In-plane Biaxial Compressive Strain Research Articles

Theoretical predicted Janus SrAlGaTe4: effects of strain and electric field and its topological properties

The asymmetric Janus SrAlGaTe4, constructed from its parent SrGa2Te4 monolayer, was predicted theoretically by first principle calculations. Its stability was confirmed by phonon structure without imaginary frequency and ab initio molecular dynamics (AIMD) simulations. The Janus structure reduces the symmetry of SrGa2Te4 monolayer, which causes the absence of topological states in free-standing Janus SrAlGaTe4. To explore the possible electronic and topological properties, the effects of strain and external electric field, working as effective modulation methods for the electronic properties, were investigated. The SrAlGaTe4 monolayer undergoes a direct-to-indirect bandgap transition when the in-plane biaxial compressive strain is −8%. When the tensile strain is 9% or the electric field is 0.5 V Å−1, the Janus SrAlGaTe4 monolayer exhibits topological insulator (TI) characters, which was confirmed by the evolution of the Wannier charge centers (WCC). And the critical values for the topological transition are 2% for the biaxial tensile strain, and 0.2 V/Å for the applied electric field. The asymmetric Janus structure induces a Rashba spin splitting not only in the valence band but also in the conduction band near the Fermi level when the spin–orbit coupling (SOC) is present. Our findings offer theoretical insights into the exotic physical properties of SrAlGaTe4 and also provide guides to new spintronic device designs.

Multiple control of few-layer Janus MoSSe systems

In this computational work based on density functional theory we study the electronic and electron transport properties of asymmetric multi-layer MoSSe junctions, known as Janus junctions. Focusing on 4-layer systems, we investigate the influence of electric field, electrostatic doping, strain, and interlayer stacking on the electronic structure. We discover that a metal to semiconductor transition can be induced by an out-of-plane electric field. The critical electric field for such a transition can be reduced by in-plane biaxial compressive strain. Due to an intrinsic electric field, a 4-layer MoSSe can rectify out-of-plane electric current. The rectifying ratio reaches 34.1 in a model junction Zr/4-layer MoSSe/Zr. This ratio can be further enhanced by increasing the number of MoSSe layers. In addition, we show a drastic sudden vertical compression of 4-layer MoSSe due to in-plane biaxial tensile strain, indicating a second phase transition. Furthermore, an odd-even effect on electron transmission at the Fermi energy for Zr/$n$-layer MoSSe/Zr junctions with $n=1, \, 2,\, 3, \,\dots,\, 10$ is observed. These findings reveal the richness of physics in this asymmetric system and strongly suggest that the properties of 4-layer MoSSe are highly tunable, thus providing a guide to future experiments relating materials research and nanoelectronics.

A two-dimensional electron gas based on a 5s oxide with high room-temperature mobility and strain sensitivity

The coupling of optical and electronic degrees of freedom together with quantum confinement in low-dimensional electron systems is particularly interesting for achieving exotic functionalities in strongly correlated oxide electronics. Recently, high room-temperature mobility has been achieved for a large bandgap transparent oxide – BaSnO3 upon extrinsic La or Sb doping, which has excited significant research attention. In this work, we report the realization of a two-dimensional electron gas (2DEG) on the surface of transparent BaSnO3 via oxygen vacancy creation, which exhibits a high carrier density of ~7.72 × 1014 cm−2 and a high room-temperature mobility of ~18 cm2·V−1·s−1. Such a 2DEG is rather sensitive to strain and a less than 0.1% in-plane biaxial compressive strain leads to a giant resistance enhancement of ~350% (more than 540 kΩ/□) at room temperature. Thus, this work creates a new path to exploring the physics of low-dimensional oxide electronics and devices applicable at room temperature.

Understanding the anisotropic strain effects on lithium diffusion in graphite anodes: A first-principles study

The lithium diffusion in graphite anode, which is the most widely used commercial electrode material today, affects the charge/discharge performance of lithium-ion batteries. In this study, the anisotropic strain effects on lithium diffusion in graphite anodes are systematically investigated using first-principles calculations based on density functional theory (DFT) with van der Waals corrections. It is found that the effects of external applied strains along various directions of LixC6 (i.e., perpendicular or parallel to the basal planes of the graphite host) on lithium diffusivity are different. Along the direction perpendicular to the graphite planes, the tensile strain facilitates in-plane Li diffusion by reducing the energy barrier, and the compressive strain hinders in-plane Li diffusion by raising the energy barrier. In contrast, the in-plane biaxial tensile strain (parallel to the graphite planes) hinders in-plane Li diffusion, and the in-plane biaxial compressive strain facilitates in-plane Li diffusion. Furthermore, both in-plane and transverse shear strains slightly influence Li diffusion in graphite anodes. A discussion is presented to explain the anisotropic strain dependence of lithium diffusion. This research provides data for the continuum modelling of the electrodes in the lithium-ion batteries.

