Amand George

Direct evidence of dislocation transmission through ∑= 9 grain boundaries in germanium and silicon by in situ high-voltage electron microscopy observations

Abstract In situ straining experiments of Si and Ge bicrystals in the high-voltage electron microscope have clearly revealed that dislocations can be transmitted across ∊ = 9 coincidence tilt boundaries even when their Burgers vector is not a lattice vector of the second grain. Observed transmitted dislocations are either perfect dislocations with ½‘110’ Burgers vector or, more often, Shockley partial dislocations trailing behind them extended stacking faults linked to the grain boundary. Dissociation of dislocations into DSC dislocations glissile in the boundary was also observed. Observations of repeated transmission events at a given area of the grain boundary suggest that residual dislocations can be removed from the impact point. The main-but not all-observed features are explained by a simple model based on energy calculations of the possible dislocation configurations formed at successive steps of the transmission process.

Structure and characterization of the dislocations in tilt grain boundaries between Σ = 1 and Σ = 3; a high resolution electron microscopy study

Abstract It is shown that the displacement shift complete (DSC) lattice is no longer necessary to define the Burgers vector associated with an interfacial dislocation. In Si and Ge [011] perfect or imperfect bicrystals, the grain boundary atomic structure is made of a limited number of structural units (SUs). Using high resolution electron microscopy (HREM), it is shown that one SU can be considered as the core of a grain boundary dislocation whose Burgers vector can be easily defined on the HREM image. The limited number of SUs strongly restricts the possible Burgers vectors compared with the DSC lattice vectors. These experimental results provide a link between the geometrical approach developed by Pond and the energetic approach developed by Sutton and Vitek.

Surface damage of CdTe produced during sample preparation, and determination of dislocation types near microhardness indentations

Abstract The influence of cutting, grinding and polishing during sample preparation of CdTe Bridgman monocrystals has been investigated by an etch-pit technique. Dislocations, mainly produced by cutting the sample with a diamond wire saw, were found to move several hundreds of micrometres from the surface into the bulk of the samples. The structure of dislocation loops near microhardness indentations was analysed by the same etch-pit technique and by birefringence observations. The Burgers vectors of the dislocations were determined by X-ray topography. Long dislocation segments were found to be screw dislocations.

Dislocations at grain boundaries in deformed silicon

Abstract Regions adjacent to Σ = 9 and Σ = 25 boundaries in silicon bicrystals are found to be in an advanced stage of hardening. Electron microscopy shows multiple slip, cross-slip, network formation and some transmission of slip across the boundary.

Plastic deformation of gallium antimonide single crystals : yield stresses, activation parameters and dislocation structures

Abstract Single crystals of undoped GaSb were deformed by compression at low strain rates from 0.48 Tm to 0.3 T m (where T m is the melting temperature). The activation enthalpy derived by standard techniques at the yield stress in pre-strained GaSb is about 1.6eV in fair agreement with activation energies for dislocation velocities. The dislocation structures were estimated by transmission electron microscopy. A progressive transition is observed from curved dislocations and edge-type lenticular loops and dipoles at high temperatures and low stresses to predominant long screw dislocations at low temperatures and high stresses. This transition is explained in terms of dislocation-dislocation interactions as against Peierls lattice friction, the onset of thermally activated cross-slip and the large anisotropy of dislocation velocities. Quantitative estimates show that screw dislocations are rate controlling and give the major contribution to the plastic strain at T 10MPa.

Plastic deformation, extended stacking faults and deformation twinning in single crystalline indium phosphide. 2. S doped InP

Abstract Sulphur doped InP single crystals were deformed by compression in dual and multislip conditions at a low shear strain rate (∼2 × 10−5 s−1) and moderate temperature (∼ 0.4T m). Hardening curves show a very different behaviour from pure InP, although the S does not have a net hardening effect at the yield point for the investigated testing conditions, contrarily to what is observed at higher temperatures. Transmission electron microscope observations of deformed samples have revealed extended stacking faults similar to those observed in pure InP, but also numerous twins or microtwins which were not observed in the latter case. Partial dislocation configurations suggest that pole mechanisms are operative. No evidence could be found that extrinsic stacking faults act as twin nuclei. The role of S in twinning is discussed.

On the yield point of floating-zone silicon single crystals II. A quantitative analysis of the dislocation structure at the lower yield point

Abstract The dislocation structures in silicon single crystals deformed up to the lower yield point at moderate temperatures (823 K ≲ T ≲ 1073 K) and low shear strain rates (γ<2×10−4s−1) were estimated by transmission electron microscopy. Both the total dislocation density ρt and the mobile-dislocation density ρm could be measured rather accurately, at least when the dislocations lie preferentially along ⟨110⟩ orientations. Local shear stresses were estimated from the radius of curvature at dislocation bends. With the distribution of local stresses and the known dislocation velocities, an average velocity v and the corresponding effective stress τeff could be derived. The internal consistency of the measured ρm and τeff was checked using Orowans relation where b is the Burgers vector, and was found to be good at sufficiently high stresses (τ1y ≳ 10 MPa), giving support to the view that, in those conditions, τeff is probably much closer to τly than predicted by current models.

Mechanical testing device for in situ experiments on reversibility of dislocation motion in silicon

Abstract We present a general-purpose mechanical testing device built and tested for in situ experiments using synchrotron radiation. A new setup was used in order to investigate the reversibility of dislocation motion under stress reversal. The observations were done by in situ transmission X-ray topography, at the ID 19 beamline of the ESRF. Specimens of FZ silicon single crystals, with a [ 1 1 4 ] axis (double slip orientation), were submitted to load cycles in tension and compression (τ=10–20 MPa) at high temperature (650–750 °C). Dislocations were created at surfaces by Vickers indents. They were observed using a monochromatic beam (λ=0.035 nm) and a 220 diffraction vector. The images were recorded both on X-ray films and using the FRELON X-ray camera. The first observations show that dislocations may behave in different ways within the same specimen, and that their motion can be partially reversible.

Plastic deformation of semiconductors: some recent advances and persistent challenges

Abstract Some recent results regarding the mechanical behaviour of semiconductors, mainly elemental, are critically reviewed. Topics under consideration are the core structures of dislocations in relation with glide processes and dislocation mobilities and the nucleation of dislocations, a problem encountered in specific cases in semiconductors. These new results were obtained thanks to unproved experimental techniques (e.g. direct kink imaging by HREM) or in recently appeared new materials (oriented bicrystals, Si-Ge alloys…). The rising importance of computer simulation of atomistic processes is acknowledged. Limits of present understanding are underlined and possible new experiments are suggested.

TEM observations of dislocations in aluminium nitride after high temperature deformation

Abstract AlN ceramics were deformed up to 10% strain in compression at elevated temperatures (1820–1920 K) under constant stress in the range 150–250 MPa or at a constant fixed strain rate of 5×10−6 s−1. Several kinds of dislocations were identified by TEM in deformed crystals. Most of them have a → type Burgers vectors and were seen to glide in the basal plane or in prismatic planes. They looked usually undissociated but large dissociations were observed after deformation tests of long durations. In such cases, a → dislocations could be dissociated either by pure glide in two Shockley partials or by combined glide plus climb in two Frank–Shockley partials. In any case, the enclosed stacking fault was in the basal plane. c → + a → dislocations were also observed but seldom. Different kinds of faulted loops are reported, delineated by Shockley, Frank or Frank–Shockley partials.Indications are given that oxygen segregation should play a role in the dissociation of dislocations.