Dislocation Glide Kinetics Research Articles

Optimizing misfit dislocation glide kinetics for enhanced threading dislocation density reduction in Si1−xGex/Si(001) layers through dynamic growth rate control

Relaxed Si1−xGex layers on Si(001) serve as virtual substrates for strained Si or Ge layers. However, plastically relaxed layers inevitably contain misfit and threading dislocations, negatively affecting devices. Deposition of a SiGe layer on the backside of the substrate introduces a dislocation reservoir at the wafer edge that can reduce the threading dislocation density (TDD) of Si0.98Ge0.02/Si layers, as these preexisting dislocations start gliding toward the wafer center upon reaching the critical thickness. Here, we show that this low-strain system can be used effectively to study dislocation glide kinetics. In agreement with the literature, dislocation glide is a thermally activated process with an activation energy of 2.12–2.16 eV. Near the critical thickness, relaxation is sluggish and inefficient due to the linear dependence of the glide velocity on excess stress. At lower growth rates, dislocations from the edge reservoir are activated in a lower density due to the increase in the critical thickness through partial strain relaxation by already activated dislocations. Contrary to common models, here, the lowest possible growth rate is not essential for minimizing the TDD. Instead, a careful balance between low and high growth rates is beneficial. Overcoming the initial sluggish and inefficient relaxation phase is critical while also avoiding accumulation of strain energy, and, therefore, the activation of dislocation sources. Only in a later stage of buffer growth, the growth rate should be reduced to a minimum. With this method, the TDD of strain relaxed Si0.84Ge0.16 layers is reduced to 7 × 104 cm−2.

Strength of Dry and Wet Quartz in the Low-Temperature Plasticity Regime: Insights From Nanoindentation.

At low‐temperature and high‐stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, that is, low‐temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz with electron backscatter diffraction and secondary‐ion mass spectrometry measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20–100 wt ppm H2O) crystals exhibit comparable indentation hardness. Quartz yield strength, which is proportional to indentation hardness, seems to be unaffected by the intracrystalline H2O content when deformed under room temperature, high‐stress conditions. Pre‐indentation intracrystalline microstructure may have provided a high density of dislocation sources, influencing the first increments of low‐temperature plastic strains. Our results have implications for fault strength at the frictional‐viscous transition and during transient deformation by LTP, such as seismogenic loading and post‐seismic creep.

Kinetically limited misfit dislocations formed during post-growth cooling in III–V lasers on silicon

Misfit dislocations (MDs) classically form at interfaces when an epitaxially strained film exceeds a critical thickness. We show that metastable MDs also form between layers nominally below critical thickness with respect to each other when externally driven threading dislocations (TDs) have significant dissimilarities in dislocation mobility in these layers controlled by glide kinetics. The InAs quantum dot laser on silicon presents a technologically important case for this phenomenon where TDs are pinned by indium-containing regions but glide in GaAs or AlGaAs cladding regions driven by thermal expansion mismatch strain with silicon during sample cool down following growth. This generates long MDs adjacent to the active region that is responsible for gradual degradation in performance. We calculate the driving force for MD formation and its dynamics in model structures, building up to full lasers, and describe the design of intentionally introduced indium-containing trapping layers that displace the MDs away from the active region, which is key to long laser lifetime. We show that factors controlling dislocation glide kinetics: doping, indium alloying, and dislocation core character have a strong influence on the final structure of defects. Yet, the introduction of indium must be done with care, illustrated using two cases where indium is not useful to overall device defect engineering.

Elevated-temperature Deformation Mechanisms in a CrMnFeCoNi High-Entropy Alloy

Stress reduction creep experiments were performed on a CrMnFeCoNi high-entropy alloy at 1073 K to characterize the steady-state and transient creep properties of this material. From measurements of constant-structure creep, the activation area for deformation of CrMnFeCoNi was determined to be ∼100 b2 and to decrease with increasing applied stress, indicating the presence of both concentrated solid solution and forest dislocation control of high temperature plastic deformation. With the aid of a recent solid solution theory for high entropy alloys, quantitative separation of the two mechanisms was carried out using a Haasen plot and the results show that creep in CrMnFeCoNi relies heavily on thermal activation with the majority of creep strength coming from solid solution hardening, especially at low applied stresses. The overall analysis conducted in this study reveals that steady-state creep deformation of CrMnFeCoNi at 1073 K can be adequately described by existing concentrated solid solution hardening models and forest dislocation hardening models. This observation suggests it may be possible to develop a unified treatment of the dislocation glide kinetics for this alloy from cryogenic to elevated temperatures.

