不同应变对Ge的光学性质影响的第一性原理研究

研究了不同方向、不同强度的应变对Ge光学性质的影响。结果表明,Ge在单轴张应变和双轴张应变的调控下,均可由间接带隙转向直接带隙,其中,单轴应变有更低的转变点。Ge在常用波段处(0.4 eV)的介电函数实部和虚部在张应变作用下,均急速上升而后在一定应变范围内下降。对Ge进行[111]单轴应变调控能表现出更好的光学性能以及更便捷的器件设计(较低的应变量)。     

数字图像相关中基于位移场局部最小二乘拟合的全场应变测量

为了从含噪声的位移场中计算得到可靠的应变场,提出一种基于位移场局部最小二乘拟合的全场应变求解方法。介绍了数字图像相关方法的原理,阐述了基于位移场局部最小二乘拟合的全场应变求解方法,并讨论了计算区域边界、孔洞及裂纹附近区域等情况下的应变计算。对均匀变形和中心带圆孔的薄铝板拉伸实验的计算结果表明,该方法能有效地从原始位移场数据中提取全场应变信息。在均匀变形情况下应选择大的应变计算窗口,计算结果更逼近真值;在非均匀变形情况下,如果位移场中包含较强的噪声,则应选择较大的应变计算窗口,而位移场精度很高时可选择更小的应变计算窗口。     

Valence band structure and density of states effective mass model of biaxial tensile strained silicon based on k·p theory

After constructing a stress and strain model, the valence bands of in-plane biaxial tensile strained Si is calculated by k·p method. In the paper we calculate the accurate anisotropy valance bands and the splitting energy between light and heavy hole bands. The results show that the valance bands are highly distorted, and the anisotropy is more obvious. To obtain the density of states (DOS) effective mass, which is a very important parameter for device modeling, a DOS effective mass model of biaxial tensile strained Si is constructed based on the valance band calculation. This model can be directly used in the device model of metal-oxide semiconductor field effect transistor (MOSFET). It also a provides valuable reference for biaxial tensile strained silicon MOSFET design.     

Growth of High Quality Strained-Si on Ultra-Thin SiGe-on-Insulator Substrate

Ultra-thin and near-fully relaxed SiCe substrate is fabricated using a modified Ce condensation technique, and then a 25-nm-thiek biaxially tensile strained-Si with a low rms roughness is epitaxially deposited on a SiGe- on-Insulator （SGOI） substrate by ultra high vacuum chemical vapor deposition （UHVCVD）. High-Resolution cross-sectional transmission electron microscope （HR-XTEM） observations reveal that the strained-Si/SiGe layer is dislocation-free and the atoms at the interface are well aligned. Furthermore, secondary ion mass spectrometry （SIMS） results show a sharp interface between layers and a uniform distribution of Ge in the SiCe layer. One percent in-plane tensile strain in the strained-Si layer is confirmed by ultraviolet （UV） Raman spectra, and the stress maintained even after a 30-s rapid thermal annealing （RTA） process at 1000℃. According to those results, devices based on strained-Si are expected to have a better performance than the conventional ones.     

Strained layer quantum well semiconductor optical amplifiers: Polarization insensitive amplification

Polarization insensitive optical amplification was demonstrated in newly developed semiconductor optical amplifiers that have strained GalnAsP quantum well structures. We tailored the active region of the quaternary strained layer quantum well structure with a small biaxially tensile strain of 0.2% in the well layers for polarization insensitive operation.     

Hyperfine fields in ZnO studied under uni- and biaxial pressure

The II-VI semiconductor ZnO has many potential applications in optoelectronic and sensor devices. When used as a transparent conducting contact it is often grown epitaxially onto a different substrate with the consequence that the layers are biaxially strained due to lattice mismatch. Similarly, impurity-implanted layers can lead to the development of local strain fields. Strain usually changes the electronic properties of layers and/ or implanted crystal regions. Detailed knowledge about local strain and its influence on the crystal fields is therefore helpful in predicting changes in crystal properties. The perturbed angular correlation technique was applied to study the electric field gradient (EFG) at the site of implanted In dopants in ZnO under uniaxial and biaxial strain. The observed linear change of the EFG with pressure and a change in symmetry due to compression perpendicular to the c-axis could be well reproduced by theoretical calculations using the point charge model.     

