Partial Dislocation Loops Research Articles

On the mechanism of {111}-defect formation in silicon studied byin situelectron irradiation in a high resolution electron microscope

Abstract The initial stages of point defect cluster formation on {111} habit planes in Si crystals have been investigated during in situ electron irradiation in a high resolution electron microscope to elucidate their nature and origin. It was observed that 〈110〉 interstitial chains located in {111} planes at regular spacing are formed by the agglomeration of self-interstitial atoms to the core of vacancy or interstitial Frank partial dislocation loops and by the insertion of interstitial chains between two perfect {111} planes. Based on experimental and calculated high resolution electron microscopy images a structural model for the {111}-defect is proposed which includes a regular sequence of double five-membered and single eight-membered rings in which no dangling bonds are involved. A dependence of the displacement vector of the {111}-defect on the formation mechanism is observed. An isolated {111}-defect is characterized by the fully relaxed atomic structure with a displacement vector of (a/5)〈111〉. ...

A modeling of radiation induced microstructural evolution under applied stress in austenitic alloys

Effects of applied stress on interstitial type Frank loop evolution at early stages of irradiation were investigated by both numerical calculation and irradiation experiments. In the experimental part of this work, microstructural inspection has been made by transmission electron microscopy with a special emphasis on Frank loops and perfect loops on every {111} plane. The results of the TEM observation revealed that Frank loop concentration on a {111} plane increased as the resolved normal stress to a {111} plane increased and that small perfect loops were more likely produced on {111} planes where larger resolved shear stress was applied. The model of a stress effect on Frank loop unfolding was provided, which is triggered by nucleation of a Shockley partial dislocation loop in a Frank loop, was proposed. The results of the numerical calculation was successful to predict the strong dependence of Frank loop concentration on the resolved normal stress to {111} plane, which was the characteristic feature seen in the irradiation experiments.

Field ion microscopy observation of intrinsic stacking faults in iridium

An intrinsic stacking fault in Ir is characterized using FIM techniques. Lattice distortion in the vicinity of the partial dislocation loop bounding the fault is evaluated. The maximum radius of visible distortion in the vicinity of the partial dislocation is found to be ∼4.6 nm. This result provides experimental data for the assessment of the magnitude of lattice disturbance caused by crystal defects, which can be useful in the computer modeling of crystal defects.

Growth and intercalation of silver deposited onto Nb 1+ xS 2

Growth of silver thermally deposited at temperatures between 300 and 570 K onto non-stoichiometric single crystals of 3R-Nb 1+ x S 2 ( x ≈ 0.09) was studied by transmission electron microscopy and scanning tunnelling microscopy. The observed reduced amount of silver deposited at elevated temperatures and the appearance of partial dislocations and dislocation loops not present in the as-grown disulphide crystals indicated that silver intercalated into their van der Waals gaps during the very deposition. Morphology of silver islands and intercalation rates were dependent on the deposition temperature. Post-deposition intercalation of silver by gradual disappearance of hexagonally shaped islands could be observed even at room temperature.

Three-dimensional epitaxy: Thermodynamic stability range of coherent germanium nanocrystallites in silicon

We study the stability range of coherent Ge quantum dots with an epitaxial Si shell. The critical radius is evaluated as a function of Si shell thickness and Ge nanocrystallite radius by comparing the energy of the system in the coherent and incoherent state. We find that the system is coherent up to a Ge nanocrystallite radius of about 100 Å, irrespective of the Si shell thickness. Nanocrystallites of radii larger than 270 Å lose coherency by the generation of perfect dislocation loops. In nanocrystallites of intermediate radii (between 100 and 270 Å), the coherency is lost by the introduction of partial dislocation loops enclosing a stacking fault. As the shell thickness decreases, the critical radius increases.

