Tem Of Dislocations Under High Stress In Germanium And Doped Silicon

The stacking fault energies y of silicon (58 ± 6 mJ m^?2) and germanium (75 ± 10 mJ m^?2) were determined. Within the limits of accuracy γ was not found to change on doping with (13·8 mol m^?3 (8 × 10¹⁸ cm^?3) boron, and 1·17 mol m^?3 (7 × 10¹⁷ cm^?3) phosphorus). Freezing in dislocations under high shear stress reveals a different behaviour of screw dislocations: whereas these dislocations become wider in pure and p-silicon, they become narrower in n-silicon. From this we conclude the ratio of mobilities of the two 30° partials to be different in n- and p-silicon. Other observations on frozen dislocations are mentioned.     

Electron Microscope Studies Of Vpe Gainas Layer Structures Suitable For Use As Infrared Leds

Electron microscope investigations have been carried out on vapour grown (100) GaAs/GaInAs structures designed for use as infrared emitters of wavelength 1·06 μm. The structures consist of a GaAs substrate, a graded layer in which the indium concentration is increased from zero to 17 atomic %, and a constant composition Ga_0·83In_0·17As layer which contains a p-n junction. X-ray microprobe analysis of cross-sections of the slices established the uniformity of the grading. TEM analysis showed a dense and extensive asymmetric network of misfit dislocations (1 × 10¹² m^?2 (10⁸ cm^?2)) in the graded layer, threading dislocations and other anomalous contrast features extending from the graded layer through the p-n junction to the surface (local densities of 1 × 10¹¹–1 × 10¹² m^?2 (10⁷–10⁸ cm^?2)), and a planar network of dislocations just below the surface (spacing 0·2–2 μm). SEM EBIC and CL studies of the layer above the junction revealed dark spots, and a cross-grid of dark lines, which could be correlated with the threading defects, and the dislocation network just below the surface, respectively. The SEM results showed that these defects had a deleterious effect on the luminescent and electrical properties of the material in the vicinity of the p-n junction, and would therefore impair the performance of devices made from these layer structures.     

Nonradiative recombination at dislocations in III–V compound semiconductors

Carrier recombination at individual dislocations is investigated for the case of misfit dislocations at a heterojunction between Ga_1–xAl_xAs_1–yP_y epitaxial layers. Through the combined use of scanning transmission electron microscopy, electron beam induced current and cathodoluminescence analysis, it is shown that the non-radiative and radiative recombination properties of dislocations are associated with their fine crystallographic configuration. The edge Lomer-Cottrell dislocation is found to be electrically neutral. The absence of carrier recombination strongly suggests that core reconstruction may be important in eliminating dangling bonds and kink sites along the core of these dislocations. An undissociated dislocation with a Burgers vector b =1/2a〈110〉 is proposed as the more likely configuration for the edge Lomer-Cottrell dislocation. An asymmetry in the nonradiative recombination properties and crystallographic structure of the 60° dislocation is also reported and discussed.     

Investigation of dislocation geometries in the diamond cubic structure

The weak-beam technique of electron microscopy (Cockayne, Ray & Whelan, 1969) has been applied to the examination of dislocations in germanium. These are shown to be dissociated into partial dislocations with a separation in the edge orientation of 5.5 ± 1·0 nm. A value for the stacking-fault energy of γ = 60 ± 8 mJ m^?2 (erg cm^?2) is deduced from the measured dissociation width as a function of orientation, using anisotropic elasticity theory.     

Combined HREM and STEM microanalysis on decorated dislocation cores

Impurity precipitation at dislocation cores in silicon and germanium have been identified by lattice imaging and electron energy loss spectroscopy on a STEM. The results lead to the conclusion that oxygen segregates at dislocations with large Burgers vectors forming a crystalline phase in the form of a continuous cylinder all along the dislocation line. The ceosite phase is tentatively proposed as being this crystalline phase. Severe radiation-induced damage has been observed under the high flux employed in a STEM. This results in complete disappearance of the precipitates by desorption of oxygen at the surfaces of the thin foil after an integrated dose of 10¹⁰ C/m².     

High-temperature dislocation plasticity in the single-crystal superalloy LEK94

The evolution of the dislocation structure that forms during uniaxial creep deformation in the single‐crystal superalloy LEK94 of low density and with Re additions was analysed using transmission electron microscopy. The material has a γ/γ′‐microstructure consisting of γ′‐cubes (L12 phase, 80 vol.%) separated by thin γ‐channels (face‐centred cubic). <100> tensile creep tests were performed at 980 and 1020 °C at stresses of 200 and 240 MPa. The microstructure was investigated at three characteristic stages of creep (directly after loading, at 5% strain and after rupture) to show the evolution of the dislocation structure during high‐temperature creep. It was found that in the early stages of creep, a₀/2<011> dislocations form within the γ‐channels. Later on, dislocation networks form and γ′ cutting processes with a₀/<001> superdislocations are observed. The results are in line with observations made for other superalloy single crystals in the high‐temperature low‐stress creep regime.     

