YBa2Cu3O7-x/SrTiO3//LaAlO3 heterostructures obtained by injection MOCVD

Abstract

YBa₂Cu₃O_7-x /Ba_xSr_1-xTiO₃ /LaAlO₃ heterostructures can be used as a basis for devices with voltage control in microwave circuits.

Ba_xSr_1-xTiO₃ (x=0–0.1) (BST) thin films have been epitaxially grown on LaAlO₃ substrates using injection MOCVD. The excellent crystalline quality of the obtained BST films can be proven by a FWHM of <0,2° for the rocking curve of the (002) BST reflection. An AFM study revealed flat surfaces, showing a surface roughness R_s as low as 1nm. YBa₂Cu₃O_7-x/Ba_xSr_1-x TiO₃//LaAlO₃ heterostructures were than optimised. The YBa₂Cu₃O_7-x (YBCO) layers obtained on Ba_xSr_1-xTiO₃ films are epitaxial with a FWHM of 0.45° for the (005) YBCO rocking curve and display very promising superconducting properties of T_c=92K.

Finally the microwave properties of the superconducting films were studied. For YBa₂Cu₃O_7-x layers directly deposited on LaAlO₃, surface resistance values of 0,32mΩ were obtained, while for YBa₂Cu₃O_7-x /SrTiO₃//LaAlO₃ heterostructures, higher values of 1mΩ at 8.5GHz were measured.     

Effect of substrate surface modification using Ag nano-dots on the improvement of J c and microstructures in YBa2Cu3O7 thin films grown on LaAlO3 (100) by pulsed laser deposition

YBa₂Cu₃O₇ (Y123) thin films were grown by pulsed laser deposition (PLD) on LaAlO₃ (100) substrates whose surfaces were modified by a discontinuously layer of Ag nano-dots. The Y123 films were characterised by atomic force microscope, X-ray diffraction, scanning electron microscope, and DC magnetization measurements. Effect of substrate surface modification using various densities of the Ag nano-dots on the improvements of critical current density J _c and microstructures in the Y123 films has been studied systematically. The results showed that at fixed physical deposition conditions J _c increased with the number of Ag shots, n. Zero field J _c at 77 K increased from 10⁶ to 3.3 × 10⁶ A/cm², and from 1.5 × 10⁷up to 4 × 10⁷A/cm² for 5 K as the number of Ag shots increased from zero to 150. However, a fluctuation of J _c was observed for n < 60 at 77 and 40 K in both low and high fields. Detailed microstructure analysis revealed that ab misalignment was gradually improved as Ag nano-dots density gradually increased and believed to be responsible for the J _c enhancement.     

BaTiO 3 on YBa 2 Cu 3 O 7 High T C Superconductors-Microwave Properties

We have made both patterned and unpatterned BaTiO 3 on YBa 2 Cu 3 O 7 microwave test structures as well as identical YBa 2 Cu 3 O 7 test structures and characterized them at 3.3 and 35 GHz. In both cases we found that the best unloaded Q values of the test resonators was for devices made from BaTiO 3 coated YBa 2 Cu 3 O 7 . This had led us to conclude that, at low temperatures, BaTiO 3 does not add additional losses to the system and that it may, in addition to being a tunable ferroelectric material, aid in passivating the YBa 2 Cu 3 O 7 ' surface. We present here preparation and measurement details and discuss the direction of future work in this area.     

PZT thick films by diol chemical solution deposition

Process optimization and properties of lead zirconate titanate (PZT) films for piezoelectric micromachined ultrasonic transducers (pMUTs) for scanning probe devices will be presented. The goal of the work was a replacement of the tetragenic and mutagenic solvent and a decrease of time-consuming PZT 2–methoxy ethanol (2MOE) route. An alternative diol-based solution synthesis process was developed and “Design Of Experiment” (DOE) was used to achieve processing optimization for thick and crack free films. Tight parameter control allowed to develop a highly reproducible PZT diol process. The crystallization behaviour of crack-free PbZr_0.53Ti_0.47O₃ films (1–5 μm) with oriented perovskite structure was examined by X-ray diffraction and surface analysis using scanning electron microscopy. Piezoelectric and dielectric properties were examined. The effective transverse piezoelectric coefficient e _31,f of sol–gel processed films was investigated for 4 μm thick layers. Best properties were achieved with {1 0 0}-textured films, where a remanent e _31,f value of −7.3 C/m² was measured for 4.1 μm thick films.     