Reduction of pulsed-laser surface irradiation induced residual stress using ultrasonic vibration method

The ultrasonic vibration method was introduced to relieve the residual stress in the small AISI 1045 steel specimen subjected to the pulsed-laser surface irradiation. The ultrasonic vibration plasticity framework was developed based on the thermal activation theory and the dislocation glide kinetics theory in order to reveal the double dynamic stress effect and the time effect of the ultrasonic stress relief. To validate the proposed model, the ultrasonic stress relief experiments were carried out using the AISI 1045 steel specimens treated by the pulsed-laser surface irradiation based on the Nd:YAG pulsed-laser machine. The results show that the ultrasonic stress relief has good effects on eliminating residual stress. Furthermore, the residual stress relaxation due to the double dynamic stress effect can be improved by increasing the ultrasonic vibration power and the initial magnitude of residual stress. The residual stress decrease due to the time effect can be improved by increasing the ultrasonic vibration time. The unified ultrasonic vibration plasticity model can accurately account the double dynamic stress effect and the time effect. The findings confirm that the ultrasonic stress relief can be used to eliminate residual stress in small workpieces and provide a basis to understand the underlying mechanism of the ultrasonic stress relief.

Kinetics of dislocations in pure Fe. Part I. In situ straining experiments at room temperature

In situ straining experiments have been carried out in pure Fe, in order to determine the geometry and the kinetics of dislocation glide at room temperature. Straight screw dislocations glide slowly in {1 1 0} elemental slip planes, at a velocity proportional to their length, whereas curved non-screw parts are highly mobile. The exact loop shape can yield the local stress as well as the difference of core energy between pure screw and near-screw orientations. The velocity–stress dependence of screws has been measured at the scale of a single dislocation source, and compared with macroscopic activation areas. The results are discussed in terms of the kink-pair mechanism.

Comparison of thermally activated overcoming of barriers in creep of aluminum and its solid solutions

The creep behavior of two aluminum solid solutions (Al-5.5 wt.% Mg and Al-13.7 wt.% Zn) was tested at temperatures from 573 to 773 K. Besides conventional creep experiments, constant structure creep was also studied by means of sudden change of the applied stress in the steady-state creep region. According to Nakayama and Gibeling, results of both types of experiments can be rationalized on the basis of the kinetics of dislocation glide controlled by overcoming of obstacles. Nakayama and Gibeling have also suggested a procedure that makes it possible to determine the basic characteristics of the process, i.e., the Helmholtz free energy, the Gibbs free energy and the obstacle strength, by means of stress-change experiments at different temperatures. The results of the procedure applied to the solid solutions under consideration can be compared with results of a similar study of creep of pure aluminum published previously by the present authors.

Evolution of dislocation glide kinetics during cyclic deformation of copper

Strain rate change tests were performed during low cycle fatigue of polycrystalline copper using plastic strain as the control variable. The evolution of dislocation interactions was observed by evaluating the activation area and true stress as a function of cumulative plastic strain. Activation areas at each of three plastic strain amplitudes, Δε p/2=0.2, 0.4, and 0.6%, have initial values of approximately 2000 b 2 which decrease to 600 b 2 during cyclic loading to saturation. This observation suggests a transition from forest dislocation cutting to increasing contributions of cross-slip as the predominant rate-controlling mechanisms of dislocation motion. Haasen plots of normalized inverse operational activation area ( b 2/ Δa) for specimens cycled to saturation exhibit a deviation from linearity similar to that observed for monotonic deformation. This nonlinearity corresponds to a failure of the Cottrell–Stokes law that correlates with the development of characteristic dislocation structures during cyclic deformation. Tests performed at various stresses at saturation reveal a linear dependence of b 2/Δ a on true stress. The athermal stress, σ b=86.5 MPa, measured at saturation by extrapolating the activation area data compares favorably with the value determined from a Bauschinger analysis, σ b=80 MPa, at a plastic strain amplitude of 0.6%. In addition, athermal stress values vary with plastic strain amplitude as expected, resulting in a constant value of approximately σ b/ σ=0.5.