Dislocation multiplication behind the shock front

A new model of fast operated dislocation source based on multiplication and motion of partial dislocations bounding the stacking fault is suggested. Stress-activated stretching of lateral branches of the partial dislocation bowed-out segment results in collapse of these branches with subsequent restoration of the ‘‘initial’’ dislocation half-loop and generation of a ‘‘fresh’’ partial dislocation loop, both capable to produce the next multiplication act. The multiplication results in the exponential increase of both dislocations and stacking faults concentration and is accompanied with the plastic deformation having a strain rate dε/dt∼2t/ΔT. The characteristic time of the process, ΔT, ranges from 10−9 to 10−10 s. The model explains the variations of x-ray diffraction pattern for the material undergoing shock compression, the shock-induced formation of twins, and shear bands and pre-fracture voids nucleation in the rarefaction wave.

A dislocation in a compositionally graded epilayer

The mismatch stress field in a graded epilayer and its substrate is analyzed by analogy to thermal stresses. In particular, a straight dislocation line and a half-rectangular dislocation loop in the graded epilayer are investigated. For the straight dislocation line, there are two positions in the epilayer at which the resultant force acting on the dislocation is null. If the dislocation is generated from the surface and passes the first zero force position, it will continue moving toward the second zero force position, where the dislocation is at a stable equilibrium. It is the stable equilibrium position that determines the dislocation formation energy and then the critical thickness of the graded epilayer. For a half-rectangular dislocation loop, its optimum shape occurs at a saddle point of the total energy profile as a function of the dislocation width and length. The activation energy for emitting a perfect or a partial dislocation loop, the dislocation width, and the aspect ratio of dislocation length to width are all related to the graded layer thickness. All three, the activation energy, the dislocation width, and the aspect ratio, decrease quickly when the thickness increases from its critical value and then gradually approach extreme value as the thickness approaches infinity. The extreme values of the activation energy, the dislocation width, and the aspect ratio correspond to those for a dislocation loop emitted from the surface of an epilayer with a uniform composition. The results indicate that 1. dislocation emission is more difficult in a compositionally graded layer than in a compositionally uniform layer and that 2. the aspect ratio of threading dislocation density to mismatch dislocation density is lower in a compositionally graded layer than in a compositionally uniform layer. The theoretical results agree with experimental observations.

Deformation behavior of [001] Ti-56Al single crystals

Specimens of an [001]-oriented Ti-56at.%Al single crystal were tested at room temperature (RT) and 873 K. The elevated temperature sample displayed anomalous yield behavior, confirming the previous findings. Dislocation structures in the deformed samples at RT and 873 K, respectively, were observed by TEM. The micrographs for both RT and 873 K show long, partial dislocation loops of 〈101] superdislocations. However, dislocation dissociation behavior at the two temperatures was quite different. At room temperature, the 〈101] dislocations dissociated into a 1 6 [112] plus a 1 6 [5 14] dislocation connected by an SISF, whereas at elevated temperature, a dissociation involving two 1 2 〈101] dislocations linked by an anti-phase boundary (APB) was observed. These observations suggest that at elevated temperature, a Kear-Wilsdorf or a “roof-type” barrier is responsible for the anomalous flow stress. These results and their impact on the overall deformation behavior of gamma titanium aluminides will be discussed.

Dislocation multiplication in f.c.c. metals at presence of high shear stress

A new model of shock-induced stress relaxation based on multiplication and motion of partial dislocations bounding the stacking fault is suggested. Shock-generated high shear stress leads to stretching of lateral branches of a bowed out dislocation segment (half-loop) followed by collapse of those branches. The result of the collapse is the forming of the “fresh” partial dislocation loop bounding the stacking fault area and “initial” dislocation half-loop, both being capable of the next multiplication act. After every collapse time interval ΔT the process leads to the doubling of both the dislocation concentration and stacking faults total area. The energy dissipation rate behind the shock front increases exponentially, d W/d t ≈ 2 t/ ΔT but it should be limited as soon as the cross-slip of the “fresh” loops will be impended and the loops doubling will be stopped. An explanation of twinning and shear bands formation in shocked material is proposed.