Growth and characterization of epitaxial SrBi2Ta2O9 films on (110) SrTiO3 substrates

Abstract

SrBi₂Ta₂O₉ (SBT) is an attractive material for nonvolatile ferroelectric memory applications. In this paper we report on the deposition of highly epitaxial and smooth SrBi₂Ta₂O₉ films on (110) SrTiO₃substrates. The films were grown by pulsed laser deposition at temperatures ranging from 600 to 800°C and at various laser fluences from a Bi-excess SBT target. The background oxygen pressure was maintained at 28 Pa during the film deposition. Structural characterization of the films was performed by x-ray diffraction. Atomic force microscopy was used to investigate morphology and growth of the films. The films grew with preferred (115) or (116) orientation. The roughness was of the order of unit cell height. The films display a growth pattern resulting in corrugated film morphology.     

Fatigue and aging in ferroelectric PbZr0.2Ti0.8O3/YBa2Cu3O7 heterostructures

We have carried out fatigue, aging and retention studies of epitaxial ferroelectric PbZr_0.2Ti_0.8O₃/YBa₂Cu₃O₇ heterostructures, grown by pulsed laser deposition. The capacitors show fatigue lifetime of better than 10¹⁰ cycles. We find that the fatigue process is reversible by the application of a 2 second “pulsed poling” treatment at the fatigue voltage. These results suggest that the loss of switchable polarization with cycling is due to domain wall pinning; however, the role of space charge effects at the electrodefilm interfaces cannot be ruled out. Indeed, the overall fatigue performance may be controlled by a combination of these two factors. These heterostructures show excellent resistance to aging (extrapolated lifetime of better than 10¹¹ seconds and good logic state retention characteristics (10¹¹ seconds).     

Effect of Ultra-Thin SrTiO3 Seed Layer on the Microwave Surface Resistance of YBa2Cu3O7 Films Deposited on (1 0 0) MgO

RHEED controlled ultra-thin buffer layers of SrTiO₃ have been deposited on (1 0 0) MgO by pulsed laser deposition to grow YBa₂Cu₃O₇ films for microwave applications. A buffer layer with a thickness from 5 to 40 unit cells of SrTiO₃ is sufficient to expand to more than 60°C the range of deposition temperatures for which a low microwave surface resistance [Rs(10 GHz, 77 K)<0.5 m] is obtained. The Rs values are as low as those obtained on LaAlO₃ substrates, furthermore they present a slightly lower magnetic field dependency. The XRD -scans show that the SrTiO₃ seed layer induces an oriented epitaxial growth with the [1 0 0] axis of YBCO parallel to the one of MgO over this broadened range of deposition temperatures. This seed layer promoting effect is not observed with other oxides such as Ba₀.15Sr_0.85TiO₃, CeO₂, Ce₁–x _{La x} O₂.     

基于YBa2Cu3O7薄膜的超导故障限流器设计与试验分析

针对直流电力系统短路电流分断困难的问题,提出了基于YBa2Cu3O7薄膜的电阻型超导故障限流器(SFCL)保护方案,设计了300V/200A的SFCL模型机。试验分析了不同的电源电压、负载电感和负载电阻等系统参数对薄膜限流效果及失超特性的影响。试验表明,YBa2Cu3O7薄膜能在短路电流到达临界电流Ic后准确动作,并在0.1ms时间里将短路电流峰值限制在2Ic以内。随着电源电压的增加,薄膜的失超传播速度和失超电阻值迅速增加。推导出限流器重要指标Umax、Imax与线路参数(L、R、Ue)、材料特性(Ic)的数值关系。     

High quality lead zirconate titanate films grown by organometallic chemical vapour deposition

Abstract

Organometallic chemical vapour deposition is a suitable technique for the deposition of thin films of oxidic compounds such as lead zirconate titanate, PbZr_xTi_1?xO₃. Above a deposition temperature of about 600°C stoichiometric PbZr_xTi_1?xO₃ films can be grown on platinized silicon wafers within a large process window, independent of the precursor partial pressures and the deposition temperature. This is the result of a self-regulating mechanism. The PbZr_xTi_1?xO₃ films have excellent ferroelectric properties exhibiting high values, up to 60μC/cm², for the remanent polarisation. The value of the coercive field strength varies between 50 and 180 kV/cm, dependent on the composition. Layers with comparable properties can also be grown at lower temperatures, down to 500°C. In this case careful control of the gas-phase composition is required to obtain films with the correct stoichiometry.     

