Effect Of Uniaxial Strain Research Articles

Strain-Dependent Polar Optical Phonon Scattering and Drive Current Optimization in Nanoscale Monolayer MoS2 FETs

Recently, microelectromechanical system has been used to dynamically strain atomically thin materials including MoS2. While strain can significantly modulate the electronic and phonon structures of monolayer MoS2, its impact on electron transport, especially in the dissipative regime, has not been well explored. In this paper, using a three-dimensional particle-based quantum-corrected Monte Carlo device simulator, the effects of uniaxial and biaxial strain on room-temperature electron transport in a model monolayer molybdenum disulphide (MoS2) based field-effect transistor have been investigated. In the beginning, the simulator has been validated against recently published experimental results. Overall, strain in monolayer MoS2 strongly affects the polar optical phonon modes as well as the electronic bandstructure. Uniaxial strain breaks the degeneracy of E′ Raman mode and results in phonon softening. In this case, our results show that, for both E+ and E− Raman modes, ON current first increases for up to 3.7% of applied strain and then decreases as the strain is increased further. As for biaxial strain, we consider the effects of both tensile and compressive stresses. We find that the application of biaxial tensile strain boosts the ON current for up to 4% of strain. Especially, biaxial tensile strain leads to ~ 15.56% increase in the ON current, which is highest for any type of applied stress.

Ab initio calculations on oxygen vacancy defects in strained amorphous silica**Project supported by the Science Challenge Project, China (Grant No. TZ2016003-1-105) and the CAEP Microsystem and THz Science and Technology Foundation (Grant No. CAT201501).

The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica (a-SiO2) are systematically investigated using ab-initio calculation based on density functional theory. Four types of positively charged oxygen vacancy defects, namely the dimer, unpuckered, and puckered four-fold (4 ×), and puckered five-fold (5 ×) configurations have been investigated. It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures, which can be identified from the fluctuations of the curves of relative total energy versus strain. Driven by strain, a positively charged dimer configuration may relax into a puckered 5× configuration, and an unpuckered configuration may relax into either a puckered 4× configuration or a forward-oriented configuration. Accordingly, the Fermi contacts of the defects remarkably increase and the defect levels shift under strain. The Fermi contacts of the puckered configurations also increase under strain to the values close to that of center in a-SiO2. In addition, it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain, that is, those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain, both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap. As strain induces more puckered configurations with the transition levels in Si band gap, it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation. This work sheds a light on understanding the strain effect on ionization damage at an atomic scale.

Effect of Uniaxial Tensile Strains at Different Orientations on the Characteristics of AlGaN/GaN High-Electron-Mobility Transistors

The influence of uniaxial tensile strains with different orientations to the conduction channel on the electrical and physical properties of AlGaN/GaN high-electron-mobility transistors (HEMTs) was investigated. The output current decreases with the increase of the tensile strain as the orientation of strain changes from 0° to 90° with respect to the conduction channel. The results of the measured ${C}$ – ${V}$ curves show that for the same strain level the two-dimensional electron gas (2DEG) density decreases monotonically to different extents depending on the strain orientation. A conversion of the strain at different orientations to an equivalent strain parallel to the conduction channel shows that the theoretical resultant changes of the 2DEG density are consistent with the experimental results. The corresponding electron mobility is also calculated, which shows a decreasing trend under the tensile strain. Furthermore, using the transient current method, it is established that the detrapping time constant increases as a result of the tensile strain, which is ascribed to the movement of the trap level away from the conduction band.

Topological Features in a Chiral p-wave Superconductor under Uniaxial Strain

Uniaxial strain effects in the superconducting phase are investigated assuming chiral p-wave pairing for the unconventional superconductor Sr2RuO4. The Fermi surface structure of the γ-band is espe...

Tunable electronic properties of the dynamically stable layered mineral Pt2HgSe3 (Jacutingaite).