Three-Dimensional Epitaxy: Thermodynamic Stability Range of Coherent Germanium Nanocrystallites in Silicon Host

AbstractWe study the stability range for a coherent interface between Ge quantum dots and an epitaxial Si shell. The critical radius at which coherency is lost is evaluated as a function of Si shell thickness and Ge nanocrystallite radius by comparing the energy of the system in the coherent and incoherent state. We find that the system is coherent up to a Ge nanocrystallite radius of about 100 Å, irrespective of the Si shell thickness. Nanocrystallites of radii larger than 270 Å lose coherency by the generation of perfect dislocation loops. In nanocrystallites of intermediate radii ( between 100 Å and 270 Å), the coherency is lost by the introduction of partial dislocation loops enclosing a stacking fault. As the shell thickness decreases, the critical radius increases.

Emission of a Half-Rectangular Dislocation Loop from the Surface of a Compositionally Graded Epilayer

AbstractWhen a half-rectangular perfect or partial dislocation loop is emitted from the surface of a strained epilayer, the optimum dislocation shape occurs at a saddle point of the total energy profile as a function of the dislocation width and length. The activation energy and the optimum shape are calculated for either a compositionally graded or uniform epilayer. The results indicate that 1) dislocation emission is more difficult in a compositionally graded layer than in a compositionally uniform layer; and 2) the ratio of threading dislocation density to mismatch dislocation density is lower in a compositionally graded layer than in a compositionally uniform layer. Theoretical results agree with experimental observations [1,2].

Stacking Fault Tetrahedra Formation During Growth of Si1-xGex Strained Layers on 〈111〉 Oriented Si Substrates: Tem Observations and Defect Modeling

AbstractWe report the observation of stacking fault tetrahedra (SFT) in strained Si1-xGex layers grown via rapid thermal CVD on (111) Si substrates. It is shown that these defects provide a mechanism for strain relief in films strained in compression due the presence of bounding edge-type stair rod partials whose Burgers vectors lie parallel to the strained layer interface. Cross section and plan view TEM were used to characterize this defect structure in epilayers (30 to 650nm thick) of Si1-xGex (0 < xGe < 0.27) grown on 〈111〉 oriented Si wafers. Stacking fault tetrahedra were observed only in alloys in the compositional range xGe ≥ 0.13 and only when growth proceeded on the 〈111〉 surface. A critical strain energy model that identifies conditions for the stable growth of stacking fault tetrahedra in a strained layer is presented. The model was based on conventional strain energy considerations where the energy of the stacking fault area plus the bounding dislocation network (including dislocation interactions but neglecting the free surface) was balanced against the strain energy released by the introduction of the defect. In addition, a formation mechanism consistent with these observations is described that involves the dissociation of Frank partial dislocation loops bounding stacking faults lying in the growth plane.

Dislocation configurations and internal stresses in the creep of Nimonic 91

The internal stress, σi, developed during the creep of Nimonic 91 was determined as a function of applied stress, σa, using the strain transient dip technique. Transmission electron microscope observations of thin films of crept specimens showed Orowan dislocation loops to exist aroundγ′ phase particles at low stresses with partial dislocation loops around faultedγ′ particles at high stresses. The numbers of loops per specimen volume were counted and the resulting internal stress calculated. The results indicate that a significant part of the mechanically measured internal stress can be attributed to Orowan loops aroundγ′ particles which are stabilized against climb by the superlattice fault resulting from partial penetration of theγ′ particle by the dislocation. The variation of internal stress with applied stress can be accounted for qualitatively by the variation of loop density with stress at low stresses and the initiation of relaxation processes involving partial or complete shearing ofγ′ particles by loops at high stresses. It is suggested that creep in Nimonic 91 is dependent on the magnitude of the effective (σa-σi) and that the internal stress is determined largely by the density of dislocation loops aroundγ′ particles.