Enhancement of superconductivity in FeSe thin crystals induced by biaxial compressive strain

We report on the enhancement of superconductivity in FeSe thin crystals induced by in-plane biaxial compressive strain, with an underlying scotch tape as an in-situ strain generator. It is found that, due to the compressive strain, the superconducting transition temperature Tc ≈ 9 K of FeSe is increased by 30%–40% and the upper critical field Hc2(0) ≈ 14.8 T is increased by ∼ 20%. In parallel, the T*, which characterizes an onset of enhanced spin fluctuations, is raised up from 69 K to 87 K. On the other hand, the structural transition temperature Ts ≈ 94 K, below which an orthorhombic structure and an electronic nematic phase settle in, is suppressed down by ∼ 5 K. These findings reveal clear evolutions of the orders/fluctuations under strain effect in FeSe, the structurally simplest iron-based superconductor where the lattice/spin/charge degrees of freedom are closely coupled to one another. Moreover, the presented research provides a simple and clean way to manipulate the superconductivity in the layered iron compounds and may promote applications in related materials.

Spin-phonon coupling in R2CoMnO6 (R = Pr, Nd, Sm) thin films under biaxial compressive strain

The R2CoMnO6 (R = Pr, Nd, Sm) thin films on (001) LaAlO3 substrates were fabricated successfully by a simple polymer assisted deposition method, confirming by atomic force microscopy, X-ray diffraction, and Raman spectroscopy. The study on the magnetic properties indicates that all the films are B-site ordering and show a single ferromagnetic transition at Curie temperature (Tc) with decreasing temperature. Meanwhile, an anomalous phonon softening related to the spin-phonon coupling is observed at around Tc in all the temperature-dependent Raman spectra. With the decreases of R-site ionic radius (rR) and/or the in-plane biaxial compressive strain, both the turning point TR of the temperature-dependent Δω(T) and Curie temperature Tc decrease, revealing the existence of spin-phonon coupling and its intensity in the decreasing order of Pr2CoMnO6 > Nd2CoMnO6 > Sm2CoMnO6 accompanied with the Co-O-Mn bond angle decreasing and the Co(Mn)-O bond length elongating. It is suggested that the existence of the biaxial compressive strain may be helpful to form the B-site ordered phase and suppress the formation of the disordered phase by decreasing the oxygen vacancies. Both the chemical strain and the biaxial compressive strain are cooperating on the magnetic interaction and spin-phonon coupling in the system, which are of special interest for application.

Hole Effective Mass of Strained Ge1-X Sn x Alloys P-Channel Quantum-Well MOSFETs on (001), (110), and (111) Ge Substrates

