Dislocation Generation Mechanisms Research Articles

Dislocation structure formation in SiGe/Si(001) heterostructures withlow-temperature buffer layers

Mechanisms of misfit dislocation generation as well as peculiarities of dislocationstructure formation in Si0.7Ge0.3/Si multilayer heterostructures grown bymolecular beam epitaxy with low-temperature (LT) SiGe and (SiGe + Si) bufferlayers were studied. Full strain relaxation in the heterostructures with 200–250 nmthick buffer layers has been achieved. In the heterostructures with both types ofLT layer, misfit dislocation generation is found to proceed similarly toheterostructure growth at high temperatures; however, the rate of dislocationnucleation is higher due to the high vacancy concentration near the interface.Threading dislocations, which are not connected with misfit dislocationnetwork, are generated in the epitaxial layers apart from the SiGe–Siinterface to form a dislocation density of 107–108 cm−2 on the surface.

Alternative mechanism for misfit dislocation generation during high-temperature Ge(Si)/Si (001) island growth

The misfit dislocations in [001] Ge(Si)/Si islands grown at 700 °C were investigated using transmission electron microscopy. 30° partial misfit dislocations are found both in the island/substrate interface and near the island surface. Since the 30° partial leads the movement of the 60° dissociated misfit dislocation in a (001) compressively strained system such as (001) GeSi/Si, a generation mechanism of misfit dislocations through partial misfit dislocations half loops is proposed.

Examination of Dislocations in Ice

Three techniques have been used to study dislocations in ice: etch pitting-replication, transmission electron microscopy, and X-ray topography (XT). Each is considered, and it is shown that the most useful is XT. This is because ice has low absorption of X-rays and can be produced with a low dislocation density, thus, allowing relatively thick specimens to be studied. The many useful observations that have been made with conventional XT are presented. However, the introduction of high-intensity synchrotron radiation showed that conventional XT observations are of dislocations that have undergone recovery. Thus, the important dynamic observations and measurements that have been made using synchrotron XT are also outlined. In single crystals, it has been shown that slip mainly occurs by the movement of screw and 60° a/3(1120) dislocations on the basal plane. In addition, the operation of Frank-Read sources has been clearly demonstrated, and dislocation velocities have been measured. In contrast, in polycrystals, dislocation generation has been observed to occur at stress concentrations at grain boundaries, and this completely overwhelms any lattice dislocation generation mechanisms. The nature of faulted dislocation loops has been determined in both polycrystals and single crystals.

Constitutive description of dynamic deformation: physically-based mechanisms

The response of metals to high-strain-rate deformation is successfully described by physically-based mechanisms which incorporate dislocation dynamics, twinning, displacive (martensitic) phase transformations, grain-size, stacking fault, and solution hardening effects. Several constitutive equations for slip have emerged, the most notable being the Zerilli–Armstrong and MTS. They are based on Becker's and Seeger's concepts of dislocations overcoming obstacles through thermal activation. This approach is illustrated for tantalum and it is shown that this highly ductile metal can exhibit shear localization under low temperature and high-strain-rate deformation, as predicted from the Zerilli–Armstrong equation. A constitutive equation is also developed for deformation twinning. The temperature and strain-rate sensitivity for twinning are lower than for slip; on the other hand, its Hall–Petch slope is higher. Thus, the strain rate affects the dominating deformation mechanisms in a significant manner, which can be quantitatively described. Through this constitutive equation it is possible to define a twinning domain in the Weertman–Ashby plot; this is illustrated for titanium. A constitutive description developed earlier and incorporating the grain-size dependence of yield stress is summarized and its extension to the nanocrystalline range is implemented. Computational simulations enable the prediction of work hardening as a function of grain size; the response of polycrystals is successfully modeled for the 50 nm–100 m range. The results of shock compression experiments at pulse durations of 3–10 ns (this is two–three orders less than gas-gun experiments) are presented. They prove that the defect structure is generated at the shock front; the substructures observed are similar to the ones at much larger durations. A mechanism for dislocation generation is presented, providing a constitutive description of plastic deformation. The dislocation densities are calculated which are in agreement with observations. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle–Grady relationship.

Strain-rate dependent mechanism of cooperative dislocation generation: application to the brittle–ductile transition

A strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature is described. The massive dislocation activity, which commences near the crack tip at the brittle-to-ductile transition temperature is modeled in terms of this mechanism. The strain-rate dependence of the critical temperature arises from the glide of both pre-existing dislocations and dislocations which are ‘thermally nucleated’ below the critical temperature by the cooperative process. Depending on their relative contributions, the apparent activation energy associated with the brittle-to-ductile transition temperature is either equal to or larger than the activation energy for dislocation motion. We compare the predictions of the model with observations in TiAl.

