Deformation-induced Point Defects Research Articles

Dislocation States and Deformation-Induced Point Defects in Plastically Deformed Germanium

We used the DLTS and photoluminescence (PL) techniques to study the deep states due to dislocations and deformation-induced point defects (PDs) in plastically deformed p-type germanium single crystals containing predominantly 60 dislocations with density ND, ranging from 105 to 106 cm-2. The narrow line near the temperature 140K dominates in the DLTS spectra. The ionization enthalpy and the capture cross section for holes traps indicate that the substitution copper atoms Cus are the main type of PDs. A decrease of the Cus atoms concentration and redistribution of the intensity in the PL spectra after the heat treatment of deformed samples at a temperature 500 °C are attributed to the diffusion of copper atoms to dislocations resulting in the appearance of “dirty” regular segments of 60 dislocations.

Dislocation photoluminescence in plastically deformed germanium

Abstract Dislocation photoluminescence ( PL ) was measured at 4.2 K in p-type germanium single crystals (acceptor concentration N a ∼10 14 cm −3 and density of as-grown dislocations N D ∼10 cm −2 ) deformed at temperatures 450 and 500 °C. The effect of isochronal heating of deformed samples on the spectral distribution of the recombination radiation due to regular segments of dissociated 60° dislocations of “relaxed” morphology was studied over the range from 130 to 700 °C. The substantial redistribution of the PL intensity after the 500 °C heating of deformed samples correlates with a decrease of a major portion of deformation-induced point defects ( PDs ). This suggests that the PDs annealing is accompanied by gathering of impurities near 60° dislocation cores and changing of the radiation energy of regular segments of dissociated 60° dislocations.

Factors which define the α/γ ratio in CsI:Tl crystals

Dependences of light yield and α/γ ratio on the Tl concentration have been studied within a wide range of shaping times. It is shown that the α/γ ratio essentially depends on the Tl concentration. Proper combination of the Tl concentration and optimum shaping time in electronics allows to obtain detectors with rather high light output for γ-rays, α-particles and α/γ ratio values. It has been shown that both the light yield at α-excitation and the α/γ-ratio depend on the time of crystal storage after polishing. On the basis of the idea of the formation of deformation-induced point defects in a thin surface-adjacent layer, the causes of the temporary increasing in light yield for α-particle are explained. An explanation is given of the results obtained by Gwin and Murray concerning the fact that the α/γ ratio is practically independent on Tl concentration.

Point Defect Mechanisms in Deformation and Phase Transformation of Titanium Aluminide Alloys

ABSTRACTPoint defects in intermetallic compounds are subjected to significant constraints due to the directional bonding, long-range order and off-stoichiometric deviation of the materials. This leads to a variety of defects with distinct differences in morphology, concentration and mobility. In the present study the implications of these defect characteristics on the mechanical properties of two-phase titanium aluminide alloys will be investigated. The major areas of the study are: (i) deformation induced point defects that contribute to work hardening; (ii) dislocation locking due to the formation of defect atmospheres; (iii) transport processes involved in phase transformation and recrystallization occurring during long-term creep. The applied methods include mechanical testing, static strain aging and structural characterisation by high resolution electron microscopy.

Structural, thermal and deformation-induced point defects in PdIn

Perturbed angular correlation of gamma rays (PAC) was applied to measure concentrations of point defects in quenched and milled intermetallic compounds. Two systems with high ordering energies were studied, PdIn and NiAl, with principal results obtained for PdIn. The defects, lattice vacancies and antisite atoms, were detected by quadrupole interactions induced at nuclei of nearby In/Cd probe atoms. Measurements on annealed, quenched and milled samples are compared to identify quadrupole interaction signals with specific configurations of defects near probe atoms. Muller and Hahn had previously identified signals with a Pd-vacancy in the first near-neighbor shell of the In probe and a Pd-antisite atom in the second shell. Six additional signals are identified in this work, including two distinct configurations of two Pd-vacancies in the first shell, a Pd-vacancy in the fourth shell, an In-vacancy in the second or third shell, and an In-antisite atom in the first shell. After quenching, observation of both Pd and In-vacancies after quenching from 1100-1500 K demonstrates that the high temperature equilibrium defect is the Schottky vacancy pair. An activation energy of 0.91(18) eV was determined for formation of a Schottky pair. After milling, the site fraction of probes having one Pd-vacancy in the first shell increased rapidly and saturated at a value of about 20%. This implies a Pd-vacancy concentration of about 3.3 at.%, with a corresponding enthalpy of the order of 3 kJ mol−1, making a significant fraction of the excess enthalpy in the milled intermetallic. After milling NiAl, a Ni-vacancy concentration of order 1–10 at.% was observed. A comparison is made of defect combinations observed after mechanical milling of PdIn and NiAl by PAC and of other intermetallics by Mossbauer spectroscopy. Dominant defects produced by milling are the Schottky pair for PdIn, triple defect for NiAl and FeRh and antisite atom pair for FeAl.

