Microwave surface impedances of BCS superconducting thin films

The microwave propagation-dominated problem in the multilayer structure made of a BCS superconducting film and a dielectric substrate is investigated theoretically by using the modified two-fluid model and transmission line theory. The effective microwave surface impedances are studied as functions of temperature, frequency, and film thickness, as well as substrate thickness. Special attention is paid to the substrate resonance phenomenon in the resonant structure. The influence of BCS coherence effects on surface impedance and resonant behavior is clearly demonstrated. The resonant effect in the stack structure is well interpreted with the help of the transverse resonance technique in the microwave theory     

Measurement of microwave surface resistance of patterned superconducting thin films

A simple and versatile measurement technique has been demonstrated for characterization of thin film superconductors and the substrates on which they are deposited. Unlike other state of the art measurements that characterize only the top layer of unpatterned thin films in end-plate replacement or perturbation-type cavity arrangements, we test the sample under conditions very close to those of actual microwave circuit applications. Measurements on a laser deposited, thin-film YBCO superconductor, patterned in the form of a meander line on a LaAlO₃ substrate, show a microwave surface resistance one order of magnitude lower than that of copper at 79 K and 11 GHz. At 1.4 GHz and 79 K, an improvement by over two orders of magnitude is observed. This is the first demonstration of superior microwave performance of a comb filter structure using a HTS thin film. The research portion performed at Sarnoff was supported by the Strategic Defense Initiative Organization under Contract SDIO84-88-C-0044.     

Fabrication of niobium titanium nitride thin films with high superconducting transition temperatures and short penetration lengths

We report a systematic study of the superconducting and normal state properties of reactively sputtered Nb/sub 0.62/Ti/sub 0.38/N thin films deposited on thermally oxidized Si wafers. The superconducting transition temperature (T/sub c/) was found to increase from 12 K for films prepared on unheated substrates to over 16 K for films prepared on substrates maintained at 450/spl deg/C. A Nb buffer layer was found to improve T/sub c/ by /spl sim/0.5 K for growths at lower substrate temperatures. The films fabricated at 450/spl deg/C have an amply smooth surface (1.5/spl plusmn/0.25 nm root mean square roughness), a sufficiently high T/sub c/, and sufficiently small penetration depth (200/spl plusmn/20 nm at 10 K) to be useful as ground planes and electrodes for current-generation 10 K rapid single-flux quantum circuit technology.     

Anisotropic Jc-ϑ measurements at low fields on a-MoSi thinfilms: evidence of a two-dimensional regime

Transport critical current versus applied field inclination (Jc-ϑ plots) have been made on a-MoSi thin films in low fields of order 0.04 T. The applied field was rotated (at fixed magnitude) in a plane both parallel (φ=0°) and perpendicular (φ=90°) to the film's longitudinal axis. The results indicate a Jc governed by the perpendicular component of the applied field as is sometimes observed in the layered high-temperature superconducting (HTS) thin films. These results also suggest that the peaks in the Jc-ϑ occur when the vortex density coming from external sources becomes comparable to that induced internally from the self field of the transport current. A broad peak in the Jc when the field is incident perpendicular to the film surface is observed on the silicon substrate film but not on the sapphire substrate film. Possible reasons for this broad peak are discussed     

高温超导膜微波滤波器

我们用氧化锆单晶衬底GBCO高温超膜研制了超导膜悬微波低通滤波器。在77K测试获得:通带插损在0—2GHz,0—4GHz和0—6GHz通带内,分别为0.326dB,0.507dB和1.393dB。文中阐明悬置微带电路用于研制高介电常数衬底的高温超导微波无源器件是适宜的。     

MgO缓冲层对Si衬底上制备Fe3Si薄膜性能的影响

采用磁控溅射方法和热加工工艺在n型Si衬底上溅射不同厚度的MgO层并制备Fe-Si薄膜层,退火后形成Fe3Si/MgO/Si多层膜结构.利用MgO缓冲层对退火时Si衬底扩散原子进行屏蔽,并分析MgO层对Fe3Si薄膜结构和电学性质的影响.通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)和四探针测试仪对Fe3Si薄膜的晶体结构、表面形貌、断面形貌和电阻率进行表征与分析.研究结果表明:当MgO层厚度为20 nm时生成Fe0.9Si0.1薄膜,当厚度为50,100,150和200 nm时都生成了Fe3Si薄膜,生成的Fe3Si和Fe0.9Si0.1薄膜以(110)和(211)取向为主.随MgO缓冲层厚度增加,Si衬底扩散原子对Fe3Si薄膜的影响减小,Fe3 Si薄膜的晶格常数逐渐减小,晶粒大小趋向均匀,平均电阻率呈现先增大后减小趋势.研究结果为后续基于Fe3 Si薄膜的器件设计与制备提供了参考.     