- The dependence of effective mass on Sn content, pseudomorphic strain, cap layer thickness, and substrate orientation is theoretically studied for Ge-cap/Ge1-x Sn x /Ge substrate p-channel MOSFETs through six-band k·p method. The Luttinger-like and deformation potential parameters were obtained from the fit of Ge1-x Sn x band structures by the empirical pseudopotential method (EPM). The valence band alignment is theoretically determined by the EPM and the model-solid theory. The high Sn content and thin Ge-cap layer both result the small conductivity mass along <110> of the first subband. Introduction Sn incorporating into Ge makes Ge1-x Sn x alloys as a direct band gap material that is good for nFETs.[1] For Ge1-x Sn x p-channel, several works show the high hole mobility on Ge (001)[2], (110)[3], and (111)[4] substrates. The strain effect to reduce the conductivity mass (m c) might be the main factor for the mobility enhancement.[5] However, there are few theoretical reports to address the dependence. In addition, few works discuss the m c on the Ge1-x Sn x quantum well (QW) heterostructures. In this work, the m c of Ge-cap/Ge1-x Sn x QW/Ge substrate FETs dependent with different Sn content, strain, cap thickness, and substrate orientations is theoretically studied. Models The dependence of valence band structures near the Γ point and effective mass of Ge1-x Sn x alloys along [100], [110], and [111] on Sn content and pseudomorphic strain on (001), (110), and (111) substrates are calculated by the six-band k·p model with the fitted Luttinger-like and deformation potential parameters derived from the band structure calculated by EPM[1]. Note that the fit of band structure of spin-orbital is ingnored for better fit of the other two valence bands. The EPM claculated valence band strucutres (solid line) of relxed Ge (Fig. 1(a)) and Ge0.9Sn0.1 (Fig. 1(b)) and strained Ge (Fig. 1(c)) and Ge0.9Sn0.1 (Fig. 1(d)) under (110) in-plane biaxial compressive strain of -1 % (ε|| = -1%) are compared to the band strucutres calculated by six-band k·p model (dash line). To simulate the p-channel inversion layer of Ge-cap/10 nm Ge1-x Sn x QW layer on (001), (110), and (111) Ge substrates, the six-band k·p model was used in the self-consistently calculations of Poisson and Schrödinger equations.[6] The valence band alignment of the heterostructure is theoretically determined by the EPM and the model-solid theory[7]. The inverse of m c along channel direction <110> of 1stsubband will be calculated as Ref. [6]. Results and Discussions Fig. 2(a) shows the calculated valence band diagram of 1st subband hole distribution of Ge-cap/Ge0.9Sn0.1 QW on Ge (001) substrate at N inv = 5×1012 cm-2 with cap of 4 nm and 2 nm. For the cap of 4 nm, the 1st subband energy (E 1st ) cross the cap and QW layers results the ~30% and ~70% carriers of 1st subband located at cap and QW layer, respectively. Note that the Fermi level energy is set as 0 eV. For the cap of 2 nm, there is only ~10% carriers located at cap and ~90% at QW layer. Fig. 2(b) shows the confined 2D energy contours of the 1st subband of Ge0.9Sn0.1 QW for cap of 4 nm (solid line) and 2 nm (dashed line). The larger of cap of 2 nm than that of 4 nm shows the small m c of Ge-cap/Ge0.9Sn0.1QW/Ge (001) with thin cap layer. For the Sn content up to 15 % (Fig. 2(c)), there is only ~9% and ~5% carriers located at cap layer for cap of 4 nm and 2 nm, respectively, due to the increasing valence band offset by increasing Sn content. The comparable of cap of 2 nm and 4 nm are presented in Fig. 2(d). Conclusion The m c of Ge-cap/Ge1-x Sn x /Ge substrates are theoretically studied through six-band k·p method calibrated by EPM. The m c of 1st subband is composed of the m c of the cap and QW layers due to the small valence band offset with low Sn content. High Sn content and thin cap layer keep the carriers of 1st subband located in the QW layer to result small m c. Reference [1] H.-S. Lan and C. W. Liu, Appl. Phys. Lett., 104, p. 192101, 2014. [2] Dian Lei et al., J. Appl. Phys., 119, p. 024502, 2016. [3] Chunlei Zhan et al., in Symp. VLSI Technol., Dig. Tech. Pap., 2012. [4] Yan Liu et al., Semicond. Sci. Technol., 29, p. 115027, 2014. [5] Suyog Gupta et al., I EEE Electron Device Lett., 34(7), p. 831, 2013. [6] H.-S. Lan et al., ECS Transactions, 50, p. 151-155, 2013. [7] Chris G. Van de Walle, Phys. Rev. B, 39, p. 1871, 1989. Figure 1

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 &amp;gt; 20 nm with a roughness of 1.00 ± 0.05 nm at d = 47.9 nm.

InGaN/GaN light-emitting diode with a polarization tunnel junction

We report InGaN/GaN light-emitting diodes (LED) comprising in situ integrated p+-GaN/InGaN/n+-GaN polarization tunnel junctions. Improved current spreading and carrier tunneling probability were obtained in the proposed device architecture, leading to the enhanced optical output power and external quantum efficiency. Compared to the reference InGaN/GaN LEDs using the conventional p+/n+ tunnel junction, these devices having the polarization tunnel junction show a reduced forward bias, which is attributed to the polarization induced electric fields resulting from the in-plane biaxial compressive strain in the thin InGaN layer sandwiched between the p+-GaN and n+-GaN layers.

