S.Z. Yang

HgBa2CaCu2Oy superconducting thin films prepared by laser ablation

Abstract HgBa 2 CaCu 2 O y superconducting thin films were prepared on SrTiO 3 (100) and LaAlO 3 (100) substrates using laser ablation and post-Hg-vapor annealing. X-ray diffraction and electromagnetic measurements show that these films are dominated by c -axis-oriented Hg-1212 phase. For the Hg-1212/SrTiO 3 sample, a zero-resitance superconducting transition temperature ( T c0 ) of 110 K and critical current density ( J c ) of up to 10 6 A/cm 2 at 77 K and zero field have been obtained. The Hg-1212/LaAlO 3 sample exhibits slightly lower T c0 ) ( = 100 K) and J c (= 2 × 10 5 A/cm 2 ). Our investigations suggest that by using superconducting HgBaCaCuO target mercury can be incorporated into the films easily during post annealing.

Epitaxial growth of YBa2Cu3O7/CeO2/YSZ thin films on silicon-on-insulator substrates

A silicon-on-insulator (SOI) substrate is a compliant type of substrate because it is possible to grow a wide variety of materials on it with a small lattice mismatch and a small thermal expansion mismatch. It is quite promising in microelectronics applications, such as hybrid semiconductor/superconductor devices. In this paper, a SOI substrate has been used to grow a superconducting thin film. Using in situ pulsed laser deposition, epitaxial YBaCuO (>4000 A) thin films have been grown on SOI (100) substrates via a multiple buffer layer of CeO2 and yttrium-stabilized zirconium oxide (YSZ). X-ray diffraction analyses have shown that both the YBaCuO films and the multiple buffer layers are c-axis orientated. We have obtained YBa2Cu3O7/CeO2/YSZ multilayers with a critical temperature, Tc, of 88 K and a critical current density, Jc, of 2 × 106 A cm−2 at 77 K.

Fabrication and characterization of NbN, AlN and NbN/AlN/NbN on MgO substrates

At ambient substrate temperatures, NbN and AlN thin films as well as NbN/AlN/NbN sandwiches are prepared on single crystal MgO substrates, using direct current (dc) or radio frequency (RF) magnetron sputtering techniques. Excellent single crystal orientations of these structures are revealed by x-ray diffraction and transmission electron microscopy (TEM), while x-ray photoelectron spectroscope (XPS) shows that the stoichiometric composition of the NbN films is 1: 0.9 (Nb:N). Furthermore, AFM (atomic force microscope) scans indicate a root mean square (rms) roughness of 0.755 nm for AlN/NbN, and 0.83 nm for AlN, both over an area of 5 µm × 5 µm.

Fabrication of AlN thin films on different substrates at ambient temperature

Aluminium nitride (AlN) is very useful as a barrier in superconductor?insulator?superconductor (SIS) device or as an insulating layer in many other applications. At ambient temperature, we deposit AlN thin films onto different substrates (such as MgO, LaAlO3 and Si) by using radio-frequency magnetron sputtering and pure Al target. X-ray diffraction (XRD) and ?-scan patterns show that the films grown on MgO substrates are excellent epitaxial films with (101) orientation of a hexagonal lattice. A possible structure of the interface between the film and the substrate is suggested and discussed.

Local microwave characterization of metal films using a scanning microwave near-field microscope

In microwave integrated circuits, the performance of devices is often influenced by the homogeneity of microwave properties of metal thin films in the circuits. In this paper, a scanning microwave near-field microscope (SMNFM) has been designed using a coaxial resonator as the probe, to study the local microwave properties of the electronic devices. Several typical samples of metal films have been measured and the results have shown that we can image the local microwave properties by recording the changes of the resonant frequency and quality factor of the coaxial resonator probe. The spatial resolution of the SMNFM has been improved to be better than 5 μm by utilizing a probe with a structure similar to that used in a scanning tunnelling microscope. To demonstrate the ability of local microwave characterization, the surface resistances of Cr thin film and Ag/Cr multilayer have been imaged by the SMNFM.

Laser annealing of SrBi2Ta2O9 thin films

A laser annealing technique was used to fabricate SrBi 2 Ta 2 O 9 (SBT) thin films at a low temperature. Amorphous SBT thin films were prepared on Pt/Ti/SiO 2 /Si substrates by a metalorganic decomposition (MOD) technique. A two-step process was used to crystallize the amorphous thin films: the films were annealed at 550°C for 0.5 h to initiate the nucleation of the SBT phase, and then annealed with a KrF excimer laser. X-Ray diffraction (XRD) patterns showed that the laser-annealed thin films were transformed into a SBT perovskite structure without the presence of a second phase. The films irradiated by laser with a lower energy density and a greater pulse number had a better grain and a rougher surface.

Magnetic response of disk shaped YBa2Cu3O7−δ thin film to a perpendicular magnetic field

Abstract Magnetic measurements have been performed on a disk shaped YBa2Cu3O7−δ thin film in a perpendicular field. The critical state model developed by Mikheenko and Kuzovlev and Zhu et al. is extended to analyze the demagnetizing effects of the film. Using the field-dependent critical current density through the two-current model, the virgin magnetic moment as a function of the applied field and the hysteresis loops at several temperatures can be well fit by the extended model. The current density profiles and the flux density profiles of the film have been obtained and discussed.

Pulsed laser deposition of YBa2Cu3O7−δ/I/LaNiO3 trilayers (I = SrTiO3, CeO2 or Eu2CuO4) on (100) SrTiO3 substrates

To look at a quasi-particle injection into high Tc superconductors, YBa2Cu3O7−δ/insulator/LaNiO3 (YBCO/I/LNO) trilayers have been grown on (100) SrTiO3 substrates in situ by pulsed laser deposition (PLD), where, an insulator (I), SrTiO3 (STO), CeO2 (CO) or Eu2CuO4 (ECO) are used. All the deposited films are highly oriented. The surface morphology of LNO and the insulating films is investigated during fabrications after the films are made, revealing small roughness and low droplet density. The top YBCO layer exhibits good superconducting properties with a critical temperature above 82 K and a transition width as sharp as 1.5 K.

Nondestructive evaluation of the dielectric properties of the substrate materials for high-Tc superconducting microwave devices

In this paper, microwave near-field scanning technique has been employed to study the local dielectric properties of the substrate materials for superconducting microwave devices. A coaxial cavity together with a niobium tip has been used as a probe in our scanning microwave near-field microscope (SMNFM). Using the SMNFM, quantitative measurement of both the relative permittivity and the loss tangent of various substrate materials for the high-temperature superconducting thin films have been obtained, which coincide with the data reported in the literature quite well. Nondestructive imaging of the dielectric properties of the substrate is also given with a spatial resolution of several micrometres. We believe that the SMNFM is a reliable method for nondestructively evaluating the dielectric properties of the substrates in the microwave circuit devices. It will be helpful to better understand the influences of the variations and defects in the substrates on the designing and performance of the superconducting microwave devices.

Nondestructive imaging of the microwave properties of superconducting thin film devices with a scanning microwave near-field microscope

In this paper, nondestructive microwave scanning techniques have been used to image the local microwave properties of planar microwave devices. A coaxial cavity, together with a niobium tip, has been used as a probe in the scanning microwave near-field microscope. Quantitative images of the dielectric constant of the substrates as well as the variations of the surface resistance of the line patterns in the metal multilayers have been obtained. Local surface resistances at 3GHz have been measured on microwave devices prepared with laser ablated YBaCuO thin films, which helps in evaluating the performance of the devices.