Microwave properties and applications of Y–Ba–Cu–O thin films grown on various substrates

Microwave properties of YBa₂Cu₃O_7-δ (YBCO) films grown on (100) LaAlO₃ (LAO), (110) NdGaO₃ (NGO) and (001) SrLaAlO₄ (SLAO) substrates were studied in the form of a microstrip ring resonator at temperatures above 20 K. The YBCO resonator on a SLAO substrate showed microwave properties better than or comparable to other YBCO resonators on LAO substrates. For the YBCO resonators on LAO and SLAO substrates, both Q_U and f₀ appeared to decrease as the temperature was raised. Meanwhile the resonator on a NGO substrate showed different behaviors with Q_U showing a peak at ∼70 K, which are attributed to the unique temperature dependence of the loss tangent of the NGO substrate. An X-band oscillator with a YBCO ring resonator coupled to the circuit was prepared and its properties were investigated at low temperatures. The frequency of the oscillator signal appeared to change from 7.925 GHz at 30 K to 7.878 GHz at 77 K, which was mostly attributed to the change in f₀ of the YBCO ring resonator. The signal power appeared to be more than 4.5 mW at 30 K and 2.1 mW at 77 K, respectively. At 55 K, the frequency of the oscillator signal was 7.917 GHz with the 3 dB-linewidth of 450 Hz.     

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa₂Cu₃O_7−δ (YBCO) high-T_c superconductor. It was found that the transverse voltage V_x was proportional to the longitudinal temperature difference Δ T_z, measured directly with a pair of differential thermocouples, for textured samples with their c-axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V_x against the sample’s thickness d, at a given Δ T_z, deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V_x(d) was fitted to obtain a reasonably reliable ∣ S_c−S_ab∣ value of 12 μ V K⁻¹.     

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.     

Depression of the superconducting critical temperature and finite-size scaling relation in YBa2Cu3O7−δ/La2/3Ca1/3MnO3 bilayers

High-quality YBa₂Cu₃O_7?δ/La_2/3Ca_1/3MnO₃ (YBCO/LCMO) bilayers were fabricated on (0 0 1)-oriented SrTiO₃ (STO) substrates by dc-sputtering technique. Bottom layer was always LCMO since it grows better on STO than on YBCO. The thickness of the ferromagnetic layer varied between 5 and 35 monolayers (～2–13 nm) and that of the top YBCO was fixed at 10 monolayers (～12 nm). The transport properties of the YBCO layers as well as the magnetic properties of the LCMO counterparts were studied as a function of the LCMO layer thickness. A sizeable depression of the Curie temperature (T_C) of the LCMO layers from the bulk to lower temperatures is observed when decreasing their layer thickness d_LCMO, which might be ascribed to intrinsic dimensionality effects or strain-induced phenomena. On the contrary, the superconducting critical temperature of the YBCO layer T_S displays a sudden strong decrease at a critical LCMO thickness of ～12 nm. Since no dramatic change of the structural and morphological quality of the YBCO top layers with increasing d_LCMO is observed, the suppression of T_S of the YBCO layers should take place via proximity effect due to the increasing magnetization strength in the LCMO layer. However, extrinsic factors like interface strain, interdiffusion of cations between YBCO and LCMO or injection of spin-polarized carriers from the magnetic into the superconducting layer could also play an important role or even to be directly responsible for the observed depressed T_S.     

Melt texturing of YBCO for high current applications

In recent years significant progress has been made in J_c enhancement in high T_c superconductors using melt texturing techniques. Among the many melt texturing methods and modifications, seeding and directional solidification techniques offer extensive control over the location of the growth front and the growth direction over long distances during melt texturing which makes these techniques most attractive from the standpoint of long conductor fabrication. These processes have the capability of producing J_cs of about 45 000 A cm⁻² across single domains of YBCO. A novel variant of the conventional melt texturing process called the liquid phase removal method provides a means to improve the grain boundary coupling in melt textured bulk polycrystalline HTS. Grain boundaries in samples processed by this technique with misorientation angles as high as 54° have demonstrated J_cs as high as 18 000 A cm⁻². Recent developments in texturing of RE-123 compounds (Nd and Yb) at high growth rates give promise for considerable reduction in processing times in directional solidification. Texturing has been observed even in samples processed at rates as high as 100 mm h⁻¹. With these advances in melt texturing methods, utilization of bulk HTS in practical applications such as high capacity current leads etc., appears to be a distinct possibility of the near future.     

