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).     

Preparation of in-plane textured buffer layers for YBa2Cu3Oy film growth by modified bias sputtering

A modified bias sputtering technique has been proposed to grow in-plane textured yttria-stabilized zirconia buffer layers on polycrystalline metallic substrates for deposition of YBa₂Cu₃O_y films. The principle of developing an in-plane texturing by this technique is basically the same as that of ion beam assisted deposition; an in-plane texturing occurs by off-normal ion beam bombardment because of the higher sputtering yields of all orientations other than the channelling direction. In our process, however, a flux of energetic particles impinging on the growing film is generated using specially devised negatively biased electrodes installed in a magnetron sputtering system instead of a separate ion-source in IBAD. So far an X-ray phi-scan width of 18° was attained for YSZ films on Hastelloy tapes. Epitaxial YBCO films grown on these buffer layers using pulsed laser deposition showed the J_c’s exceeding 10⁵ A cm⁻² (77 K, 0 T). In this paper, we present variation of the bias sputtering technique also used to obtain the textured films on large area substrates. Although the proposed process offers a very convenient method to grow textured films, more efforts must be made to increase growth rates of the films (currently ∽0.1 nm s⁻¹) for large-scale applications of YBCO films.     

Film thickness effect on the properties of interconnection between YBCO and Si for superconductor and semiconductor integration

YBa₂Cu₃O_7−δ (YBCO) films were grown on Si using a buffer layer of yttria-stabilized zirconia (YSZ) by pulsed laser deposition. Interconnections between the YBCO film and the Si substrate using Au have been fabricated by lithography. The YBCO films showed zero-resistance critical temperatures in the range of 80–85 K and were found to be grown in the c–axis orientation. The structural properties of the films of various thicknesses were analyzed by XRD, SEM and Raman spectroscopy. Because of very different thermal expansion coefficients of Si and YBCO, there is a tendency to crack formation for films. The crack formation interrupts the current flow and increases the normal state resistance of the films. The effect of cracks on the contact resistances between YBCO films of various thicknesses and Si substrates through Au interconnections is reported.     

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.     

Structural and magnetic properties of silver doped melt-textured YBCO prepared in a solar furnace

Magnetic measurements and structural investigation have been performed on melt-textured YBCO and AgYBCO HTS. The “sun” technique produces very dense YBCO (ρ=5.86 g cm⁻³) and AgYBCO [ρ=6.36 g cm⁻³; 10% b/w (by weight) silver. This technique renders samples with a large volume fraction of the Y₂BaCuO₅ (211) phase. The material is characterized by very high J_c values, as compared with bulk polycrystalline YBCO prepared by other methods. This feature is attributed to the enhanced amount of 211 particles which serve as pinning centers. Additional significant densification of the structure due to silver incorporation is obtained, and a reduction of the size of 211 inclusions is also observed. Silver doped samples show “butterfly”-like hysteresis loops at relatively high temperatures (T≥60K). This feature is probably associated with oxygen deficiency which arises from the slower oxygen diffusion into silver doped samples. J_c values enhancement was obtained in silver doped “sun” samples at high temperatures (T≥60K) and fields of 20–30 kOe. The temperature dependence of effective activation energy of pinning, U_eff, was measured for YBCO and AgYBCO materials. U_eff is higher in silver doped samples in the high temperature region T≥60K.     

Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell

Wide band gap and highly conducting n-type nano-crystalline silicon film can have multiple roles in thin film solar cell. We prepared phosphorus doped micro-crystalline silicon oxide films (n-μc-SiO:H) of varying crystalline volume fraction (X_c) and applied some of the selected films in device fabrication, so that it plays the roles of n-layer and back reflector in p-i-n type solar cells. It is generally understood that a higher hydrogen dilution is needed to prepare micro-crystalline silicon, but in case of the n-μc-SiO:H an optimized hydrogen dilution was found suitable for higher X_c. Observed X_c of these films mostly decreased with increased plasma power (for pressure<2.0 Torr), increased gas pressure, flow rate of oxygen source gas and flow rates of PH₃>0.08 sccm. In order to determine deposition conditions for optimized opto-electronic and structural characteristics of the n-μc-SiO:H film, the gas flow rates, plasma power, deposition pressure and substrate temperature were varied. In these films, the X_c, dark conductivity (σ_d) and activation energy (E_a) remained within the range of 0–50%, 3.5×10⁻¹⁰ S/cm to 9.1 S/cm and 0.71 eV to 0.02 eV, respectively. Low power (30 W) and optimized flow rates of H₂ (500 sccm), CO₂ (5 sccm), PH₃ (0.08 sccm) showed the best properties of the n-μc-SiO:H layers and an improved performance of a solar cell. The photovoltaic parameters of one of the cells were as follows, open circuit voltage (V_oc), short circuit current density (J_sc), fill-factor (FF), and photovoltaic conversion efficiency (η) were 950 mV, 15 mA/cm², 64.5% and 9.2% respectively.     

High performance dye-sensitized solar cells (DSSCs) achieved via electrophoretic technique by optimizing of photoelectrode properties

This paper describes a simple method utilizing electrophoretic deposition (EPD) of commercial P25 nanoparticles (NPs) films on fluoride-doped tin oxide (FTO) substrate. In this process, voltage and the number of deposition cycles are well controlled to achieve TiO₂ film thickness of around 1.5–26 μm, without any mechanical compression processing. The experimental results indicate that the TiO₂ film thickness plays an important role as the photoelectrode in DSSCs because it adsorbs a large number of dye molecules which are responsible for electrons supply. Furthermore, it was found that effects of the bulk traps and surface states within the TiO₂ films on the recombination of the photo-injected electrons (electron–hole pairs) strongly depend on the TiO₂ electrode annealing temperature. Finally, a DSSC with a 24 μm thick TiO₂ film and annealed at 500 °C produced the highest conversion efficiency (η=6.56%, I_SC=16.4, V_OC=0.72, FF=0.55) with an incident solar energy of 100 mW/cm².     

Sputtered oxides used for passivation layers of amorphous InGaZnO thin film transistors

Four sputtered oxide films (SiO₂, Al₂O₃, Y₂O₃ and TiO₂) along with their passivating amorphous InGaZnO thin film transistors (a-IGZO TFTs) were comparatively studied in this paper. The device passivated by an Al₂O₃ thin film showed both satisfactory performance (μ_FE=5.3 cm²/V s, I_on/I_off>10⁷) and stability, as was probably related to smooth surface of Al₂O₃ thin films. Although the performance of the a-IGZO TFTs with a TiO₂ passivation layer was also good enough (μ_FE=3.5 cm²/V s, I_on/I_off>10⁷), apparent V_th shift occurred in positive bias-stress tests due to the abnormal interface state between IGZO and TiO₂ thin films. Sputtered Y₂O₃ was proved no potential for passivation layers of a-IGZO TFTs in this study. Despite unsatisfactory performance of the corresponding a-IGZO TFT devices, sputtered SiO₂ passivation layer might still be preferred for its high deposition rate and excellent transparency which benefit the mass production of flat panel displays, especially active-matrix liquid crystal displays.     

Single-walled carbon nanotube transistors fabricated by advanced alignment techniques utilizing CVD growth and dielectrophoresis

Single-walled carbon nanotube field effect transistors (SWNT-FETs) are fabricated by two different alignment techniques. The first technique is based on direct synthesis of an aligned SWNTs array on quartz wafer using chemical vapor deposition. The transistor with three SWNTs and atomic layer deposited (ALD) Al₂O₃ gate oxide shows a contact resistance of 280 KΩ, a maximum on-current of ?7 μA, and a high I_on/I_off ratio (>10³). The second technique is based on room temperature self-assembly of SWNT bundles using dielectrophoresis. By applying AC electric fields, we have aligned nanotube bundles between drain and source contact patterns of a transistor at room temperature. Transistors based on twisted bundle of SWNTs show high contact resistance (MΩ range) and low current drive in the order of tens of nA.     

