Integrated YBa2Cu3O7 magnetometers for biomagnetic applications

Three concepts of integrated magnetometers intended for biomagnetic multichannel systems are investigated: The Ketchen-type magnetometer, the multiloop magnetometer and the magnetometer with integrated multiloop pickup coil (IMPUC). The devices are fabricated from YBa₂Cu₃O₇-SrTiO₃-YBa₂Cu₃O₇ thin-film multilayers, and step-edge grain-boundary Josephson junctions as well as ramp-edge junctions with PrBa₂Cu₃O₇ barriers are employed. The magnetometers are compared regarding their effective flux collecting area, inductance and noise performance. With an optimized layout for the multiloop device, we achieve an effective flux collecting area of 2.3 mm² at an inductance of 145 pH. At 77 K we measure a magnetic flux density noise down to √S_B=60 fT/√Hz at 1 Hz and of 17 fT/√Hz in the white noise regime. Due to the low noise levels of the magnetometers high quality magnetocardiograms were recorded inside a magnetically shielded room without signal averaging. The noise performance of the magnetometers is also analyzed in unshielded environment and in static magnetic fields.     

Metal–organic chemical vapor deposition as an emerging technique for the fabrication of YBa2Cu3Ox tapes

The deposition of YBa₂Cu₃O_x thin films on metal substrates is currently being investigated as the basis for future tape applications. The YBa₂Cu₃O_x coatings, discussed in this report, are deposited by metal–organic chemical vapor deposition (MOCVD). MOCVD is a versatile technique with many opportunities in terms of reaction chamber design and shapes of substrates to be coated. MOCVD equipment for fabrication of YBa₂Cu₃O_x coatings on long length, flexible metal tapes is currently being developed. Tapes of various materials, such as polycrystalline stainless steel, have been investigated. The growth of biaxially aligned buffer layers between the stainless-steel tapes and YBa₂Cu₃O_x coatings is necessary, to prevent interdiffusion, to reduce interfacial reactions and to allow the succeeding growth of aligned YBa₂Cu₃O_x thin films. The superconducting properties of the resulting YBa₂Cu₃O_x/buffer/substrate thin film systems, T_c≈90 K and J_c>10⁵ A cm⁻², are very promising. the major drawbacks in MOCVD of YBa₂Cu₃O_x thin films are the thermo-chemical properties of MOCVD precursors. For example, the traditional solid precursors have limited volatility. The development of new precursor delivery systems using precursor solutions and new solid and liquid precursors have resulted in improved process reliability. Finally, in the standard MOCVD process, which is thermally activated, the reported growth rates are approximately 0.5 μm h⁻¹. An increase by one order of magnitude can be achieved in a photo-assisted MOCVD process.     

Up-scalable synthesis of size-controlled copper ferrite nanocrystals by thermal treatment method

Close-packed cubic copper ferrites (CuFe₂O₄) nanoparticles were synthesized using an effective thermal-treatment method directly from an aqueous solution containing copper and iron nitrates as metal precursors and poly(vinyl pyrrolidone) as a capping agent. The FTIR spectra of the calcined samples revealed the vibration bands of Fe–O and Cu–O at 315 and 535 nm respectively. The structural, morphological, optical and magnetic properties of the nanocrystal powder samples were analyzed using various characterization techniques. The powder X-ray diffraction unveiled the formation of spinel phase of CuFe₂O₄ with the average particle size determined from TEM images increased from 24 to 34 nm at the calcination temperatures between 773 and 1173 K. The band gap calculated using Kubelka–Munk function from the UV–visible diffuse reflectance spectra decreased from 2.64 to 2.45 eV with increasing calcination temperature. The electron spin resonance (ESR) spectroscopy confirmed the presence of unpaired electrons in the calcined samples. The g-factor increased from 2.10497 to 2.57056 and the resonance magnetic field decreased from 3.11599×10⁻⁷ to 2.55161×10⁻⁷ A/m with increasing calcination temperature.     