Density functional theory calculations are performed in order to study the structural and electronic properties of monolayer Pt2HgSe3. Our results show that the dynamically stable monolayer Pt2HgSe3 is a topological insulator with a band gap of 160 meV. In addition, the effect of layer thickness, strain and electric field on the electronic properties are systematically investigated using fully relativistic calculations. We find that the electronic properties are sensitive to the applied electric field. With increasing electric field strength up to 0.5 V Å-1, the band gap decreases from 160 to 10 meV at 0.5 V Å-1. Interestingly, upon further increasing the electric field up to 1.0 V Å-1, the band gap opens again and reaches its bare value (160 meV) at 1.0 V Å-1, which indicates that the band gap is reversibly controllable via the applied external electric field. Moreover, the electronic properties are also examined under uniaxial and biaxial strain. Our results reveal that the band gap value can be tuned to 150 meV (at 1%) and to 92 meV (at 6%) under uniaxial strain, while under biaxial tensile strain, it increases to 170 meV at 5% and fluctuates between 150 and 100 meV in the range of 5-10%. In contrast, the biaxial-compressive strain is found to drive the semiconducting-to-metallic transition for sufficiently large compressions (over 8%). On the other hand, we find that increasing the thickness of Pt2HgSe3 modifies the band gap to 150 meV (for the bilayer) and 140 meV (for the trilayer). In the bilayer Pt2HgSe3 structure, we further investigated the effect of out-of-plane pressure, both compressive and tensile, and our results show that the electronic structure of bilayer Pt2HgSe3 is largely preserved. Our study provides new insight into the modification of the electronic structure of monolayer Pt2HgSe3 upon application of external fields and variation in the layer thickness.

Changing the magnetic states of an Fe/BaTiO3 interface through crystal field effects controlled by strain.

The search for better and inexpensive magnetoelectric materials is now commonplace in solid state physics. Intense coupling between technologically viable electric and magnetic properties, embedded in a single material, is still an attribute greatly pursued by the scientific community. Following this line of thought, using DFT, the study of a specific interface between the TiO2 layer of BaTiO3 and a monolayer of Fe atoms is presented, probing different uni-axial strain effects of the considered supercell. Depending on the strain, several different metastable magnetic states are predicted: a perfectly balanced antiferromagnetic state, an unbalanced ferrimagnetic state, a ferromagnetic state, and a non-magnetic state where each atom has its total magnetic moment quenched. Since these multiple magnetic states can be reversibly controlled by strain, under optimized conditions, this interface can switch from the ferromagnetic state (μ≈ 2.2 μB per Featom) to the non-magnetic state (μ = 0 μB per Featom), enabling enticing prospects for technological applications.

Effects of external mechanical or magnetic fields and defects on electronic and transport properties of graphene

We report on the results obtained modelling the electronic and transport properties of single-layer graphene subjected to mechanical or magnetic fields and containing point defects. Reviewing, analyzing, and generalizing our findings, we claim that effects of uniaxial tensile strain or shear deformation along with their combination as well as structural imperfections (defects) can be useful for achieving the new level of functionalization of graphene, viz. for tailoring its electrotransport properties via tuning its band gap value as much as it is enough to achieve the graphene transformation from the zero-band-gap semimetal into the semiconductor and even reach the gap values that are substantially higher than for some materials (including silicon) typically used in nanoelectronic devices.

Influence of uniaxial strain on the electronic properties of doped graphene mono-sheets: a theoretical study

In this study, we investigated the effect of uniaxial strain (compression and elongation) on the band gap values of pristine graphene mono-sheet as well as boron-doped, nitrogen-doped, and boron-nitrogen co-doped graphene mono-sheet. Although a clear band gap opening was observed in all graphene mono-sheet structures, the rate of band gap opening in pristine graphene mono-sheet is higher than the rate of band gap opening in doped graphene mono-sheet with symmetric behaviour was observed when the applied strain is in the armchair direction. However, a clear discrepancy was observed in the calculated band gap values when the applied strain is in the zigzag direction. Additionally, we found that the rate of change in the Fermi energy with respect to the applied strain depends crucially not only on the dopant type but also on the strain type whether it is compression or elongation.