Elevated temperature deformation behavior in an AZ31 magnesium alloy

An AZ31 magnesium alloy was tested at constant temperatures ranging from 423 to 473 K (0.46 to 0.51T _m ) under constant stresses. All of the creep curves exhibited two types depending on stress levels. At low stress (σ/G<4×10⁻³), the creep curve was typical of class A (Alloy type) behavior. However, at high stresses (σ/G>4×10⁻³), the creep curve was typical of class M (Metal type) behavior. At low stress level, the stress exponent for the steady-state creep rate was of 3.5 and the true activation energy for creep was 101 kJ/mole which is close to that for solute diffusion. It indicates that the dominant deformation mechanism was glide-controlled dislocation creep. At low stress level wheren=3.5, the present results are in good agreement with the prediction of Fridel model.     

The dissociation of (a+c) misfit dislocations at the InGaN/GaN interface

In hexagonal materials, ( a+c ) dislocations are typically observed to dissociate into partial dislocations. Edge ( a+c ) dislocations are introduced into (0001) nitride semiconductor layers by the process of plastic relaxation. As there is an increasing interest in obtaining relaxed InGaN buffer layers for the deposition of high In content structures, the study of the dissociation mechanism of misfit ( a+c ) dislocations laying at the InGaN/GaN interface is then crucial for understanding their nucleation and glide mechanisms. In the case of the presented plastically relaxed InGaN layers deposited on GaN substrates, we observe a trigonal network of ( a+c ) dislocations extending at the interface with a rotation of 3° from <1 $\overline{1}$00> directions. High-resolution microscopy studies show that these dislocations are dissociated into two Frank–Shockley 1/6<2 $\overline{2}$03> partial dislocations with the I₁ BSF spreading between them. Atomistic simulations of a dissociated edge ( a+c ) dislocation revealed a 3/5-atom ring structure for the cores of both partial dislocations. The observed separation between two partial dislocations must result from the climb of at least one of the dislocations during the dissociation process, possibly induced by the mismatch stress in the InGaN layer.     

Evidence of dislocation dissociation in nearly stoichiometric tantalum carbide using the weak-beam technique

The dissociation of dislocations in nearly stoichiometric tantalum carbide has been revealed by the weak-beam technique; this is in agreement with the assumptions of a model of dislocation glide which had been derived for this compound. An order of magnitude of the partial spacing for the screw and the edge dislocation is 2·7 and 5.2 nm respectively, which leads to a stacking-fault energy of about 170 mJ m^?2 (erg cm^?2).     

Direct imaging of paracrystalline phospholipid structure in the electron microscope

A long chain amphiphilic molecule—the phospholipid 1,2-dihexadecyl sn glycerophosphoethanolamine—has been crystallized epitaxially so that the interlamellar molecular periodicity is parallel to the substrate and hence normal to the electron beam in the electron microscope. This has permitted the direct resolution of the 55·6 Å lamellae in unstained crystals at room temperature. The lattice images have shown the presence of line dislocations and lenticular cracks in the crystals. Of significance to their biological properties is that the lattice is undulating with a periodicity of 0·1–0·5 μm. This would also account for the difficulties encountered by X-ray and electron diffraction techniques when examining these crystals.     

分子动力学仿真过程中硅晶体位错模型的构建

基于位错形成机理,在单晶硅晶体结构基础上描述了硅晶体位错形成的过程。应用偶极子模型,构建了60°滑移位错芯和螺旋位错芯,进而得到硅晶体含有60°滑移位错的模型和含有螺旋位错的模型。对含有螺旋位错的硅晶体模型进行了分子动力学仿真计算,分析了含有螺旋位错的硅晶体超精密磨削的加工过程,研究了含有螺旋位错缺陷的硅晶体纳米级磨削机理。     

Modification of Tribological Properties of Silicon by Boron Ion Implantation

Friction and wear properties of silicon used in the fabrication of microelectromechanical systems (MEMS) are important for their long-term reliability. In the present study, the authors have implanted single-crystal and polycrystalline silicon wafers with boron ions to improve their mechanical and tribological properties. The authors have studied the effects of ion implantation on the crystallinity, microstructure, nanohardness, and friction and wear properties and have found that silicon remains crystalline after ion bombardment at doses up to 2 × 10¹⁷ ions.cm^?2 but with a large amount of defects. The ion bombardment modifies elastic/plastic deformation characteristics and crack nucleation that occurs during indentation. There is a minor increase, ? 10-15 percent, in the nanohardness as a result of boron-ion implantation. Ion bombarded single-crystal silicon exhibits very low friction (0.05) and low wear factor (10^?6 mm³·N^?1m^?1) while slid against a 52100 steel ball. The coefficient of friction of bombarded silicon in dry air and dry nitrogen is even lower.     