Improved flux pinning in melt textured YBa2Cu3O7−δ through chemical additions

The effect of Ce-based additions on Y₂BaCuO₅ (211) particle coarsening and flux pinning in YBa₂Cu₃O_7−δ (123) has been examined. BaCeO₃ is found to react with the Ba-Cu-O rich peritectic liquid. The limited Ce dissolution results in a decreased 211 coarsening rate. In addition, the effects of Ce-based additions (CeO₂ and BaCeO₃) in combination with MgO additions on the magnetic properties of melt textured YBa₂Cu₃O_7−δ have been investigated. The additions lead to improvements in the magnetic properties of YBa₂Cu₃O_7−δ compared to samples with either addition alone or with no additions. The Ce-Mg addition combination produces a “peak effect” in the magnetic hysteresis loop without significant T_c degradation. This is postulated to be due to the formation of a new type of pinning center. Both Ce and Mg ions are thought to substitute in the YBa₂Cu₃O_7−δ lattice, creating defects that produce a “peak effect” in the magnetic hysteresis loop. Mg additions alone lead to a reduced T_c, while Ce additions restore the T_c and enhance the magnitude of the peak.     

Microstructure of high Jc NdBa2Cu3Ox/Nd4Ba2Cu2O10 composite superconductor

Single crystalline NdBa₂Cu₃O_x (Nd123) superconductors with dispersed Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were produced by the floating zone partial melting and solidification (FZPMS) method. The initial composition of the precursor material was Nd_1.8Ba_2.4Cu_3.4O_x with or without 0.1 wt% Pt addition. FZPMS was carried out in a low oxygen partial pressure atmosphere. Microstructure of quenched samples were investigated by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Furthermore, superconductive properties were measured by superconducting quantum interference device (SQUID). The results of SQUID measurements indicate that the critical temperature (T_c) of the samples with Pt addition so produced with the oxygen heat treatment (623 K for 300 h) was 94 K and the critical current density (J_c) was 2.3×10⁴ A/cm² at 77 K, 0.2 T.     

Ultra-low-energy ion-beam-synthesis of Ge nanocrystals in thin ALD Al2O3 layers for memory applications

Structural and electrical properties of ALD-grown 5 and 7 nm-thick Al₂O₃ layers before and after implantation of Ge ions (1 keV, 0.5–1 × 10¹⁶ cm^?2) and thermal annealing at temperatures in the 700–1050 °C range are reported. Transmission Electron Microscopy reveals the development of a 1 nm-thick SiO₂-rich layer at the Al₂O₃/Si substrate interface as well as the formation of Ge nanocrystals with a mean diameter of ～5 nm inside the implanted Al₂O₃ layers after annealing at 800 °C for 20 min. Electrical measurements performed on metal–insulator–semiconductor capacitors using Ge-implanted and annealed Al₂O₃ layers reveal charge storage at low-electric fields mainly due to location of the Ge nanocrystals at a tunnelling distance from the substrate and their spatial dispersion inside the Al₂O₃ layers.     

YBa2Cu3O7−x dc squid magnetometers with bicrystal junctions for biomagnetic multichannel applications

We have fabricated magnetometers based on direct-coupled YBa₂Cu₃O_7−x superconducting quantum interference devices (SQUID) with SQUID inductances of 50 pH and 100 pH on 10×10 mm² 24° SrTiO₃ bicrystal substrates. The thin films were deposited by hollow cathode discharge sputtering and patterned using conventional photolithography and Ar ion beam etching. The optimization of the fabrication process was performed utilizing design of experiment methodology. The SQUID magnetometers were operated with an improved direct-coupled flux-locked loop electronics with a preamplifier voltage noise of 0.44 nV Hz^−1/2 and bias reversal (f_b=0.1–500 kHz). The best sensor had a white noise of 41 fT Hz^−1/2 and a 1/f corner at 2 Hz, including electronics and environmental noise. High-quality magnetocardiograms recorded in the Berlin magnetically shielded room (BMSR) illustrate that the sensors are suitable for biomagnetic multichannel systems.     

Characteristics of ZnOV2O5MnO2Nb2O5Er2O3 semiconducting varistors with sintering processing

The effects of sintering temperature on the microstructure, electrical properties, and dielectric characteristics of ZnOV₂O₅MnO₂Nb₂O₅Er₂O₃ semiconducting varistors have been studied. With increase in sintering temperature the average grain size increased (4.5–9.5 μm) and the density decreased (5.56–5.45 g/cm³). The breakdown field decreased with an increase in the sintering temperature (6214–982 V/cm). The samples sintered at 900 °C exhibited remarkably high nonlinear coefficient (50). The donor concentration increased with an increase in the sintering temperature (0.60×10¹⁸–1.04×10¹⁸ cm^?3) and the barrier height exhibited the maximum value (1.15 eV) at 900 °C. As the sintering temperature increased, the apparent dielectric constant increased by more than four-fold.     

