Base Electrode Research Articles

Properties of perpendicular-anisotropy magnetic tunnel junctions fabricated over the bottom electrode contact

Perpendicular-anisotropy magnetic tunnel junctions (MTJs) were prepared on four substrate geometries, i.e., directly on the axis of the bottom electrode contact, directly off the axis of the bottom electrode contact, on the axis of the bottom electrode contact with a polished bottom electrode, and off the axis of the bottom electrode contact with a polished bottom electrode. Electrical shorts were observed for direct on-axis geometry at a certain extent, whereas there were no electrical shorts for the other three geometries. The MR ratio/σR, JC0, and thermal stability factor of the devices for polish on-axis geometry were almost the same as those for polish off-axis geometry. From TEM observations of the polish on-axis device, the interface between the bottom contact and the base electrode was determined to be rough, whereas the MgO barrier layer was determined to be smooth, indicating that the polish process was effective for smooth magnetic tunnel junction fabrication over the bottom contact. MTJs for polish on-axis geometry eliminated the base electrode resistance and increased the magnetoresistance ratio. This technology contributes to the higher density of spin transfer torque magnetic random access memory.

Development of Array Detectors with Three-Dimensional Structure toward 1000 Pixels of Superconducting Tunnel Junctions

Superconducting tunnel junction (STJ) array detectors can exhibit excellent performance with respect to energy resolution, detection efficiency, and counting rate in the soft X-ray energy range, by which those excellent properties STJ array detectors are well suited for detecting X-rays at synchrotron radiation facilities. However, in order to achieve a high throughput analysis for trace impurity elements such as dopants in structural or functional materials, the sensitive area of STJ array detectors should be further enlarged up to more than 10 times larger by increasing the pixel number in array detectors. In this work, for a large STJ-pixel number of up to 1000 within a 10 mm- square compact chip, we have introduced three-dimensional (3D) structure by embedding a wiring layer in a SiO2 isolation layer underneath a base electrode layer of STJs. The 3D structure is necessary for close-packed STJ arrangement, avoiding overlay of lead wiring, which is common in conventional two-dimensional layout. The fabricated STJ showed excellent current-voltage characteristics having low subgap currents less than 2 nA, which are the same as those of conventional STJs. An STJ pixel has an energy resolution of 31 eV (FWHM) for C-Kα (277 eV).

Immobilization of nanobeads on a surface to control the size, shape, and distribution of pores in electrochemically generated sol–gel films

Electrochemically assisted deposition of an ormosil film at a potential where hydrogen ion is generated as the catalyst yields insulating films on electrodes. When the base electrode is modified with 20-nm poly(styrene sulfonate), PSS, beads bound to the surface with 3-aminopropyltriethoxysilane (APTES) and using (CH3)3SiOCH3 as the precursor, the resulting film of organically modified silica (ormosil) has cylindrical channels that reflect both the diameter of the PSS and the distribution of the APTES-PSS on the electrode. At an electrode modified by a 20-min immersion in 0.5 mmol dm-3 APTES followed by a 30-s immersion in PSS, a 20-min electrolysis at 1.5 V in acidified (CH3)3SiOCH3 resulted in an ormosil film with 20-nm pores separated by 100 nm. Cyclic voltammetry of Ru(CN)64- at scan rates above 5 mVs-1 yielded currents controlled primarily by linear diffusion. Below 5 mVs-1, convection rather than the expected factor, radial diffusion, apparently limited the current.

