MgO衬底上YBa2Cu3O7-δ台阶边沿型约瑟夫森结的制备及特性

MgO衬底上的YBa2Cu3O7-δ(YBCO)台阶边沿型约瑟夫森结(台阶结)在高灵敏度高温超导量子干涉器(superconducting quantum interference device,SQUID)等超导器件研制方面具有重要的应用价值和前景.本文对此类YBCO台阶结的制备和特性进行了研究.首先利用离子束刻蚀技术和两步刻蚀法在MgO(100)衬底上制备陡度合适、边沿整齐的台阶,然后利用脉冲激光沉积法在衬底上生长YBCO超导薄膜,进而利用紫外光刻制备出YBCO台阶结.在结样品的电阻-温度转变曲线中,观测到低于超导转变温度时的电阻拖尾现象,与约瑟夫森结的热激活相位滑移理论一致.伏安特性曲线测量表明结的行为符合电阻分路结模型,在超导转变温度TC附近结的约瑟夫森临界电流密度TC随温度T呈现出(TC-T)^2的变化规律,77 K时JC值为1.4×10^5 A/cm^2.利用制备的台阶结,初步制备了YBCO射频高温超导SQUID,器件测试观察到良好的三角波电压调制曲线,温度77 K、频率1 kHz时的磁通噪声为250μΦ0/Hz^1/2.本文结果为进一步利用MgO衬底YBCO台阶结研制高性能的高温超导SQUID等超导器件奠定了基础.     

First principles simulation of a ceramic /Metal interface with misfit

The relaxed atomic structure of a model ceramic/metal interface, 222MgO/Cu, is simulated, including lattice constant mismatch, using first principles local-density functional theory plane wave pseudopotential methods. The 399-atom computational unit cell contains 36 O and 49 Cu atoms per layer in accordance with the 7/6 ratio of MgO to Cu lattice constants. The atomic layers on both sides of the interface warp to optimize the local bonding. The interface adhesive energy is calculated. The interface electronic structure is found to vary appreciably with the local environment.     

MgO单晶基片上YBCO高温超导薄膜的制备

在2英寸MgO(001)单晶基片上,采用直流溅射法,通过基片高温退火,成功制备了性能优越的YBa2C3O7-δ(YBCO)双面超导薄膜,能够满足超导滤波器的设计要求。X射线衍射(XRD)分析表明经过退火的基片上生长的YBCO薄膜与基片有单一的外延取向关系;用原子力显微镜(AFM)和高能电子衍射(RHEED)分析高温退火对基片表面状况的改变。结果表明制备的YBCO薄膜具有很好的超导电性,薄膜临界电流密度Jc(77K,0T)≈2.5×106A/cm2,微波表面电阻Rs(10GHz,77K)≈0.16mΩ。     

Surface and interface properties of epitaxial Fe3O4 films studied by Mössbauer spectroscopy

Stoichiometric Fe₃O₄ films have formed epitaxially on -Al₂O₃ and MgO single-crystal substrates by a reactive vapor deposition method. In order to apply conversion electron Mössbauer spectroscopy depth-selectively, a 5–7 Åthick probe layer containing ⁵⁷Fe was formed at various depths in inactive ⁵⁶Fe₃O₄ matrix films. At the topmost surfaces and also at the interfaces, the essential electronic features of bulk Fe₃O₄ are retained, including a rapid electron hopping between the Fe²⁺ and Fe³⁺ ions at B sites. Minor depth-dependent changes are confined to a few outermost atomic layers, and the changes depend on the orientation and the lattice mismatch with the substrate. For (111) growth on -Al₂O₃, the surface layer seems to be strongly relaxed to reduced the electric polarization, while a high density of defects seems to be concentrated at the interface with -Al₂O₃. For (001) growth on MgO, the surface retains the spinel lattice though slightly oxidized, while the interface with MgO has good crystallinity and stoichiometry. An enhanced thermal fluctuation of the Fe³⁺-spins in contact with the MgO substrate and in the topmost surface layer can be seen in their reduced magnetic hyperfine field at 300 K.     

