In-plane Biaxial Strain Research Articles

Band gap modulation of transition-metal dichalcogenide MX2 nanosheets by in-plane strain

The electronic properties of quasi-two-dimensional honeycomb structures of MX2 nanosheets (M=Mo, W and X=S, Se) subjected to in-plane biaxial strain have been investigated using first-principles calculations. We demonstrate that the band gap of MX2 nanosheets can be widely tuned by applying tensile or compressive strain, and these ultrathin materials undergo a universal reversible semiconductor-metal transition at a critical strain. Compared to WX2, MoX2 need a smaller critical tensile strain for the band gap close, and MSe2 need a smaller critical compressive strain than MS2. Taking bilayer MoS2 as an example, the variation of the band structures was studied and the semiconductor-metal transition involves a slightly different physical mechanism between tensile and compressive strain. The ability to tune the band gap of MX2 nanosheets in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of application.

High Ms Fe16N2 thin film with Ag under layer on GaAs substrate

(001) textured Fe16N2 thin film with Ag under layer is successfully grown on GaAs substrate using a facing target sputtering (FTS) system. After post annealing, chemically ordered Fe16N2 phase is formed and detected by X-ray diffraction (XRD). High saturation magnetization (Ms) is measured by a vibrating sample magnetometer (VSM). In comparison with Fe16N2 with Ag under layer on MgO substrate and Fe16N2 with Fe under layer on GaAs substrate, the current layer structure shows a higher Ms value, with a magnetically softer feature in contrast to the above cases. In addition, X-ray photoelectron spectroscopy (XPS) is performed to characterize the binding energy of N atoms. To verify the role of strain that the FeN layer experiences in the above three structures, Grazing Incidence X-ray Diffraction (GIXRD) is conducted to reveal a large in-plane lattice constant due to the in-plane biaxial tensile strain.

Controlling the cation distribution and electric polarization with epitaxial strain in Aurivillius-phase Bi5FeTi3O15

This work explores the impact of in-plane bi-axial (epitaxial) strain on the cation distribution and electric polarization of the Aurivillius-phase compound Bi5FeTi3O15 using first-principles electronic structure calculations. Our calculations indicate that the site preference of the Fe3+ cation can be controlled via epitaxial strain. Tensile strain enhances the preference for the inner sites within the perovskite-like layers of the Aurivillius-phase structure, whereas compressive strain favors occupation of the outer sites, i.e., the sites close to the Bi2O2 layer. Controlling the distribution of the magnetic cations offers the possibility to control magnetic order in this magnetically dilute system. Furthermore, the magnitude of the electric polarization is strongly strain-dependent, increasing under tensile strain and decreasing under compressive strain. We find strongly anomalous Born effective charges, both of the Bi3+ and the Ti4+ cations.

Insight into the band structure engineering of single-layer SnS2 with in-plane biaxial strain

The effects of in-plane biaxial strain on the electronic structure of a photofunctional material, single-layer SnS2, were systematically investigated using hybrid density functional calculations. The bonding diagram for the band gap was firstly proposed based on the crystal orbital overlap population analysis. The conduction band-edge of single-layer SnS2 is determined by the anti-bonding interaction between Sn-5s and S-3p orbitals, while the valence band-edge comes from the anti-bonding between the neighboring S atoms. It is found that the compressive strain not only decreases the indirect band gap of single-layer SnS2, but also effectively promotes the band-edges of the conduction band to realize the overall water splitting. Besides, the dispersion of the valence band of single-layer SnS2 becomes weaker with increasing tensile strain which is beneficial for the photo-excitation through direct transitions.

