Application Of Compressive Strain Research Articles

(Invited) Mobility Enhancement Technology of Extremely-Thin Body Ge-on-Insulator Channel Mosfets

An extremely thin body (ETB) channel is mandatory for scaled nano-sheet structures to realize continuous miniaturization of CMOS devices. Here, mobility booster technologies to maintain high channel mobility are strongly needed, because surface roughness scattering significantly reduces mobility in such ETB channels. From this viewpoint, Ge-On-Insulator (GOI) or SiGe-On-Insulator (SGOI) channels are promising because of the low electron and hole effective mass along channel directions. It is theoretically shown that (111) GOI n-MOSFETs can hold high electron mobility even in the body thickness down to 2 nm. Thus, (111) GOI structures and n-MOSFETs are fabricated, and the electrical characteristics are studied. For p-MOSFETs, on the other hand, (110) surface orientation and application of compressive strain are still effective in boosting hole mobility in ETB channels. Here, (110) asymmetric compressive strain SGOI p-MOSFETs are experimentally demonstrated, and the body thickness dependencies of the effective mobility are examined.

Theoretical study on photoelectric properties of ferroelectric photovoltaic perovskite CsGeBr3 based on first-principle calculations

Ferroelectric photovoltaic materials have attracted great attention because of their unique photoelectric conversion mechanism, high photo-generated voltage, and adjustable polarization intensity. Traditional ferroelectric oxide perovskites such as BaTiO3, BiFeO3, and Pb(ZrTi)O3 have attracted much attention but they are not suitable as light absorbing layers in solar cells, due to the large optical bandgap, low light absorption rate, and small photogenerated current. Therefore, it is necessary to seek prominent materials with both ferroelectric and suitable band gaps. Recently, the evidence of ferroelectricity in the typical three-dimensional all-inorganic halide perovskites CsGeX3, with band gaps of 1.6 eV to 2.3 eV has been confirmed. However, the spontaneous polarization of ferroelectric perovskite CsGeX3 is ∼10 to 20 μc cm−2 which is weaker than that of ABO3 (∼26 to 75 μc cm−2). Strain engineering has a significant influence on the properties of semiconductor materials by controlling the lattice scaling and the internal atomic spacing. Hence, in this work, strain engineering is introduced to adjust the ferroelectric polarization and the photoelectric properties of ferroelectric perovskite CsGeBr3. The calculated results show that when the applied compressive strain increases from 0% to −4%, the spontaneous polarization of ferroelectric perovskite CsGeBr3 increases from 14.23 μc cm−2 to 51.61 μc cm−2, and the band gap reduces from 2.3631 eV to 1.5310 eV. The effective mass of electrons and holes gradually reduces, exciton binding energies decrease from 48 meV to 5 meV, and the optical absorption coefficient is strongly enhanced from 3 × 105 cm−1 to 5 × 105 cm−1 in the visible range. Besides, the power conversion efficiency(PCE) of CsGeBr3 is significantly increased from 16.95% to 26.77%. Therefore, the results indicate that the application of compressive strain can increase the ferroelectric polarization and enhance the original photovoltaic performance of ferroelectric perovskite CsGeBr3. Our theoretical calculations can provide useful insights and beneficial guidance into experimental studies of ferroelectric perovskites in photoelectric applications.

Anderson Localization of Phonons in Thermally Superinsulating Graphene Aerogels with Metal-Like Electrical Conductivity.

In the quest to improve energy efficiency and design better thermal insulators, various engineering strategies have been extensively investigated to minimize heat transfer through a material. Yet, the suppression of thermal transport in a material remains elusive because heat can be transferred by multiple energy carriers. Here, the realization of Anderson localization of phonons in a random 3D elastic network of graphene is reported. It is shown that thermal conductivity in a cellular graphene aerogel can be drastically reduced to 0.9mWm-1K-1 by the application of compressive strain while keeping a high metal-like electrical conductivity of 120Sm-1 and ampacity of 0.9A. The experiments reveal that the strain can cause phonon localization over a broad compression range. The remaining heat flow in the material is dominated by charge transport. Conversely, electrical conductivity exhibits a gradual increase with increasing compressive strain, opposite to the thermal conductivity. These results imply that strain engineering provides the ability to independently tune charge and heat transport, establishing a new paradigm for controlling phonon and charge conduction in solids. This approach will enable the development of a new type of high-performance insulation solutions and thermally superinsulating materials with metal-like electrical conductivity.

