. Experimente werden angegeben, die die photoplastische Response in Quecksilber–Kadmiumtelluridkristallen demonstrieren. Diese Experimente zeigen eine Reihe von Zeitverzogerungen vor der Spannungsanderung, wenn (Hg1–xCdx)Te-Kristalle mit Licht bestrahlt werden. Es werden positive oder negative Spannungsanderungen beobachtet. Zusatzlich beeinflust das Anlegen einer elektrischen Spannung – konstant oder zeitlich veranderlich – die plastische Response des sich deformierenden Kristalls. Alle diese Ergebnisse werden mit einem Modell erklart, in dem geladene Punktdefekte uber Coulombkrafte mit geladenen Versetzungen wechselwirken. Die durch das Licht eingefuhrten Elektronen und Locher diffundieren entweder zu Punktdefekten oder Versetzungen, und fuhren zu Anderungen in der Wechselwirkung zwischen Punktdefekten und Versetzungen.

Two trapping centers in the photoplastic effect in CdS

Two trapping centers in the photoplastic effect in CdS

Deformation mechanism and photoplastic effects in CuBr single crystals

Abstract Single crystals of CuBr with the zincblende structure, grown by the Bridgman method with KBr flux, have been deformed in compression between room and liquid-nitrogen temperatures. The resolved yield stress at room temperature is as low as 1.0 MN m−2, but increases steeply below about 130K with decreasing temperature. The activation volume for plastic deformation decreases to less than 10 b3 (b is the magnitude of the Burgers vector) at shear stresses higher than 10 MNm−2. These facts suggest that the Peierls mechanism controls the deformation at low temperatures. Illumination with light during plastic deformation causes almost reversible hardening, the amount of which is undetectable at room temperature, increases with decreasing temperature and levels off below about 130 K. The effect is maximum for a wavelength slightly longer than the fundamental absorption edge. Below 130 K, illumination brings about another effect, namely irreversible softening, which is possibly caused by enhanced cracking.

Mechanical properties of anthracene crystals under illumination: Photoplastic effect

The mechanical properties of the photoplastic effect (PPE) in anthracene crystals were extensively studied. Detailed results of its dependence on strain rates ε̇, strain-rate sensitivity, temperature, and wavelength of exciting light have been obtained, and also the photomemory effect of PPE has been observed. Temperature dependence of PPE showed that the formation energy of physical dimers (photoinduced products) was estimated to be about 0.2 eV. Moreover, the strain-rate dependence of PPE indicated that the change in stress Δτs decreased with increasing strain rates, namely, the strain-rate sensitivity m=Δτs/Δ ln ε̇<0. In addition, the photomemory effect shows that physical dimers were stable at dislocation cores with a long life.

Dislocation Photoconduction and Related Dislocation Memory Effects in KCl Crystals

AbstractTwo types of memory phenomena are discussed, both related to dislocation photoconduction of highly pure and coloured KCl crystals, using the combination of Lyman‐ and F‐illuminations. A very simplified explanation of the dislocation memory effects is given taking into consideration possible changes in the direction of the photocurrents. Some considerations may be of interest for the explanation of photoplastic effects.

On the time delay of the photoplastic effect

Photoplasticity in mercury cadmium telluride, Hg1-x Cdx Te with x = 0.3, has been studied as a function of light frequency and deformation temperature. We show that there is an easily measurable time delay accompanying irradiation of the crystal and the change in stress. This time delay is temperature dependent, suggesting a diffusion of charge carriers, introduced by the light, to the interior of the crystal. A simple analysis is given of the observed temperature dependence that is consistent with the experiments.

Properties of II–VI semiconductors associated with moving dislocations

Abstract Moving dislocations in II–VI semiconductors carry a large electric charge. This charge is not in thermal equilibrium, but is due to the sweeping up of electrons from point defects. Its movement produces a dislocation current during plastic deformation, and conversely, the application of an external field changes the flow stress. This paper reviews the structure and properties of these dislocations, the theory of their charge and the phenomena which are a consequence of the strong mutual interactions of the dislocation and electronic sub-systems in these crystals. The materials show a large photoplastic effect (a change in flow stress under illumination), and related effects due to the injection of electrons at an electrode. Deformation produces reversible changes in the conductivity, pulsed and continuous luminescence and the emission of electrons from the surface.