薄膜绝缘层对陶瓷厚膜电致发光显示器件的影响

采用丝网印刷技术，在Al2O3陶瓷板上印刷、高温烧结内电极及绝缘层制备出陶瓷厚膜基板，进而制备了新型厚膜电致发光显示器（TDEL），整个器件结构为陶瓷基板／内电极／厚膜绝缘层／发光层／薄膜绝缘层／ITO透明电极。对用不同薄膜绝缘材料制备的显示器件的特性进行测试、比较、分析，结果表明薄膜绝缘介质层对器件的阈值电压、发光亮度均有一定的影响，以复合绝缘层的性能最优。最后对器件的衰减特性进行了初步分析。     

High-Tc superconducting microwave phase shifter

The relation between the surface impedance Z_S(=R_S?jX_S) of superconductors and DC current I has been derived to determine the propagation constant of a superconducting microstrip line, and the relation between the phase delay ? of the superconducting microstrip line and DC current I is also presented. On the basis of these analytical results, a microwave phase shifter has been made by using YBCO superconducting thin film. In this paper, we describe the design principle of phase shifter, its fabrication, and present the experimental results. These measured values agree well with the theoretical curves.     

Properties of an electromagnetically coupled small antenna with asuperconducting thin-film radiator

This paper describes the structure and properties of a 900-MHz-band small antenna with a superconducting thin-film radiator operated by electromagnetic coupling. The radiation efficiency of the antenna with a λ/38 radiator reached 74% at 79 K. The loss in this antenna comprised mismatching loss and radiator loss caused by the surface resistance of the superconducting thin film. A simulation model with a meander-line radiator and a parallel line is also proposed. This model makes it possible to design a superconducting thin-film radiator which can be fabricated on a dielectric substrate by using the moment method without taking account of the dielectric substrate     

Full-wave characterization of high-Tcsuperconducting transmission lines

A full-wave numerical analysis is applied to accurately characterize superconducting transmission lines embedded in a layered dielectric medium. A volume integral equation formulation is developed by using a spectral domain dyadic Green's function for stratified media. Galerkin's method with rooftop basis functions for the electric field distribution inside the superconductor is then employed to solve the complex propagation constant. The thickness of the superconducting film is arbitrary in this analysis, and the formulation rigorously accounts for the anisotropy of the superconducting film. The propagation characteristics of a superconducting microstrip transmission line with a thin dielectric buffer layer are investigated. A superconducting stripline configuration with an air gap is also studied     

Propagation characteristics of superconducting microstrip lines

The modified spectral-domain approach is applied to study the propagation characteristics of high temperature superconducting microstrip lines whose signal strip and ground plane are of arbitrary thickness. In this study, numerical results for effective dielectric constant, attenuation constant, and strip current distribution are presented to discuss the effects due to frequency, temperature, strip thickness, and substrate loss tangent. In particular, the conductor and dielectric attenuation constants of superconducting microstrip line are depicted separately to discuss the mechanism of the line losses. A comparison with published theoretical and experimental results is also included to check the accuracy of the new approach's results     

Electrical properties of superconducting micro-contact junction made of Y−Ba−Cu−O thin film

We have carried research for the electrical properties of the superconducting micro-contact junction made of Y?Ba?Cu?O thin film. Y?Ba?Cu?O thin film was deposited on SrTiO₃ substrate by using the RF magnetron sputtering methode. Under 70GHz millimeter wave radiation, we observed the bolometric response by use of the fabricated micro-contact junctions.     

改善玻璃衬底上ZnO薄膜特性的方法

利用磁控溅射法在玻璃衬底上淀积铝掺杂氧化锌(AZO)薄膜作为缓冲层,在其上制备了ZnO薄膜。重点研究了AZO薄膜作为缓冲层对玻璃衬底上ZnO薄膜特性的影响。扫描电子显微镜(SEM)图像和X射线衍射(XRD)图谱分析结果表明,玻璃衬底上加入厚度为1μm的AZO缓冲层后,提高了衬底材料和ZnO薄膜之间的晶格匹配程度,有助于增大ZnO薄膜晶粒尺寸,提高其(002)取向择优生长特性、薄膜结晶特性及晶格结构完整性。室温下的透射光谱结果表明玻璃/AZO和玻璃衬底上ZnO薄膜的透光特性没有显著不同。光致发光(PL)谱研究结果表明AZO缓冲层可以有效阻止衬底表面硅原子从ZnO薄膜中"俘获"氧原子,减少ZnO薄膜中的缺陷,改善ZnO薄膜的结晶质量。     

基于不同金属衬底制备碳膜的场发射研究

利用磁控溅射的方法,在相同的陶瓷衬底上面分别镀上三种不同金属,形成三种不同的金属衬底,对金属层进行相同的表面处理后,放入微波等离子体化学气相沉积腔中,制备出三种碳膜。对制备出不同的碳膜用扫描电镜、拉曼光谱、X射线衍射仪进行结构分析,并用二极管型结构测试了它们的场致发射电子的性能。找到了最适合场发射的金属衬底,进一步对不同金属衬底制备碳膜的场发射特性不同的原因进行了初步的研究。     

ITO衬底上LiTaO3薄膜的制备与介电特性

用溶胶凝胶法在ITO衬底上制备了钽酸锂(LiTaO3)薄膜,利用XRD、SEM和AFM对薄膜的晶向、表面形态等作了表征;研究了不同溶剂对LiTaO3溶胶稳定性的影响和不同退火条件对LiTaO3薄膜结晶的影响;利用Al/LiTaO3/ITO结构,测试了薄膜的介电系数和介电损耗.结果表明:每层薄膜都晶化退火比交替使用焦化、结晶退火能生长出质量更好的LiTaO3薄膜;频率1KHz时,介电损耗约0.4,相对介电系数约53.并讨论了介电损耗增大的原因.     