Biaxially aligned YBCO film tapes fabricated by all pulsed laser deposition

Biaxially aligned yttria-stabilized zirconia (YSZ) films on Ni-based alloy substrates were realized with high deposition rate of 0.5 μm min⁻¹ by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing effect in the ISD process. To raise I_c values, YBCO thickness was increased up to 5 μm. Thick YBCO films with high J_c values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high I_c value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved.     

Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory

We have investigated the contact resistivity of GeCu₂Te₃ (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρ_c of as-deposited amorphous GCT to W was 3.9×10⁻² Ω cm². The value of ρ_c drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρ_c of 4.8×10⁻⁶ Ω cm². The ρ_c contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge₂Sb₂Te₅ (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell.     

Growth features and intergranular connectivity of melt processed YBCO

Large grain melt processed Y-Ba-Cu-O (YBCO) samples have been prepared by seeded and unseeded growth techniques. The current carrying ability within individual grains and across grain boundaries has been investigated and correlated with features in the microstructure of samples fabricated by both techniques. The development of an inhomogeneous, cell-like growth microstructure in seeded samples at distances ≈4 mm from the seed is related to a saturation in inclusion density and volume proportion of Y₂BaCuO₅ (Y211) particles. A significant decrease in critical current density is associated with this change over a wide temperature range. The resistance of high angle c-axis grain boundaries is observed to depend critically on the magnitude of the injection current whereas the intra-granular resistance is not influenced significantly by this variable. J_c of a grain boundary fabricated by unseeded melt growth is estimated to be less than 100 A cm⁻² at 77 K in zero applied field, which is more than two orders of magnitude lower than the intragranular J_c. Field screening measurements suggest that low angle grain boundaries do not form weak links between grains in modest magnetic fields and hence do not present a significant barrier to current flow.     

Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

Thermal stability of lead-free CH3NH3SnxI3 systems (0.9 ≤ x ≤ 1.1) for photovoltaics

Methylammonium-tin-iodide (MASn_xI₃, 0.9 ≤ x ≤ 1.1) systems were prepared by precipitation process in aqueous solutions. The “as prepared” MASn_xI₃ systems exhibited a tetragonal crystalline phase (space group I4cm) with polyhedral crystallites (length 50–400 µm). The as prepared samples were annealed at T = 150 °C for t = 8 h under nitrogen and synthetic air. Under nitrogen, the CH₃NH₃Sn_xI₃ systems adopt nearly-cubic tetragonal structure (space group P4mm) with crystallites of 2–4 µm length whereas a degradation process with formation of non-crystalline phases occurred in air. The differential thermal analysis (DTA) profile in nitrogen revealed events at T = 247 °C, T = 297 °C (decomposition of CH₃NH₃Sn_xI₃ systems into methylamine (CH₃NH₂), hydroiodic acid (HI) and SnI₂), and in the range T = 342–373 °C (melting of SnI₂) respectively. The thermal profile in air showed endothermic events at T = 139 °C and T = 259 °C with additional events at onset temperatures of T = 337 °C and T = 423 °C respectively which correspond to the formation of Sn(IV)-O binds and to the decomposition of methylamine. Static thermogravimetry analysis (TG), performed at T = 85 °C and T = 150 °C for t = 2 h, revealed a linear weight loss as a function of the time. The optical absorption spectra displayed absorbance edges in near infrared range, at 1107.0 nm (x = 0.9), 1098.6 nm (x = 1.0) and 1073.2 nm (x = 1.1) respectively.     