Prediction of two-dimensional materials: TeXO6 (X = transition metals):structure and electronic properties

Due to the extensive application of two-dimensional (2D) semiconductor materials in spintronics and tunable nano-mechanical devices, many researchers have invested in the research of 2D semiconductor materials, and they have obtained many excellent research results. Utilizing the first-principles calculations and Green’s function, we forecast the monolayer TeXO6 (X = Cr, Fe, Zn) as new 2D materials with various advantageous functional features. The calculation results for TeCrO6, TeFeO6, and TeZnO6 show good stability and promise prospective application potential if the materials are experimental synthesis. With applied strain, TeCrO6 possesses an indirect band gap of 1.096 eV, which is predicted to be strongly manipulabe in this work. We use the density functional theory to investigate the structure and electrical characteristics of the monolayer (X = transition metals). Detailed simulations of their energetics, atomic structures, and electronic structures under the impact of a biaxial strain have been performed. It is discovered that whereas TeCrO6 does not, TeXO6 (with X = Sc, Ti, V, Mn, Fe, Co, Ni, Cu, and Zn) exhibit spin splitting at the ground state. With the application of compressive strain, the electronic band gap narrows. These materials’ band topologies have been discovered to be more susceptible to biaxial stresses. If a strain of 8% is applied, TeFeO6 may change from semimetal to semiconductor. Under baxial strain, TeCrO6 exhibits interesting Van Hove singularities and Mexican-hat-like bands. Due to all these desirable characteristics, 2D TeXO6 is a prospective option for use in a number of technologies, particularly for spintronic and electrical devices.

Self-Powered Sb2Te3/MoS2 Heterojunction Broadband Photodetector on Flexible Substrate from Visible to Near Infrared

Van der Waals (vdWs) heterostructures, assembled by stacking of two-dimensional (2D) crystal layers, have emerged as a promising new material system for high-performance optoelectronic applications, such as thin film transistors, photodetectors, and light-emitters. In this study, we showcase an innovative device that leverages strain-tuning capabilities, utilizing a MoS2/Sb2Te3 vdWs p-n heterojunction architecture designed explicitly for photodetection across the visible to near-infrared spectrum. These heterojunction devices provide ultra-low dark currents as small as 4.3 pA, a robust photoresponsivity of 0.12 A W−1, and reasonable response times characterized by rising and falling durations of 0.197 s and 0.138 s, respectively. These novel devices exhibit remarkable tunability under the application of compressive strain up to 0.3%. The introduction of strain at the heterojunction interface influences the bandgap of the materials, resulting in a significant alteration of the heterojunction’s band structure. This subsequently shifts the detector’s optical absorption properties. The proposed strategy of strain-induced engineering of the stacked 2D crystal materials allows the tuning of the electronic and optical properties of the device. Such a technique enables fine-tuning of the optoelectronic performance of vdWs devices, paving the way for tunable high-performance, low-power consumption applications. This development also holds significant potential for applications in wearable sensor technology and flexible electro-optic circuits.

Bandgap engineering in BP/PtO2 van der Waals (vdW) hetero-bilayer using first-principles study

Based on the motivation of the recent advancement of the van der Waals heterostructure (vdW HBL), we have studied the tunable optoelectronic properties of the two-dimensional (2D) boron phosphide–platinum di-oxide (BP/PtO2) heterostructure using dispersion corrected density functional theory (DFT). Six different stackings are considered to stack the 2D BP upon 2D PtO2 and are tested through DFT. Phonon spectra and binding energy calculation validate the dynamical and chemical stability of the constructed heterostructures. It is found that HBL1, HBL3, and HBL4 have type-II indirect band gaps of 0.001, 0.027, and 0.021 eV, respectively whereas the other HBLs 2, 4, and 5 show a semiconductor–metal transition. The variation in the interlayer distances, cross-plane electric field, and biaxial strain can effectively tune the bandgap, although type-II band alignment remains unaffected in all cases. A large built-in electric field, of ∼15 eV in electrostatic potential between the 2D structures and type-II band alignment of the HBL, suggests efficient separation of charges in all the HBLs. The bandgaps are highly responsive to the interlayer distances, electric field, and biaxial strain in the HBL. It is found that the bandgap increases under the application of compressive strain and an external electric field along the negative z-direction up to −0.4 V/Å. Interestingly, a semiconductor–metal transition occurs for tensile strain and the external electric field along the positive z-direction. All HBLs have efficient optical absorption in the visible and UV portions of the solar spectra, which is highly anticipated for optoelectronics applications. These unrivaled properties of the vdW BP/PtO2 HBL that we have explored make them a promising candidate for nano-electronic devices and infrared detector applications.