Atomic structure of extended defects in boron-implanted silicon layers

The structure of extended defects introduced into Si by means of boron implantation followed by thermal annealing at T = 900 °C is studied by the method of high-resolution transmission electron microscopy and computer modeling for different values of the implantation dosage (D) and concentration of boron atoms in substitutional positions B₀ \((C_{B_0 } )\) injected into the Si lattice before implantation. It is shown that the Frank dislocation loops of both interstitial (I) and vacancy (V) type at a ratio of 4: 1 are observed in Si samples with D = 10¹⁶ cm^?2 up to \(C_{B_0 } \) = 0.8·10²⁰ cm^?3. The atomic structure of the I-type Frank dislocation loops is heavily deformed, which suggests segregation of finely dispersed boron in the defect plane. At the same time, the structure of the V-type Frank dislocation loops tends to be reconstructed due to interaction with self-interstitials. At \(C_{B_0 } \) = 2.5·10²⁰ cm^?3, the I-type Frank dislocation loops are found to transform to perfect dislocation loops, and boron precipitates with a high density appear in Si. Based on the results obtained, probable reasons for vacancy deficit formation in boron-implanted Si are discussed.     

Observations of lattice defects using the weak-beam technique

The advantages of the weak-beam technique of electron microscopy for the study of small defects and dislocations are illustrated by micrographs taken of small loops in aluminium and Al–Ag alloys, GP zones and θ″ precipitates in Al + 4% Cu, and dislocation networks in Cu + 20% Zn. An estimate of 19·5 mJ m^?2 (erg cm^～2) is made for the stacking-fault energy of Cu–20% Zn from the size of extended nodes and the width of the dissociated dislocations imaged under weak-beam conditions.     

Resolution Of M23C6/Austenite Phase Boundary Defect Structures By Weak Beam Microscopy

The weak beam dark field imaging technique has been successfully applied to the problem of imaging misfit dislocations at the interface between M₂₃C₆ precipitates and an austenite matrix. The technique eliminates Moiré fringe contrast which otherwise obliterates the contrast from interfacial dislocations. Three sets of parallel dislocations were observed with spacings and line vectors consistent with them being edge dislocations with Burgers vectors of a/3<111>.     

Dislocations in nanostructured two‐phase Fe30Ni20Mn20Al30

In a previous study, the dislocations in Fe₃₀Ni₂₀Mn₂₅Al₂₅ (at. %), which consist of 50 nm wide alternating b.c.c. and B2 phases, were shown to have a/2<111> Burgers vectors after room temperature deformation. The dislocations were found to glide in pairs on both {110} and {112} slip planes and were relatively widely separated in the b.c.c. phase, where the dislocations were uncoupled, and closely spaced in the B2 phase, where the dislocations were connected by an anti‐phase boundary. In this article, we analyze the dislocations in the two ～5 nm‐wide B2 phases in a related two‐phase alloy Fe₃₀Ni₂₀Mn₂₀Al₃₀, with compositions Fe‐23Ni‐21Mn‐24Al and Fe‐39Ni‐12Mn‐34Al, compressed to ～3% strain at a strain rate 5 × 10^?4 s^?1 at 873 K (the lowest temperature at which substantial plastic flow was observed). It is shown that slip occursby the glide of a<100> dislocations. A review of the literature suggests that the differences in the observed slip vector between these B2 phases could be due to the differences in composition, differences in deformation temperature, or possibly both. Microsc. Res. Tech. 76:263–267, 2013. © 2013 Wiley Periodicals, Inc.     

Strengthening caused by power law creep deformation of dispersed particles in an elastic matrix

The interaction energy is approximated between an edge dislocation and a particle deformable by power law creep in an elastic matrix. The stress required to overcome the interaction energy barrier is found to be greater than the Orowan stress, and the dislocation bulges to escape the particle. If the ratio of the shear modulus of the matrix to the viscosity of the particle (μt^m/σ₀) is large, the stress required to climb over the particle is larger than the Orowan stress and the dilocation bulges before it climbs. It is concluded that even if the particle is soft enough to exhibit creep, the strengthening of alloys can be achieved by an Orowan mechanism. The critical resolved shear stress (CRSS) of Cu-B₂O₃, obtained experimentally by Onaka et al. [11], agrees closely with that obtained in our analysis. This supports our analysis that the strength of Cu-B₂O₃ alloy at high temperature may be accounted for by the Orowan mechanism and the attraction between a dislocation and viscous particles. The energy and the force to overcome the energy barrier increases significantly with decrease of m, the strain rate exponent associated with the power law creep particle. It is found through analysis that for m < 1.0 and for certain values of μt^m/σ₀ > 1, the particle repulses the dislocation, while for m = 1.0 and for all values of μt^m/σ₀ > 1, the particle attracts the dislocation, which is the expected interaction between an elastic particle and a dislocation in an elastic matrix.     

A spectroscopic study of some high-temperature superconductors using a low-temperature STM

We used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high-transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu₂O_x show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa₂Cu₃O_6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa₂Cu₃O_6·5+x and Y_0·5Al_0·05Ba₂Cu₃O_6·5+x, may indicate the presence of a lower temperature phase in these samples.     

The determination of the 1/2<lll> {110} antiphase boundary energy of NiAl

A value for the 1/2 <111> {110} antiphase boundary (APB) energy in stoichiometric NiAl has been obtained by employing the weak-beam technique to resolve the separation of the two 1/2 <111> partial dislocations composing the <111 superdislocation. A separation of 4·5 nm is obtained corresponding to an APB energy of 200 ± 40 mJ m^?2 (erg cm²).