Strain-driven Process Research Articles

High Mobility Cd3As2(112) on GaAs(001) Substrates Grown via Molecular Beam Epitaxy

The three-dimensional Dirac semimetal Cd3As2 exhibits ultrahigh electron mobilities that are attractive for optoelectronic devices. However, its strong propensity to grow in the (112) orientation limits the feasibility to epitaxially integrate it into semiconductor structures that are conventionally grown in the (001) orientation. Here, we demonstrate a route to epitaxially growing high mobility Cd3As2(112) layers on GaAs(001) substrates, opening up possibilities for device design. The (001) crystallographic orientation of the GaAs substrate is switched to the (111) orientation through a strain-driven process at a CdTe/GaAs interface, resulting in a CdTe(111) buffer layer on top of which Cd3As2 can be grown. Although the CdTe(111) buffer layer templates Cd3As2 in the (112) orientation, it is not sufficient for producing Cd3As2 with high electron mobility. We therefore demonstrate additional buffer layer design principles for realizing Cd3As2(112) epilayers with similar electron mobilities to those grown on lattice-mismatched III–V (111) substrates. Finally, we outline a pathway to use this approach to grow Cd3As2(112) epilayers on Si(001) substrates, further expanding the potential to integrate Cd3As2 into electronic devices.

Quasi-static versus dynamic stability associated with local damage models

We investigate the stability of dynamic and quasi-static strain-driven processes for a large class of micromechanics-based models of damage in brittle solid materials. A special attention is paid to the compatible models having the same equilibrium states where the dynamic damage law is constructed from a quasi-static one. We deduce a simple stability criterion for non-associated damage models. The unstable equilibria of quasi-static models are related to the solution multiplicity, shocks and non-smooth stress-strain curves. Also, we deduce the stability of the dynamic models from the linear stability of the associated damage differential system. For compatible models the dynamic and quasi-static stability criteria coincide. We use the stability criteria to analyse mechanically non-interacting cracks with self-similar growth, and find a critical crack density parameter that distinguishes between stable and unstable behaviours. It depends on the chosen configuration and Poisson’s ratio, but it is independent of the other micromechanical properties. The configurations with the crack densities smaller than the critical value are mechanically unstable but cracks are activated at high stress levels, generating a sharp stress drop in the stress-strain diagram. For crack densities above the critical value, the model is stable and the stress varies smoothly. For dynamic loadings of unstable configurations a high strain rate is associated with a smooth stress-strain curve, while a low strain rate induces an abrupt stress drop in the stress-strain relation. This kind of instability is qualitatively the same for porous brittle plates under uni-axial compression and for the self-consistent effective elasticity and the wing crack models.

Variational inequality in classical plasticity. Applications to Armstrong–Frederick elasto-plastic model

In this paper the quasi-static initial and boundary value problem for an elasto-plastic mixed hardening material is reformulated within the constitutive framework of small strains. The plastic factor plays the basic role in describing the rate independent evolution equations for the plastic strain and hardening variables. The plastic factor is equivalently represented as the solution of an appropriate local inequality involving the yield function. The main idea was to introduce the variational inequality at any time t to be solved for the velocity field and the complementary plastic factor. There is the plastic factor in a strain-driven process. The solution procedure proposed here to solve the initial and boundary value problem is based on the solutions of the variational inequality at time t, coupled with an update algorithm in order to evaluate the current state of the material for an incremental deformation process. This time the return mapping algorithm is avoided as the values of the plastic factor and the velocity are known at time t. As we developed a procedure to simultaneously solve the equilibrium equation coupled with the rate-independent evolution equations, no necessity to compute the algorithmic elasto-plastic tangent moduli occurs. The numerical simulations are done for the mixed hardening elasto-plastic model involving Armstrong–Frederick kinematic hardening. To validate the proposed numerical algorithms, we compare the solutions based on the variational inequality and those based on return mapping algorithm, computed for the same Prager kinematic hardening law.

External-field-induced crystal structure and domain texture in (1−x)Na0.5Bi0.5TiO3–xK0.5Bi0.5TiO3 piezoceramics

Lead-free perovskites based on Na0.5Bi0.5TiO3 (NBT) are being considered as viable alternatives to lead-containing piezoelectric materials. The piezoelectric response of morphotropic compositions of these bismuth-based piezoelectrics during the application of external stimuli are governed by an intrinsic local disorder, ferroelectric and antiferrodistortive instabilities, and domain texturing. Understanding the coupling between these mechanisms is of crucial importance for the development of new, environmentally friendly, piezoelectric materials. In this investigation we applied in-situ transmission electron microscopy and high-energy X-ray diffraction to study the changes in the crystal structure and domain texturing during the application of mechanical stresses and electric fields to the morphotropic composition of (1-x)Na0.5Bi0.5TiO3–xK0.5Bi0.5TiO3 (NBT-KBT) piezoceramics. It was found that the mechanisms involved largely depend on the materials' initial structural state and that phase transitions and domain texturing dominate the polarization- and strain-driven processes.