Mixing Y(RE)BCO (RE=Nd,Sm) superconductors by crystal pulling method

Y_1−xRE_xBa₂Cu₃O_7−δ [Y(RE)BCO or REsBCO] superconductors were prepared by the crystal pulling method. The RE mixing content x in the crystal can be effectively controlled by the processing temperature. With an increase in processing temperature, the RE concentration in the liquid increased, which resulted in the higher RE substitution content in single crystals. The higher critical temperatures T_c above 92 K could be obtained by an optimized oxygenation treatment. The T_c value of Y(RE)BCO tends to be insensitive to the growth atmosphere of the oxygen partial pressure under a certain limit of the RE content. The Y_.727Nd_.273Ba₂Cu₃O_7−δ and the Y_.941Sm_.059Ba₂Cu₃O_7−δ samples show values of the critical current density J_c are about 2×10⁴ Acm⁻² at 1.2 T and 2.8×10⁴ Acm⁻² at 1.1 T for H//c, respectively, indicating that mixing REs 123 have obvious effects on J_c–H curves since RE ions are likely to substitute at both Y²⁺ and Ba²⁺ sites.     

Growth of high quality YbBa2Cu3O7−δ thin films by pulsed laser deposition

YbBa₂Cu₃O_7−δ (Yb-123) films are deposited for the first time using Pulsed Laser Deposition (PLD) method at three different substrate temperatures, viz. 675°C, 700°C and 725°C. Films are characterized using XRD, dc electrical resistivity, critical current density (J_c) and microstructural study by Atomic Force Microscopy (AFM) techniques. It is found that 700°C is the optimum growth temperature for growing high quality Yb-123 films. The best T_c and J_c values obtained at optimum growth conditions are 88 K and 2.6×10⁶ A cm⁻² at 77 K, respectively. AFM photographs provide evidence in confirming the relation between growth temperature and superconducting properties.     

ZrO2 insulator modified by a thin Al2O3 film to enhance the performance of InGaZnO thin-film transistor

A high-performance InGaZnO (IGZO) thin-film transistor (TFT) with ZrO₂–Al₂O₃ bilayer gate insulator is fabricated. Compared to IGZO-TFT with ZrO₂ single gate insulator, its electrical characteristics are significantly improved, specifically, enhancement of I_on/I_off ratios by one order of magnitude, increase of the field-effect mobility (from 9.8 to 14 cm²/Vs), reduction of the subthreshold swing from 0.46 to 0.33 V/dec, the maximum density of surface states at the channel-insulator interface decreased from 4.3 × 10¹² to 2.5 × 10¹² cm⁻². The performance enhancements are attributed to the suppression of leakage current, smoother surface morphology, and suppression of charge trapping by using Al₂O₃ films to modify the high-k ZrO₂ dielectric.     

Preparation and physical properties of the layered niobate Cu0.5Nb3O8: Application to photocatalytic hydrogen evolution

The new layered niobate Cu_0.5Nb₃O₈ is synthesized by soft chemistry in aqueous electrolyte via Cu²⁺→H⁺ exchange between copper nitrate and HNb₃O₈·H₂O. The characterization of the exchanged product is made by means of thermal gravimetry, chemical analysis, X-ray diffraction and IR spectroscopy. Thermal analysis shows a conversion to anhydrous compound above 500 °C. The oxide displays a semiconductor like behavior; the thermal variation of the conductivity shows that d electrons are strongly localized and the conduction is thermally activated with activation energy of 0.13 eV. The temperature dependence of the thermopower is indicative of an extrinsic conductivity; the electrons are dominant carriers in conformity with an anodic photocurrent. Indeed, the Mott–Schottky plot confirms n-type conduction from which a flat band potential of −0.82 V_SCE, an electronic density of 8.72×10¹⁹ m⁻³ and a depletion width of 4.4 nm are determined. The upper valence band, located at ~5.8 eV below vacuum is made up predominantly of Cu²⁺: 3d with a small admixture of O²⁻: 2p orbitals whereas the conduction band consists of empty Nb⁵⁺: 5s level. The energy band diagram shows the feasibility of the oxide for the photocatalytic hydrogen production upon visible light (29 mW cm⁻²) with a rate evolution of 0.31 mL g⁻¹ min⁻¹.     