Molecular dynamics simulation of atomic hydrogen diffusion in strained amorphous silica

Understanding hydrogen diffusion in amorphous SiO_2(a-SiO_2),especially under strain,is of prominent importance for improving the reliability of semiconducting devices,such as metal-oxide-semiconductor field effect transistors.In this work,the diffusion of hydrogen atom in a-SiO_2 under strain is simulated by using molecular dynamics(MD) with the ReaxFF force field.A defect-free a-SiO_2 atomic model,of which the local structure parameters accord well with the experimental results,is established.Strain is applied by using the uniaxial tensile method,and the values of maximum strain,ultimate strength,and Young's modulus of the a-SiO_2 model under different tensile rates are calculated.The diffusion of hydrogen atom is simulated by MD with the ReaxFF,and its pathway is identified to be a series of hops among local energy minima.Moreover,the calculated diffusivity and activation energy show their dependence on strain.The diffusivity is substantially enhanced by the tensile strain at a low temperature(below 500 K),but reduced at a high temperature(above500 K).The activation energy decreases as strain increases.Our research shows that the tensile strain can have an influence on hydrogen transportation in a-SiO_2,which may be utilized to improve the reliability of semiconducting devices.     

Strained layer quantum well semiconductor optical amplifiers: Polarization insensitive amplification

Abstract

Polarization insensitive optical amplification was demonstrated in newly developed semiconductor optical amplifiers that have strained GalnAsP quantum well structures. We tailored the active region of the quaternary strained layer quantum well structure with a small biaxially tensile strain of 0.2% in the well layers for polarization insensitive operation.     

Theoretical studies of polarization dependent electro-optical modulation in lattice matched and strained multi-quantum well structures

We report on polarization dependent optical absorption for excitonic and interband transitions in lattice matched (GaAs/AlGaAs) and strained (biaxial tensile strain - GaAsP/AlGaAs; biaxial compressive strain - InGaAs/AlGaAs) multiquantum well structures in the presence of transverse electric fields. The hole states are solved by using the Kohn-Luttinger Hamiltonian and using an eigenvalue technique. The effect of heavy-hole and light-hole mixing due to the strain, electric field and quantization is studied. Under biaxial tensile strain the heavy-hole and light-hole transition can coincide, leading to interesting polarization dependent effects. Results are presented for excitonic and interband transitions.     

Digital image correlation using iterative least squares and pointwise least squares for displacement field and strain field measurements

Digital image correlation (DIC) method using iterative least squares algorithm (ILS) for displacement field measurement and pointwise least squares algorithm (PLS) for strain field measurement is proposed in this paper. A more general and practical intensity change model is employed with consideration of the linear intensity change of the deformed image, followed by an iterative least squares algorithm for calculating displacement field with sub-pixel accuracy. The concept of correlation function is not used in the ILS method, even though we prove that the algorithm is actually equivalent to the optimization of the sum of squared difference correlation function using improved Newton–Raphson method. Besides, different from the conventional strain estimation approaches based on smoothing the displacement fields first and followed by differentiation of the smoothed displacement fields, a simple yet effective PLS algorithm is proposed for extracting strain fields from the computed displacement fields. The effectiveness and accuracy of the proposed techniques is verified through numerical simulation experiments. A practical application of the algorithms to residual plastic deformation field measurement of GH4169 alloy subjected to tensile fatigue is also presented.     

Tuning electronic properties of the S_2/graphene heterojunction by strains from density functional theory

Based on the density functional calculations, the structural and electronic properties of the WS_2/graphene heterojunction under different strains are investigated. The calculated results show that unlike the free mono-layer WS_2, the monolayer WS_2 in the equilibrium WS_2/graphene heterojunctionis characterized by indirect band gap due to the weak van der Waals interaction. The height of the schottky barrier for the WS_2/graphene heterojunction is 0.13 eV, which is lower than the conventional metal/MoS_2 contact. Moreover, the band properties and height of schottky barrier for WS_2/graphene heterojunction can be tuned by strain. It is found that the height of the schottky barrier can be tuned to be near zero under an in-plane compressive strain, and the band gap of the WS_2 in the heterojunction is turned into a direct band gap from the indirect band gap with the increasing schottky barrier height under an in-plane tensile strain. Our calculation results may provide a potential guidance for designing and fabricating the WS_(2~-)based field effect transistors.     