Material Effects on Stress-Induced Defect Generation in Trenched Silicon-on-Insulator Structures

We have investigated the influence of the material properties of the silicon device layer on the generation of defects, and in particular slip dislocations, in trenched and refilled fusion-bonded silicon-on-insulator structures. A strong dependence of the ease of slip generation on the type of dopant species was observed, with the samples falling into three basic categories; heavily boron-doped silicon showed ready slip generation, arsenic and antimony-doped material was fairly resistant to slip, while silicon moderately or lightly doped with phosphorous or boron gave intermediate behavior. The observed behavior appears to be controlled by differences in the dislocation generation mechanism rather than by dislocation mobility. The introduction of an implanted buried layer at the bonding interface was found to result in an increase in slip generation in the silicon, again with a variation according to the dopant species. Here, the greatest slip occurred for both boron and antimony-implanted samples. The weakening of the implanted material may be related to the presence of a band of precipitates observed in the silicon near the bonding interface. © 2001 The Electrochemical Society. All rights reserved.

A new mechanism for misfit dislocation generation: superdislocations associated with Ruddlesden–Popper planar defects

We have investigated by high-resolution transmission electron microscopy SrTiO 3/SrRuO 3 bilayer films epitaxially grown on LaAlO 3 substrate. At the SrRuO 3/LaAlO 3 interface we frequently observe dissociated superdislocations in connection with Ruddlesden–Popper-type planar defects. These superdislocations can contribute to misfit relaxation equivalent to a perfect misfit dislocation with Burgers vector a〈1 1 0〉. The formation and the geometry of this defect arrangement is discussed in detail. The Sr-rich Ruddlesden–Popper-defect can be introduced by composition fluctuations during film deposition. It can be concluded that it can act as a source for the generation of misfit dislocations.

Properties of GaN epitaxial layers grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

The structural, electrical, and optical properties of GaN grown on 6H-SiC(0001) substrates by molecular beam epitaxy are studied. Suitable substrate preparation and growth conditions are found to greatly improve the structural quality of the films. Threading dislocation densities of about 3×109 cm−2 for edge dislocations and <1×106 cm−2 for screw dislocations are achieved in GaN films of 0.8 μm thickness. Mechanisms of dislocation generation and annihilation are discussed. Increasing the Ga to N flux ratio used during growth is found to improve the surface morphology. An unintentional electron concentration in the films of about 5×1017 cm−3 is observed, and is attributed to excess Si in the films due to a Si-cleaning step used in the substrate preparation. Results from optical characterization are correlated with the structural and electronic studies.

Generation and propagation of dislocations during crystal growth

Abstract In the analysis of growth defects two categories of dislocations are distinguished: (i) ‘growth dislocations’ which are connected with the growth front; (ii) ‘post-growth dislocations’ which are generated ‘behind’ growth front, either still during the growth experiment or after growth, e.g. during cooling to room temperature. Inclusions are the main sources of dislocations. Two mechanisms of dislocation generation are active: (i) when the growth front closes the crystal lattice behind an inclusion ‘lattice closure errors’, resulting in dislocations, may occur, in which case these dislocations proceed with the advancing growth front (‘growth dislocations’); and (ii) inclusions ‘emit’ dislocations loops or half loops (stress relaxation by plastic processes). In crystals grown on planar faces and in the brittle state, growth dislocations are straight-lined and adopt more or less sharply defined directions which depend on the Burgers vector of the dislocation and the growth direction of the face on which they end. These directions are characterized by a minimum of the dislocation line energy per growth distance, or, in another approach, by the vanishing of the (image) force exerted by the growth surface upon the dislocations lines. The typical ‘as-grown’ dislocation geometry is observed in crystals grown in their brittle state at relatively low temperatures, e.g. from an aqueous solution. In crystals grown under thermal stress in their plastic region the ‘as-grown’ geometry may be drastically changed by post-growth movement of growth dislocations and the generation of post-growth dislocations (dislocation glide). The processes of dislocation generation, dislocation propagation and post-growth movement are demonstrated by selected X-ray diffraction topographs (conventional Lang technique) of various crystals grown from solutions and melts.

