Mobility enhancement of strained GaSb p-channel metal oxide semiconductor field-effect transistors with biaxial compressive strain

Various biaxial compressive strained GaSb p-channel metal–oxide–semiconductor field-effect transistors(MOSFETs)are experimentally and theoretically investigated. The biaxial compressive strained GaSb MOSFETs show a high peak mobility of 638 cm~2/V·s, which is 3.86 times of the extracted mobility of the fabricated GaSb MOSFETs without strain.Meanwhile, first principles calculations show that the hole effective mass of Ga Sb depends on the biaxial compressive strain.The biaxial compressive strain brings a remarkable enhancement of the hole mobility caused by a significant reduction in the hole effective mass due to the modulation of the valence bands.     

Substrate-induced stress in silicon nanocrystal/SiO₂ multilayer structures

A Raman frequency upshift in the nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in the Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO2 (3 nm) layers are deposited by high-vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N2 atmosphere at 1100°C for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high growth temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in the thermal expansion coefficient between the substrate and the Si/SiO2 SL film. Such a substrate-induced stress indicates a new method for tuning the optical and electronic properties of Si nanocrystals for strained engineering.     

Substrate-induced stress in silicon nanocrystal/SiO2 multilayer structure

A Raman frequency upshift of nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO₂ (3 nm) layers are deposited by high vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N₂ atmosphere at 1100 ℃ for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high grown temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in thermal expansion coefficient between the substrate and the Si/SiO₂ SL film. Such a substrate-induced stress indicates a new method to tune the optical and the electronic properties of Si nanocrystals for strained engineering.     

Uniaxial strain-modulated electronic structures of CdX (X = S,Se, Te) from first-principles calculations:A comparison between bulk and nanowires

Using first-principles calculations based on density functional theory, we systematically study the structural deformation and electronic properties of wurtzite CdX(X = S, Se, Te) bulk and nanowires(NWs) under uniaxial [0001] strain. Due to the intrinsic shrinking strain induced by surface contraction, large NWs with {10ˉ10} facets have heavy hole(HH)-like valence band maximum(VBM) states, while NWs with {11ˉ20} facets have crystal hole(CH)-like VBM states. The external uniaxial strain induces an HH–CH band crossing at a critical strain for both bulk and NWs, resulting in nonlinear variations in band gap and hole effective mass at VBM. Unlike the bulk phase, the critical strain of NWs highly depends on the character of the VBM state in the unstrained case, which is closely related to the size and facet of NWs. The critical strain of bulk is at compressive range, while the critical strain of NWs with HH-like and CH-like VBM appears at compressive and tensile strain, respectively. Due to the HH–CH band crossing, the charge distribution of the VBM state in NWs can also be tuned by the external uniaxial strain. Despite the complication of the VBM state, the electron effective mass at conduction band minimum(CBM) of NWs shows a linear relation with the CBM–HH energy difference, the same as the bulk material.     

Strain-tuned magnetic properties in (Ga,Fe)Sb:First-principles study

In view of the importance of enhancing ferromagnetic(FM) coupling in dilute magnetic semiconductors(DMSs),the effects of strain on the electronic structures and magnetic properties of(Ga,Fe)Sb were examined by a first-principles study.The results of the investigation indicate that Fe_(Ga) substitution takes place in the low-spin state(LSS) with a total magnetic moment of 1μB in the strain range of-3% to 0.5%,which transitions to the high-spin state(HSS) with a total magnetic moment of 5μB as the strain changes from 0.6% to 3%.We attribute the changes in the amount and distribution of the total moment to the influence of the crystal field under different strains.The FM coupling is strongest under a strain of about0.5%,but gradually becomes weaker with increasing compressive and tensile strains.The magnetic coupling mechanism is discussed in detail.Our results highlight the important contribution of strain to magnetic moment and FM interaction intensity,and present an interesting avenue for the future design of high Curie temperature(T_C) materials in the(Ga,Fe)Sb system.     

Impact of GaNAs strain compensation layer on the electronic structure of InAs/GaAs quantum dots

The strain and electron energy levels of InAs/GaAs(001) quantum dots (QDs) with a GaNAs strain compensation layer (SCL) are investigated. The results show that both the hydrostatic and biaxial strain inside the QDs with a GaNAs SCL are reduced compared with those with GaAs capping layers. Moreover, most of the compressive strain in the growth surface is compensated by the tensile strain of the GaNAs SCL, which implies that the influence of the strain environment of underlying QDs upon the next-layer QDs’ growth surface is weak and suggests that the homogeneity and density of QDs can be improved. Our results are consistent with the published experimental literature. A GaNAs SCL is shown to influence the strain and band edge. As is known, the strain and the band offset affect the electronic structure, which shows that the SCL is proved to be useful to tailor the emission wavelength of QDs. Our research helps to better understand how the strain compensation technology can be applied to the growth of stacked QDs, which are useful in solar cells and laser devices.     