Measurements of n-InGaAs with MBE layers: Relevance of negative magnetoresistance in the investigated samples

Measurements of n_H were performed. n_H values showed a distinct increase at temperatures below ~90 K (1.1 μm n-InGaAs samples) and a decrease at temperatures below ~30 K (7 μm n-InGaAs samples), depending on the doping level. These trends might be related to the magnetoresistance (MR) of the n-InGaAs samples. The MR behavior of the n-InGaAs samples with respect to magnetic field and temperature was apparently dependent on the doping level. Two n-InGaAs samples, one of which had a thin InGaAs epilayer (1.1 μm) and the other with a thicker (7 μm) epilayer, showed interesting behavior at low temperature. Their behavior at magnetic fields of approximately −15000 to +15,000 G were determined. The resistivity ((ρ_G – ρ₀)/ρ₀) of the 1.1 μm sample was negative at temperatures lower than 30 K.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Effect of magnetic field on galvanomagnetic properties of mica/Bi/EuS heterostructures

The dependences of the Hall coefficient R_H and magnetoresistance Δρ/ρ on magnetic field (B=0.01?1.0 T) were obtained in the temperature range 77–300 K for thin Bi films with thicknesses d=40–250 nm, grown on mica substrates and covered by a EuS layer. It was established that in the entire temperature range for all Bi films the criterion of weak field was fulfilled at magnetic fields up to 1 T: R_H remained practically constant in the entire range of magnetic field and Δρ/ρ for all investigated samples changed with changing magnetic field according to a parabolic law.     

Direct observation of changes in the effective minority-carrier lifetime of SiNx-passivated n-type crystalline-silicon substrates caused by potential-induced degradation and recovery tests

We directly observed reductions in the effective minority-carrier lifetime (τ_eff) of n-type crystalline silicon (c-Si) substrates with silicon-nitride passivation films caused by potential-induced degradation (PID). We prepared PID-test samples by encapsulating the passivated substrates with standard photovoltaic-module encapsulation materials. After PID tests applying − 1000 V to the c-Si samples from the glass surface, the τ_eff was decreased, which probably pertains to Na introduced into the c-Si. After PID tests applying + 1000 V, the sample, on the other hand, showed a considerably rapid τ_eff reduction, probably associated with the surface polarization effect. We also performed recovery tests of predegraded samples, by applying a bias opposite to that used in a degradation test. The τ_eff of a sample predegraded by applying + 1000 V was rapidly completely recovered by applying − 1000 V, while those of predegraded by applying − 1000 V show only slight and insufficient τ_eff recovery.     

Structural and electrical properties of Pr1−xSrxMnO3 (x=0.25, 0.3, 0.35 and 0.4) manganites

In this work, we have studied the structural features and electrical conductivity of the polycrystalline Pr_1−xSr_xMnO₃ (x=0.25, 0.3, 0.35, 0.4) manganites. All the compounds have orthorhombic structure with space group Pbnm. The electrical resistivity of all the samples exhibits metal–insulator transition T_MI for both H=0 T and H=8 T. At H=8 T, ρ (T) drops down considerably which indicates the existence of the CMR effect. The electron–phonon, electron–electron and electron–spin fluctuation interactions are effective to describe the resistivity behavior for temperature less than the metal–insulator transition (T_MI). For temperatures, T>T_MI, the insulating nature is discussed with a small polaron conduction (SPC) model. The resistivity of all the samples shows a field-dependent minimum at low temperature. The above is due to the Coulomb interaction between carriers strongly enhanced by disorder and Kondo-like spin dependent scattering.     

Structural,optical and antibacterial properties of nanocrystalline Zn1–xLaxO compound semiconductor

The Zn_1–xLa_xO powders were synthesized by the planetary ball milling method. An accumulated milling time of 15 h with a milling speed of 400 rpm were found to be the optimum milling conditions. The crystal structure, morphology, selected area electron diffraction, and elemental analysis were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The solubility limit of La that could substitute at the Zn sites was only 5 mol% or x=0.05. The dependence of the crystallite size on the La doping content can be explained by the Zener pinning effect. The particle size of the milled ZnO powder was about 34.03 nm and the particle size was reduced to 30.90 nm when doped with 10 mol% La. The particles of all milled samples could agglomerate as a cluster. The largest E_g value of 3.14 eV was obtained from the Zn_0.97La_0.03O powder due to this sample having the smallest crystallite size. The Zn_1–xLa_xO powders can inhibit Staphylococcus aureus better than Escherichia coli due to the presence of an extra lipopolysaccharide layer on the outer surface of the latter.     