Dislocations in semiconductors

Glide in elemental semiconductors or III–V compounds takes place by dissociated 1/2〈110〉 dislocations on {111} glide planes. Structural models and valence force calculations suggest that the cores of the basic partial dislocation may be reconstructed. Kinks may be reconstructed or have dangling bonds. The former will be associated with fairly shallow levels, the latter with deep acceptor and donor levels in the bandgap. Deep–level transient spectroscopy, electrical, optical, and electron paramagnetic resonance data give information on energy levels and concentrations of dangling bonds and deep levels, but the identification with particular sites on the dislocations is very difficult. The electronic states also lead to the dislocations acting as recombination centres, and give rise to photoplastic effects. There are strong interactions with impurities, which have a profound effect on the electronic properties. Dislocation velocities at relatively low temperatures are controlled by the Peierls force, and motion occurs by the generation and motion of double kinks. There is a pronounced dependence of dislocation velocity on doping, for both elemental semiconductors and III–V compounds. A recent theory attributes the doping effect to a dependence of the concentration of charged dangling bond kinks on the Fermi level, and of the kink velocity on the charge state. The doping effect is also reflected in mechanical properties, for example the variation of yield stress with temperature, rosette diameters, and radial cracking behaviour around hardness indentations, and values of hardness. In III–V compounds different mobilities of α– and β–dislocations give rise to anisotropies in hardness, rosette configurations, and associated cracking, and for {111} indentations different hardness values, slip patterns, and cracking behaviour result on the group III and V crystal faces.MST/269

Dislocations in semiconductors

AbstractGlide in elemental semiconductors or III–V compounds takes place by dissociated 1/2〈110〉 dislocations on {111} glide planes. Structural models and valence force calculations suggest that the cores of the basic partial dislocation may be reconstructed. Kinks may be reconstructed or have dangling bonds. The former will be associated with fairly shallow levels, the latter with deep acceptor and donor levels in the bandgap. Deep–level transient spectroscopy, electrical, optical, and electron paramagnetic resonance data give information on energy levels and concentrations of dangling bonds and deep levels, but the identification with particular sites on the dislocations is very difficult. The electronic states also lead to the dislocations acting as recombination centres, and give rise to photoplastic effects. There are strong interactions with impurities, which have a profound effect on the electronic properties. Dislocation velocities at relatively low temperatures are controlled by the Peierls force...

Theory of statistical dislocation dynamics in photomechanical effects

A simple theory is proposed for the photoplastic effect in alkali halides containing F-centers based on statistical dislocation dynamics. The present theory depicts well several dependences of flow stress increment on, e.g., defect concentration, strain rate and light intensity.

Effect of specimen size on photoplastic effects in anthracene crystals

The photoplastic effect (PPE) in anthracene crystals, which is a photoinduced hardening, has been studied by shear deformation. The PPE is produced at wavelengths ranging from less than 300 to 440 nm, with a maximum effect at 430 nm. The PPE is a bulk effect; it can be produced by light that has travelled more than 2 mm into a crystal. When PPE occurs with short wavelength (≤400 nm) small attenuation length (∼10−4 cm) illumination, it is not produced directly by the intrinsic light absorption, but rather by longer wavelength fluorescence radiation from the near-surface region. The photoproducts that interact with dislocations to increase the flow stress are most likely physical dimers near defect sites.

Investigations of well defined dislocations in silicon

The velocity v of dislocation half-loops introduced into swirl-free floating-zone grown undoped silicon has been measured at 420°C in the resolved shear stress range 30 <τ<300 MPa. Clearly impurity atoms interact with dislocations in this material. Using the starting value of v we found the two types of 60° dislocations, which are distinguished by the sequence of their partials, to have different velocities. Furtheron the velocity depends not only on τ, but also on the elastic strain of the lattice.In the second part the papers review EPR spectroscopy of plastically deformed silicon and collects new results on the activity of dislocations in this material as trapping / recombination centers (decay of photo-EPR, photoluminescence, EBIC microscopy and photoplastic effect).

Photoplastic effect in silicon

The influence of illumination (hv=1.17 eV, P=1 W/cm2 on the crystal surface) on the motion of dislocations in silicon is investigated in the temperature range 325°C–422°C using the double etching technique.The screw dislocation and one type of 60° dislocation (30/90 dislocation) exhibit a strong photoplastic effect : the activation energy of the 30/90 dislocation is changed by ΔE=0.68 eV.The different behaviour of the two types of 60° dislocations points to recombination enhancement of the dislocation motion in contrast to the effect of a changed charge state.

