AIN/Si3N4 纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN,Si3N4薄膜和不同调制周期的AlN/S3N4纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si3N4层的晶体结构和多层膜的硬度依赖于Si3N4层的厚度.当AlN层厚度为4.0 nm、Si3N4层厚度为0.4nm时,AlN和Si3N4层共格外延生长,多层膜形成穿过若干个调制周期的柱状晶结构,产生硬度升高的超硬效应.随着Si3N4层厚的增加,Si3N4层逐步形成非晶并阻断了多层膜的共格外延生长,多层膜的硬度迅速降低,超硬效应消失.采用材料热力学和弹性力学计算了Si3N4层由晶态向非晶转变的临界厚度.探讨了AlN/Si3N4纳米多层膜出现超硬效应的机理.     

YBa_2Cu_3O_(7-δ)/BaHfO_3/YBa_2Cu_3O_(7-δ)复合多层涂层导体的织构外延及临界电流特性北大核心CSCD

通过脉冲激光沉积技术(PLD),在氧化物缓冲层合金基底上外延生长一系列具有相同厚度,不同BaHfO3插层数(N=0、1、2、3)的YBa_2Cu_3O_(7-δ)/BaHfO_3/YBa_2Cu_3O_(7-δ)复合多层膜.X光衍射分析表明:所有样品都是c轴取向,随着层数的增加,薄膜的面内和面外织构有一定的退化.拉曼光谱图也可以看出除了N=3的样品有微弱的a轴峰,其它样品都只有c轴峰.通过扫描电镜观察发现,BaHfO_3插层的引入没有对YBa_2Cu_3O_(7-δ)超导薄膜表面产生明显的影响,薄膜表面光滑无裂痕.通过原子力显微镜发现YBa_2Cu_3O_(7-δ)超导薄膜的粗糙度在引入单层BaHfO3插层时最大,之后随着插层数的增加而减小.感应法测试表明,BaHfO3插层数为N=1时,YBa_2Cu_3O_(7-δ)超导薄膜的临界电流密度Jc(77K,自场)达到2.86 MA/cm2.在场临界电流表明,引入较少BaHfO_3插层(N=1、2)表现相对好的磁场依赖关系.     

Si3N4的晶体化和ZrN/Si3N4纳米多层膜的超硬效应

研究了Si3N4层在ZrN/Si3N4纳米多层膜中的晶化现象及其对多层膜微结构与力学性能的影响.一系列不同Si3N4层厚度的ZrN/Si3N4纳米多层膜通过反应磁控溅射法制备.利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.结果表明,由于受到ZrN调制层晶体结构的模板作用,溅射条件下以非晶态存在的Si3N4层在其厚度小于0.9 nm时被强制晶化为NaCl结构的赝晶体,ZrN/Si3N4纳米多层膜形成共格外延生长的柱状晶,并相应地产生硬度升高的超硬效应.Si3N4随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.     

TiN/Al2O3纳米多层膜的共格外延生长及超硬效应

采用多靶磁控溅射法制备了一系列具有不同Al2O3调制层厚度的TiN/Al2O3纳米多层膜.利用X射线能量色散谱、X射线衍射、扫描电子显微镜、高分辨透射电子显微镜和微力学探针表征了多层膜的成分、微结构和力学性能.研究结果表明,在TiN/Al2O3纳米多层膜中,单层膜时以非晶态存在的Al2O3层在厚度小于1.5 nm时因TiN晶体层的模板效应而晶化,并与TiN层形成共格外延生长,相应地,多层膜产生硬度明显升高的超硬效应,最高硬度可达37.9 GPa.进一步增加多层膜中Al2O3调制层的层厚度,Al2O3层逐渐形成非晶结构并破坏了多层膜的共格外延生长,使得多层膜的硬度逐步降低.     

AlN/Si3N4纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN, Si₃N₄薄膜和不同调制周期的AlN/Si₃N₄纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si₃N₄层的晶体结构和多层膜的硬度依赖于Si₃N₄层的厚度.当AlN层厚度为4.0nm、 Si₃N₄层厚度 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 应力场 超硬效应     

Si3N4在h-AlN上的晶体化与AlN/Si3N4纳米多层膜的超硬效应

采用反应磁控溅射法制备了一系列具有不同Si3N4层厚度的AlN/Si3N4纳米多层膜,利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.研究了Si3N4层在AlN/Si3N4纳米多层膜中的晶化现象及其对多层膜生长结构与力学性能的影响.结果表明,在六方纤锌矿结构的晶体AlN调制层的模板作用下,通常溅射条件下以非晶态存在的Si3N4层在其厚度小于约1nm时被强制晶化为结构与AlN相同的赝形晶体,AlN/Si3N4纳米多层膜形成共格外延生长的结构,相应地,多层膜产生硬度升高的超硬效应.Si3N4随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.分析认为,AlN/Si3N4纳米多层膜超硬效应的产生与多层膜共格外延生长所形成的拉压交变应力场导致的两调制层模量差的增大有关.     