Deformation-induced point defects in NiAl single crystals

NiAl single crystals, oriented for single slip, were deformed at room temperature to a strain of 2%, and were subsequently annealed in the temperature range of 673–873 K ( T/T m =0.35–0.45). In as-deformed samples, dislocation substructures consist of jogged edge and screw dislocations, and prismatic loops. Densities of vacancy- and interstitial-type loops are about equal. Annealing causes shrinkage and disappearance of the interstitial loops and significant growth of the vacancy loops. These observations suggest that excess vacancies are present after room temperature deformation. These non-equilibrium point defects may result from non-conservative motion of jogged screw dislocations.

Short-range order kinetics in α-AgZn for various states of post-deformation defect annealing after cold-rolling

Kinetics of short-range ordering (SRO) in Ag with 21, 23 and 28 at% Zn is investigated by residual resistometry during isochronal and isothermal heat treatment for different states of post-deformation defect annealing after cold-rolling to about 30 and 60% thickness reduction. Resistivity changes due to pure ordering can be separated from the as-measured total resistivity change which includes defect annealing. Although the initial state of SRO of the as-rolled material can be estimated to be comparably low, for as-rolled and partially annealed states by appropriate thermal treatment evolution of SRO is achieved which corresponds quite well to that of recrystallized samples. It is observed, however, that quenched-in surplus vacancies contribute considerably to the ordering process for the recrystallized state and that this contribution is still increased by the grain growth during the final stage of annealing. It therefore turns out that SRO-kinetics under equilibrium vacancy conditions can be better observed in a state of post-deformation annealing, for which deformation induced point defects are annealed-out, but a relatively high dislocation density is still present to act as a vacancy sink.

Scanning deep-level transient spectroscopy investigations on gallium arsenide

The present state of the art of the application of the scanning deep-level transient spectroscopy (SDLTS) technique is reported. Special attention is given to the theoretical modelling of the signal generation process, yielding important information on the factors influencing the spatial resolution to be attained. The experimental possibilities of the dedicated SDLTS equipment available now are described. This system was used to investigate point defects generated by plastic deformation in GaAs p+-n junctions induced by diamond indentations made at room temperature. A cloud of point defects (hole traps at Ev + 0.5 eV) up to a distance of approximately 20 μm from the indentations has been detected by SDLTS. Interestingly, the deformation-induced point defects are localized in regions outside the slip bands rather than within the slip bands of the deformation-induced dislocations. Annealing experiments seem to contradict their interpretation as the second charge state of the EL2 level.

Point-defect-driven dislocation patterning during deformation under irradiation

Dislocation patterning driven by interactions of dislocations with deformation-induced point defects is considered. The effect of concurrent irradiation-induced production of point defects is also included. The uniform time-dependent solution of the set of equations describing the evolution of the system is “probed” by small periodic perturbations. A linear stability condition obtained in this way as well as the preferred wavelength of the emerging pattern depend on the values of the parameters reflecting biases in the production and annihilation of vacancies and interstitial atoms. It is proposed that by studying the effect of different types of radiation and different irradiation intensities on the occurrence and the wavelength of the dislocation pattern information about the deformation-induced point defect production bias may be obtained.

The yield point of as-grown and pre-deformed semiconductors

Abstract The strain-rate and temperature dependence of the lower yield stress of silicon and several III-V compounds is different for as-grown and pre-deformed crystals. While the behaviour of as-grown material is rather uniform among these semiconductors, a pre-deformation of the specimens gives rise to various effects. Intrinsic and deformation-induced point defects clustering into small voids and precipitates may be responsible for such differences.

Inhomogeneities in plastically deformed silicon single crystals. II. Deep-level transient spectroscopy investigations of p- and n-doped silicon.

The deep-level transient spectroscopy (DLTS) spectra of p- and n-type silicon samples, which were plastically deformed in the temperature range 390 \ifmmode^\circ\else\textdegree\fi{}C\ensuremath{\le}${\mathit{T}}_{\mathit{d}}$\ensuremath{\le}800 \ifmmode^\circ\else\textdegree\fi{}C with different resolved shear stresses 8\ensuremath{\le}\ensuremath{\tau}\ensuremath{\le}200 MPa, can be quantitatively related to the deformation-induced ESR spectra. Deformation-induced point defects are inhomogeneously distributed and produce dominant energy levels in the lower half of the band gap. A smaller amount of dislocation-related defects causes energy levels in the upper part and in the middle of the band gap. The deformation-induced defects compensate p- and n-type material. The DLTS linewidths can be simulated by the introduction of a broadening parameter \ensuremath{\delta}, which describes deformation-induced disorder. From its dependence on the applied deformation stress (21.5 meV/GPa in n-type material) and the similarity of the value to the pressure derivative of the conduction-band edge, we conclude that part of the disorder is related to fluctuations of the band edges. Contributions of fluctuating Coulomb potentials and variations of the capture cross sections caused by compensation probably contribute to the broadening of the energy levels. Similarities to polycrystalline materials are discussed.