Strain effects on the electronic and optical properties of Van der Waals heterostructure MoS2/WS2: A first-principles study

Abstract In the present work, the effect of biaxial and uniaxial strains on the electronic and optical properties of heterostructure MoS2/WS2 has been studied by density functional theory. Our numerical calculations demonstrate that the heterostructure MoS2/WS2 is an indirect semiconductor with a type-II band alignment and its energy gap is 1.45 eV. Besides, the semiconductor–metal transition can be found at large biaxial strain of 14% in the van der Waals heterostructure MoS2/WS2. Influence of the strain engineering on the optical spectra of the heterostructure MoS2/WS2 are also considered in this work. Our calculations indicate that, while the effect of uniaxial strain on the optical properties is quite weak, the optical parameters of the heterostructure MoS2/WS2 depend strongly on the biaxial strain. Our predicted results may be a reference for future experimental and theoretical studies of the heterostructures.

Strain effects on the quantum capacitance of graphene nanoribbon devices

One of the most important characteristics of graphene-based nanodevices is the quantum capacitance (QC) of the channel which plays a dominant role in governing the device performance. Moreover, QC can be utilized to realize various types of high performance sensors and energy storage devices. In the present work, the effect of in-plane uniaxial strain on the QC of GNR-based devices is investigated utilizing a compact analytical model which accounts for several finite-size and edge effects. Carrier density and QC are calculated for the three distinct families of armchair GNRs. The strong modulation of QC by uniaxial strain is related to strain-induced changes in both bandgap and effective mass of GNRs. Our study could be helpful for designing GNR-based nanodevices in the Quantum Capacitance Limit.

Soliton Photoconduction in the Charge-Density-Wave Conductor Orthorhombic TaS3

The effect of uniaxial strain on low-temperature conduction and photoconduction of the Peierls conductor orthorhombic TaS3 is studied. A consistent increase in the conductance and photoconductance at T 60 K, was found, contradicting the collisional recombination model, which well describes photoconduction in unstrained samples. The increase in conductance is accompanied by an increase in the threshold field for onset of charge-density-wave sliding, ET, and the relationship between ET and the conductance is similar to that observed in the case of photoconduction. This relationship is caused by a change in the screening conditions of the charge density wave owing to a change in the current carrier concentration. A change in the character of the charge-density-wave pinning from three-dimensional at T 60 K to one-dimensional at T 45 K under e ≈ 0.5% elongation is observed. The effects are explained by the fact that under the strain, the contribution of solitons to both low-temperature conduction and photoconduction becomes dominant.

Displacement Cascades in Monocrystalline Silicon: Effects of Temperature, Strain, and PKA Energy

With large-scale molecular dynamics, we investigate displacement cascades in monocrystalline silicon with regard to the effects of temperature, strain, and primary knock-on atom energy on defect generation and evolution. With temperature increasing, both the thermal spike region and the peak defect count increase, while the effect of temperature on the surviving defect number is negligible. Nevertheless, higher temperature shows negative effect on clustering of vacancy. The effects of uniaxial strain on defect production and clustering is negligible, while its hydrostatic counterpart is evident. With the increment of hydrostatic strain, both the peak and surviving defect count increase (decrease) under tensile (compressive) hydrostatic loading. Meantime, tensile hydrostatic strain will promote defect clustering. More defects and larger defect clusters are produced at higher energy. Otherwise, interstitials are hard to form clusters under different conditions.