Optical absorption and intrinsic recombination in relaxed and strained InAs1–xSbx alloys for mid-wavelength infrared application

The intrinsic carrier recombination lifetime in relaxed and strained InAs1−xSbx alloys is investigated using the full-band Green's function theory. By computing the phonon-perturbed electron self-energy of the system, both direct and phonon-assisted indirect Auger and radiative processes are studied as functions of antimony molar fractions, lattice temperatures and applied in-plane biaxial strains. To improve the overall accuracy of the calculation, an empirical pseudopotential band structure for the alloy is also fitted to the measured band extrema and effective masses under different biaxial strains. A set of effective screened potentials valid for all the needed antimony fractions x and biaxial strains ϵ, therefore, is obtained and applied to the calculation. The results showed reduced total Auger recombination rates and enhanced radiative recombination rates in InAsSb alloys at room temperature when a compressive strain is applied. Furthermore, the study on the widely employed mid-wavelength infrared detector material, InAs0.91Sb0.09, strained by an InAs substrate, demonstrated that much longer minority carrier lifetime can be achieved compared to that in the lattice-matched situation when the lattice temperature is above 200 K.

Structural phase transitions of phosphorene induced by applied strains

The effects of normal compressive strain and in-plane strain on the structures of phosphorene have been investigated by using first-principles calculations. It is quite intriguing to find that a structural transformation from pristine Z-phosphorene to a new A-phosphorene occurs under the normal compressive strain of $\ensuremath{\varepsilon}=48%$ or the anisotropic biaxial in-plane strain of ${\ensuremath{\varepsilon}}_{\mathrm{x}}=\ensuremath{-}16%$ and ${\ensuremath{\varepsilon}}_{\mathrm{y}}=54%$. In the extreme case where the pucker structure is flattened into a plane, the phosphorene structure is quite unstable at finite temperatures, transforming into another new H-phosphorene phase. The anisotropic structure of A-phosphorene gives rise to its direction-dependent mechanical properties whereas H-phosphorene exhibits isotropic mechanical properties. Both A-phosphorene and H-phosphorene are semiconductors with indirect band gaps of about 0.42 and 1.94 eV that use the Perdew-Burke-Ernzerhof exchange-correlation functional, respectively. The electronic properties of the two new phases are found to be sensitive to the magnitude and direction of the applied strains, which offer an effective method to modulate them in future device engineering.

Epitaxial growth of tungsten layers on MgO(001)

Smooth single crystal W(001) layers were grown on MgO(001) substrates by magnetron sputtering at 900 °C. X-ray diffraction ω–2θ scans, ω-rocking curves, pole figures, and reciprocal space maps indicate a 45°-rotated epitaxial relationship: (001)W‖(001)MgO and [010]W‖[110]MgO, and a relaxed lattice constant of 3.167 ± 0.001 nm. A residual in-plane biaxial compressive strain is primarily attributed to differential thermal contraction after growth and decreases from −0.012 ± 0.001 to −0.001 ± 0.001 with increasing layer thickness d = 4.8–390 nm, suggesting relaxation during cooling by misfit dislocation growth through threading dislocation glide. The in-plane x-ray coherence length increases from 3.4 to 33.6 nm for d = 4.8–390 nm, while the out-of-plane x-ray coherence length is identical to the layer thickness for d ≤ 20 nm, but is smaller than d for d ≥ 49.7 nm, indicating local strain variations along the film growth direction. X-ray reflectivity analyses indicate that the root-mean-square surface roughness increases from 0.50 ± 0.05 to 0.95 ± 0.05 nm for d = 4.8–19.9 nm, suggesting a roughness exponent of 0.38, but remains relatively constant for d > 20 nm with a roughness of 1.00 ± 0.05 nm at d = 47.9 nm.

Electronic properties of monolayer and bilayer arsenene under in-plain biaxial strains

Using first-principles density functional theory, we have investigated the mechanical and electronic properties of monolayer and bilayer arsenenes under in-plain biaxial strains. It is interesting to find that under large enough tensile strains, the monolayer arsenene transfers from buckled honeycomb structure to planar honeycomb phase while the interlayer distance D of bilayer arsenene maintains at about 1.371Å. Both monolayer and bilayer arsenenes possess indirect band gaps and their electronic properties can be tuned via in-plane biaxial strains. The variations of the band gap energy are diverse with respect to the compressive and tensile biaxial strains. Under compressive strains, the band gap of both monolayer and bilayer arsenenes initially increases, and then rapidly decreases. In addition, the monolayer arsenene exhibits an indirect-to-direct band gap transition when the compressive strains reach to −10%. However, under tensile strains, the band gap of monolayer arsenene monotonously decreases with the strains, while the band gap of bilayer arsenene reduces quickly to zero under small tensile strains. Our present results will be conductive to design strain-based arsenene materials for applications in nanoelectronic and optoelectronic devices.