Growth and characterization of indium oxide diodes prepared by reactive magnetron sputtering

The electrical properties of Cr/Pt/Au and Ni/Au ohmic contacts with unintentionally doped In₂O₃ (U-In₂O₃) film and zinc-doped In₂O₃ (In₂O₃:Zn) prepared by reactive magnetron sputtering deposition are described. The lowest specific contact resistance of Cr/Pt/Au and Ni/Au is 2.94 × 10⁻⁶ and 1.49 × 10⁻² Ω-cm², respectively, as determined by the transmission line model (TLM) after heat treatment at 300 °C by thermal annealing for 10 min in nitrogen ambient. The indium oxide diodes have an ideality factor of 1.1 and a soft breakdown voltage of 5 V. The reverse leakage current prior to breakdown is around 10⁻⁵ A.     

Synthesis and characterization of the semiconductor Li0.93Cu0.07Nb3O8: Application to oxygen evolution under visible light

The semiconductor Li_0.93Cu_0.07Nb₃O₈ is prepared by soft chemistry in aqueous electrolyte via Cu²⁺ → Li⁺ exchange between copper nitrate and LiNb₃O₈. The substituted niobate crystallizes in an orthorhombic symmetry and the semiconducting and photo-electrochemical properties are investigated for the first time. The oxide exhibits a dark brown color and the UV–Visible spectroscopy gives an optical gap of 1.42 eV, due to the crystal field splitting of Cu²⁺ in octahedral site. The thermal variation of the conductivity shows that Nb: 4d-electrons are localized and the data are fitted by a small-polaron hopping model σ = σ_o exp {−0.053 eV/kT} with a carrier density thermally activated. The capacitance measurement done in ionic electrolyte (Na₂SO₄, 10⁻² M) indicates n type semiconductor with mixed valences Nb^5+/4+, due to the hetero-valent substitution Li⁺/Cu²⁺, with a flat band potential of 0.28 V_SCE and electrons density of 2.17×10¹⁷ cm⁻³. The Nyquist diagram shows mainly the bulk contribution with a diffusion process. The valence band (6.39 eV below vacuum) derives from O^2-: 2p orbital with a small admixture of Cu²⁺: 3d character while the conduction band is made up of Nb⁵⁺: 4d orbital. The material is successfully tested for the oxygen generation with an evolution rate of 87 µmol mn⁻¹ g⁻¹ under visible light (29 mW cm⁻²) and a quantum yield of 0.35%.     

YBCO/YSZ coated conductors on flexible Ni alloy substrates

A coating system for the deposition of in-plane oriented yttria-stabilized zirconia (YSZ) template films on 1 cm wide flexible metal substrates is presented. In static mode, the system is capable of producing high quality template films on 20 cm substrate lengths. In a continuous coating mode, the system is capable of producing good quality template films on 1.1 m substrate lengths. Superconducting YBa₂Cu₃O_7−δ (YBCO) films subsequently deposited onto these template films have demonstrated critical currents (I_c) of 200 A (1.5 cm length), 70 A (12 cm length) and 4 A (1 m length).     

Rapid synthesis of Y- and Bi-based high-Tc cuprates by microwave irradiation

High-T_c superconducting samples of YBa₂Cu₃O_7−x with T_c∼90 K and Bi_2.2Sr_1.8Cu_1.05O_x with T_c∼9 K have been prepared for several ten min, using a domestic microwave oven operated at 2.45 GHz, without any post-heat-treatment. Post-heat-treatment is not necessary due to improvement of the sample environment during the microwave process. That is, a pellet of mixed powder of starting materials is surrounded by mixed powder of starting materials and, subsequently, wrapped in glass wool in order to suppress rapid dissipation of heat from the surface of the pellet. This leads to successful preparation of homogeneous and oxygenated samples. In addition, we have attempted to prepare Bi-2212 and Bi-2223 phases. A sample whose major phase was Bi-2212 was obtained. However, no sample with the Bi-2223 phase could be obtained.     