Graphene Quantum Strain Transistors

There is a wide range of science and applications accessible via the strain engineering of quantum transport in 2D materials. We propose a realistic experimental platform for uniaxial strain engineering of ballistic charge transport in graphene. We then develop an applied theoretical model, based on this platform, to calculate charge conductivity and demonstrate graphene quantum strain transistors (GQSTs). We define GQSTs as mechanically strained ballistic graphene transistors with on/off conductivity ratios $> 10^{4}$, and which can be operated via modest gate voltages. Such devices would permit excellent transistor operations in pristine graphene, where there is no band gap. We consider all dominant uniaxial strain effects on conductivity, while including experimental considerations to guide the realization of the proposal. We predict multiple strain-tunable transport signatures, and demonstrate that a broad range of realistic device parameters lead to robust GQSTs. These devices could find applications in flexible electronic transistors, strain sensors, and valleytronics.

Assessing optoelectronic properties of PbI2 monolayer under uniaxial strain from first principles calculations

Abstract In this work, the effect of vertical uniaxial strain on the electronic and optical properties of PbI2 monolayer was investigated by means of the first-principles calculations with full-potential linearized augmented plane-wave (FP-LAPW) method. The strain is applied by varying the thickness of monolayer. Our calculations asserted that unstrained PbI2 monolayer has a wide indirect band gap of 2.476 eV. The valence band and conduction band are mainly dominated by the I-5p and Pb-6p states, respectively. The band gap increases as the strains go from −12% to 12%, but this parameter shows a higher dependence on the compressive strain than on the tensile strain. Also, the optical properties of PbI2 monolayer depend strongly on the applied strain, especially in the low energy range up to deep ultraviolet. Interestingly, while the compressive strains decrease the band gap and the absorption coefficient of considered material, they can widen the absorption band, that is the capability of PbI2 monolayer of absorbing a wider range of light.

Effects of uniaxial tensile strain on the electron–phonon scattering limited carrier mobility in an n-type monolayer MoS2 at room temperature: first-principles calculations

In the present work, we theoretically study the strain effect on the room temperature mobility (RTM) of a n-type Monolayer MoS2 limited by electron–phonon (e–ph) scattering. Our numerical results indicate that the RTM along zigzag direction of such a 2D material can be efficiently modulated by a uniaxial tensile strain. Such an RTM, denoted as , has a sizable reduction (enhancement) as a moderate tensile strain is applied in a parallel (perpendicular) direction. For example, when the strain strength amounts to 7%, the two distinct cases of the strain applied in x and y directions differ from each other by roughly two times. In contrast, the RTM in armchair direction is not so sensitive to a tensile strain. The underlying mechanism for such a strain effect on the mobility is then analysed in depth. Our results are obtained completely on the level of first-principles calculations, free from any empirical simplifications. Therefore, our above findings provide reliable and detailed information for experimentally manipulating the RTM of a n-type monolayer MoS2 by simply stretching the sample.

First-principles analysis of structural stability, electronic and phonon transport properties of lateral MoS2-WX2 heterostructures

We performed first-principles-based methods to study the structural stability, electronic and phonon transport properties of lateral transition metal dichalcogenides. Specifically, we focused on the interface at the MoS2-WX2 heterostructures, where X = S or Se. The structures underwent pseudo uniaxial strain testing for compression and tension from 0 to 10% at 2% intervals. The electronic and phonon densities of states were calculated at each interval in comparison with the unstrained structure. Computational results provide insight into the effect of uniaxial strain on structure, electronic and phonon transport processes, causing a crucial impact of use of the materials in electronic devices. In addition, combining the calculated force constants with the atomistic Green's function method reveals interfacial thermal transport at the heterostructure and its underlying phonon mechanisms.