Chemisorption of Fe on Au-passivated Si（001） surface

The chemisorption of one monolayer of Fe atoms on a Au-passivated Si(001) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The Fe adatom chemisorption on an ideal Si(001) surface is also considered for comparison. The chemisorption energy and layer projected density of states for a monolayer of Fe atoms on Au-passivated Si(001) surface are calculated and compared with that of the Fe atoms on an ideal Si(001) surface. The charge transfer is investigated. It is found that the most stable position is at the fourfold hollow site for the adsorbed Fe atoms, which might sit below the Au surface. Therefore there will be a Au－Fe mixed layer at the Fe/Au－Si(100) interface. It is found that the adsorbed Fe atoms cannot sit below the Si surface, indicating that a buffer layer of Au atoms may hinder the intermixing of Fe atoms and Si atoms at the Fe/Au－Si(001) interface effectively, which is in agreement with the experimental results.     

Microstructure and Mechanical Properties of Ti3SiC2 Irradiated by Carbon Ions

Thanks to its noteworthy mechanical properties, excellent damage tolerance and good thermal stability, the Ti3SiC2 ternary compound has attracted great concern and has been considered as a potential structural component material for the 4th generation of reactors （e.g., gas fast nuclear reactors） and future fusion reactors. The outstanding properties are due to the nanolameIlar structure which imparts characteristics of both metals and ceramics to this material In our work, Ti3SiC2 samples have been irradiated by C^＋ ions with a high fluence of 1.78 × 10^17 ions/cm^2 at a range of temperatures from 120℃～850℃. Subsequently, series of characterization techniques including synchrotron irradiation x-ray diffraction, scanning electron microscopy and nano-indentation are carried out to understand the changes of microstructure and mechanical properties. The composition exhibits high damage tolerant properties and a high recovery rate through the analysis, especially at high temperature. The minimum damage to an irradiated sample appears around 350℃ in the temperature range 120℃-550℃. At a high irradiation temperature, a significant reduction in the damage can be achieved and an almost complete lack of damage compared with an un-irradiated sample is revealed at the temperature of 850℃.     

Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters,such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition,using an in-plane grazing incidence x-ray diffraction technique.The results are analysed and compared with a complementary out-of-plane xray diffraction technique.The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method.The method for directly determining the in-plane lattice parameters of the GaN layers is also presented.Combined with the biaxial strain model,it derives the lattice parameters corresponding to fully relaxed GaN films.The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established,reaching to a maximum level of-0.89 GPa.     

Electron–acoustic phonon interaction and mobility in stressed rectangular silicon nanowires

We investigate the effects of pre-stress and surface tension on the electron–acoustic phonon scattering rate and the mobility of rectangular silicon nanowires.With the elastic theory and the interaction Hamiltonian for the deformation potential,which considers both the surface energy and the acoustoelastic effects,the phonon dispersion relation for a stressed nanowire under spatial confinement is derived.The subsequent analysis indicates that both surface tension and pre-stress can dramatically change the electron–acoustic phonon interaction.Under a negative(positive)surface tension and a tensile(compressive)pre-stress,the electron mobility is reduced(enhanced)due to the decrease(increase)of the phonon energy as well as the deformation-potential scattering rate.This study suggests an alternative approach based on the strain engineering to tune the speed and the drive current of low-dimensional electronic devices.     

无铅四方相钙钛矿短周期超晶格压电效应机理研究

超晶格压电行为与内部正离子之间的内在联系尚缺乏相关的研究.本文基于密度泛函理论的第一性原理方法,研究了三种无铅四方相钙钛矿铁电超晶格(BaTiO_3/SrTiO_3,KNbO_3/KTaO_3和BaTiO_3/KNbO_3)中A,B位正离子对整体的极化和压电贡献.通过计算超晶格不同轴向应变条件下原子结构和Born有效电荷,获得了超晶格和各个正离子的极化值和压电系数.结果表明,在轴向压缩应变条件下(-0.15—0 A),无铅超晶格中的正离子位移D(A)和D(B)受到抑制,在拉应变时位移才显著增大,因此极化和压电行为不明显.在轴向拉伸应变作用下(0—0.15 A),无铅超晶格中各原子的极化贡献显著增大,特别是B位原子Ti,Nb和Ta的极化贡献使得总的极化强度也显著提高,并当拉应变达到一定值,超晶格才会出现明显的压电行为.无铅超晶格的极化和压电行为主要由B位正离子贡献.     