Fabrication of conductive oxide polycrystalline BaPbO3 films by chemical solution deposition and their electrical resistivity

BaPbO₃ films were fabricated by a chemical solution deposition on the SiO₂/Si(100) and MgO(100) substrates followed by a post-deposition annealing at the temperatures between 673 and 1073 K under oxygen flow. Polycrystalline BaPbO₃ films were formed together with secondary phases such as PbO and Pb₃O₄ onto MgO(100) substrates at around 750 K, and the films were crystallized into single phase of BaPbO₃ above 823 K. Endothermic peak in differential thermal analysis due to crystallization of BaPbO₃ was observed at 750 K, which is consistent with crystallization temperature of BaPbO₃ estimated from X-ray diffraction. The electrical resistivity depended on the annealing temperature even in the single phase BaPbO₃ films, the lowest resistivity of 3?×?10^?6 μΩ·m which was comparable to that of bulk BaPbO₃ was achieved at the annealing temperature of 873 K.     

Synthesis and properties of ferroelectric Si-doped (Bi, Nd)4Ti3O12 thin films by chemical solution deposition

Ferroelectric Si-doped (Bi,Nd)₄Ti₃O₁₂ thin films have been prepared on Pt/TiO_x/SiO₂/Si substrates through metal-organic compounds by the chemical solution deposition. The Bi_3.25Nd_0.75Ti_2.9Si_0.1O₁₂ (BNTS) precursor films were found to crystallize into the Bi-layered perovskite Bi₄Ti₃O₁₂ single-phase above 600^∘C. The synthesized BNTS films revealed a random orientation having a strong 117 reflection. The BNTS thin films prepared between 600^∘C and 700^∘C showed well-saturated P-E hysteresis loops with P _r of 13–14 μ C/cm² and E _c of 100–110 kV/cm at an applied voltage of 5 V. The surface roughness of the BNTS thin films was improved by Si doping compared with that of undoped Bi_3.35Nd_0.75Ti₃O₁₂ films.     

Effect of seed layer on electrical properties of Pb(Zr,Ti)O3 films grown by metalorganic chemical vapor deposition

Abstract

Pb(Zr_xTi_1?x)O₃ (PZT) ferroelectric thin films were prepared by metalorganic chemical vapor deposition (MOCVD) on Pt/Ti/SiO₂/Si substrate. Very thin PZT films, which were deposited at a lower temperature and post-annealed at higher temperature for crystallization, were used as a seed layer. PZT films grown on the seed layer exhibited superior characteristics in the crystalline structure and electrical properties, compared to those deposited without seed layer. Depending on the deposition conditions of PZT seed layer, a wide variation of surface morphology and stoichiometry was found between samples, whereas chemical composition was found to be very similar.     

Ferroelectric properties of Al and Nb doped PbTiO3 thin films prepared by chemical solution deposition process

Abstract

Thin films of pure and acceptor (Al) as well as donor (Nb) doped PbTiO₃ were prepared using a chemical solution deposition method. The nominal compositions of the solutions were equivalent to those of PbTiO₃, Pb(Ti_0.95Al_0.95)O₃ and Pb(Ti_0.95Nb_0.05)O₃, respectively. In the case of PbTiO₃, the remanent polarization amounted to 43 μC/cm², but the shape of the hysteresis curve was round. On the other hand, Al and Nb doped PbTiO₃ thin films exhibited well-saturated P-Ehysteresis curves. The shape of these curves was slim compared to that of PbTiO₃. The remanent polarization of the Pb(Ti_0.95Al_0.05)O₃ and Pb(Ti_0.95Al_0.05)O₃ thin films amounted to 18 and 15 μC/cm², respectively, and the coercive field decreased in comparison with that of PbTiO₃. The leakage currents of the PbTiO₃, Pb(Ti_0.95Al_0.05)O₃ and Pb(Ti_0.95Al_0.05)O₃ thin films were 7.68x10^?3, 3.21x10^?6 and 6.58x10^?4 A/cm², respectively, at+10V.     