Use of graphene-supported manganite nano-composites for methanol electrooxidation

Binary nano-composites of palladium and a metal (Fe or Cu) manganite on graphene nanosheets (GNS) have been prepared by a microwave-assisted polyol reduction method and investigated as electrocatalysts for the methanol oxidation reaction (MOR) in 1 M KOH at 25 °C. Structural and electrocatalytic surface characterizations of composites are carried out by X-ray diffraction, transmission electron microscopy, X-ray photoelectron microscopy, cyclic voltammetry and chronoamperometry. Results show that new composite catalysts, particularly 40 wt%Pd–x wt%FeMn2O4/GNS (where x = 5, 8, 10 & 15) are MOR active and that the activity is the greatest with the catalyst containing 8 wt% of the oxide. The composite, Pd–8 wt%FeMn2O4/GNS, exhibits much superior catalytic activity as well as stability compared to the base (Pd/GNS) electrode. The enhanced catalytic activity and stability of the Pd/GNS catalyst in presence of the oxide can be ascribed to increased population of adsorbed OH− ions/OH radicals at the electrode surface.

Electron backscatter diffraction applied to lithium sheets prepared by broad ion beam milling.

Due to its very low hardness and atomic number, pure lithium cannot be prepared by conventional methods prior to scanning electron microscopy analysis. Here, we report on the characterization of pure lithium metallic sheets used as base electrodes in the lithium-ion battery technology using electron backscatter diffraction (EBSD) and X-ray microanalysis using energy dispersive spectroscopy (EDS) after the sheet surface was polished by broad argon ion milling (IM). No grinding and polishing were necessary to achieve the sufficiently damage free necessary for surface analysis. Based on EDS results the impurities could be characterized and EBSD revealed the microsctructure and microtexture of this material with accuracy. The beam damage and oxidation/hydration resulting from the intensive use of IM and the transfer of the sample into the microscope chamber was estimated to be <50 nm. Despite the fact that the IM process generates an increase of temperature at the specimen surface, it was assumed that the milling parameters were sufficient to minimize the heating effect on the surface temperature. However, a cryo-stage should be used if available during milling to guaranty a heating artefact free surface after the milling process.

Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes

We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore, Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and a b-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I– V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I– V characteristics and I c R n– T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I c R n product of the interface-engineered barrier junction was 12 μV and the planar junction with Au barrier showed 18 μV, we could increase the I c R n to 90 μV for planar TiO x barrier junctions.

Ultrasensitive electrochemical immunoassay for CEA through host–guest interaction of β-cyclodextrin functionalized graphene and Cu@Ag core–shell nanoparticles with adamantine-modified antibody

A novel non-enzymatic immunoassay was designed for ultrasensitive electrochemical detection of carcino-embryonic antigen (CEA) using β-cyclodextrin functionalized Cu@Ag (Cu@Ag-CD) core–shell nanoparticles as labels and β-cyclodextrin functionalized graphene nanosheet (CD-GN) as sensor platform. CD-GN has excellent conductivity which promoted the electric transmission between base solution and electrode surface and enhanced sensitivity of immunosensor. In addition, owing to supramolecular recognition of CD-GN for the guest molecule, quite a few synthesized adamantine-modified primary antibodies (ADA-Ab1) were immobilized on the CD-GN by supramolecular host–guest interaction between CD and ADA. Cu@Ag-CD as a signal tag could be captured by ADA-modified secondary antibody (ADA-Ab2) through a host–guest interaction, leading to a large loading of Cu@Ag nanoparticles with high electrical conductivity and catalytic activity. The fabricated immunosensor exhibits excellent analytical performance for the measurement of CEA with wide range linear (0.0001–20ng/mL), low detection limit (20fg/mL), good sensitivity, reproducibility and stability, which provide an enormous application prospect in clinical diagnostics.