Strain-tunable topological quantum phase transition in buckled honeycomb lattices

Low-buckled silicene is a prototypical quantum spin Hall insulator with the topological quantum phase transition controlled by an out-of-plane electric field. We show that this field-induced electronic transition can be further tuned by an in-plane biaxial strain ε, owing to the curvature-dependent spin-orbit coupling (SOC): There is a Z2 = 1 topological insulator phase for biaxial strain |ε| smaller than 0.07, and the band gap can be tuned from 0.7 meV for ε=+0.07 up to 3.0 meV for ε=−0.07. First-principles calculations also show that the critical field strength Ec can be tuned by more than 113%, with the absolute values nearly 10 times stronger than the theoretical predictions based on a tight-binding model. The buckling structure of the honeycomb lattice thus enhances the tunability of both the quantum phase transition and the SOC-induced band gap, which are crucial for the design of topological field-effect transistors based on two-dimensional materials.

Strain-Engineered Biaxial Tensile Epitaxial Germanium for High-Performance Ge/InGaAs Tunnel Field-Effect Transistors

The structural, morphological, and energy band alignment properties of biaxial tensile-strained germanium epilayers, grown in-situ on GaAs via a linearly graded InxGa1−xAs buffer architecture and utilizing dual chamber molecular beam epitaxy, were investigated. Precise control over the growth conditions yielded a tunable in-plane biaxial tensile strain within the Ge thin films that was modulated by the underlying InxGa1−xAs “virtual substrate” composition. In-plane tensile strains up to 1.94% were achieved without Ge relaxation for layer thicknesses of 15 to 30 nm. High-resolution x-ray diffraction supported the pseudomorphic nature of the Ge/InxGa1−xAs interface, indicating a quasi-ideal stress transfer to the Ge lattice. High-resolution transmission electron microscopy revealed defect-free Ge epitaxy and a sharp, coherent interface at the Ge/InxGa1−xAs heterojunction. Surface morphology characterization using atomic force microscopy exhibited symmetric, 2-D cross-hatch patterns with root mean square roughness less than 4.5 nm. X-ray photoelectron spectroscopic analysis revealed a positive, monotonic trend in band offsets for increasing tensile strain. The superior structural and band alignment properties of strain-engineered epitaxial Ge suggest that tensile-strained Ge/InxGa1−xAs heterostructures show great potential for future high-performance tunnel field-effect transistor architectures requiring flexible device design criteria while maintaining low power, energy-efficient device operation.

Strain dependence of antiferromagnetic interface coupling inLa0.7Sr0.3MnO3/SrRuO3superlattices

We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg1/3Nb2/3O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu0HAF = 2.8 T is found to decrease by deltaHAF/delta epsilon ~ -520 mT %-1 under reversible biaxial strain mu0HAF at 80 K in a [La0.7Sr0.3MnO3(22)/SrRuO3(55)]15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO3 layers at the interface. Strain-dependent orbital occupation in a single-ion picture cannot explain the sign of the observed strain dependence, whereas the enhanced Mn order at the interface is qualitatively in line with it.

Strain-induced modulation of perpendicular magnetic anisotropy in Ta/CoFeB/MgO structures investigated by ferromagnetic resonance