Layer by layer growth of ZnO on (0001) sapphire substrates by radical-source molecular beam epitaxy

Layer by layer growth of ZnO epilayers on (0001) Al₂O₃ substrates is achieved by radical-source molecular beam epitaxy. A thin MgO buffer, followed by a low-temperature ZnO buffer was used in order to accommodate the lattice mismatch between ZnO and sapphire. Reflection high-energy electron diffraction intensity was employed for the optimization of the ZnO growth. The surface morphology of the samples was studied with atomic force microscopy. Investigation of the nature of the influence of the MgO buffer layer on the formation of ZnO on sapphire substrate was carried out using Transmission Electron Microscopy. For the first time it was shown that a thin spinel (magnesium aluminum oxide) layer is formed on the interface of the sapphire substrate and MgO buffer layer leading to the crystalline quality improvement of the ZnO main layer. X-Ray diffractometry measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO heteroepitaxially grown on sapphire. The crystalline quality of the ZnO layers is even better than that of our previously reported layers grown employing hydrogen peroxide as an oxidant. The full width at half maximum of the XRD (0002) rocking curve is as low as 25 arc s. The influence of growth parameters (Zn/O flux ratio, temperature, etc.) on the structural properties as well as on the surface morphology of the zinc oxide layers on sapphire is investigated and discussed.     

Highly adhesive calcium phosphate layer on UHMWPE prepared by IBAD

A study has been made on adhesive strength and chemical states of calcium phosphate layer deposited on the surface of ultra-high molecular weight polyethylene (UHMWPE) by ion beam assisted deposition (IBAD). IBAD is well known to be able to give an atomic intermixed zone at layer/substrate interface during deposition. The X-ray diffraction (XRD) results demonstrated that the main structure is amorphous calcium phosphate (ACp) in the layer. Tape test and scratch test were used to measure the adhesive strength between layer and substrate. Both results show that the adhesive strength between layer and substrate of samples prepared by IBAD is much stronger than that of contrast samples prepared by ion beam sputtering deposition (IBSD). It has been found by XPS that there are much more stable covalent bonds between layer and substrate in IBAD samples than that in IBSD samples. The results above lead us to a conclusion that besides the atomic intermixed zone at the interface, the increment of covalent bonds between layer and substrate also enhanced the adhesive strength between layer and substrate in IBAD samples.     

Attenuation of interfacial pinning enhancement in YBCO using a PrBCO buffer layer

Numerous experimental results have suggested that the J_c of YBa₂Cu₃O₇ (YBCO) films is significantly higher near the film–substrate interface than in the remainder of the film. We previously proposed that this effect is due to interfacial pinning enhancement caused by stress and the resulting misfit dislocations at the heteroepitaxial interface. To test this hypothesis we have used a non-superconducting PrBa₂Cu₃O_7?δ (PrBCO) buffer layer to minimize the lattice mismatch with YBCO. We find that the PrBCO layers lower J_c of the 0.4 μm YBCO films in a predictable way, and that, if sufficiently thick (～0.5 μm), they eliminate interfacial enhancement altogether. Our interpretation of this result is that the defects responsible for interfacial enhancement of flux pinning originate at the bottom of the non-superconducting PrBCO layer, which screens the pinning centers from vortices in YBCO. This result demonstrates that the pinning enhancement arises from stress at the film–substrate interface.     

Enhanced strain relaxation induced by epitaxial layer growth mode of MgO thin films

Growth of MgO films on silicon substrate was conducted by KrF excimer pulsed-laser ablation system. Two kinds of growth mode were revealed in situ by reflection high energy electron diffraction. It was found that the layer growth mode of MgO thin films could remarkably reduce the misfit strain originated from the different lattice constant and thermal expansion coefficiency between MgO films and Si. An enhanced strain relaxation was discovered for MgO films, which were grown with the layer growth mode, in the film thickness range of 40-100 nm. The value of critical thickness for the formation of misfit dislocation agrees well with the calculated one. This exceptional phenomenon should be ascribed to the layer growth mode of epitaxial MgO films.     

用高分辨透射电镜对MOCVD生长的ZnSe1-xSx-ZnSe应变超晶格结构的研究

本文利用高分辨电子显微镜(TEM)从原子尺度对MOCVD生长的ZnSe_1-xS_x-Znse应变超晶格的精细结构进行了细致观察.通过对缺陷的种类、分布的分析提出缺陷的产生原因与过渡层质量有直接关系,通过改善过渡层的成份及各层间的厚度可制备出结构较完整以及较平整的超晶格薄层材料.     

Effect of strain on geometric and electronic structures of graphene on Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations. Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately. Considering the strain induced by the lattice mismatch, we optimize the atomic structures and investigate the electronic properties of the graphene. Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N×N superlattice and the substrate. The strain does not induce significant changes in electronic structure. Furthermore, the results calculated in the local density approximation (LDA) are compared with those obtained in the generalized gradient approximation (GGA), showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.     