Negative photoplastic effect due to the relaxed excited state of the F-center in alkali halides

Abstract To make clear the mechanism of the photoplastic effect (PPE) in alkali halides containing F-centers, on the basis of the photochemical reaction of the F-center, temperature dependence of the PPE in KC1, and KBr and NaCl was studied from 95 K up to RT. The characteristic critical points θ ts in colored KC1 and KBr were determined to be 110 and 190 K, respectively. Below θ t the sign of the PPE in both crystals was negative, i.e., decrement of the flow stress during light illumination was observed. Moreover, the θ ts were in good agreement with the temperature where the photoconductivities show remarkable increase in the course of the measurements. The negative PPE is explained in terms of the rotation of the principal strain axis of the relaxed state of the F-center having ⟨100⟩ tetragonarity as to relax the stress around the edge dislocation coming closely.

Two photoplastic effects and their temperature dependence in alkali halides

The characteristic critical temperatures for the sign of the photoplastic effect (PPE) in KCl and KBr, using also photoconductivity measurements, were determined to be 110 K and 190 K, respectively. Negative PPE is explained in terms of elastic interaction between a moving dislocation and the relaxed excited state of the F-center.

Photoplastic Effects in II-VI Crystals

ABSTRACTA reversible change in the flow stress with the illumination of a band gap light (photoplastic effect: PPE) is observed commonly in II-VI semiconducting compounds both with the zincblende and the wurtzite structures. After reviewing the experimental results accumulated so far concerning the usually observed PPE, detailed microscopic experiments on CdTe (partly on CdS) single crystals are described. In-situ TEM straining experiments with a laser illumination system clarified that the dislocation mobility, evidently controlled by the Peierls mechanism, is not affected by light illumination.The investigation of the dislocation glide behavior over a longer range using the etch pit method and the cathodoluminescence microscopy with a SEM revealed that the dislocations are apt to become immobilized after traveling a certain distance from the sources with a high velocity.The dislocation loops and cusps observed by TEM suggest that the immobilization is caused by jog formation along screw segments. Those results suggest that the positive PPE is caused by the decrease in the mean free path of multiplied dislocations due to the enhanced jog formation by illumination. This viewpoint is supported by the high density of dislocation debris left in specimens deformed under illumination, the enhanced work hardening rate under illumination and the reduction of the PPE when the dislocation mean free path becomes determined by other photoinsensitive factors such as forest dislocation cutting and specimen size. The microscopic mechanism of the enhanced jog formation is discussed in terms of electrostatic interaction between charged point defects and dislocation charge which is increased with the illumination.

Plastic Deformation of CdTe Single Crystals II. Photoplastic Effect of II-VI Compounds

In-situ observations by TEM reveal that dislocation velocity is not influenced by light illumination, indicating that the cause of the photoplastic effect (PPE) in II-VI compounds is not a change in the dislocation mobility. Densities of dislocations and loops left after plastic deformation in light are about an order of magnitude higher than those in darkness. These results together with the results of mechanical tests and those given in Part I (J. Phys. Soc. Jpn. 49 (1980) 1909) lead to a conclusion that the PPE is caused by the decrease in mean free path of multiplied dislocations. Jog formation by cross slip resulting in immobilization of glide dislocations is considered to be enhanced by their interaction with charged point defects that trap minority carriers generated by illumination.

Photoplastic effect in anthracene crystals

The photoplastic effect is observed in anthracene single crystals under illumination with near-ultraviolet light. The overall features of the action spectrum of hardening bear a close resemblance to the absorption spectrum. The maximum hardening due to illumination reaches as much as 10% of yield stress at 120 W/m2.

Cathodoluminescence studies of dislocation motion in IIb?VIb compounds deformed in SEM

Continuous observation of dislocation motion on the specimen surface of IIb–VIb semiconducting compounds (CdS and CdTe) has been made using a scanning electron microscope (SEM) with the cathodoluminescence (CL) by use of a deformation apparatus installed in the SEM chamber. With the application of stress, SEM-CL patterns revealed an increasing density of dark spots that formed dark stripes along slip traces. For both CdS and CdTe crystals, regardless of the slip system involved, the dark spots corresponding to individual dislocations appeared in the SEM field without displaying their moving state and seldom disappeared, indicating that dislocations are immobilized after they travel a certain distance from the source with a high velocity. The investigation of slip-band growth in CdS showed that screw bands on the basal plane exhibit the photoplastic effect, whereas those on the prismatic plane do not, in accordance with the macroscopic deformation tests.