Dislocation glide and blocking kinetics in compositionally graded SiGe/Si

The effects of growth temperature, substrate offcut, and dislocation pileup formation on threading dislocation density (TDD) in compositionally graded SiGe buffers are explored. To investigate dislocation glide kinetics in these structures, a series of identical samples graded to 30% Ge were grown at temperatures between 650 and 900 °C on (001)-, (001) offcut 6° towards an in-plane 〈110〉-, and (001) offcut 6° towards an in-plane 〈100〉-oriented Si substrates. The field threading dislocation density (field TDD) in the on-axis samples varied exponentially with temperature, from 3.7×106 cm−2 at 650 °C to 9.3×104 cm−2 at 900 °C. The activation energy for dislocation glide in this series, calculated from the evolution of field TDD with growth temperature, was 1.38 eV, much lower than the expected value for this composition. This deviation indicates that strain accumulating during the grading process at low growth temperatures is forcing further dislocation nucleation, resulting in a deviation from pure glide-limited relaxation. The TDD of samples grown on offcut substrates exhibited a more complicated temperature dependence, likely because films grown on offcut substrates have an increased tendency towards saturation in dislocation reduction reactions at high temperature. Dislocation reduction processes were further explored by initiating compositional grading up to 15% Ge at 650 °C and continuing the grade to 30% Ge at 900 °C. The low temperature portion of this growth provided an excess concentration of threading dislocations which could subsequently be annihilated during the high temperature portion of the growth, enabling a comparison of reduction rates for different substrate offcuts. Combining these results with threading dislocation densities in a variety of other samples, a complete picture of strain relaxation kinetics in compositionally graded SiGe/Si emerges. Generally, strain relaxation in these structures is limited by dislocation glide, and threading dislocation densities are independent of final Ge content. However, we theorize that dislocation pileup formation inhibits the strain relaxation process and is therefore accompanied by a rise in field threading dislocation density. Based on these results, we now have a predictive model for TDD in compositionally graded SiGe/Si over a wide range of growth conditions.

Evolution of microstructure and dislocation dynamics in InxGa1−xP graded buffers grown on GaP by metalorganic vapor phase epitaxy: Engineering device-quality substrate materials

This study explores the dislocation dynamics of strain relaxation in graded composition buffers of InxGa1−xP grown on GaP (InxGa1−xP/GaP) by metalorganic vapor phase epitaxy. Transmission electron microscopy, cathodoluminescence imaging, atomic force microscopy, and triple-axis x-ray diffraction are applied to the characterization of InxGa1−xP/GaP with final compositions ranging from x=0.09 to x=0.39 and growth temperatures ranging from 650 to 810 °C. The previously reported escalation of defect density with continued grading of InxGa1−xP/GaP beyond x∼0.3 is discovered to be due to the formation of dislocation pileups. A new defect microstructure with a branching morphology and featuring sharp local strain fields, hereafter referred to as branch defects, is observed to pin dislocations and cause the dislocation pileups. Branch defect morphology varies strongly with growth temperature, becoming significantly stronger with increasing growth temperature and causing severe material degradation above 700 °C. Further experiments show that branch defects evolve during growth and that the onset of branch defect formation is delayed by increasing growth temperature. Comparison with the literature suggests that the evolution of branch defects may control the microstructure of indium-bearing phosphides and arsenides over a very wide range of conditions. In the absence of branch defects at high growth temperatures and low indium compositions near x∼0.1, nearly ideal dislocation dynamics dominated by dislocation glide kinetics are recovered, providing the first experimental proof of a kinetic model for graded buffer relaxation. This new understanding of the evolution of microstructure and dislocation dynamics in InxGa1−xP/GaP suggests that growth temperature must be optimized as a function of composition for optimal material quality. A simple process optimization in InxGa1−xP/GaP graded to x=0.39 results in an overall threading dislocation density of 4.7×106 cm−2, which is the lowest reported value to date for x&amp;gt;0.3. Combining the new observations with earlier findings, we present three basic design rules for producing practical, device-quality graded buffers: branch defects must be avoided or suppressed, growth temperature must be maximized, and surface roughness must be minimized. Using these design rules, we also present optimization strategies for achieving device-quality substrate materials. Applying these design rules and optimization strategies, we hope to achieve threading dislocation densities of &amp;lt;106 cm−2 in InxGa1−xP/GaP over the full range of useful compositions.