Enhanced Electrical Transport and Photoconductivity of ZnO/ZnS Core/Shell Nanowires Based on Piezotronic and Piezo-Phototronic Effects

We report a significant enhancement in the electrical transport and photoconductivity of ZnO/ZnS core/shell nanowires (NWs) compared to those of ZnO NWs via the application of compressive strain. Under a compressive strain of −0.15%, the output current of the ZnO/ZnS core/shell NWs increases by 91.1% compared to that under the no-strain condition, whereas that of the ZnO NWs under the same condition is 42.7%. The significant increase in the output current of the ZnO/ZnS core/shell NWs is attributed to the type-II band alignment and strain-induced piezopotential changes at the junction interface, which induce a reduction in the barrier height to enable efficient charge carrier transport. Furthermore, under UV illumination and a compressive strain of −0.15%, although the photocurrent of the ZnO/ZnS core/shell NWs increases by 4.5 times compared to that of the ZnO NWs, the relative increase in the photocurrent of the ZnO/ZnS core/shell NWs is 11.7% compared to that under the no-strain condition, while the photocurrent of the ZnO NWs increases by 32.3% under the same condition. A decrease in the increase rate in the photocurrent of the ZnO/ZnS core/shell NWs with a change in strain under UV light compared to that under the dark condition can be explained by the piezoelectric screening effect induced by photogenerated carriers. By calculating the change in the Schottky barrier height (SBH), we demonstrate that the piezoelectric potential with a change in strain decreased the SBH, thus increasing the current level. Lastly, we propose a mechanism of the piezotronic and piezo-phototronic effects under applied strain and their effects on energy-band diagrams.

Magnetization Reversal and Domain Structures in Perpendicular Synthetic Antiferromagnets Prepared on Rigid and Flexible Substrates

Ferromagnetic (FM) layers separated by nonmagnetic metallic spacer layers can exhibit Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, which may lead to a stable synthetic antiferromagnetic (SAF) phase. In this article, we have studied magnetization reversal by varying the number of bilayer stacks [Pt/Co] as well as thicknesses of Ir space layer t\(_{Ir}\) on rigid Si(100) and flexible polyimide substrates. The samples with t\(_{Ir}\) = 1.0 nm show a FM coupling, whereas samples with t\(_{Ir}\) = 1.5 nm show an antiferromagnetic (AFM) coupling between the FM layers. At t\(_{Ir}\) = 2.0 nm, it shows a bow-tie shaped hysteresis loop indicating a canting of magnetization at the reversal. Higher anisotropy energy compared to the interlayer exchange coupling (IEC) energy is an indication of the smaller relative angle between the magnetization of lower and upper FM layers. We have also demonstrated the strain-induced modification of IEC as well as magnetization reversal phenomena. The IEC shows a slight decrease upon application of compressive strain and increase upon application of tensile strain, which indicates the potential of SAFs in flexible spintronics.

Strain induced valley degeneracy: a route to the enhancement of thermoelectric properties of monolayer WS2.

Two-dimensional transition metal dichalcogenides show great potential as promising thermoelectric materials due to their lower dimensionality, the unique density of states and quantum confinement of carriers. Here the effects of mechanical strain on the thermoelectric performances of monolayer WS2 have been investigated using density functional theory associated with semiclassical Boltzmann transport theory. The variation of the Seebeck coefficient and band gap with applied strain has followed the same type of trend. For n-type material the relaxation time scaled power factor (S2σ/τ) increases by the application of compressive strain whereas for p-type material it increases with the application of tensile strain due to valley degeneracy. A 77% increase in the power factor has been observed for the n-type material by the application of uniaxial compressive strain. A decrease in lattice thermal conductivity with the increase in temperature causes an almost 40% increase in ZT product under applied uniaxial compressive strain. From the study, it is observed that uniaxial compressive strain is more effective among all types of strain to enhance the thermoelectric performance of monolayer WS2. Such strain induced enhancement of thermoelectric properties in monolayer WS2 could open a new window for the fabrication of high-quality thermoelectric devices.