IGZO thin film transistors with Al2O3 gate insulators fabricated at different temperatures

Thin film transistors (TFTs) with bottom gate and staggered electrodes using atomic layer deposited Al₂O₃ as gate insulator and radio frequency sputtered In–Ga–Zn Oxide (IGZO) as channel layer are fabricated in this work. The performances of IGZO TFTs with different deposition temperature of Al₂O₃ are investigated and compared. The experiment results show that the Al₂O₃ deposition temperature play an important role in the field effect mobility, I_on/I_off ratio, sub-threshold swing and bias stability of the devices. The TFT with a 250 °C Al₂O₃ gate insulator shows the best performance; specifically, field effect mobility of 6.3 cm²/Vs, threshold voltage of 5.1 V, I_on/I_off ratio of 4×10⁷, and sub-threshold swing of 0.56 V/dec. The 250 °C Al₂O₃ insulator based device also shows a substantially smaller threshold voltage shift of 1.5 V after a 10 V gate voltage is stressed for 1 h, while the value for the 200, 300 and 350 °C Al₂O₃ insulator based devices are 2.3, 2.6, and 1.64 V, respectively.     

Rationalization of dielectric properties of nano-sized iron doped yttrium copper titanate using impedance and modulus studies

Iron doped Yttrium Copper Titanate nano-ceramic with composition, Y_2/3Cu₃Ti_3.90Fe_0.10O₁₂ (YCTFO) was prepared by semi-wet route. It displays all normal XRD peaks of Y_2/3Cu₃Ti₄O₁₂ (YCTO) along with a few secondary peaks of CuO. Stoichiometric purity of the composition was ascertained by EDX spectral analysis. The distribution of bimodal spherical grains confirms 0.5–1.5 µm size limit along with a few irregular shaped large grains with size 1.5–2.8 µm. The impacts of acceptor type of hetero-valent substitution of Ti⁴⁺ by Fe³⁺ in Y_2/3Cu₃Ti₄O₁₂ (YCTO) were reflected as decrease in grain size and broadening of ε_r–T peak with simultaneous decrease in ε_r value. The presence of temperature dependent relaxation was also rationalized by impedance and modulus spectroscopic studies which confirm ferroelectric to paraelectric phase transition at 348 K.     

Effect of gate insulator thickness on device performance of InGaZnO thin-film transistors

Top-contact thin-film transistors (TFTs) are fabricated in this work using atomic layer deposition (ALD) Al₂O₃ as the gate insulator and radio frequency sputtering InGaZnO (IGZO) as the channel layer so as to investigate the effect of Al₂O₃ thickness on the performance of IGZO-TFTs. The results show that TFT with 100-nm-thick Al₂O₃ (100 nm-Al₂O₃-TFT) exhibits the best electrical performance; specifically, field-effect mobility of 5 cm²/Vs, threshold voltage of 0.95 V, I_on/I_off ratio of 1.1×10⁷ and sub-threshold swing of 0.3 V/dec. The 100 nm-Al₂O₃-TFT also shows a substantially smaller threshold voltage shift of 1.1 V after a 10 V gate voltage is applied for 1 h, while the values for TFTs with an Al₂O₃ thickness of 220 and 280 nm are 1.84 and 2 V, respectively. The best performance of 100 nm-Al₂O₃-TFT can be attributed to the larger capacitance and the smaller amount of total trap centers possessed by a thinner insulator compared to the thicker ones.     

Electrical and optical properties of Cu2ZnSnS4 grown by a thermal co-evaporation method and its diode application

Cu₂ZnSnS₄ (CZTS) is low cost and constitutes non-toxic materials abundant in the earth crust. Environment friendly solar cell absorber layers were fabricated by a thermal co-evaporation technique. Elemental composition of the film was stated by energy dispersive spectroscopy (EDS). Some optical and electrical properties such as absorption of light, absorption coefficient, optical band gap charge carrier density, sheet resistance and mobility were extracted. Optical band gap was found to be as 1.44 eV, besides, charge carrier density, resistivity and mobility were found as 2.14×10¹⁹ cm⁻³, 8.41×10⁻⁴ Ω cm and 3.45×10² cm² V⁻¹ s⁻¹, respectively. In this study Ag/CZTS/n-Si Schottky diode was fabricated and basic diode parameters including barrier height, ideality factor, and series resistance were concluded using current–voltage and capacitance–voltage measurements. Barrier height and ideality factor values were found from the measurements as 0.81 eV and 4.76, respectively, for Ag/CZTS/n-Si contact.