Strain-Rate Dependence of the Brittle-to-Ductile Transition Temperature in TiAl

ABSTRACTThe brittle-to-ductile transition (BDT) and the strain-rate dependence of the brittle-to-ductile transition temperature (BDTT) have been recently investigated in single crystals of TiAl [1]. It was found that the activation energy associated with the BDTT is 1.4 eV when the slip is dominated by ordinary dislocations and 4.9 eV when it is dominated by superdislocations. Despite this difference in the activation energies, the BDTT, while varying with the strain-rate, remains in the same temperature range, viz., between 516–750C and 635–685C for ordinary and superdislocations, respectively. In this paper, we examine how the activation energy of the BDTT can vary with the type of dislocation activity and explain why it can attain values which are clearly much larger than the activation energy for dislocation motion. We describe a strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature and use it to explain the characteristics of the BDT in TiAl. We show that the activation energy associated with the BDTT is a composite value determined by two or more inter-dependent thermally activated processes and its magnitude can be much larger than the activation energy for dislocation motion in certain materials. The predictions of the model are in good agreement with observations in TiAl.

Modeling the generation, equilibrium, and emission of dislocations at elastically misfitted particles

The generation, equilibrium, and emission of dislocations from an elastically mismatched particle–matrix interface under the combined effects of thermal and mechanical loading have been studied. Energy considerations are used to compute the generation, equilibrium, and instability conditions, and force balance is used to find the equilibrated loop shape. When the mismatch strain exceeds the generation limit, which is particle size dependent, the generation of dislocations lowers the system energy, and a stable dislocation loop segment will be generated and bulge out from the particle–matrix interface. Upon reaching instability, also particle size dependent, the dislocation will be emitted from the particle. Analytic expressions describing the generation, equilibrium, and instability are derived. A dislocation generation mechanism map is obtained for the analysis and design of particle-reinforced composites under thermo-mechanical loading.

Anisotropic misfit dislocation nucleation in two-dimensional grown InAs/GaAs(001) heterostructures

We report on structural investigations of the plastic strain relief in the highly mismatched InAs/GaAs(001) system grown in the layer-by-layer growth mode. The misfit dislocation generation mechanism is observed to be anisotropic in the two perpendicular 〈110〉 directions. We explain this result by the interplay between the chemically inequivalent dislocation types present in compound semiconductors and the applied growth condition.

Effect of Oxide Precipitate Sizes on the Mechanical Strength of Czochralski Silicon Wafers

The effect of oxide precipitate sizes on the mechanical strength of Czochralski silicon (CZ-Si) wafers has been studied with emphasis on the mechanism of slip dislocation generation by oxide precipitates. Thermal stresses, which are larger than those set up in wafers during actual device processes, were applied to two-step annealed wafers. It was determined by X-ray topography and transmission electron microscopy observations that both platelet and polyhedral precipitates can generate slip dislocations when their size is larger than approximately 200 nm. With further experiments, it is concluded that the precipitates cannot generate slip dislocations during actual device processing when the precipitate size is smaller than 200 nm, and this conclusion is independent of the precipitate density. The stress concentration of compressive thermal stresses applied to oxide precipitates should be the cause of slip dislocation generation. Three critical stress curves of slip dislocation generation were obtained for the wafers, in which the platelet sizes are approximately 70 nm, 200 nm and from 330 nm to 490 nm.

Mechanism of yielding in dislocation-free crystals at finite temperatures—Part I. Theory

A new mechanism of dislocation generation that can be activated suddenly above a critical temperature is proposed. Unlike dislocation generation by Frank-Read type sources, this process is a thermally driven, stress-assisted cooperative instability of many dislocation loops (dipoles in two dimensions). The dislocation loops are formed by thermal fluctuations and are sub-critical in size at low temperatures. The small plastic strain associated with the loops invokes an effective decrease in the moduli that describe the linear relation between the applied stress and the total strain. The self-energy of a loop which forms in the presence of other loops is proportional to these effective moduli and, consequently, it is lower than the energy of an isolated loop. As the temperature increases, the density of thermally generated loops increases and the concomitant lowering of the effective moduli and the self-energy provides a feedback for its further increase. Ultimately, above a critical temperature, the free energy of the loops becomes negative and a collective unstable expansion of many loops occurs. In a solid which is not loaded, this instability corresponds to the Kosterlitz-Thouless model of a defect-mediated phase transition that occurs just below the melting temperature. However, under large applied loads, the instability appears well below the melting temperature. Above the critical temperature, the density of glissile dislocations increases precipitously and extensive yielding occurs. This paper analyzes this mechanism in two dimensions where dislocation loops are replaced by dipoles.