DIELECTRIC, PIEZOELECTRIC, AND ELASTIC PROPERTIES OF BaTiO3/SrTiO3 FERROELECTRIC SUPERLATTICES FROM FIRST PRINCIPLES

The effect of epitaxial strain on the phonon spectra, crystal structure, spontaneous polarization, dielectric, piezoelectric, and elastic properties of (001)-oriented ferroelectric ( BaTiO 3)m/( SrTiO 3)n superlattices (m = n = 1-4) was studied using the first-principles density functional theory. The ground state of free-standing superlattices is the monoclinic Cm polar phase. Under the in-plane biaxial compressive strain, it transforms to tetragonal P4mm polar phase, and under the in-plane biaxial tensile strain, it transforms to orthorhombic Amm2 polar phase. When changing the in-plane lattice parameter, a softening of several optical and acoustic modes appears at the boundaries between the polar phases, and corresponding components of dielectric, piezoelectric, and elastic tensors diverge critically. The comparison of the mixing enthalpy of disordered Ba0.5Sr0.5TiO3 solid solution modeled using two special quasirandom structures SQS-4 with the mixing enthalpy of the superlattices reveals a tendency of the BaTiO3–SrTiO3 system to short-range ordering and shows that these superlattices are thermodynamically quite stable.

Platinum Germanosilicide Contacts Formed on Strained and Relaxed $\hbox{Si}_{1 - x}\hbox{Ge}_{x}$ Layers

Contact resistivity is a key contributor to the parasitic series resistance of nanoscale MOSFETs. Since the contact resistivity is an exponential function of the Schottky barrier height, new contact materials that can provide smaller barrier heights to source-drain junctions are needed. Platinum germanosilicide (PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) is of interest as a contact material to the recessed Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> junctions of p-channel MOSFETs due to the large work function of platinum silicide (PtSi). In this paper, we explore the impact of in-plane biaxial compressive strain in Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layers on PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> formation and the impact of the PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> on the strain in Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . The parameters considered in this paper include the Ge content, the thickness of the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> epitaxial layer, and the PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> thickness. The results show that the resistance, surface morphology, and the crystalline structure of the PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> films are independent of the strain in the original Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer. The results also indicate that PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> does not influence the strain in the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer. The barrier-height measurements suggest the presence of Fermi-level pinning, and the pinning position is independent of the strain in the alloy, and it is primarily determined by the Ge concentration. As a result of Fermi-level pinning, hole Schottky barrier height of PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> contacts is 0.1-0.2 eV higher than that of the PtSi-Si contacts.

Thickness dependence of the magnetic anisotropy in La2/3Ca1/3MnO3 thin films grown on LaAlO3 substrates

Magnetic anisotropy of sputtered epitaxial La2/3Ca1/3MnO3 (LCMO) thin films grown on top of LaAlO3 (LAO) substrates has been studied as a function of thickness. A series of films with thickness ranging from 20 to 150 nm have been prepared. LCMO films grown on LAO substrates are subjected to an in-plane biaxial compressive strain that progressively relaxes as film thickness increases as deduced from the variation of the c cell parameter. Nevertheless, some strain is observed even in 150-nm-thick films. The relaxation process is accompanied by an improvement of the magnetic and transport properties approaching that of the bulk material. The observed magnetic easy axis evolves from perpendicular to plane for thin films towards in plane as film thickness increases. Values of the effective anisotropy constant of K1eff≈2.2×107 erg/cm3, at T=10 K, and magnetostriction constant of λ≈2×10−4 are found.

Chemical Ordering and Epitaxy in Relaxor and Ferroelectric PbSc 0.5 Nb 0.5 O 3 Thin Films

Thin films of relaxor ferroelectric PbSc 0.5 Nb 0.5 O 3 were deposited by in situ pulsed laser ablation at 825-975 K onto a La 0.5 Sr 0.5 CoO 3 (001) conducting oxide bottom electrode on MgO(001). Crystalline phases, epitaxial relations, crystal perfection, and chemical B-site ordering were studied by x-ray diffraction θ-2θ-, φ- and y -scans. The perovskite films were pseudocubic and with 'cube-on-cube' epitaxy. At low deposition temperatures, also other epitaxial relations were obtained. All films showed NaCl type (1:1) B-site ordering that did not depend markedly on the deposition temperature. The perfection of epitaxy, in-plane biaxial compressive strain, and lateral dimensions of crystal columns increased with deposition temperature. The correlation between microstructural features of the films and the relaxor-like or ferroelectric-type dielectric behavior of the films in the heterostructures was discussed.

Effects of surface stress on the elastic moduli of thin films and superlattices.

A thermodynamic model which predicts a significant sample-size effect on the elastic properties of very thin films and small-period superlattices is presented. Compressive surface stresses cause the in-plane interatomic distances in a thin metal film to decrease as the thickness decreases. For copper films with a thickness of 0.75 nm, a 1% in-plane biaxial compressive strain is obtained which gives rise to a 50% increase in the biaxial modulus. This model also predicts a similar modulus enhancement (supermodulus effect) in multilayered thin films due to strains caused by incoherent interfacial stress.