Design of a Compact Biaxial Tensile Stage for Fabrication and Tuning of Complex Micro- and Nano-scale Wrinkle Patterns

Wrinkling of thin films is a strain-driven process that enables scalable and low-cost fabrication of periodic micro- and nano-scale patterns. In the past, single-period sinusoidal wrinkles have been applied for thin-film metrology and microfluidics applications. However, real-world adoption of this process beyond these specific applications is limited by the inability to predictively fabricate a variety of complex functional patterns. This is primarily due to the inability of current tools and techniques to provide the means for applying large, accurate, and nonequal biaxial strains. For example, the existing biaxial tensile stages are inappropriate because they are too large to fit within the vacuum chambers that are required for thin-film deposition/growth during wrinkling. Herein, we have designed a compact biaxial tensile stage that enables (i) applying large and accurate strains to elastomeric films and (ii) in situ visualization of wrinkle formation. This stage enables one to stretch a 37.5 mm long film by 33.5% with a strain resolution of 0.027% and maintains a registration accuracy of 7 μm over repeated registrations of the stage to a custom-assembled vision system. Herein, we also demonstrate the utility of the stage in (i) studying the wrinkling process and (ii) fabricating complex wrinkled patterns that are inaccessible via other techniques. Specifically, we demonstrate that (i) spatial nonuniformity in the patterns is limited to 6.5%, (ii) one-dimensional (1D) single-period wrinkles of nominal period 2.3 μm transition into the period-doubled mode when the compressive strain due to prestretch release of plasma-oxidized polydimethylsiloxane (PDMS) film exceeds ∼18%, and (iii) asymmetric two-dimensional (2D) wrinkles can be fabricated by tuning the strain state and/or the actuation path, i.e., the strain history. Thus, this tensile stage opens up the design space for fabricating and tuning complex wrinkled patterns and enables extracting empirical process knowledge via in situ visualization of wrinkle formation.

Dissipation inequality-based periodic homogenization of wavy materials

In this paper we present an internal variable-based homogenization of a composite made of wavy elastic-perfectly plastic layers. In the context of a strain-driven process, the macrostress and the effective yield surface are expressed in terms of the residual stresses, which act as hardening parameters in the effective behavior of the composite. Moreover, an approximate two-steps homogenization scheme useful for composites made of matrix with wavy inclusions is proposed and a comparison with one computational and one semi-analytical homogenization method is presented.

Multifunctional nanomembranes self-assembled into compact rolled-up sensor–actuatordevices

This paper reports on the functional principle for a rolled-up microdevice family providingthe possibility to realise electrical sensor and actuator functions which are applicable tomicrosystem techniques, in the field of physical measuring techniques, microfluidics andbiological–medical analytics. The fabrication of the roll-up devices is based ona strain-driven process converting a planar two-dimensional membrane into athree-dimensional functional element. For the realisation of these roll-up structures a broadspectrum of thin film technological approaches were used, which are well established. Thespecial feature of these advanced microdevices consists in the possibility to design thelatter’s inner surface using established additive or subtractive thin film techniques. Thetechnological route elaborated in the present work could possibly be used to realise a valvefor microfluidics thanks to the combined advantages of rolled-up devices andthe unique properties of temperature-sensitive hydrogels. Due to the thermaldecoupling of the processed microheaters from the supporting substrate the obtainedmicrodevices obtained are superior to conventional miniaturised planar devices in theirgeneral performance, especially in respect to the dynamical behaviour of thethermal properties. This is a prospective basis for the conceptual creation ofcombined sensor/actuator devices with tailored electrical and thermal properties.

In-Plane Epitaxial Growth of Self-Assembled Ge Nanowires on Si Substrates Patterned by a Focused Ion Beam

We report a novel method for obtaining ordered arrays of self-assembled Ge nanowires (NWs) using Au seed catalysts, with the latter deposited using a focused ion beam (FIB). For this purpose we apply a three-step process involving first FIB nanopatterning, a second step of AuSi seed formation during UHV annealing, and third the nucleation and growth of Ge NWs by combining molecular-beam epitaxy (MBE) and the vapor–liquid–solid (VLS) process. We show that FIB allows for the local implantation of Au in the areas impacted by the ion beam; the implanted Au evolves during annealing into AuSi clusters, serving as nucleation seeds for the nucleation and growth of Ge NWs. We thus prove that FIB with gold ions is a successful method to obtain gold-catalyzed self-assembled nanowires. We obtain Ge NWs of homogeneous dimensions and oriented in-plane along [110] directions, as a consequence of a strain-driven process. Wire kinking is governed by surface morphological features. Based on our experimental results, we ela...