Comparison of IGBT short-circuit failure “ohmic mode”: Epoxy molded package versus silicone gel module for new fail-safe and interruptible power converters

An epoxy molded package is compared with a silicone gel module with IGBTs chips in short-circuit failure modes with respect to critical energy, I²T_melting and explosion energy capabilities. Special importance was attached to “ohmic mode” assessment and ageing of the failed chips. The molded technology yields a very low and stable R_sc (<10 mΩ) as a “residual ohmic value” of the dies in low energy short-circuit failure, which is analysed through a complete reverse. Continuous thermal cycling tests over a medium time duration (>1000 h) also exhibit an acceptable drift of the R_sc property (<20%). The silicone gel module clearly exhibits an unstable R_sc value due to damage of the “free moving” wire-bonding on the chips. The authors show that the paralleled wires connections and the multiple parallel melting pits allow a sort of active redundancy and a possible on-state operation. All these results are used for the design of new and original failsafe converters. These topologies use only one paralleled safety leg that is spontaneously and directly connected in series with the failed devices, through the low R_sc value of the failed chips, without any additional complexity or extra cost.     

Broadband nuclear magnetic resonance using DC SQUID amplifiers

We have constructed two pulsed NMR spectrometers in which the signal is coupled to the input coil of a low T_c DC SQUID using a superconducting flux transformer, yielding broadband response, with bandwidth determined by the SQUID electronics. A 50 kHz bandwidth commercial system has been used to observe free induction decay signals from platinum powder, bulk platinum, ³He gas and surface monolayers of ³He in the temperature range from 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal-to-noise ratio is as calculated with the noise dominated by flux noise in the SQUID in all samples but the bulk metal. A second system, which operates in flux-locked loop mode with bandwidth of 3.4 MHz using a SQUID with additional positive feedback, has been used to observe NMR signals from platinum powder at frequencies from 38 to 513 kHz and at a temperature of 4.2 K. The advantage of this technique in the study of systems with short T₂ at frequencies below 1 MHz is discussed. In addition we discuss the benefits of both broadband and tuned input circuits for NMR detection and we describe the performance of a spectrometer with a tuned input circuit which has been used to obtain signals at 1 MHz from platinum powder at 4.2 K and from ∼2 layers of ³He absorbed on a surface area of 0.11 m² at 1.7 K. The amplifier noise temperature is predicted to be 60 mK in the ³He experiment. This demonstrates the potential of the tuned set-up for measurements at low millikelvin temperatures on systems with low spin density and with T₂ greater than several hundred microseconds.     

High-performance pentacene field-effect transistors using Al2O3 gate dielectrics prepared by atomic layer deposition (ALD)

High-performance pentacene field-effect transistors have been fabricated using Al₂O₃ as a gate dielectric material grown by atomic layer deposition (ALD). Hole mobility values of 1.5 ± 0.2 cm²/V s and 0.9 ± 0.1 cm²/V s were obtained when using heavily n-doped silicon (n⁺-Si) and ITO-coated glass as gate electrodes, respectively. These transistors were operated in enhancement mode with a zero turn-on voltage and exhibited a low threshold voltage (< −10 V) as well as a low sub-threshold slope (<1 V/decade) and an on/off current ratio larger than 10⁶. Atomic force microscopy (AFM) images of pentacene films on Al₂O₃ treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray diffraction patterns showed characteristics of a “thin film” phase. Low surface trap density and high capacitance density of Al₂O₃ gate insulators also contributed to the high performance of pentacene field-effect transistors.     

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (T_s) using glass slides as substrates. At T_s≤673 K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3 μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For T_s=873 K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4 μm. The films obtained at T_s>473 K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Т_s=873 K (0.5×10⁻³) is considerably smaller than for the films deposited at Т_s=773 K (4.0×10⁻³). Increase of the value of T_s improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for T_s=873 K and can be suitable for various applications.