应力调控BlueP/XTe2(X=Mo,W)范德瓦耳斯异质结电子结构及光学性质理论研究

通过第一性原理计算探讨了蓝磷烯与过渡金属硫化物MoTe₂/WTe₂形成范德瓦耳斯异质结的电子结构和光学性质,以及施加双轴应力对相关性质的影响.计算结果表明,形成BlueP/XTe₂(X=Mo,W)异质结,二者能带排列为间接带隙type-Ⅱ并有较强的红外光吸收,同时屏蔽特性增强.随压缩应力增加,BlueP/XTe₂转变为直接带隙type-Ⅱ能带排列最后转变为金属性;随拉伸应力增加,异质结转变为间接带隙type-Ⅰ能带排列.外加应力也能有效调控异质结的光吸收性质,随压缩应力增加吸收边红移,光吸收响应拓展至中红外光谱区且吸收系数增大;BlueP/MoTe₂较BlueP/WTe₂在中红外至红外光区间表现出更强的光吸收响应;静态介电常数ε1(0)大幅增加.结果表明,压缩应力对BlueP/MoTe₂和BlueP/WTe₂能带排列、光吸收特性均有显著的调控作用,其中BlueP/MoTe₂对调控更敏感,这些特性也使BlueP/XTe₂异质结在窄禁带中红外半导体材料及光电器件具有令人期待的应用价值.     

外延压应变对BaTiO$lt;sub$gt;3$lt;/sub$gt;铁电体抗辐射性能影响的分子动力学研究

采用基于壳模型的分子动力学模拟方法, 研究了存在外延压应变时BaTiO₃铁电体的辐射位移效应, 以O原子作为初冲原子(primary knock-on atom, PKA), 能量为1 keV, 方向为[001], 分别计算了外延压应变为0, 0.4%, 0.8%, 1.2%, 1.6%, 2.0%时体系的缺陷数量、分布, 以及辐射前后的极化强度, 比较了压应变为2%以及无应变下损伤区域、缺陷离位距离和反向外电场下PKA的迁移距离. 结果表明, 随外延压应变增加体系极化近似线性增加, 辐射后极化降低幅度降低、缺陷产生的数量有所减小, 2% 压应变存在时缺陷原子的离位距离、PKA在反向外电场作用下的迁移距离和损伤区域都小于无应变的情况, 说明外延压应变的存在对辐射造成的晶格损伤具有抑制作用, 对辐射损伤具有改善作用, 可以通过引入外延压应变来调控BaTiO₃的辐射损伤. 关键词： 应变 3')" href="#">BaTiO₃ 辐射损伤 分子动力学模拟     

应力作用下EuTiO3铁电薄膜电热效应的唯象理论研究

基于Laudau-Devonshire的热动力学模型, 计算了EuTiO₃铁电薄膜材料的电热效应. 结果显示在外加应力的调控下, 电极化、电热系数以及绝热温差都会随之变化. 外加垂直于表面的张应力加大, 薄膜的相变温度升高, 绝热温差增加, 最大绝热温差所对应的工作温度向高温区移动. 对于二维平面失配应变u_m =-0.005的薄膜, 当外加张应力σ₃ = 5 GPa时, 其最大电热系数为1.75×10^-3 C/m²·K, 电场变化200 MV/m 时室温下绝热温差ΔT 的最大值可达到14 K 以上, 绝热温差ΔT ≥13 K 的工作温区超过120 K, 表明可以通过调控外部应力来获取室温时较大的绝热温差. 此结果预示着铁电EuTiO₃ 薄膜在室温固态制冷方面可能具有较好的应用前景.     

Effect of biaxial strain and hydrostatic pressure on the magnetic properties of bilayer CrI3

Two-dimensional van der Waals magnetic materials are intriguing for applications in the future spintronics devices, so it is crucial to explore strategy to control the magnetic properties. Here, we carried out first-principles calculations and Monte Carlo simulations to investigate the effect of biaxial strain and hydrostatic pressure on the magnetic properties of the bilayer CrI₃. We found that the magnetic anisotropy, intralayer and interlayer exchange interactions, and Curie temperature can be tuned by biaxial strain and hydrostatic pressure. Large compressive biaxial strain may induce a ferromagneticto-antiferromagnetic transition of both CrI₃ layers. The hydrostatic pressure could enhance the intralayer exchange interaction significantly and hence largely boost the Curie temperature. The effect of the biaxial strain and hydrostatic pressure revealed in the bilayer CrI₃ may be generalized to other two-dimensional magnetic materials.     