Overdamped NbN junctions with NbNx barriers formed by plasma nitridation

We have fabricated NbN Josephson junctions with NbN_x barriers formed by plasma nitridation on the surface of the base superconductive NbN layer. Both nonhysteretic junctions (overdamped junctions) and small-hysteretic ones have been obtained at 4.2 K by changing plasma nitridation conditions. An overdamped junction showed a product of the critical current I_c and junction resistance R_n (an I_cR_n product) of 0.95 mV at 4.2 K and the I_c exponentially decreased with increasing temperature. On the other hand, a small-hysteretic junction showed Superconductor-Insulator-Superconductor (SIS)-like characteristics in the temperature dependence of I_c and its current–voltage characteristics changed to nonhysteretic ones at the temperature more than 8 K. The I_cR_n product for the junction was 0.91 mV at 8.2 K.     

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.     

Microwave performance of field-plate 0.13-μm MOS transistors with varying field-plate extension

Si-based field-plate 0.13 μm gate length metal-oxide-semiconductor field effect transistor (Si MOSFET) with field-plate (FP) lengths of 0.1 μm, 0.2 μm, and 0.3 μm have been fabricated and investigated. The field-plate metals were connected to gate electrode in this study to improve device gate resistance (R_g) resulting in the better microwave performance. By increasing the length of field-plate metal extension (L_FPE), the off-state drain-to-source surface leakage current can be suppressed. Besides, low surface traps in FP NMOS also leads to a higher drain-to-source current (I_ds) especially at high current regime compared to standard device. The power added efficiency (PAE) was 56.3% for L_FPE of 0.3 μm device, and these values where 54.7% and 53.8% for L_FPE of 0.2 μm and 0.1 μm devices, respectively. Wider field-plate metal extension exhibits highly potential for low noise amplifier and high efficiency power amplifier applications.     

Properties of high sensitivity ZnO surface acoustic wave sensors on SiO2/(1 0 0) Si substrates

The properties of ZnO/SiO₂/Si surface acoustic wave (SAW) Love mode sensors were examined and optimized to achieve high mass sensitivity. SAW devices A and B, were designed and fabricated to operate at resonant frequencies around 0.7 and 1.5 GHz. The ZnO films grown by pulsed laser deposition on SiO₂/Si demonstrated c-axis growth and the fabricated devices showed guided shear horizontal surface acoustic wave (or Love mode) propagation. Acoustic phase velocity in the ZnO layer was measured in both devices A and B and theoretical and experimental evaluation of the mass sensitivity showed that the maximum sensitivity is obtained for devices with ZnO guiding layer thicknesses of 340 nm and 160 nm for devices A and B, respectively. The performance of the SAW sensors was validated by measuring the mass of a well-characterized polystyrene–polyacrylic acid diblock copolymer film. For the optimized sensors, maximum mass sensitivity values were as high as 4.309 μm²/pg for device A operating at 0.7477 GHz, and 8.643 μm²/pg for device B operating at 1.5860 GHz. The sensors demonstrated large frequency shifts per applied mass (0.1–4 MHz), excellent linearity, and extended range in the femto-gram region. The large frequency shifts indicated that these sensors have the potential to measure mass two to three orders of magnitude lower in the atto-gram range.     

Realizing high breakdown voltage for a novel interface charges islands structure based on partial-SOI substrate

A novel interface charge islands partial-SOI (ICI PSOI) high voltage device with a silicon window under the source and its mechanism are studied in this paper. ICI PSOI is characterized by a series of equidistant high concentration n⁺-regions on the bottom interface of top silicon layer. On the condition of high-voltage blocking state, inversion holes located in the spacing of two n⁺-regions effectively enhance the electric field of the buried oxide layer (E_I) and reduce the electric field of the silicon layer (E_S), resulting in a high breakdown voltage (V_B). It is shown by the simulations that the enhanced field ΔE_I and reduced field ΔE_S by the accumulated holes reach to 449 V/μm and 24 V/μm, respectively, which makes V_B of ICI PSOI increase to 663 V from 266 V of the conventional PSOI on 5 μm silicon layer and 1 μm buried oxide layer with the same silicon window length. On-resistance of ICI PSOI is lower than that of the conventional PSOI. Moreover, self-heating-effect is alleviated by the silicon window in comparison with the conventional SOI at the same power of 1 mW/μm.     