绿色GaP-LED中NH3掺杂的动力学

本文研究了NH3作为气相掺杂剂,在GaP-LED层中氮结合的动力学过程;分析讨论了影响外延层中氮浓度的诸因素.     

大模场Er3+/Yb3+共掺锥形微结构光纤1.5μm激光特性

采用全矢量有限元法设计了一种大模场双包层微结构光纤及锥形波导结构.采用复丝拉制方法制备了成分为45Bi2 O3-29GeO2-15Ga2 O3-10Na2 O-1CeO2的大模场双包层微结构光纤,纤芯直径为70μm,内包层的占空比为0.25,模场面积约为3014.8μm2.采用熔融拉锥技术制备了纤芯直径为17.5μm、...     

Si3N4在h-AlN上的晶体化与AlN/Si3N4纳米多层膜的超硬效应

采用反应磁控溅射法制备了一系列具有不同Si₃N₄层厚度的AlN/Si₃N₄纳米多层膜,利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.研究了Si₃N₄层在AlN/Si₃N₄纳米多层膜中的晶化现象及其对多层膜生长结构与力学性能的影响.结果表明,在六方纤锌矿结构的晶体AlN调制层的模板作用下,通常溅射条件下以非晶态存在的Si₃N₄层在其厚度小于约1nm时被强制晶化为结构与AlN相同的赝形晶体,AlN/Si₃N₄纳米多层膜形成共格外延生长的结构,相应地,多层膜产生硬度升高的超硬效应.Si₃N₄随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.分析认为,AlN/Si₃N₄纳米多层膜超硬效应的产生与多层膜共格外延生长所形成的拉压交变应力场导致的两调制层模量差的增大有关. 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 赝晶体 超硬效应     

单轴压缩下Ti3B4的力学、电学性能及变形机制的第一性原理研究

Ti3B4作为一种重要的钛硼化合物,被广泛应用于工业生产和国防军事中.但是有关Ti3B4在外载荷下的变形行为却鲜有报道,这在很大程度上限制了它的应用.本文采用基于密度泛函理论的第一性原理方法研究了Ti3B4在不同方向单轴压缩下的力学行为、电子结构以及变形机制.结果表明,在不同方向单轴压缩下,Ti3B4的变形行为表现出很强的各向异性.a轴压缩下,层内Ti-Ti键减弱使Ti3B4承载能力降低,最终层间Ti-Ti键和沿b轴B-B键断裂造成压缩应力突降;b轴压缩下,层内Ti-B键减弱和层间Ti-B键增强导致Ti3B4承载能力逐渐降低,B-B键断裂导致结构破坏;c轴压缩下,层内Ti-B键断裂和层间Ti-B键形成使结构稳定性降低.由态密度分布可知,在单轴压缩下,变形后的Ti3B4仍然呈现金属性,但是其共价性能降低.通过讨论Ti3B4在不同方向单轴压缩下的力学行为与微观变形机制可以为改善其宏观性能提供一定的理论指导.     

Bi2Te3/Sb超晶格纳米线的外延生长和热电测量

通过脉冲电沉积,外延生长出小单元长度的Bi₂Te₃/Sb超晶格纳米线.借助哈曼方法,测量了超晶格纳米线阵列的热电性能,330 K时的ZT值可达0.15.研究了Bi₂Te₃/Sb超晶格纳米线阵列器件的制冷或者加热能力,发现器件的上下表面的最大温差可以达到6.6 K.     

Superconducting spin valve effect of a v layer coupled to an antiferromagnetic [Fe/V] superlattice

We studied superconducting V layers deposited on an antiferromagnetically coupled [Fe(2)V(11)](20) superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T(S) decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe2 layer is reduced if at a distance of 1.5 nm a second Fe2 layer with antiparallel spin orientation exists.     

The hyperfine properties of a hydrogenated Fe/V superlattice

We study the effect of hydrogen on the electronic, magnetic and hyperfine structures of an iron-vanadium superlattice consisting of three Fe monolayers and nine V monolayers. The contact charge density (ρ), the contact hyperfine field (B_hf) and the electronic field gradient (EFG) at the Fe sites for different H locations and H fillings are calculated using the first principle full-potential linear-augmented-plane-wave (FP-LAPW) method. It is found that sizeable changes in the hyperfine properties are obtained only when H is in the interface region.     