The atomic and electronic structure of the TiO2- and BaO-terminated BaTiO3(0 0 1) surfaces in a paraelectric phase

Relaxation and rumpling of BaTiO₃(0 0 1) surface with two different terminations have been investigated from ab initio local density approximation calculations. Large displacements of ions deviated from their crystalline sites have been obtained. These kinds of displacements lead to the formation the surface rumpling, dipole moments and electric field in the near-surface region. Band structures, density of states, bond population and electronic density redistributions have been obtained. Considerable enhancement of Ti–O chemical bond covalency nearby the surface, especially for the TiO₂ termination surface, has been found.     

多孔钛酸钡陶瓷制备及其增强的压电灵敏性

在压电陶瓷中增加孔洞数量,可以有效改善陶瓷的静水压优值,提高其压电灵敏性.考虑到铅基压电陶瓷对环境和人体的危害,本文以糊精为造孔剂,采用传统固相烧结法制备无铅钛酸钡(BaTiO_3)多孔压电陶瓷.研究烧结温度(1250,1280,1300℃)和糊精含量(5%,10%,15%)对BaTiO_3陶瓷晶体结构、孔隙率以及孔形貌特征的影响,探索孔隙率与BaTiO_3陶瓷介电、压电、声阻抗以及静水压优值等性能之间的相关性.结果表明:所有多孔陶瓷表现出三维贯通的开气孔,尺寸约为1—7μm.烧结温度强烈影响BaTiO_3陶瓷的孔隙率,加入10%低沸点的糊精时,1250℃和1280℃烧结均获得孔隙率高达58%的多孔BaTiO_3陶瓷;然而1300℃烧结,陶瓷孔隙率急速下降到13%.1250℃烧结10%糊精含量的陶瓷表现出高的静水压优值(约8376×10~/(-15)Pa~(-1))和低的声阻抗(约2.84MRrayls(1Rayl=10Pa·s/m)).与1250℃相比,1280℃烧结的陶瓷晶粒之间的结合力明显增强,有利于提高陶瓷的力学强度.这些优异的性能预示着多孔钛酸钡陶瓷在传感器和水听器领域有着潜在的应用前景.     

Epitaxial growth and transport properties of compressively-strained Ba2IrO4 films

Ba₂IrO₄ is a sister compound of the widely investigated Sr₂IrO₄ and has no IrO₆ octahedral rotation nor net canted antiferromagnetic moment, thus it acts as a system more similar to the high-T_c cuprate. In this work, we synthesize the Ba₂IrO₄ epitaxial films by reactive molecular beam epitaxy and study their crystalline structure and transport properties under biaxial compressive strain. High resolution scanning transmission electron microscopy and x-ray diffraction confirm the high quality of films with partial strain relaxation. Under compressive epitaxial strain, the Ba₂IrO₄ exhibits the strain-driven enhancement of the conductivity, consistent with the band gap narrowing and the stronger hybridization of Ir-t_2g and O-2p orbitals predicted in the first-principles calculations.     

立方相Na_(1/2)Bi_(1/2)TiO_3和K_(1/2)Bi_(1/2)TiO_3的电子结构和结构不稳定性的第一性原理比较研究

采用基于密度泛函理论的第一性原理方法比较研究了Na1/2Bi1/2TiO3和K1/2Bi1/2TiO3的电子结构、离子位移势能面和Γ声子等性质.结果表明,Na1/2Bi1/2TiO3和K1/2Bi1/2TiO3的电子结构很相似,价带由O 2p电子态主导并包含部分Ti 3d和Bi 6p电子态,导带低能部分由Ti 3d空轨道构成;K取代Na后其Ti—O和Bi—O键的键强略有增加.两者的离子位移势能面也很接近,O离子的偏心位移对结构不稳定性起主导作用,且K取代Na后其作用增强.Γ声子都存在3个软模,分析表明软模主要来自O6基团的振动,K取代Na后A2u软模发生硬化.     

Possible ferrimagnetism and ferroelectricity of half-substituted rare-earth titanate: A first-principles study on Y0.5La0.5TiO3

Titanates with the perovskite structure, including ferroelectrics (e.g., BaTiO₃) and ferromagnetic ones (e.g., YTiO₃), are important functional materials. Recent theoretical studies predicted multiferroic states in strained EuTiO₃ and titanate superlattices, the former of which has already been experimental confirmed. Here, a first-principles calculation is performed to investigate the structural, magnetic, and electronic properties of Y half-substituted LaTiO₃. Our results reveal that the magnetism of Y_0.5La_0.5TiO₃ sensitively depends on its structural details because of the inherent phase competition. The lowest energy state is the ferromagnetic state, resulting in 0.25 μB/Ti. Furthermore, some configurations of Y_0.5La_0.5TiO₃ exhibit hybrid improper polarizations, which can be significantly affected by magnetism, resulting in the multiferroic properties. Because of the quenching disorder of substitution, the real Y_0.5La_0.5TiO₃ material with random A-site ions may exhibit interesting relaxor behaviors.