On interactions of superlattice dislocations in ni3(al, ti) a weak-beam analysis

Abstract A single crystal of L12 ordered Ni3(Al, Ti) has been deformed in compression at 450°C which is the peak temperature of the flow stress anomaly. The dislocation structure in the cube (010) planes is analysed by using the weak-beam imaging technique. Superlattice dislocations (b = ± α[10]) glissile on (010) planes are found to interact with superlattice dislocations of the primary cube ± α[10](001) slip system. Dislocation reactions are produced by the intersecting slip systems. The analysis of such interactions which act as locked structures is the major subject of this paper. Characteristic resultant features are dislocation dipoles which break up into partial dipoles and dislocation loops. They are attached to the superlattice dislocations in a very specific manner. A model is proposed to explain the formation of such configurations. From the fact that these reactions are formed by intersecting slip systems we assume that they determine work-hardening properties. A more common meaning is derived...

The effect of interstital Frank partial dislocations on the gradual degradation of 1.3-μm double-channel planar buried heterostructure laser diodes

The structure of defects occurring in degraded 1.3-μm double-channel planar buried heterostructure type laser diodes was studied in detail by conventional diffraction contrast, weak-beam dark-field microscopy and high-resolution lattice imaging in a transmission electron microscope. The electrical activity of the defects was analyzed by means of the electron-beam-induced current. Device degradation is associated with the presence of interstitial Frank partial dislocation loops with a Burgers vector of (a/3) {111} at the interfaces between the InGaAsP active layer exposed after the channel etching and the first p+-InP layer of the liquid-phase-epitaxy-grown current-blocking layers. No other types of defects were observed in the course of the present study. The implications of the latter finding are far-reaching and lead to a new degradation model for the microstrctural changes associated with the gradual degradation of long-wavelength lasers. A new degradation model is proposed, and laser diode design considerations for degradation-free operation are given. These consdierations are twofold: the device structure should (a) not favor Frank partial dislocation loop nucleation and (b) suppress the dissociation of the Frank partial dislocations.

Influence of the Temperature of Implantation on the Morphology of Defects in MeV Implanted GaAs.

AbstractWe have studied the influence of the temperature of implantation on the morphology of the defects created during 1-MeV implantation of Si into GaAs, using RBS-channeling and TEM. The annealing behavior of the disorder has also been investigated.Implantation at liquid-nitrogen temperature results in the amorphization of the implanted sample for doses of 2×1014 cm−2 and larger. Subsequent rapid thermal annealing at 900°C for 10 seconds leads to partial epitaxial regrowth of the amorphous layer. Depending on the implantation dose, the regrowth can proceed from both the front and back ends of the amorphous region or only from the deep end of the implanted zone. Nucleation and growth of a polycrystalline phase occurs concurrently, limiting the extent of the epitaxial regrowth. After implantation at room temperature and above, two distinct types of residual defects are observed or inferred: point defect complexes and dislocation loops. Most of the point defects disappear after rapid thermal annealing at temperatures ≥ 700°C. The effect of annealing on the dislocation loops depends on the distance from the surface of the sample. Those in the near surface region disappear upon rapid thermal annealing at 700°C, whereas the loops located deeper in the sample grow in size and begin to anneal out only at temperatures in excess of 900°C. Implantation at temperatures of 200 - 300°C results in a large reduction in the number of residual point defects. Subsequent annealing at 900°C leads to a nearly defect-free surface region and, underneath that, a buried band of partial dislocation loops similar to those observed in the samples implanted at room temperature and subsequently annealed.