Possible Role of Deformation‐Induced Point Defects in Dislocation Patterning

AbstractThe evolution of dislocation density is considered by taking into account the effect of deformationinduced point defects on the dislocation annihilation (recovery) rate. It is shown by linear stability analysis of the coupled equations describing the evolution of the dislocation density and the diffusion of point defects that a periodic spatial pattern can form during plastic deformation. The preferred wavelength of the emerging periodic structure is evaluated as a function of the production and annihilation bias parameters reflecting certain asymmetries in the generation and annihilation of interstitials and vacancies in the course of straining.

Hydrogen desorption from crystalline silicon and its modification due to the presence of dislocations

Hydrogen effusion spectra of oxidized Si samples are investigated and characteristic activation energies associated with the rupture of hydrogen bonds are estimated. Dislocation-enhanced solubility of hydrogen is found in deformed crystals and macroscopic diffusion depth (mm) can be realized. Hydrogenation is performed by exposure of the crystals to H2 gas at 800 °C. Beside the desorption of hydrogen bound close to the Si surface, the rupture of hydrogen-hydrogen bonds of molecules stored in deformed crystals is observed. The storage of the H2 molecules requires the presence of dislocations or deformation-induced point defects, and the H:H binding energy ranges from 2.7 to 3.8 eV. Most likely this energy variation comes from different local strain around dislocations. Atomic hydrogen is found to dominate the effusion kinetics.

Dislocation Dissociation and Electrical Properties of Plastically Deformed Germanium Single Crystals

The dissociation of dislocations in at 680°C plastically deformed germanium single crystals of n-type is studied using the weak-beam technique of electron microscopy. Both, the Hall e.m.f. and de conductivity are measured within the temperature range 30 to 300 K. A dependence of the dissociation width of dislocations and electrical properties of deformed specimens on their cooling rate after deformation is observed. The “anomalous” dissociation of dislocations with a predominant screw component in rapidly cooled specimens and the different temperature dependences of concentrations and mobilities of the current carriers in rapidly and slowly cooled deformed specimens are connected with the presence of deformation-induced point defects in rapidly cooled specimens. According to quantitative estimations the “anomalous” dissociation of the above mentioned dislocations may be caused by the Coulomb repulsion of partial dislocations enforced by point defects localized near them. [Russian Text Ignored].

Deformation-induced point defects in germanium

Abstract Using a transient capacitive method (deep-level transient spectroscopy), we have studied defects introduced into n-type germanium by plastic deformation at temperatures below 0·6T m (where T m is the melting temperature). From the annealing behaviour of a defect with a level at E c - 0·29 eV, we conclude that point-defect clouds surround the dislocation formed during plastic deformation, and that these anneal out at high temperatures.

DISLOCATION RELAXATION PEAKS IN MOLYBDENUM

In high purity molybdenum single crystals measurements of the damping and modulus are reported in the temperature range of the α and γ-relaxation. Activation volumes of the α-subpeaks were determined from the amplitude dependence of the Internal Friction. The results clearly support the explanation of the α-relaxation in terms of kink pair fromation in 71°-dislocations and diffusion of different types of geometrical kinks in screw dislocations. Deformation induced point defects interact with these dislocation segments giving rise to the β-relaxation. Some characteristics of the γ-peak (kink pair formation on screw dislocations) are discussed.

Internal friction peaks of cold-worked Cu + 0.1 at.% Ag in the kHz range

Damping and elastic modulus of deformed Cu + 0.1 at.% Ag have been measured as functions of temperature at about 5 kHz. Two peaks are found which could be correlated to the peaks P a and P b found in previous 1 Hz-experiments. In some specimens a new peak P c is observed in the kHz-range. By connecting the 1 Hz and the new 5 kHz peak temperatures, the effective activation energies and the frequency factors have been determined. Furthermore, the amplitude dependence, the recovery-behaviour, the peak-width and the relations between damping and modulus defect are discussed and compared to the corresponding quantities determined from the previous 1 Hz measurements. Possible mechanisms for these peaks based on the interaction between dislocations with silver atoms or deformation induced point defects are discussed.

Recovery of damping and modulus changes following plastic deformation

A theory is developed which assumes that changes with time in the decrement and modulus of a crystalline material following plastic deformation are a result of dislocation pinning by deformation—induced point defects. This time dependence is based upon the Cottrell-Bilby t 2 3 law for strain-aging. Comparison of the results of the theory with available data shows that the measured time law is that predicted by the theory for specimens which have been deformed between about 0.4 and 4.0 per cent. For smaller and larger deformations, deviations are obtained. Although measurements so far available permit only a qualitative check of the predictions of the theory with regard to the dependence of the recovery rate on purity and deformation, a check of the temperature dependence is afforded by recovery measurements of Young's modulus for copper. From these an activation energy for the migration of vacancies in copper is determined to be 1.0 eV. A number of experiments needed for checking the theory further are proposed.