Effects of uniaxial strain on the performance of armchair graphene nanoribbon resonant tunneling diode

Electronic performance of armchair graphene nanoribbon (AGNR) resonant tunneling diodes (RTDs) is influenced by strain effects, when they are mounted on the stretchable substrates or mechanically deformed due to real working conditions. Therefore, it is important to investigate how uniaxial strain can impact the performance of AGNR RTDs. In this paper, two platforms of AGNR RTD namely width-modified AGNR RTD and field-modified AGNR RTD are introduced and they are under both compressive and tensile uniaxial strain. It is found that the characteristics of AGNR RTD change considerably under either compressive or tensile strain. In particular, peak to valley ratio (PVR) can be totally deteriorated upon strong enough whole-body strain. However, local strain in the channel and barrier regions, in contrast to whole-body strain, can even improve the efficiency of AGNR RTD devices. Furthermore, the behavior of strained AGNR RTD is investigated while the width of device is modified. Numerical tight binding coupled with non-equilibrium Green’s function is derived for this study to calculate corresponding Hamiltonian matrices and transport properties.

A metal-semiconductor-metal transform appeared in the metallic single-walled carbon nanotubes, controlled by the combined effect of magnetic field and uniaxial strain

As known to us, the metallic zigzag tubes will open a band gap by applying the magnetic field or uniaxial strain. The exciton states of the nearest band to Fermi level (M00) in the metallic carbon nanotubes are calculated by a tight-binding model, under the combined effect of the magnetic field and uniaxial strain. It is found that the excitation energy and binding energy of the metallic zigzag carbon nanotube (24, 0) have the similar variation tendency, which are split into M00+ and M00− excitons by the applied magnetic field and strain. When the uniaxial strain is applied, for M00+, it increases with increasing magnetic field, while for M00−, it decreases. However, it is interesting to find that as the magnetic field increases, the M00− exciton will decrease to be zero and a metal-semiconductor-metal change will appear, which is expected to be confirmed by experiments. The obtained result provides some useful information for future THz applications by controlling the magnetic field and uniaxial strain.

The effect of channel uniaxial strain on thermal conductivity of graphene nano-ribbon field effect transistor

The effect of channel uniaxial strain on thermal conductivity of graphene nano-ribbon field effect transistor (GNRFET) is analyzed by self-consistent Hückel method. The supposed strains are tensile and its values are 2% to 24% of lattice constant. All of the assumed strains are applied to the channel length direction. Energy band gap, density of states (DOS), phonon transmission, thermal conductivity, and I–V characteristics of the GNRFET have been calculated. The results show that by increasing the strain, the energy band gap of the channel is increased and the drain current is decreased. Also by increasing the band gap, phonon transmission is decreased. Maximum phonon transmission occurs in 8% strain. By considering all of these parameters, the results show that there is a maximum thermal conductivity versus temperature in 8% uniaxial strain that is more than the bare one and its value is decreased intensively in 16% and 24% strain. This is due to maximum phonon transmission that is observed in 8% strain and increasing the DOS around the energy band gap in this value. Also, it is observed that in the energy range of more than 0.75 eV, by increasing the strain, thermal conductivity is increased.

Valley engineering by strain in Kekulé-distorted graphene

A Kekul\'e bond texture in graphene modifies the electronic band structure by folding the Brillouin zone and bringing the two inequivalent Dirac points to the center. This can result, in the opening of a gap (Kek-O) or the locking of the valley degree of freedom with the direction of motion (Kek-Y). We analyze the effects of uniaxial strain on the band structure of Kekul\'e-distorted graphene for both textures. Using a tight-binding approach, we introduce strain by considering the hopping renormalization and corresponding geometrical modifications of the Brillouin zone. We numerically evaluate the dispersion relation and present analytical expressions for the low-energy limit. Our results indicate the emergence of a Zeeman-like term due to the coupling of the pseudospin with the pseudomagnetic strain potential which separates the valleys by moving them in opposite directions away from the center of the Brillouin zone. For the Kek-O phase, this results in a competition between the Kekul\'e parameter that opens a gap and the magnitude of strain which closes it. While for the Kek-Y phase, in a superposition of two shifted Dirac cones. As the Dirac cones are much closer in the supperlattice reciprocal space that in pristine graphene, we propose strain as a control parameter for intervalley scattering.