Effects of oxidants on the deposition and dielectric properties of the SrTiO3 thin films prepared by liquid source metal-organic chemical vapor deposition (MOCVD)

Abstract

SrTiO₃ thin films are deposited by a liquid source metal-organic chemical vapor deposition (MOCVD). The effects of oxidants on the deposition characteristics and dielectric properties of the films are mainly tested. O₂, N₂O and O₂ + N₂O gases are used as the oxidants and the films with Ti-rich and Sr-rich compositions are obtained when O₂ and N₂O is used, respectively. Deposition of thin initial layer under O₂ atmosphere is very effective to obtain large dielectric constant of the SrTiO₃ thin film when the main layer is deposited under O₂ + N₂O atmosphere. The dielectric constants of 40 nm thick SrTiO₃ films with thin O₂, N₂O initial layers and without the initial layers are 235, 145 and 210, respectively.     

Enhancement of the surface and structural properties of ZnO epitaxial films grown on Al2O3 substrates utilizing annealed ZnO buffer layers

ZnO films were grown on Al₂O₃ (1000) substrates without and with ZnO buffer layers by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the surface roughness of the ZnO films grown on ZnO buffer layers annealed in a vacuum was decreased, indicative of an improvement in the ZnO surfaces. X-ray diffraction patterns showed that the crystallinity of the ZnO thin films was enhanced by using the annealed ZnO buffer layer in comparison with the film grown on without a buffer layer. The improvement of the surface and structural properties of the ZnO films might be attributed to the formation of the Zn-face ZnO buffers due to annealing in a vacuum. These results indicate that the surface and structural properties of ZnO films grown on Al₂O₃ substrates are improved by using ZnO buffer layers annealed in a vacuum.     

Metalorganic chemical vapor deposition of ferroelectric Pb(Zr,Ti)O3 thin films

Abstract

The controllability of PZT film properties and the possibility of the scaling up the MOCVD to the commercial based production were briefly reviewed. The film composition and crystalline phase of the PZT films were easily controlled using the MOCVD process. The electrical properties were also controlled by changing the growth parameters. A low processing temperature was achieved using a new Pb precursor, tryethyl n-pentoxy lead. Large area growth of PZT and PLZT films on a 6–8 inch wafer was also achieved.     

基于YBa2Cu3O7块材的高温超导限流器的限流特性研究

电阻型高温超导限流器(HTSFCL)利用超导体的超导态-正常态转变来限制短路电流,无需短路故障检测电路,具有体积小、重量轻的优点。为此在国内提出了基于YBa2Cu3O7高温超导材料的电阻型HTSFCL方案;开展了电阻型HTSFCL限流特性的实验研究,得到了不同电压下,HTSFCL的限流效果;分析了超导体失超电阻的传播特性。实验结果表明:电阻型HTSFCL可将预计200A(交流峰值)的短路电流限到120A,限流效果明显。这对进一步开展基于YBCO材料的电阻型HTSFCL研究具有重要的指导意义。     

The electrical properties of Ba1-x SrxTiO3 thin films grown by sputter deposition

Abstract

Ba_1-x Sr_xTiO₃ (BST) thin films were deposited by reactive rf-magnetron sputtering onto Si substrates. The influence of the deposition parameters such as temperature and oxygen ambient on the dielectric constant of the films is presented. BST films deposited at 450°C and optimum conditions exhibited a dielectric constant of approximately 200 at frequencies as high as 1GHz. In addition, the films were found to have leakage current densities of <0.1μAmp/cm² at fields of 5×10⁵V/cm. An extrapolated lifetime greater than 10 years was obtained from stress tests at elevated temperatures and fields. These films compared favorably with published data.     

Structure and dielectric properties of Ba(Sn0.2Ti0.8)O3 thin films grown on different substrates by a sol-gel process

Ba(Sn_0.2Ti_0.8)O₃ (BTS) thin films were deposited on Pt/Ti/SiO₂/Si and LaNiO₃/Pt/Ti/SiO₂/Si substrates by a sol-gel processing technique, respectively. The influences of substrates on the phase and microstructure of the thin films were examined. Dielectric properties of the thin films were investigated as a function of frequency and direct current electric field. The results showed that the substrates strongly influenced the microstructure and the dielectric properties of the films. The properties of BTS thin films on LaNiO₃/Pt/Ti/SiO₂/Si substrates were superior to that of the films grown on Pt/Ti/SiO₂/Si substrates.