Influence of MgO surface conditions on the in-plane crystal orientation and critical current density of epitaxial YBCO films

The effect of magnesium oxide (MgO) surface conditions on in-plane grain orientation and critical current density of epitaxial YBa₂Cu₃O₇ (YBCO) films was systematically investigated. The MgO substrates were either “as received” or stored for some time, cleaned using different methods and lithographically prepared for our step-edge junction devices. The YBCO films were grown via reactive thermal co-evaporation by Theva, GmbH. The surface characterisation of MgO substrates was studied using X-ray photoelectron spectroscopy (XPS). The in-plane grain orientation of the YBCO films was studied by means of X-ray diffraction (XRD) φ-scan and the critical current density was measured for the XRD scanned samples. The surface condition of the MgO substrates was found to have a strong influence on the in-plane grain orientation and the critical current density of the YBCO films. The MgO substrates with a degraded or contaminated surface gave rise to 45° grain misorientation in YBCO films and reduced the critical current density. A final process step using a low energy Ar ion beam etching (IBE) of the MgO substrates prior to the YBCO film deposition was found effective in removing the in-plane grain misorientation and promoting the growth of perfectly aligned c-axis YBCO films.     

The effect of chemical polishing on the interface structure and electrical property of Au/Cd0.9Zn0.1Te contact

The interface structures between Au electrode and Cd_0.9Zn_0.1Te wafer with different surface treatments are studied by means of transmission electron microscopy. Before the preparation of the Au film, atomic force microscopy and scanning electron microscopy are employed to investigate the surface morphology and elemental concentration before and after the chemical polishing process. It is found that an amorphous layer with the thickness of approximately 5 nm exists at the interface area for the only mechanical polished samples. As the chemical polishing process goes on, the interfaces become flatter and smoother. A thinner lattice mismatch layer instead of the amorphous layer after the chemical polishing process is found between Au and Cd_0.9Zn_0.1Te. The formation mechanism for the amorphous layer is considered to be the large lattice mismatch between Au and matrix. Furthermore, current–voltage (I–V) measurement is also carried out to investigate the relationship between the interface structure and electrical properties. The ohmic contact coefficient is calculated to increase from 0.4609 to 1.0904 after 4 min chemical polishing corresponding to the I–V test. It is indicated that the charges become easier to move across the interface, which has no amorphous layer, due to the weaker blocking effect to the charges for the thinner and ordered interface region.     

InAs/(GaIn)Sb superlattices for IR optoelectronics: Strain optimization by controlled interface formation

The structural properties of InAs/(GaIn)Sb and (InGa)As/GaSb superlattices (SLs), grown by solid-source molecular-beam epitaxy on GaAs substrates using a strain relaxed GaSb or InAs buffer layer or directly on InAs substrates, were analyzed by high-resolution X-ray diffraction and Raman spectroscopy. The residual strain within the SL was found to depend critically on the type of interface bonds, which can be either InSb- or GaAs-like. Thus, to achieve lattice matching to the buffer layer or substrate by strain compensation within the SL stack, the controlled formation of the interface bonds is vital. On the other hand, minimization of the residual strain is shown to be a prerequisite for achieving a high photoluminescence yield and high responsivities for InAs/(GaIn)Sb SL based IR detectors.     

Amorphous hydrogenated carbon films on semiconductors

The adhesion quality of amorphous hydrogenated carbon films (a-C:H) on semiconductor substrates depends to a large degree on the properties of the interface. The present work complements the photoemission results of the preceding paper with a detailed investigation of the atomic structure of the a-C:H/Si and a-C:H/GaAs interfaces. We show that the method of substrate cleaning and the deposition parameters affect the thickness of the interfacial layer and the interface roughness. The carbide compounds that form in the interfacial layer are found to be amorphous and we present evidence for the precipitation of metallic Ga at the a-C:H/GaAs interface. Finally, we have determined the extent of atomic intermixing in the interfacial region and compare our results with different mechanisms of adhesion.     

Preparation and structural properties of YBCO films grown on GaN/c-sapphire hexagonal substrate

Epitaxial YBCO thin films have been grown on hexagonal GaN/c-sapphire substrates using DC magnetron sputtering and pulsed laser deposition. An MgO buffer layer has been inserted between the substrate and the YBCO film as a diffusion barrier. X-ray diffraction analysis indicates a c-axis oriented growth of the YBCO films. Φ-scan shows surprisingly twelve maxima. Transmission electron microscopy analyses confirm an epitaxial growth of the YBCO blocks with a superposition of three a-b YBCO planes rotated by 120° to each other. Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal no surface contamination with Ga even if a maximum substrate temperature of 700 °C is applied.     