Strain effects on optical properties of pyramidal InAs/GaAs quantum dots

Strain distribution and optical properties in a self-assembled pyramidal InAs/GaAs quantum dot grown by epitaxy are investigated. A model, based on the theory of linear elasticity, is developed to analyze three-dimensional induced strain field. In the model, the capping material in the heterostructure is omitted during the strain analysis to take into account the sequence of the fabrication process. The mismatch of lattice constants is the driving source of the induced strain and is treated as initial strain in the analysis. Once the strain analysis is completed, the capping material is added back to the heterostructure for electronic band calculation. The strain-induced potential is incorporated into the three-dimensional steady-state Schrödinger equation with the aid of Pikus–Bir Hamiltonian with modified Luttinger–Kohn formalism for the electronic band structure calculation. The strain field, the energy levels and wave functions are found numerically by using of a finite element package FEMLAB. The energy levels as well as the wave functions of both conduction and valence bands of quantum dot are calculated. Finally, the transition energy of ground state is also computed. Numerical results reveal that not only the strain field but also all other optical properties from current model show significant difference from the counterparts of the conventional model.     

Structural defects influence on the conductance of strained zigzag graphene nanoribbon

In this paper, we investigate the influence of point structural defects on the transport properties of zigzag graphene nanoribbons (ZGNRs) under uniaxial strain field, using the numerical studies based on the ab-initio calculation, the standard tight-binding model and Green's functions. The calculation results show that the direction of applied strain and defect type significantly affect the conductance properties of ZGNRs. The conductance of the defective nanoribbons generally decreases and some dips corresponding to complete electron backscattering is appeared. This behavior is originated from the different coupling between the conducting electronic states influenced by the wave function modification around the Fermi energy which depends on the defect type. We show that the presence of defects leads to a significant increase in local current. Furthermore, we have investigated the strain-tunable spin transport of defective ZGNRs in the presence of the exchange magnetic field and Rashba spin-orbit coupling (RSOC).     

应变对两层半氢化氮化镓薄膜电磁学性质的调控机理研究

采用基于密度泛函理论的第一性原理模拟计算,研究了在应变作用下两层半氢化氮化镓纳米薄膜的电学和磁学性质.没有表面修饰的两层氮化镓纳米薄膜的原子结构为类石墨结构,并具有间接能隙.然而,当两层氮化镓纳米薄膜的一侧表面镓原子被氢化时,该纳米薄膜却依然保持纤锌矿结构,并且展示出铁磁性半导体特性.在应变作用下,两层半氢化氮化镓纳米薄膜的能隙可进行有效调控,并且它将会由半导体性质可转变为半金属性质或金属性质.这主要是由于应变对表面氮原子的键间交互影响和p-p轨道直接交互影响之间协调作用的结果.该研究成果为实现低维半导体纳米材料的多样化提供了有效的调控手段,为其应用于新型电子纳米器件和自旋电子器件提供重要的理论指导.     

Electronic structure and optical properties of CuAlO2 under biaxial strain

An ab initio calculation has been carried out to investigate the biaxial strain ( - 10.71% < ε < 9.13%) effect on elastic, electronic and optical properties of CuAlO(2). All the elastic constants (c(11), c(12), c(13), c(33)) except c(44) decrease (increase) during tensile (compressive) strain. The band gap is found to decrease in the presence of tensile as well as compressive strain. The relative decrease of the band gap is asymmetric with respect to the sign of the strain. Significant differences between the parallel and perpendicular components of the dielectric constant and the optical properties have been observed due to anisotropic crystal structure. It is further noticed that these properties are easily tunable by strain. Importantly, the collective oscillation of the valence electrons has been identified for light polarized perpendicular to the c-axis. From calculations, it is clear that the tensile strain can enhance the hole mobility as well as the transparency of CuAlO(2).     