Graphene and 2-Dimensional Materials for Nanoelectronics Applications

Graphene science and technology has evolved substantially over the past 10 years. It is now entering a phase, where researchers and industry start to collaborate more closely on applications and to address issues of scalability and manufacturability. In addition to the semimetal graphene, recent years have seen the emergence of a number of related two-dimensional materials. These include insulators such as hexagonal boron nitride, semiconducting transition metal dichalcogenides (TMDs) like molybdenum disulfide (MoS2) or tungsten diselenide (WSe2) or silicene and germanene, similar in structure but less stable than graphene.The talk will focus on graphene-based hot electron transistors (Graphene Base Transistors, GBTs) for analog radio frequency applications [1], [2]. The operating principle of GBTs is based on vertical current flow by quantum mechanical tunneling through an insulating barrier (Figure 1). The current (red arrow) can be controlled over several orders of magnitude by a base electrode made of graphene (Figure 2). The advantage of graphene over a metal base electrode is its ultimate thinness while maintaining good conductivity. If the barrier materials are optimized, GBTs have the potential to reach cut off frequencies (fT) on the order of 1 THz [3].In the second part of the talk, options for optoelectronic devices based on 2D materials will be discussed. These include photodetectors / phototransistors made from graphene [4], where the photoresponse can be tuned by the voltage applied to a gate. In addition, photodiodes based on MoS2/silicon heterojunctions will be discussed.[1] W. Mehr, J. C. Scheytt, J. Dabrowski, G. Lippert, Y.-H. Xie, M. C. Lemme, M. Ostling, and G. Lupina, “Vertical Transistor with a Graphene Base,” IEEE Electron Device Lett., vol. 33, pp. 691–693, 2012.[2] S. Vaziri, G. Lupina, C. Henkel, A. D. Smith, M. Östling, J. Dabrowski, G. Lippert, W. Mehr, and M. C. Lemme, “A Graphene-based Hot Electron Transistor,” Nano Lett., vol. 13, p. 1435−1439, 2013.[3] F. Driussi, P. Palestri, and L. Selmi, “Modeling, simulation and design of the vertical Graphene Base Transistor,” Microelectron. Eng., vol. 109, pp. 338–341, Sep. 2013.[4] M. C. Lemme, F. H. L. Koppens, A. L. Falk, M. S. Rudner, H. Park, L. S. Levitov, and C. M. Marcus, “Gate-Activated Photoresponse in a Graphene pn Junction,” Nano Lett., vol. 11, pp. 4134–4137, 2011.

Bio-electrochemical reduction of nitrate utilizing MWCNT supported on carbon base electrodes: A comparison study

In the present study, carbon felt, carbon cloth and graphite were evaluated as a support material for immobilization of multi-wall carbon nanotube (MWCNT) in a dual chamber bio-electrochemical reactor (BER). Effect of operating parameters, such as current density, pH, and retention time on reduction efficiency were assessed. The results of the study showed that in the optimum operating conditions, nitrate reduction efficiency were about 83.58%, 79.66% and 63.98% for MWCNT/carbon felt, MWCNT/carbon cloth and MWCNT/graphite cathodes, respectively. Overall results indicate that carbon felt can serve as a support for modification of cathode by MWCNT in a BER.

Hybrid superconducting mesa-heterostructure with manganite-ruthenate interlayer

We present experimental data on Josephson effect in hybrid superconducting mesa-heterostructures (HSMH) with composite manganite-ruthenate interlayer. The HSMH base electrode consisted of the cuprate superconductor YBa2Cu3O7 grown epitaxially on a NdGaO substrate using laser ablation. The interlayer was composed from in-situ deposited SrRuO3 (F1) and La0.7Sr0.3MnO3 (F2) thin films, each characterized by different directions of magnetization. The top electrode was Nb/Au thin film. A superconducting current was observed when the interlayer thickness was well above the correlation length, determined by the exchange field in F1 and F2. Obtained IC(H) dependences and non-sinusoidal current-phase relation evaluated from microwave measurements are discussed in terms of generation of long-range spin triplet superconducting current component in heterostructures with interfaces of singlet superconductors and bilayer ferromagnetic materials with different spatial directions of magnetization.

Influence of the spreading resistance on the conductance spectrum of planar hybrid thin film SNS' junctions based on iron pnictides

To investigate the superconducting properties of iron pnictides we prepared planar hybrid SNS' junctions in thin film technology with a pnictide base electrode, a gold barrier layer and a lead counter electrode. Our design allows characterization of the electrodes and the junction independently in a 4-probe method. We show how both electrodes influence the measured spectra due to their spreading resistance. While the Pb electrode has a constant resistance above its Tc, the contribution of the pnictide electrode is clearly current-dependent and thus it needs a more advanced method to be corrected. We present an empirical method, which is simple to apply and allows to deal with the spreading resistance in our junctions to recalculate the actual conductance and voltage of one junction at given temperature.