Structural and Thermoelectric Properties of Nanocrystalline Bismuth Telluride Thin Films Under Compressive and Tensile Strain

To investigate the effect of strain on bismuth telluride films, we applied different compressive and tensile strains to thin films by changing the bending radius of a flexible substrate so the strain ranged from −0.3% (compressive) to +0.3% (tensile). The structural properties of the strained thin films, composed of nanosized grains, were analyzed by x-ray diffraction and scanning electron microscopy. For all samples the main peak was the (015) diffraction peak; crystal orientation along the (015) growth direction was slightly enhanced by application of compressive strain. The thermoelectric properties of strained bismuth telluride thin films were evaluated by measurement of electrical conductivity, Seebeck coefficient, and power factor. The magnitude and direction of the applied strain did not significantly affect the power factor, because when the strain changed from compressive to tensile the electrical conductivity increased and the absolute Seebeck coefficient decreased.

Effect of a Compressive Uniaxial Strain on the Critical Current Density of Grain Boundaries in SuperconductingYBa2Cu3O7−δFilms

The mechanism by which grain boundaries impede current flow in high-temperature superconductors has resisted explanation for over two decades. We provide evidence that the strain fields around grain boundary dislocations in YBa2Cu3O7-delta thin films substantially suppress the local critical current density Jc. The removal of strain from the superconducting grain boundary channels by the application of compressive strain causes a remarkable increase in Jc. Contrary to previous understanding, the strain-free Jc of the grain boundary channels is comparable to the intrinsic Jc of the grains themselves.

Low-temperature stress-assisted germanium-induced crystallization of silicon–germanium alloys on flexible polyethylene terephtalate substrates

The application of mechanical-compressive stress during low-temperature annealing has been investigated for the crystallization of SiGe alloys on plastic substrates. It was observed that crystallization of an amorphous Ge/Cu/Ge “sandwich” can occur at temperatures as low as 130 °C with the application of an equivalent compressive strain of 0.05%. By using this sandwich as a seed for crystallization of an underlying amorphous SiGe film, partial crystallization of the film was observed to occur at a temperature of 180 °C, again under an equivalent compressive strain of 0.05%. Without the application of the compressive strain, crystallization was not observed for either system at the temperatures investigated. The atomic percentage of Si in the SiGe alloy was 35% as confirmed by Rutherford backscattering spectroscopy and the partial crystallization of the SiGe layer was verified by scanning electron microscopy, x-ray diffraction, and transmission-electron microscopy analyses.

Crosslinking density influences the morphology of chondrocytes photoencapsulated in PEG hydrogels during the application of compressive strain

Chondrocyte deformation, which occurs during mechanical loading, is thought to play an important role in the mechanotransduction pathway. In designing a scaffold that can be gelled in situ for cartilage tissue engineering, an important consideration is the influence of mechanical loading. This study tested the hypothesis that changes in the crosslinking density of a hydrogel scaffold influence the morphology of encapsulated chondrocytes in response to an applied load. Chondrocytes were entrapped in photocrosslinkable hydrogel scaffolds based on poly(ethylene glycol) (PEG) with two crosslinking densities, 0.119 and 0.376 mol/l, with the higher density having a 11-fold higher compressive modulus. The cell-embedded hydrogels were subjected to static compressive strains between 0% and 20% after 1 and 6 days of culture. Using confocal laser scanning microscopy, chondrocytes in the highly crosslinked gel at day 1 deformed more than gels in the more loosely crosslinked gel. By day 6, this finding was reversed. When single cells within a region were followed, heterogeneities in cell deformation were observed on both a macroscopic and microscopic scale. These heterogeneities were greater in the highly crosslinked gel. These findings demonstrate that different levels of cell deformation and heterogeneity may be obtained by varying the crosslinking density in PEG hydrogels.

The Effect of Compressive Strain and Stress on Electrical Conductivity of Conductive Rubber Composites

Abstract The electrical conductivity changes significantly when compressive strain and stress are applied on the conductive rubber composites derived from acrylonitrile—butadine rubber (NBR), ethylene—propylene—diene rubber (EPDM) and their 50:50 blend. The resistivity increases during application of compressive strain. However, the change of electrical resistivity depends on strain amplitude, amount of filler loading and type of polymer matrix. The change of resistivity with time for compressed samples has also been registered. But, during application of compressive stress (pressure), the resistivity is found to decrease for NBR and increase for EPDM and dual behavior is observed in blend. All the increase and decrease of resistivity with the application of stress and strain is explained on the basis of formation and destruction of a conductive network, which further depends on viscosity (stiffness) of the matrices. The observations are supported by the similar experiments with conductive silicone rubber composites.