Tem Study of the Annealing Behavior of Secondary Defects in L MeV Si Implanted Gaas

The annealing behavior of the lattice damage induced by 1 MeV Si + implantation, at a dose of 3 x 10 15 cm -2 , into (100) GaAs has been examined by high resolution TEM, RBS and SIMS. The samples were annealed at temperatures ranging from 500 to 900oC for 10 sec by rapid thermal annealing (RTA). Two types of defects, perfect and partial dislocation loops, lying in the GaAs {111} planes, were found throughout the as-implanted region, with the highest defect density concentrated in a buried band centered at 1 μm below the surface. After annealing at 500oC, the total defect density showed little change, but the defects had become partial dislocation loops. With further increase in the annealing temperature, the surface region became increasingly defect free as the remaining individual dislocations increased in size, accompanied by a decrease in their number density. RBS dechanneling analysis showed that after annealing at 900oC, the crystal lattice of the near-surface region had recovered to the level of a pristine GaAs. SIMS measurement of the Si profile showed little movement of the implant species upon RTA processing, and the Si peak position correlated well with the maximum defect density in the defect band.

Relationship Between Electrical Activation and Residual Defects in MeV Si Implanted GaAs.

ABSTRACTWe have investigated the relationship between electrical activation and residual defects in GaAs implanted with 1 MeV Si ions to a fluence of 3×1015 cm−2 and subsequently annealed, using rapid thermal annealing (RTA) for 10 seconds, at temperatures up to 1050°C. Hall measurements show n-type activation that increases in magnitude with increasing annealing temperature to reach a sheet carrier concentration of 2.6×1014 cm−2 after RTA at 1050°C. Two main types of extended defects, perfect and partial dislocation loops, had previously been identified in the as-implanted samples. In addition, a large number of discrete point defect complexes are certainly created but escape detection. We show here a correlation between the annealing of the defects and the recovery of the electrical transport properties. Most of the point defects anneal out between room temperature and 700°C, and it is in this temperature regime that the transport properties show their most significant improvement. For annealing temperatures between 700°C and 1050°C, the mobility of the carriers decreases slightly while the carrier concentration gradually approaches saturation. Concurrently, the loops grow in size while decreasing in number and eventually anneal out. However, after annealing at 1050°C, while most of the extended defects have disappeared, only about 9% of the implanted atoms had been activated, suggesting that residual point defect centers control the activation of Si dopants in MeV implanted GaAs.

Water precipitation and diffusion in wet quartz and wet berlinite AIPO4

Annealing wet crystals of quartz and berlinite AlPO4 induces water precipitation. The evolution of precipitation has been studied by transmission electron microscopy on samples annealed in various conditions of temperature, pressure and duration. At the beginning one observes unresolved small contrast features, then tiny bubbles connected to sessile dislocation loops become visible. These dislocations are nucleated to relax the fluid pressure in the bubbles. To minimize the corresponding nucleation energy a partial dislocation loop is first nucleated, then a second partial with a complementary Burgers vector rapidly grows in the same plane and joins the first one. By measuring the mean distance X between precipitates for various annealing conditions one deduces the diffusion coefficient D of the water point defects with the help of the relation X/2 = (2D.t)1/2 where t is the duration of the annealing. One finds in the temperature range (350-1000 °C) for quartz D(m2s-1) = 10-12 exp (—95 kJ mole-1/RT). There is no visible effect of the confining pressure, at least between atmospheric pressure and 700 MPa. The α-β transition slightly affects the D values between 500 and 600 °C but except for this temperature range all the other experimental data fit well the above equation. Two theoretical models of water precipitation are developed. They are based on two contrasted hypotheses about the initial mode of water incorporation. In the first case all the water content is assumed to be initially dissolved as a highly supersaturated concentration of substitutional (4H)Si point defects. Water precipitation would thus occur by homogeneous nucleation of critical embryos, this first stage being followed by a stage of growth while new critical embryos would be continuously nucleated. In contrast, in the second case one assumes that the concentration of point defects is initially the equilibrium concentration at the growth conditions which is very low. Almost all the water would thus be incorporated as tiny clusters of water molecules too small to be detected and precipitation would occur by intercluster diffusion, the bigger clusters growing at the expense of the smaller ones which would progressively redissolve. In fact none of these models fully renders account of the experimental results. These results can only be interpreted by an intermediate situation. Water is incorporated during growth as tiny clusters and as a supersaturated concentration of point defects as well. In the quartz studied extensively in this article this concentration of point defects becomes approximately equal to the equilibrium one only at P ⋍ 700 MPa, T ⋍ 550 °C.