Optimization of Microstructure and Dislocation Dynamics in InxGa1-xP Graded Buffers Grown on GaP by Movpe

AbstractTo engineer high-quality InxGal-xP graded buffers on GaP substrates (InxGa1-xP/GaP), we have explored the evolution of microstructure and dislocation dynamics in these materials. We show that the primarily limiting factor in obtaining high-quality InxGa1-xP/GaP is a new defect microstructure that we call branch defects. Branch defects pin dislocations and result in dislocation pileups that cause an escalation in threading dislocation density with continued grading. The morphology of branch defects is dominated by growth temperature, which can be used to suppress the strength or density of branch defects. In the absence of branch defects, we observe nearly ideal dislocation dynamics that are controlled by the kinetics of dislocation glide. This new understanding results in two primary design rules for achieving high-quality materials: 1) control branch defects, and 2) maximize dislocation glide kinetics. Combining these design rules into optimization strategies, we develop and demonstrate processes based on single and multiple growth temperatures. With optimization, threading dislocation densities below 5 × 106 cm−2 are achieved out to x = 0.39 and a nearly steady-state relaxation process is recovered.

Constant structure creep of copper at 973K

The results from studies indicate there is no change in the constant structure glide kinetics of copper across the 673 to 973K temperature range. The values of the obstacle strength parameter, g{sub o}, are in the range of 0.701 and 0.785, which is characteristic of medium strength obstacles such as forest dislocations. Also, the true activation area results are in agreement with the results of other FCC metals,a nd are of the order expected for forest dislocation cutting. The linear relationship between {beta} and {mu}(T)/T{sigma} does not change in this temperature range, and is in good agreement with the results from other FCC metals. Most importantly, the values of {Delta}G are similar across all of the temperatures and lie in the range of 112 to 123 kJ/mole. A good correlation exists between {Delta}G and the activation energies for creep reported by Barrett and Sherby, 117 kJ/mole, and Feltham, and Meakin, 125.5 kJ/mole, for the temperature range of 673 to 873K. The results of these constant structure creep tests demonstrate that there is no change in the dislocation glide kinetics over the temperature rage 673 to 973K.

Deformation of clinopyroxenite: Evidence for a transition in flow mechanisms and semibrittle behavior

A systematic suite of constant strain rate experiments was performed on a vacuum‐dried, high‐purity, fine‐grained clinopyroxenite using NaCl and NaF as confining media in a Griggs‐type piston‐cylinder apparatus. The experiments were carried out over a range of temperatures from 400° to 1100°C, strain rates from 10−3 to 10−7 s−1, and confining pressures from 170 to 1990 MPa. At T = 600°C and = 1.1 × 10−5 s−1, three modes of deformation occur with increasing confining pressure: (1) Macroscopic faulting associated with low strength and stress drops, (2) stable microfracturing and plastic deformation associated with pressure‐dependent strength, and (3) plastic deformation (mechanical twinning and 〈001〉 slip) with high strengths which are insensitive to pressure variations. In experiments at P = 1500 MPa, within this high‐pressure plastic mode, two regimes of flow are clearly defined. At low to intermediate temperatures and high strain rates, flow strengths are insensitive to changes in strain rate and temperature. Optical and transmission electron microscope observations indicate that plastic strain is accomplished by mechanical twinning on (100) and (001) and by {100}〈001〉 slip. In contrast, at high temperatures and low strain rates the flow stress is strongly dependent on temperature and strain rate. Specimens deformed in this regime show evidence of recovery, multiple slip, and recrystallization; and plastic strain is much more homogeneous. The flow data within each regime can be satisfactorily fit to thermally activated power laws. In the low‐temperature regime n (the stress exponent) = 83 ± 16 and E* (the activation energy for flow) = 220 ± 40 kJ/mol. We believe that these parameters reflect flow dominated by the kinetics of dislocation glide associated with mechanical twinning and (100)〈001〉 slip. In the high‐temperature regime, n = 5.3 ± 1.1 and E* = 380 ± 30 kJ/mol. These parameters describe creep by multiple slip accompanied by increased rates of diffusion and recovery.