We demonstrate strain-induced modulation of perpendicular magnetic anisotropy (PMA) in (001)-oriented [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT) substrate/Ta/CoFeB/MgO/Ta structures using ferromagnetic resonance (FMR). An in-plane biaxial strain is produced by applying voltage between the two surfaces of the PMN-PT substrate, and is transferred to the ferromagnetic CoFeB layer, which results in tuning of the PMA of the CoFeB layer. The strain-induced change in PMA is quantitatively extracted from the experimental FMR spectra. It is shown that both first and second-order anisotropy terms are affected by the electric field, and that they have opposite voltage dependencies. A very large value of the voltage-induced perpendicular magnetic anisotropy modulation of ∼7000 fJ/V·m is obtained through this strain-mediated coupling. Using this FMR technique, the magnetostriction coefficient λ is extracted for the ultrathin 1.1 nm Co20Fe60B20 layer, and is found to be 3.7 × 10−5, which is approximately 4 times larger than the previously reported values for CoFeB films thicker than 5 nm. In addition, the effect of strain on the effective damping constant (αeff) is also studied and no obvious modulation of the αeff is observed. The results are relevant to the development of CoFeB-MgO magnetic tunnel junctions for memory applications.

Strain effects on the electronic properties in δ-doped oxide superlattices

Strain effects on the electronic properties of (LaTiO3)1/(SrTiO3)N superlattices were investigated using density functional theory. Under biaxial in-plane strain within the range of −5% ≤ ε// ≤ 5%, the dxy orbital electrons are highly localized at the interfaces whereas the dyz and dxz orbital electrons are more distributed in the SrTiO3 (STO) spacer layers. For STO thickness N ≥ 3 unit cells (u.c.), the dxy orbital electrons form two-dimensional (2D) electron gases (2DEGs). The quantized energy levels of the 2DEG are insensitive to the STO spacer thickness, but are strongly dependent on the applied biaxial in-plane strain. As the in-plane strain changes from compressive to tensile, the quantized energy levels of the dxy orbitals decrease thereby creating more states with 2D character. In contrast to the dxy orbital, the dyz and dxz orbitals always have three-dimensional (3D) transport characteristics and their energy levels increase as the strain changes from compressive to tensile. Since the charge densities in the dxy orbital and the dyz and dxz orbitals respond to biaxial in-plane strain in an opposite way, the transport dimensionality of the majority carriers can be controlled between 2D and 3D by applying biaxial in-plane strain.

A density functional theory study of the tunable structure, magnetism and metal-insulator phase transition in VS2 monolayers induced by in-plane biaxial strain

We report a density functional theory study of a phase transition of a VS2 monolayer that can be tuned by the in-plane biaxial strain. This results in both a metal-insulator transition and a low spin-high spin magnetic transition. At low temperature, the semiconducting H-phase is stable and large strain (>3%) is required to provoke the transition. On the other hand, at room temperature (300 K), only a small tensile strain of 2% is needed to induce the phase transition from the semiconducting H-phase to the metallic T-phase together with the magnetic transition from high spin to low spin. The phase diagram dependence on both strain and temperature is also discussed in order to provide a better understanding of the phase stability of VS2 monolayers.

Strain effects on the electronic structure of ZnSnP2 via modified Becke–Johnson exchange potential

ZnSnP 2 is a promising photovoltaic absorber material with a direct band gap of 1.68 eV, further reducing the band gap of ZnSnP 2 that can achieve higher photovoltaic conversion efficiency. To achieve this target, the influence of biaxial in-plane strain (±3%) on the band gap, hole effective mass and optical properties of ZnSnP 2 were investigated by first-principles calculations via Modified Becke–Johnson exchange potential. The results indicate that the biaxial tensile strain can reduce the band gap of ZnSnP 2 from 1.3 eV to 1.0 eV and enhance the absorption of visible light of c -axis direction, while the biaxial compress strain increases the band gap of ZnSnP 2 slightly. This research provides an alternative approach to tune the band gap of ZnSnP 2 by strains. The variation of the band gap under different strains is determined by the highest-energy valance band state, and it can be explained by the redistribution of electrons under different strain. • The influence of biaxial in-plane strain (±3%) on the band gap of ZnSnP 2 were investigated by DFT calculations. • MBJ exchange potential can describe the band structure of ZnSnP 2 quite well. • Tensile strain brings a substantial decrease of the band gap, while the compress strain has no evident effect. • The calculated visible light adsorption coefficient increases along c -axis direction under tensile strain.