Spectral properties of waves in superlattices with 2D and 3D inhomogeneities

We investigate the dynamic susceptibility and one-dimensional density of states in an initially sinusoidal superlattice containing simultaneously 2D phase inhomogeneities simulating correlated rough-nesses of superlattice interfaces and 3D amplitude inhomogeneities of the superlattice layer materials. The analytic expression for the averaged Green’s function of the sinusoidal superlattice with two phase inhomogeneities is derived in the Bourret approximation. It is shown that the effect of increasing asymmetry in the peak heights of dynamic susceptibility at the Brillouin zone boundary of the superlattice, which was discovered earlier [15] upon an increase in root-mean-square (rms) fluctuations, also takes place upon an increase in the correlation wavenumber of inhomogeneities. However, the peaks in this case also become closer, and the width and depth of the gap in the density of states decrease thereby. It is shown that the enhancement of rms fluctuations of 3D amplitude inhomogeneities in a superlattice containing 2D phase inhomogeneities suppresses the effect of dynamic susceptibility asymmetry and leads to a slight broadening of the gap in the density of states and a decrease in its depth. Targeted experiments aimed at detecting the effects studied here would facilitate the development of radio-spectroscopic and optical methods for identifying the presence of inhomogeneities of various dimensions in multilayer magnetic and optical structures.     

Magnetic structure simulation of Fe/V superlattices with a variable thickness of iron layers

The model of the magnetic structure of Fe/V superlattices is discussed. The discrepancy in estimation of the critical temperature for the Fe₂/V _n /Fe₃/V _n superlattice obtained by neutron scattering and the magneto-optical Kerr effect is caused by inhomogeneity of the magnetization distribution in a finite superlattice.     

Structural and electronic properties of Fe-doped BaTiO3 and SrTiO3

We have performed first principles calculations of Fe-doped BaTiO3 and SrTiO3. Dopant formation energy, structure distortion, band structure and density of states have been computed. The dopant formation energy is found to be 6.8eV and 6.5eV for Fe-doped BaTiO3 and SrTiO3 respectively. The distances between Fe impurity and its nearest O atoms and between Fe atom and Ba or Sr atoms are smaller than those of the corresponding undoped bulk systems. The Fe defect energy band is obtained, which mainly originates from Fe 3d electrons. The band gap is still an indirect one after Fe doping for both BaTiO3 and SrWiO3, but the gap changes from Γ-R point to Γ-X point.     

Electronic stability of magnetic Fe/Co superlattices with monatomic layer alternation

We report a surprising observation that the growth of the [Fe(1 ML)/Co(1 ML)](n) superlattice of L1(0) structure on Cu(100) is stable only up to six atomic layers (n=3), which cannot be rationalized by stress arguments. Instead, first-principles calculations reveal a transition from the L1(0) to the B2 structure due to the effect of dimensionality on the stability of the electronic structure of the superlattice. Whereas the majority-spin electrons are energetically insensitive to the layer thickness, the minority-spin electrons induce the transition at n=3.     

Photo-induced dynamics of Bose-Einstein condensates in optical superlattice

The photo-induced dynamics of cold atoms in a one-dimensional optical superlattice is observed. Steady state distribution of the probability amplitudes and the site population in a one-dimensional optical superlattice is found. It is shown that this solution of the equations, which describes the temporal behavior of a Bose-Einstein condensate in a superlattice, is unstable at the sufficiently high level of boson density. The expression for the increment of modulational instability is obtained on the basis of the linear stability analysis. The numerical examples of non-stationary solutions for boson density in a superlattice for the general model are discussed as applied to both the attraction and repulsion potentials of boson interaction.     

Giant magnetoresistance of iron-chromium superlattices at microwaves

Interaction between an rf electromagnetic field and the Fe/Cr superlattice placed in a rectangular waveguide so that a high-frequency current passes in the plane of superlattice layers is considered. The transmission coefficient versus the magnetic field strength is found at centimeter waves, and a correlation between this dependence and the field dependence of the dc magnetoresistance is established. It is shown that a change in the transmission coefficient may greatly exceed the giant magnetoresistance of the superlattice. The frequency dependence of the microwave measurements has an oscillatory character. The oscillation frequencies are analyzed in terms of wavelet transformation. Two types of oscillation periods are found to exist, one of which corresponds to the resonance of waves traveling in the superlattice along the direction parallel to the narrow wall of the waveguide.     

Spin flop transition in a finite antiferromagnetic superlattice: evolution of the magnetic structure

An antiferromagnetic (AF) superlattice of Fe/Cr(211) is used as a model system to study magnetic transitions in a finite-size geometry. With polarization neutron reflectometry the magnetic structure at the surface spin-flop transition and its evolution with field is determined. A domain wall created near the surface penetrates the superlattice with increasing field, splitting it into two antiphase, AF domains. After reaching the center the spin-flopped phase spreads throughout the superlattice. The experimental results are in substantial agreement with theoretical and numerical predictions.