Growth of single unit-cell superconducting La2−xSrxCuO4 films

We have developed an approach to grow high quality ultra-thin films of La_2−xSr_xCuO₄ with molecular beam epitaxy, by adding a homoepitaxial buffer layer in order to minimize the degradation of the film structure at the interface. The advantage of this method is to enable a further reduction of the minimal thickness of a superconducting La_1.9Sr_0.1CuO₄ film. The main result of our work is that a single unit cell (only two copper oxide planes) grown on a SrLaAlO₄ substrate exhibits a superconducting transition at 12.5 K (zero resistance) and an in-plane magnetic penetration depth λ_ab(0)=535 nm.     

硅基铁电薄膜的电性能研究

以 Sol- Gel方法制备了有 Pb Ti O3 (PT)过渡层的硅基 Pb(Zr0 .53 Ti0 .47) O3 (PZT)铁电薄膜 ,底电极分别为低阻硅和硅衬底上溅射的金属钛铂。测试了铁电薄膜的电性能 ,比较了两种衬底电极对铁电薄膜性能的影响及各自的优势     

Coplanar Waveguide: A Surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications

A coplanar waveguide consists of a strip of thin metallic film on the surface of a dielectric slab with two ground electrodes running adjacent and parallel to the strip. This novel transmission line readily lends itself to nonreciprocal magnetic device applications because of the built-in circularly polarized magnetic vector at the air-dielectric boundary between the conductors. Practical applications of the coplanar waveguide have been experimentally demonstrated by measurements on resonant isolators and differential phase shifters fabricated on low-loss dielectric substrates with high dielectric constants. Calculations have been made for the characteristic impedance, phase velocity, and ripper bound of attenuation of a transmission line whose electrodes are all on one side of a dielectric substrate. These calculations are in good agreement with preliminary experimental results. The coplanar configuration of the transmission system not only permits easy shunt connection of external elements in hybrid integrated circuits, but also adapts well to the fabrication of monolithic integrated systems. Low-loss dielectric substrates with high dielectric constants may be employed to reduce the longitudinal dimension of the integrated circuits because the characteristic impedance of the coplanar waveguide is relatively independent of the substrate thickness; this may be of vital importance for Iow-frequency integrated microwave systems.     

Low-Dielectric-Loss Barium Strontium Titanate Thin Films with MgO Buffer Layer for Tunable Microwave Devices

The dielectric and microwave properties of Ba_0.6Sr_0.4TiO₃ (BST60) thin films with a MgO buffer layer deposited on Al₂O₃ substrates were investigated. Insertion of the MgO buffer layer is demonstrated to be an effective approach to fabricate low-dielectric-loss BST thin films. x-Ray pattern analysis indicates that the thin films exhibit good crystalline quality with a pure perovskite phase and that insertion of the MgO buffer layer does not change the crystal structure of BST. The nonlinear dielectric properties of the BST films were measured by using an interdigital capacitor (IDC). At room temperature, the tunability of the BST films with a MgO buffer layer was 24.1% at a frequency of 1 MHz with an applied electric field of 80 kV/cm. The dielectric loss of the BST thin films is only 0.005 to 0.007 in the frequency range from 20 Hz to 2 MHz, the same as for BST films prepared on single-crystal MgO substrates. The microwave dielectric properties of the BST thin films were also measured by a vector network analyzer from 50 MHz to 10 GHz.     

Quasi-TEM modes in parallel-line transmission structures withconductors embedded in dielectrics

A transmission structure of many parallel conductors is capable of propagating many distinct quasi-TEM modes. A field theory of such modes was developed for a theoretically infinite number of very thin conductors placed on a dielectric substrate. The current paper extends this work to include a large number of thin parallel conductors embedded within finite dielectric slabs. The theory is illustrated by the following three structures: a large number of parallel wires embedded in dielectric slabs with substrates of infinite width, both with and without a ground plane; and a large number of thin wires embedded in a finite dielectric slab without a ground plane. This extension to a finite dielectric structure is obtained using a superposition method where both vertical and horizontal surfaces are considered infinite in turn. A major application of the derived formulation is the investigation of velocity distributions and crosstalk within these structures. Comparison of these structures with ones where the conductors are on the substrate (as opposed to embedded) indicates how the placement and structure of the conducting channel affects the modal velocities and crosstalk