On voidites: a high-resolution transmission electron microscopic study of faceted void-like defects in natural diamonds

In natural diamonds of optical classification type la , nitrogen is the major identified impurity and is distributed mainly in point defects known as A defects (probably a pair of nitrogen atoms substituting for a pair of adjacent carbon atoms) and B defects (probably four substituted nitrogen atoms tetrahedrally surrounding a carbon vacancy), and also in the electron-microscopically visible platelet precipitates on {100}. This paper is concerned with other electron-microscopically detectable defects, discovered by R. F. Stephenson (Ph.D. thesis, University of Reading (1977)), that lie in {100} planes in circumstances strongly suggesting that they result from the decomposition of platelets. High-resolution electron microscopy shows these defects to be {111}-faceted cavities. They behave as pure phase-contrast objects whose interior density does not exceed about one-third that of the diamond matrix: we call them ‘voidites’. The experimental background to voidite observation is reviewed, including electron-microscopic measurements on normal {100} platelets and models of their structure, and the optical, X-ray diffraction and cathodoluminescence evidence for unusually large platelets whose presence, together with a relative richness in B defects, indicates an environment in which voidites are likely to be discovered. Almost all observed voidites are confined to sheets strictly parallel to {100}. Some voidite sheets occur in ‘ partial platelets’, where they replace part of the original area of normal platelet. Other voidite sheets occur within dislocation loops whose size and shape are similar to those of the peripheries of normal platelets in the specimen. Voidites occur in a wide range of sizes. The largest equiaxed voidites observed measure about 10 nm between opposite {111} facets, and the smallest resolved about 0.5 nm. Many voidites are elongated in one of the <110> directions in the plane of the voidite sheet: the most highly elongated voidites seen approach 100 nm in length, with diameters of a few nanometres. Variations in size, shape and number density of voidites, together with many other characteristics relevant to the microscopic processes of voidite formation, are discussed in detailed descriptions of about 40 voidite sheets occurring in partial platelets and dislocation loops in two diamond specimens. One specimen was free from both grown-in dislocations and dislocations associated with plastic deformation. It contained zones of highly elongated platelets and it appeared that transformation of a platelet into a voidite sheet surrounded by a dislocation loop was triggered by the mutual very close approach of platelets. The second voidite-containing specimen had suffered plastic deformation at some stage in its history, but did not exhibit direct evidence that glide dislocations had triggered the transformation. The Burgers vectors of 24 dislocation loops enclosing voidite sheets in the second specimen were determined. Twelve were of normal ½<110> type having a component ½a 0 normal to the voidite sheet, and twelve were non-primitive, the Burgers vector being a 0 normal to the voidite sheet (a 0 is the diamond face-centred cubic (fcc) unit cell edge). The volumes of over 2000 individual voidites, representing all or major parts of 12 voidite sheets, have been measured. Values found for the ratio ∑ V /Aa 0 (where ∑ V is the aggregate voidite volume in a sheet area A ) averaged about unity for 9 sheets of generally similar, voidite-rich appearance. Other sheets are poorer in voidites of measurable dimensions: the ratios for two such sheets averaged 0.25. In the concluding analysis, a reaction involving A and B point defects is proposed for the production of platelets. Other reactions including voidites (but no dislocations) are suggested in which both platelet production and elimination might occur. For the dominating reaction, when a platelet is replaced by a voidite sheet surrounded by an interstitial dislocation loop, models are developed for the cases when the Burgers vector component perpendicular to the loop is either a 0 or ½ a 0 , with the assumption that the platelet nitrogen is dispersed partly into B defects and partly into the voidites. The predicted values of ∑ V /Aa 0 come out as about unity and as 0.25 (or lower) for the larger and smaller Burgers vectors, respectively.