Electromigration study of SNS ramp edge Josephson junctions

We report on the effects of electromigration of basal plane oxygen vacancies on SNS ramp edge Josephson junctions where the N-layer is YBa₂Cu_2.79Co_0.21O_7−δ, a doped version of the YBCO electrodes. Through the application of a 4–10 mA (∼2–5 MA/cm²) current bias at room temperature, the basal plane oxygen order and content in the N and S layers were improved. This is demonstrated by an increase in I_cR_n from <5 μV, to as much as 205 μV. The implications of these results on SNS junction fabrication, and the nature of tunneling in such devices are discussed.     

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.     

Y-123 Films on technical substrates

For high-current applications, homogeneous well-textured high-temperature superconducting Y₁Ba₂Cu₃O_7−δ (Y-123) films on technical ceramic or metallic substrates are required. The ion-beam-assisted deposition (IBAD) of yttria-stabilized zirconia (YSZ) buffer layers, which serve as a template for Y-123, was extended to large-area substrates as well as differently shaped long substrates. TEM, as well as XRD investigations, was performed to observe the relevant growth mechanisms. The influence of nucleation, growth selection, and homoepitaxial growth on the development or the maintenance of a preferential orientation of the YSZ buffer is discussed. Y-123 films were prepared by a modified pulsed-laser-deposition (PLD) technique based on a quasi-equilibrium substrate heating and variable azimuth scanning of the target. The method allows a PLD coating of long pieces of tapes and tubular substrates because no direct mechanical contact of a substrate with the heater is required. Critical current densities, J_c, above 10⁶ A cm⁻² were achieved on polycrystalline Ni tapes as well as on polycrystalline YSZ sheets at 77 K and self fields. The dependence of J_c on magnetic field resembles that of Y-123 deposited on single-crystalline substrates and demonstrates the absence of weak links due to grain boundaries. The thickness dependence of J_c is interpreted in terms of a nucleation layer appearing in the early stage of film growth. A growth model is proposed which seems to be in agreement with the experimental observations. The measured strain tolerance of the films does not impede their application.     

105 nm Gate length pMOSFETs with high-K and metal gate fabricated in a Si process line on 200 mm GeOI wafers

We report on the fabrication and electrical characterization of deep sub-micron (gate length down to 105 nm) GeOI pMOSFETs. The Ge layer obtained by hetero-epitaxy on Si wafers has been transferred using the Smart Cut^TM process to fabricate 200 mm GeOI wafers with Ge thickness down to 60–80 nm. A full Si MOS compatible pMOSFET process was implemented with HfO₂/TiN gate stack. The electrical characterization of the fabricated devices and the systematic analysis of the measured performances (I_ON, I_OFF, transconductance, low field mobility, S, DIBL) demonstrate the potential of pMOSFET on GeOI for advanced technological nodes. The dependence of these parameters have been analyzed with respect to the gate length, showing very good transport properties (μ_h ～ 250 cm²/V/s, I_ON = 436 μA/μm for L_G = 105 nm), and OFF current densities comparable or better than those reported in the literature.     

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.     

Effect of substrate temperature on indium tin oxide (ITO) thin films deposited by jet nebulizer spray pyrolysis and solar cell application

This study focused on the effect of substrate temperature (350 °C, 400 °C, and 450 °C) on morphological, optical, and electrical properties of indium tin oxide (ITO) films deposited onto porous silicon/sodalime glass substrates through jet nebulizer spray pyrolysis for use in heterojunction solar cells. X-ray diffraction analysis confirmed the formation of pure and single-phase In₂O₃ for all the deposited films whose crystallinity was enhanced with increasing substrate temperature, as shown by the increasing (222) peak intensity. Morphological observations were conducted using scanning electron microscopy to reveal the formation of continuous dense films composed of nanograins. The UV–vis spectra revealed that the transmittance increased with increasing substrate temperature, reaching a value of over 80% at 450 °C. The photoelectric performance of the solar cell was studied using the I–V curve by illuminating the cell at 100 mW/cm². A high efficiency (η) of 3.325% with I_sc and V_oc values of 14.8 mA/cm² and 0.60 V, respectively, was attained by the ITO solar cell annealed at 450 °C.