Investigation of the effects of tensile strain on optical properties of ZnO nanowire

In this paper, the effects of mechanical tensile strain on optical properties of ZnO nanowire before and after embedding ZnS nanowire were investigated by simulation. Finite element modeling (FEM) software package ABAQUS and three-dimensional (3D) finite-difference time-domain (FDTD) methods were furnished to analyze the problems numerically, including the nonlinear mechanical behavior and optical properties of the sample, respectively. The physical deformation model was imported into the FDTD to investigate optical properties of ZnO nanowire under mechanical tensile strain. Besides, the stress-strain curve via tensile experimental was compared with stress-strain curve that was obtained from finite element modeling. The results disclosed that the mechanical strain was demonstrated to play an important role in determining the optical properties of ZnO nanowire such as absorption coefficient and optical density.     

A new endocytic model based on the co-rotational grid method

Endocytosis plays important roles in many cellular physiological processes, such as metabolism and immune. Many theoretical models have been proposed to study the endocytic process, but little has considered the tensile deformation of the membrane and the actin forces. In this paper, a new endocytic model is proposed based on the co-rotational grid method. Our model gives a direct estimation of the in-plane strain of the plasma membrane and provides a basis for the calculation of further scission process of the vesicle. The results fit well with experimental data in the literature. Moreover, it is suggested that the active forces of actin at the endocytic site is the main mechanism driving the invagination of the plasma membrane.     

First principles calculations for band-gap energy properties of non-polar and semi-polar ternary nitride alloys under in-plane strain

Strain-induced band-gap energies properties of non-polar and semi-polar ternary nitride alloys are investigated by first-principles calculation based on density functional theory. The tensile and compressive strains in non-polar and semi-polar plane of wurtzite structures are analyzed and discussed. From the calculation results, we find that the band-gap energies of both Al_0.5Ga_0.5N and In_0.5Ga_0.5N super-cells under strains in m-plane (1100) are smaller than that in a-plane (1120). In addition, m-plane (1100) Al_0.5Ga_0.5N based optoelectronic device will have more significant shift of emission wavelength than a-plane (1120) and semi-polar plane (1122) with the same strains. The tensile and compressive strains in semi-polar plane have similar magnitude of influence on the emission wavelength of In_0.5Ga_0.5N. The calculations provide a qualitative picture of the strain effects on the band-gap energy.     

体声波力传感器灵敏度的微分-综合分析法

为了分析基于应力/应变效应的体声波(BAW)力传感器的敏感机理、准确计算其灵敏度,提出了一种用于BAW力传感器灵敏度分析的微分-综合分析法。该方法借鉴了微积分的原理,在Mason等效电路模型中将一个完整的BAW谐振器替换为多个谐振器微元的并联,从而将谐振器有源区面积A上应力/应变场的有限元计算结果与压电薄膜材料的力学特性、谐振器微元的电声学特性关联起来;最后,在射频电路仿真软件中进行等效电路的综合,得到整个BAW谐振器在应力/应变场作用下的阻抗特性曲线及其串/并联谐振频率。当BAW谐振器微元的划分足够细密时,获得的灵敏度分析结果将足够精确。为了论证该方法的原理,给出了一个直观的校核案例。以一个嵌入式FBAR结构的四梁BAW加速度计表头为例,介绍了该方法用于BAW力传感器灵敏度分析的详细过程。虽然案例中只讨论了一种应力/应变型BAW力传感器的单一力敏机理,但该方法具有普适性。并且,当谐振器微元小到接近其压电材料晶格的尺度时,就能与压电薄膜的力-声-电特性的第一性原理计算结果关联起来,实现从微观材料特性到介观器件物理的多尺度计算。     

Unusual polarization patterns in flat epitaxial ferroelectric nanoparticles

We investigate the effects of a lattice misfit strain on a ground state and polarization patterns in flat perovskite nanoparticles (nanoislands of BaTiO3 and PZT) with the use of an ab initio derived effective Hamiltonian. We show that the strain strongly controls the balance between the depolarizing field and the polarization anizotropy in determining the equilibrium polarization patterns. Compressive strain favors 180 degrees stripe or tweed domains while a tensile strain leads to in-plane vortex formation, with the unusual intermediate phase(s) where both ordering motifs coexist. The results may allow us to explain contradictions in recent experimental data for ferroelectric nanoparticles.