Mechanism of Slip Dislocation Generation by Oxide Precipitates in Czochralski Silicon Wafers

Mechanism of Slip Dislocation Generation by Oxide Precipitates in Czochralski Silicon Wafers

Dislocation-Free Oxidation of Porous Silicon Formed Using Highly Phosphorus-Diffused Silicon and Its Application

Porous silicon was formed using highly phosphorous-diffused n-type silicon. Dislocation generation mechanisms due to the force caused by marked volume expansion at the early stage of porous silicon oxidation and strain generation caused by oxidation are investigated. The height of the step produced by oxidation and the relationship between the rate of increase in step height and stress are discussed. The experimental results concerning the step height versus the direct oxidation time are qualitatively explained based on the oxidation time dependence of the amount of SiO2 in oxidized porous silicon calculated using a simple model. Two dislocation-free oxidation methods are proposed. One is oxidation in wet oxygen through a SiO2 film deposited on the porous silicon by low-temperature chemical vapor deposition. The other is oxidation in dry and wet oxygen after annealing in nitrogen atmosphere. The porous silicon oxidized by the former dislocation-free oxidation method was applied to reduce the parasitic capacitance between the base electrode and the collector region in a high-speed bipolar transistor. This reduction resulted in an increase of the cut-off frequency and maximum frequency.

Nucleation and evolution of misfit dislocations in ZnSe/GaAs (001) heterostructures grown by low-pressure organometallic vapor phase epitaxy

Transmission electron microscopy and x-ray diffraction were used to study strain relaxation and the evolution of the dislocation structure in ZnSe epilayers grown by low-pressure organometallic vapor phase epitaxy on a (001) surface of semi-insulating GaAs. Before the ZnSe growth, the substrate surface was exposed to a flow of tertiarybutylarsine to promote an As-terminated surface. This surface treatment results in a low density of stacking faults; 60° misfit dislocations were observed at a layer thickness as low as 0.05 μm. This agrees well with the theoretical critical value for misfit dislocation formation in the ZnSe/GaAs system, but is much lower than experimental critical thicknesses reported earlier. Various mechanisms of misfit dislocation generation were observed at different growth stages. The evolution of the dislocation structure is discussed in relation with the morphology of the ZnSe layers.

Mechanisms of Dislocation Generation in Heterostructures Based on SiGe Alloys

Mechanisms of Dislocation Generation in Heterostructures Based on SiGe Alloys

Defects generated by misfit strain in SiGe/Si(001).

The structure of misfit dislocations in a ${\mathrm{Si}}_{0.67}$${\mathrm{Ge}}_{0.33}$/Si(001) strained-layer superlattice by transmission electron microscopy (TEM) has been analyzed. An orthogonal set of misfit dislocations, using plan-view TEM specimens, was observed at the strained-layer interfaces relaxing the strain in the structure. Conventional g\ensuremath{\cdot}b diffraction contrast experiments displayed 60\ifmmode^\circ\else\textdegree\fi{}-type misfit dislocations with a/2 〈100〉 {111} slip system. To observe the propagation and generation of misfit dislocations in the superlattice, a number of annealing experiments were carried out in a vacuum furnace at temperatures between 550 and 1050 \ifmmode^\circ\else\textdegree\fi{}C at a pressure of 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ Torr for 5 min. The experimental study was performed in the microscope taking micrographs before and after each annealing process. Consequently, two orthogonal sets of misfit dislocations were generated in the superlattice at two different temperatures (760 and 940 \ifmmode^\circ\else\textdegree\fi{}C). It was ascertained that the first set of dislocations is generated by the threading dislocations and the second set is produced by the structure. An interstitial type of dislocation loop was attained for the dislocation generation mechanism at high temperatures.

On the generation of a cross grid of extended screw-type misfit dislocations on the ZnSxSe1-x/GaAs interface

Abstract New sources for heterogeneous nucleation of both (110) and (110) extended screw-type misfit dislocations to relax the lattice shear stress on the ZnS x Se1-x/GaAs interface have been observed for the first time. These are identified as Shockley partial dislocations originating at the area close to the ZnS x Se1-x/GaAs interface. The Shockley partials are formed for the accommodation of the stacking enors produced upon island coalescence. In-situ electron beam-induced heating studies were carried out to observe the dislocation generation mechanism in the films. Our results show that the stress existing in the ZnS x Se1-x films gives rise to bowing of the threading segments of the Shockley partials. The bowing results from the fact that the dislocations are pinned at the fildsubstrate interface and at the film surface. This process involves an increase in the length of the threading segments under the stress σ ∼ 1 × 109 dyne cm−2. With further accommodation of the lattice strain, the line tension o...