Atomic resolution imaging of the (001) surface of UHV cleaved MgO by dynamic scanning force microscopy

The (001) surface of UHV cleaved single MgO crystals was imaged with dynamic mode scanning force microscopy. Large-scale images show various defects, like steps of mostly one atomic height, rectangular holes of nanometer size, and some complex adstructures. First time images with atomic resolution show one square ionic sublattice in its bulklike dimension with a corrugation of up to 40 pm along the <001> direction. Most images exhibit atomic point defects which appear as depressions including a few ionic lattice sites proving that point defects are stable on flat terraces.     

Mechanisms of amorphization-induced swelling in silicon carbide: the molecular dynamics answer

We present here the continuation of an investigation of the irradiation-induced swelling of SiC using classical molecular dynamics (CMD) simulations. Heavy ion irradiation has been assumed to affect the material in two successive steps (a) creation of local atomic disorder, modeled by the introduction of extended amorphous areas with various sizes and shapes in a crystalline SiC sample at constant volume (b) induced swelling, determined through relaxation using Molecular Dynamics at constant pressure. This swelling has been computed as a function of the amorphous fraction introduced. Two different definitions of the amorphous fraction were introduced to enable meaningful comparisons of our calculations with experiments and elastic modeling. One definition based on the displacements relative to the ideal lattice positions was used to compare the CMD results with data from experiments combining ion implantations and channeled Rutherford Backscattering analyses. A second definition based on atomic coordination was used to compare the CMD results to those yielded by a simplified elastic model. The results obtained are as follows. On the one hand, comparison of the swelling obtained as a function of the lattice amorphous fraction with the experimental results shows that the melting-quench amorphization simulates the best the irradiation-induced amorphization observed experimentally. This is consistent with the thermal spike phenomenon taking place during ion implantation. On the other hand, disorder analysis at the atomic scale confirms the elastic behavior of the amorphization-induced swelling, in agreement with the comparison with the results of an elastic model. First, no major structural reconstruction occurs during relaxation or annealing. Second, the systems with the most disordered and constrained amorphous area undergo the largest swelling. This means that the disorder and the constraints of the bulk amorphous area are the driving forces for the swelling observed. On the contrary, the nature of the interface does not affect significantly the swelling observed.     

高温超导约瑟夫森结太赫兹混频系统

在小型脉冲管制冷机（工作温度约60 K）上搭建了高温超导约瑟夫森结太赫兹谐波混频系统,并采取了一系列措施增强结与太赫兹波的耦合。所采取的措施主要有:研究不同基片对结的影响并最终选择MgO做为双晶结基片,设计平面周期对数天线,同时使用一个超半球硅透镜和两个离轴抛物面反射镜组成准光学系统。最终成功在小型脉冲管制冷机中获得结对623 GHz太赫兹波的响应,并且成功进行24次谐波混频实验。     

高温超导约瑟夫森结太赫兹混频系统

在小型脉冲管制冷机（工作温度约60 K）上搭建了高温超导约瑟夫森结太赫兹谐波混频系统,并采取了一系列措施增强结与太赫兹波的耦合。所采取的措施主要有：研究不同基片对结的影响并最终选择MgO做为双晶结基片,设计平面周期对数天线,同时使用一个超半球硅透镜和两个离轴抛物面反射镜组成准光学系统。最终成功在小型脉冲管制冷机中获得结对623 GHz太赫兹波的响应,并且成功进行24次谐波混频实验。     

LPE growth of PZNT film using superheating YBCO thin film as seed layer

A superheating YBa₂Cu₃O_7-δ (YBCO or Y123) thin film was applied as a seed layer to grow Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT) films by a liquid phase epitaxy (LPE) process. In the present work, the YBCO thin film underwent a growth temperature of 1050 °C, which was about 40 °C higher than its peritectic temperature. It is very interesting that the superheated YBCO seed film could be used to grow not only Nd_1+xBa_2-xCu₃O_7-δ (NBCO) films with similar compositions but also PZNT films with completely different compositions. The XRD analysis confirmed that the PZNT film grown on the YBCO seeded MgO substrate had a good epitaxial relationship of [100](001)_PZNT//[100](001)_YBCO//[100] (001)_MgO. Compared with the PZNT films directly grown on MgO substrates, the LPE PZNT film on YBCO/MgO presented a better surface morphology. It was found that the superheating YBCO seed film plays a crucial role for the LPE growth of PZNT in the process. Furthermore, the superheating mechanism was discussed in terms of thermodynamic theories as well. PACS 77.84.-s; 61.10.Nz; 68.37.Hk; 81.15.Lm; 82.45.Mp