Superconducting Flux Flow Transistors Research Articles

Modeling of I–V characteristics in a 3-channel SFFT with nanobridges by gate current signals

A superconducting flux flow transistor (SFFT) with three channels and nanobridges was successfully fabricated by electron beam (e-beam) lithography and an Ar ion milling technique. The SFFT is composed of three weak links with a nearby gate current line. We explain the process to obtain the equation for the current-voltage characteristics and describe the method to induce external and internal magnetic fields by Biot–Savart's law. The equation can be used to predict the current-voltage curves for the 3-channel SFFT fabricated using e-beam lithography. I–V characteristics were simulated to analyze the SFFT with three channels and nanobridges by a Matlab program. From the I–V characteristics equation of the 3-channel SFFT, the drain currents and the output voltages as the gate current is applied are graphically compared with the measured value and the simulation value. The simulated I–V curves were in good agreement with the measured curves of the 3-channel SFFT with nanobridges.

Structural Studies on Superconducting Flux Flow Transistors with Nano-Scale Channel Fabricated by Electron Beam Lithography

The superconducting flux flow transistor (SFFT) with three-channel was successfully fabricated by an electron beam lithography and Ar ion milling technique. In our paper the cross sectional analysis and topological views are investigated for the channel width and the channel height by the atomic force microscopy (AFM) image. Our result shows that the SFFT of multi-structure with a nano-channel is effectively fabricated by an electron-beam lithography method. We present a fabrication process of three-channel SFFT using the electron beam lithography. Mechanisms of nano-channel SFFT are studied.

Operational Characteristics of Superconducting Amplifier using Vortex Flux Flow

The operational characteristics of superconducting amplifier using vortex flux flow were analyzed from an equivalent circuit in which its current-voltage characteristics for the vortex motion in YBCO microbridge were reflected. For the analysis of operation as an amplifier, dc bias operational point for the superconducting amplifier is determined and then ac operational characteristics for the designed superconducting amplifier were investigated. The variation of transresistance, which describes the operational characteristics of superconducting amplifier, was estimated with respect to conditions of dc bias. The current and the voltage gains, which can be derived from the circuit for small signal analysis, were calculated at each operational point and compared with the results obtained from the numerical analysis for the small signal circuit. From our paper, the characteristics of amplification for superconducting flux flow transistor (SFFT) could be confirmed. The development of the superconducting amplifier applicable to various devices is expected.

Transistor characteristics of a parallel-channel and a serial-channel superconducting flux flow transistor fabricated by the AFM anodization process

Abstract Superconducting flux flow transistors (SFFTs) with a parallel and serial channel were fabricated on superconducting thin films. A transistor pattern with a drain, source and channel was fabricated with conventional lithography, while a weak link between the source and the drain was fabricated with the AFM anodization process as a channel. The parallel and the serial channel SFFTs showed like-transistor characteristics that the drain voltage across a channel depended on the gate current. These characteristics were also simulated with numerical methods. This study describes a novel method to fabricate SFFTs with various channels by the AFM anodization process.

Characteristics of a single-channel superconducting flux flow transistor fabricated by an AFM modification technique

The demand for high performance, integrity, and miniaturization in the area of electronic and mechanic devices has drawn interest in the fabrication of nanostructures. However, it is difficult to fabricate the channel with nano-scale using a conventional photography techniques. AFM anodization technique is a maskless process and effective method to overcome the difficulty in fabricating a nano-scale channel. In this paper, we first present a new fabrication of a single-channel SFFT using a selective oxidation process induced by an AFM probe. The modified channel was investigated by electron probe microanalyzer (EPMA) to find the compositional variation of the transformed region. In order to confirm the operation of a single-channel SFFT, we measured the voltage–current characteristics at the temperature of liquid nitrogen by an I– V automatic measurement system. Our results indicate that the single-channel SFFT having effect as a weak link is effectively fabricated by an AFM lithography process.

The analysis of superconducting thin films modified by AFM lithography with a spectroscopic imaging technique

A practical application of nanolithography using atomic force microscopy (AFM) was accomplished in fabricating superconducting flux flow transistors (SFFTs). It was found that it is essential to oxidize a superconducting thin film, grown on LaAlO 3 substrates by a thermal CVD process, by an applied bias voltage between a conducting AFM tip and the films, since I/ V characteristics of the device were mainly controlled by the modified gate area in the SFFT. After AFM lithography, the critical current of an YBCO thin film was found to be degraded. Raman lines in the modified YBCO film were observed at 340, 502, and 632 cm −1 in Ar the laser system and 142, 225, and 585 cm −1 in the He–Ne laser system. Raman fluorescence images were also produced by mapping the Raman peaks. A strain image of the peak at 142 cm −1 was most clear, which means that a surface of the YBCO thin film was changed into the YBa 2Cu 3O 6 insulator. AFM nanolithography enables us to fabricate a channel between a source and a drain in SFFT in order to get I/ V characteristics.