Growth of Low-Density Vertical Quantum Dot Molecules with Control in Energy Emission.

In this work, we present results on the formation of vertical molecule structures formed by two vertically aligned InAs quantum dots (QD) in which a deliberate control of energy emission is achieved. The emission energy of the first layer of QD forming the molecule can be tuned by the deposition of controlled amounts of InAs at a nanohole template formed by GaAs droplet epitaxy. The QD of the second layer are formed directly on top of the buried ones by a strain-driven process. In this way, either symmetric or asymmetric vertically coupled structures can be obtained. As a characteristic when using a droplet epitaxy patterning process, the density of quantum dot molecules finally obtained is low enough (2 × 108 cm−2) to permit their integration as active elements in advanced photonic devices where spectroscopic studies at the single nanostructure level are required.

Investigation of strain in self-assembled multilayer InAs/GaAs quantum dot heterostructures

The self-assembled InAs/GaAs multilayer quantum dots (QDs) are formed in a strain-driven process due to the lattice mismatch of the InAs/GaAs system. We have investigated strain interaction in 10 layer QD heterostructure with varying thicknesses of combination capping (InAlGaAs and GaAs) by means of scanning transmission electron microscopy (STEM), high-resolution X-ray diffraction (HRXRD) and Raman scattering. STEM micrographs reveal nice stacking of defect-free dots in all the layers of the sample having a thick combination capping. The periodic satellite peaks in the HRXRD rocking curve show good formation of dots and an indication of reduced compressive strain in the heterostructure with increased capping thickness. We detect an upward phonon frequency shift for InAs QDs in the low-temperature Raman study, which is believed to be due to strain relaxation as the thickness of the capping layer increases. The sample with thick combination capping showed better optical emission properties.

Geometric integrators for multiplicative viscoplasticity: Analysis of error accumulation

The inelastic incompressibility is a typical feature of metal plasticity/viscoplasticity. Over the last decade, there has been a great amount of research related to construction of numerical integration algorithms which exactly preserve this property. In this paper we examine, both numerically and mathematically, the excellent accuracy and convergence characteristics of such integrators. In order to simplify the considerations, we consider strain-driven processes without hardening effects. In terms of a classical model of multiplicative viscoplasticity, we illustrate the notion of exponential stability of the exact solution. We show that this property enables the construction of effective and stable numerical algorithms, if the incompressibility is exactly satisfied. On the other hand, if the incompressibility constraint is violated, spurious degrees of freedom are introduced. In general, this results in the loss of the exponential stability and a dramatic deterioration of convergence of numerical methods.

Evolution of composition distribution of Si-capped Ge islands on Si(001)

The evolution of the composition distribution and microstructures of Ge islands on Si(001) during the Si overgrowth was investigated by atomic force microscopy combined with selective wet etching procedures. With increasing Si coverage to 5.4 nm, the uncapped Ge islands were found to change their shapes dramatically from domes to truncated pyramids, nanorings and eventually to the fully buried islands. Different atomic composition profiles in SiGe islands were observed at different Si coverages. Especially, the nanorings were found to have a Ge-rich core with a Si-rich periphery. Based on the experimental results, the Ge redistribution in islands during Si capping is not only correlated with the intermixing between Si capping layer and Ge islands, but also a strain-driven process.

SiGe Quantum Rings by Ultra-high Vacuum Chemical Vapor Deposition

SiGe quanrum rings (QRs) grown at 500oC and 600oC were observed on SiGe quantum dots (QDs) capped with Si. Average depth and diameter are 9 nm and 185 nm, respectively, for QRs at 500oC, while those are 0.9 nm and 84 nm for QRs at 600oC. Ge out-diffusion mechanism is proposed to be responsible for nanorings formation in 500oC, and Si surface diffusion toward strain-free edges is proposed to be responsible for nanorings formation in 600oC. Raman spectroscopy demonstrates that formation of QRs in 600oC is close correlated with a strain-driven process. QRs grown in 600oC are the metastable states and can be only observed in very limited conditions. Thick cap and high thermal budget can both destroy SiGe nanorings structures.

Configuration control of quantum dot molecules by droplet epitaxy

We demonstrate that by changing the substrate temperature at which Ga droplets form and by varying the InAs deposition, we are able to control the configuration of quantum dots per GaAs mound. The size of the Ga droplets increases with increasing substrate temperature and resulting configurations show a very strong correlation with the size of initial GaAs islands. In distinction from previous reports, we attained two structures: quadmolecules and quantum rod pairs. Quadmolecules are elongated along the [011] crystallographic direction due to strain-driven processes and are directly formed at the edges of the GaAs mounds. On the other hand, quantum rod pairs formed along the [01−1] direction due to higher anisotropic diffusion.