Structural and electronic properties of LaO δ-doped SrTiO3 caused by biaxial strain

δ-doping in SrTiO3 plays a pivotal role for oxide electronics. By first-principles density functional calculations, we reveal that the electronic and structural properties of LaO δ-doped SrTiO3 can be drastically altered by compressive biaxial inplane strains. We predict the existence of a critical inplane strain (ηc) above which many interesting phenomena occur, including (i) a profound symmetry breaking to the doping-induced electron states, (ii) a dramatic decline of the electron potential over a short distance of 2.5 Å near the doping layer, (iii) a new structure pattern of cation-anion displacements, and (iv) the emergence of unusual downward dispersions in the conduction bands with negative effective masses for electrons. The microscopic insight for these interesting phenomena is provided. Our study demonstrates that biaxial inplane strain can effectively modify the electronic properties in δ-doped SrTiO3 for the purpose of oxide electronics.

Surface Strain Effects on the Water–Graphene Interfacial and Confinement Behavior

We studied via molecular dynamics the link between the strain-driven hydration free-energy changes in the association process involving finite-size graphene surfaces, the resulting water–graphene interfacial tension, and the combined effect of the surface strain and confinement on the thermodynamic response functions and the dynamics of confined water. We found that an in-plane biaxial tensile strain ε = 10% enhances significantly not only the water–graphene hydrophobicity with respect to that of the unstrained counterpart but also the confinement effect on the thermodynamic response functions and slowing down of the dynamics of water over those of the corresponding bulk counterpart. The interfacial behavior of water in contact with strained-graphene plates resembles that observed for “pp” corrugated-plate configuration, as reported earlier [Chialvo et al. J. Phys. Chem. C 2013, 117, 23875], exibiting a significant enhancement of the fluid–surface hydrophobicity and response functions relative to those of the unstrained surface. In contrast, the slowing down of the dynamics of the confined water does not show any differentiation with respect to the type of surface.

Voltage control of metal-insulator transition and non-volatile ferroelastic switching of resistance in VOx/PMN-PT heterostructures.

The central challenge in realizing electronics based on strongly correlated electronic states, or ‘Mottronics', lies in finding an energy efficient way to switch between the distinct collective phases with a control voltage in a reversible and reproducible manner. In this work, we demonstrate that a voltage-impulse-induced ferroelastic domain switching in the (011)-oriented 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) substrates allows a robust non-volatile tuning of the metal-insulator transition in the VOx films deposited onto them. In such a VOx/PMN-PT heterostructure, the unique two-step electric polarization switching covers up to 90% of the entire poled area and contributes to a homogeneous in-plane anisotropic biaxial strain, which, in turn, enables the lattice changes and results in the suppression of metal-insulator transition in the mechanically coupled VOx films by 6 K with a resistance change up to 40% over a broad range of temperature. These findings provide a framework for realizing in situ and non-volatile tuning of strain-sensitive order parameters in strongly correlated materials, and demonstrate great potentials in delivering reconfigurable, compactable, and energy-efficient electronic devices.

Effect of Strain on the Magnetism of Transition Metal-Doped ZnO: The First-Principles Calculations

The electronic structures and magnetic properties of transition metal (Mn, Fe, Co)-doped ZnO under the in-plane biaxial strains are investigated systematically by the first-principles calculations. It is found that the augmented in-plane biaxial strains can mediate the Fe-doped ZnO ferromagnetism effectively, but have less influence on Mn- and Co-doped systems. Therefore, the strain may be an effective means in inducing ferromagnetism in ZnO:Fe system.

Electronic and magnetic properties of vanadium doped AlN nanosheet under in-plane biaxial strains

First-principles calculations have been performed to investigate the electronic and magnetic properties of V-doped AlN nanosheet under in-plane biaxial strains. It is found that V atom favors to substitute for Al site with the lowest formation energy. The magnetic coupling of three different configurations are studied and configuration I is demonstrated to possess room temperature ferromagnetism. The stable ferromagnetic coupling is mediated by double exchange mechanism. In addition, the in-plane biaxial strains corresponding to tensile and compressive strains can affect and manipulate the magnetic interaction of V-doped AlN nanosheet in different ways. These results are conductive to design AlN based two dimensional diluted magnetic semiconductors.