A Study on Oxidization Processes of YBaCuO Superconducting Thin Films in Atomic Force Microscope Anodization Lithography

It is essential to elucidate processing mechanism in the fabrication of nano-electronic devices in order to improve their characteristics. In this study, we systematically investigated various experimental parameters in oxidizing a thin film of YBaCuO superconductor, one of the processing steps in producing a superconducting flux flow transistor (SFFT). An area between a gate and a drain in SFFT was selectively modified by controlling the parameters such as an applied bias voltage between a conducting atomic force microscope (AFM) tip and the films, the numbers of the lithographic process on the same area, and the contact resistance between the sample holder and a sample. The height of the lithographed surface was increased with higher applied bias voltage, more numbers of the lithographic process, but was decreased by increasing the contact resistance. The analysis of the data confirmed that the height enhancement results from growth of oxide via an electrochemical reaction. Also analysis of its Raman spectra revealed that the electrical property of the oxidized area became an insulator. The peak 632 cm-1 related with the Cu–O stretching mode in the YBCO structure is disappeared or broad in a sample lithographed above 10 V, which means that the Cu–O chain is affected in superconducting structure by the AFM lithographic process.

Nano‐oxidation of superconducting flux flow transistor using a conductive atomic force microscope tip

The atomic force microscopy (AFM) has exhibited its capability to fabricate the nano-scale electronic devices. We have fabricated superconducting flux flow transistors (SFFTs) in thin superconducting films by localized anodization using an AFM lithography method. The SFFT is composed of two strips, which is characterized as a transresistance device: one strip is as a source and drain terminal, the other is as a gate terminal. Generally, the channels of SFFT have been fabricated by chemical wet etching method and dry etching method. In this study we present a new fabrication of the SFFT with single or two channels using the artificial pattern formation method based on the AFM lithography. The critical current density in the fabricated SFFT with a multi-channel structure was decreased applying the current in the gate current line. From the measured results, it is shown that the critical current of an SFFT with two channels is larger than the critical current of single-channel due to the magnetic coupling between channels. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Current‐voltage characteristics of superconducting flux flow transistor with a multi‐channel fabricated by an inductively coupled plasma

In this paper, we have fabricated the superconducting flux flow transistor (SFFT) with two or three channels from epitaxial YBa2Cu3O7–x (YBaCuO) thin films with thickness 360 nm and channel width approximately 9.37 µm, length 5 µm, and especially the thickness 300 nm in case of etching by an inductively coupled plasma (ICP). We improved the characteristics of the SFFT by forming multi-channel in the drain current line. We performed the experiments of SFFTs controlled by the magnetic field generated by a gate current, which regulates the critical current through the channels of the drain current line and the gate current line. We also explained the process to get the I–V characteristic equations and to induce the external and internal magnetic fields by Biot-Savart's law. It was shown in this study that due to the magnetic coupling between channels, the critical current of a SFFT with three channels was higher than the critical current of two channels. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characteristic variation of superconducting thin films modified by an atomic force microscope

Superconducting thin film surfaces were modified in order to test a possibility as a channel of superconducting flux flow transistor (SFFT) by AFM lithography. A conventional lithography method has been employed for superconducting strips which were patterned on LaAlO3 substrates. A region of the strips was selectively oxidized by an AFM probe in the presence of the electric field between the probe and the strip. Then the modified surface was analyzed by AFM, EPMA, and I-V curves. The thickness of the strips increases with the bias voltage as well as the scanning number due to the oxidation. The nonsuperconducting material produced by an AFM lithography process affects the characteristics of SFFT in the flux creep mode of I–V curves. This study represents a dynamic idea to fabricate the superconducting flux flow transistors with nano-channel by the AFM lithography. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical modeling of the superconducting flux flow transistor with a nanobridge

We have fabricated superconducting flux flow transistors (SFFTs) with a nanobridge from epitaxial superconducting thin films by photolithography and electron-beam lithography. We have tried to simulate their I– V characteristics by building a model. The simulation was performed by varying the penetration depth as a function of the width and the thickness in the nanobridge and the gate current. This model showed the dependence of the critical current density on the spatial distribution of an applied magnetic field induced by the gate current. The simulated I– V curves were well in agreement with the measured curves in the flux creep regime of an SFFT with a nano-scale channel. This model is suitable for prediction of the I– V characteristics in the flux creep regime of the SFFT with a nano-scale channel.