Detection of X-ray photons by niobium Josephson tunnel junctions with trapped Abrikosov vortices

The high atomic number of niobium (Z=41) can be exploited to develop a high efficiency superconducting gamma-ray detector based on a novel detection principle, namely, the interaction of a single gamma-ray photon with Abrikosov vortices trapped inside a niobium bulk absorber. To study the feasibility of this principle, niobium type Josephson tunnel junctions with the aluminium oxide as a tunnel barrier and with a thick (0.3 mm) niobium base electrode have been fabricated. The devices have been tested at T = 4.2 K in terms of the current-voltage characteristic and of the magnetic field dependence of the Josephson critical current. The feasibility of the detection principle has been tested under X-ray irradiation from the 55Fe source. The time dependence of the Josephson critical current of the junction with trapped Abrikosov vortices has been recorded without and with X-ray irradiation. The data analysis of obtained experimental curves has confirmed the effect of the X-ray photon absorption on the Josephson critical current caused by jumping of Abrikosov vortices.

All solution-processed inorganic/organic hybrid permeable metal-base transistor.

All solution-processed inorganic/organic hybrid permeable-base transistor (PMBT) based on nickel oxide emitter and P3HT collector is developed. Due to the high charge injection properties of nickel oxide and spontaneously formed nano-pinholes in the base electrode, the devices exhibit high common-base and common-emitter current gains up to 0.98 and 304, respectively with saturated output current.

Achieving good bias stress reliability in organic transistor with vertical channel

Unlike organic field-effect transistor with accumulated channel at dielectric/semiconductor interface, vertical organic transistor exhibits bulk channel current and hence performs good bias stress reliability. Adding self-assemble-monolayer to treat vertical channel can further modulate the charge-trapping surrounding the base electrode and hence influence the bias stress reliability. During 4300-s positive/negative bias stresses, stable output current and on/off ratio are demonstrated by using octadecyltrichlorosilane (OTS)-passivated vertical channel. The good reliability together with the low operation voltage and the high output current make vertical organic transistors capable of driving organic light emitting diode.

Invisible Base Electrode Coordinates Approximation for Simultaneous SPECT and EEG Data Visualization

Abstract This work was performed as part of a larger research concerning the feasibility of improving the localization of epileptic foci, as compared to the standard SPECT examination, by applying the technique of EEG mapping. The presented study extends our previous work on the development of a method for superposition of SPECT images and EEG 3D maps when these two examinations are performed simultaneously. Due to the lack of anatomical data in SPECT images it is a much more difficult task than in the case of MRI/EEG study where electrodes are visible in morphological images. Using the appropriate dose of radioisotope we mark five base electrodes to make them visible in the SPECT image and then approximate the coordinates of the remaining electrodes using properties of the 10-20 electrode placement system and the proposed nine-ellipses model. This allows computing a sequence of 3D EEG maps spanning on all electrodes. It happens, however, that not all five base electrodes can be reliably identified in SPECT data. The aim of the current study was to develop a method for determining the coordinates of base electrode(s) missing in the SPECT image. The algorithm for coordinates approximation has been developed and was tested on data collected for three subjects with all visible electrodes. To increase the accuracy of the approximation we used head surface models. Freely available model from Oostenveld research based on data from SPM package and our own model based on data from our EEG/SPECT studies were used. For data collected in four cases with one electrode not visible we compared the invisible base electrode coordinates approximation for Oostenveld and our models. The results vary depending on the missing electrode placement, but application of the realistic head model significantly increases the accuracy of the approximation.

Nanoscale Solid-State Separator/Electrolyte for3D All-Solid-State Batteries

Nanoscale Solid-State Separator/Electrolyte for3D All-Solid-State Batteries

Impact of strained GaAs spacer between InP emitter and GaAs1−ySby base on structural properties and electrical characteristics of MOCVD-grown InP/GaAs1−ySby/InP DHBTs

Novel InP/GaAs1−ySby/InP double-heterojunction bipolar transistors (HBTs) with a GaAs spacer between the InP emitter and GaAs1−ySby base layer were grown by the metalorganic chemical vapor deposition method in order to simplify the switching sequence for forming a high-quality InP-emitter/GaAs1−ySby-base interface. After removal of the InP emitter, the top surface of the GaAs spacer exhibits smooth step-flow-like morphology with root-mean-square roughness of 0.17–0.36nm, whereas the morphology of the GaAs1−ySby base of the sample without the GaAs spacer is bumpy. Secondary ion mass spectroscopy reveals that the spacer suppresses the incorporation of excess Sb into the InP emitter around the emitter–base junction. The dependence of the current gain on the thickness of the GaAs spacer is investigated and, when the GaAs spacer is 2nm, the highest current gain is obtained. Therefore, we employ the spacer to scaled-down HBTs with a 0.25-μm-wide emitter. The scaled-down HBTs show high current gain of over 90 at collector current density JC of 10mA/μm2 even though the space between emitter and base electrodes is just 0.15μm. We obtain peak current-gain cut-off frequency of 388GHz and peak maximum oscillation frequency of 290GHz at JC=10mA/μm2. This result suggests that the presence of the GaAs spacer does not impose any penalty on the characteristics at high JC. The insertion of the GaAs spacer is a good way to obtain a high-quality E–B interface with a simple precursor-supply sequence and thereby HBTs with both high-current gain and reasonably high RF performance.

Investigation of TiOx barriers for their use in hybrid Josephson and tunneling junctions based on pnictide thin films

We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high IcRn-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes, such as lead and niobium, respectively. By tuning the barrier thickness, we were able to change the junction's behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe2As2 thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5 nm, we observe clear Josephson effects with IcRn≈90 μV at 4.2 K. These junctions behave SNS'-like (SNS: superconductor-normal conductor-superconductor) and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0 nm and higher, no Josephson but SIS'- (SIS: superconductor-insulator-superconductor) or SINS'-like (SINS: superconductor-normal conductor-insulator-superconductor) behavior with a tunnel-like conductance spectrum was observed.

Quasiparticle Freeze-Out in Superconducting Tunnel Junction X-ray Detectors with Killed Base Electrode

The current-voltage characteristics of superconducting tunnel junction (STJ) X-ray detectors were measured in the temperature range from 4.2 to 0.1 K. The freeze-out of the thermal tunneling current was compared between an STJ detector with a traditional Nb/Al/Al2O3/Al/Nb layer structure and a Ti/Nb/Al/Al2O3/Al/Nb/NbN detector whose low-gap Ti film kills the X-ray response of the base electrode. The current decrease and the linear low-temperature I(V) characteristics for the detector with the killed electrode can be qualitatively explained by tunneling current contribu- tions from the subgap states of the Ti film. The data are analyzed on the basis of the proximity theory in the dirty limit.

FE Analysis of Plasma Discharge and Sheath Characterization in Dry Etching Reactor

We present a full finite element analysis for plasma discharge in etching process of semiconductor circuit. The charge transport equations of hydrodynamic diffusion-drift model and the electric field equation were numerically solved in a fully coupled system by using a standard finite element procedure for transient analysis. The proposed method was applied to a real plasma reactor in order to characterize the plasma sheath that is closely related to the yield of the etching process. Throughout the plasma discharge analysis, the base electrode of reactor was tested and modified for improving the uniformity around the wafer edge. The experiment and numerical results were examined along with SEM data of etching quality. The feasibility and usefulness of the proposed method was shown by both numerical and experimental results.