전도성 AFM 탐침에 의한 YBa2Cu3O7-x스트립 라인의 산화피막 형성

Fundamental results obtained from an atomic force microscope (AFM) chemically-induced direct nano-lithography process are presented, which is regarded as a simple method for fabrication nm-scale devices such as superconducting flux flow transistors (SFFTs) and single electron tunneling transistors (SETs). Si cantilevers with Pt coating and with 30 nm thick TiO coating were used as conducting AFM tips in this study. We observed the surfaces of superconducting strip lines modified by AFM anodization' process. First, superconducting strip lines with scan size 2 <TEX>${\mu}{\textrm}{m}$</TEX><TEX>${\times}$</TEX>2 <TEX>${\mu}{\textrm}{m}$</TEX> have been anodized by AFM technology. The surface roughness was increased with the number of AFM scanning, The roughness variation was higher in case of the AFM tip with a positive voltage than with a negative voltage in respect of the strip surface. Second, we have patterned nm-scale oxide lines on <TEX>${YBa}-2{Cu}_3{O}_{7-x}$</TEX> superconducting microstrip surfaces by AFM conductive cantilever with a negative bias voltage. The <TEX>${YBa}-2{Cu}_3{O}_{7-x}$</TEX> oxide lines could be patterned by anodization technique. This research showed that the critical characteristics of superconducting thin films were be controlled by AFM anodization process technique. The AFM technique was expected to be used as a promising anodization technique for fabrication of an SFFT with nano-channel.

ICP 식각 시스템에 의한 초전도 스트립 라인의 임계 특성 분석

Superconducting flux flow transistor (SFFT) is based on a control of the Abrikosov vortex flowing along a channel. The induced voltage by moving of the Abrikosov vortex in an SFFT is greatly affected by the thickness, the width, and the length of channel. In order to fabricate a reproducible channel in the SFFT, we studied the variation of the critical characteristics of <TEX>${YBa}_2{Cu}_3{O}_7-\delta(YBCO)$</TEX> thin films with the etching time using ICP (Inductively coupled plasma) system. From the simulation, it was certified that the vortex velocity was increased in a low pinning energy at channel width 0,5 mm. The surfaces of YBCO thin film were etched by ICP etching system. We observed the etched channel surfaces by AFM (Atomic Force Microscope) and measured the critical current density with etching time. As a measured results, the etching thickness of channel should be optimized to fabricated a flux flow transistor with specified characteristics.

Superconducting flux flow transistor fabricated by an inductively coupled plasma etching technique

Superconducting flux flow transistors (SFFT) was successfully fabricated by an inductively coupled plasma (ICP) etching technique. YBaCuO thin films on LaAlO3 substrate were patterned as a three-terminal device by a conventional wet etching method and the ICP system. The characteristics of a fabricated device were investigated by examining the I–V curves under various applied currents. The control current dependence of the transresistance was also measured. The SFFT with a channel fabricated by the ICP system showed a transistor-like characteristic over the liquid nitrogen temperature.

Characteristics of superconducting flux flow transistors under irradiation of femtosecond laser pulses

We have fabricated superconducting flux flow transistors with YBCO thin films and investigated the vortex flow characteristics under the irradiation of femtosecond laser pulses. The vortex flow velocity was calculated by the magnetic field dependence of the flow voltage. The obtained velocity without laser irradiation is 0.08–1.37 × 10 4 m/s for the bias current I B=0.4–0.8 mA. The flow voltage increased gradually when the irradiated laser power was up to 10 mW, however, it increased drastically when the laser power was more than 10 mW. The maximum velocity of 6.3 × 10 4 m/s was obtained at the laser power of 15 mW. The result indicates that the laser pulses excite the vortices and accelerate the vortex velocity.

플라즈마 식각을 이용한 초전도 자속 흐름 트랜지스터

The channel of a superconducting flux flow transistor has been fabricated with plasma etching method using a inductively coupled plasma etching. The ICP conditions then were ICP Power of 450 W, rf chuck power of 150 W, the pressure in chamber of 5 mTorr, and Ar : Cl<TEX>$_2$</TEX>=1:1. Especially, over the 5 mTorr, the superconducting thin films were not etched. The channel etched by plasma gas showed the critical temperature over 85 K. The critical current of the SFFT was altered by varying the external applied current. As the external applied current increased from 0 to 12 mA, the critical current decreased from 28 to 22 mA. Then the obtained trans-resistance value was smaller than 0.1 <TEX>$\Omega$</TEX> at a bias current of 40 mA.

Conditions of ICP for a superconducting flux flow transistor and its etching characteristics

Superconducting flux flow transistors with a micron channel (3 μm) have been fabricated based on the flux flow by the inductively coupled plasma (ICP) etching technique. For a channel of superconducting vortex flow transistor (SFFT) with the high- T c superconducting characteristic in YBaCuO, the proper ICP etching conditions were an ICP power of 700 W, r.f. power of 150 W, pressure in chamber of 5 mTorr, and mixing rate of etching gas for Ar:Cl 2, 1:1. The sample etched by ICP produced smoother morphology than by H 3PO 4 etchant, while it made little difference with a sample by non-aqueous etchant, Br. The transresistance of an SFFT etched by the ICP technique was below 0.1 Ω at an I bdy of 40 mA. Output resistance was below 0.2 Ω. It is expected that the transresistance can be improved by decreasing the thickness of the link and introducing the multi-link structure.

Current–voltage characterization of the vortex motion in YBa2Cu3O7−δ microbridges and the implications on the development of superconducting flux flow transistors

In order to develop superconducting thin films or multilayer structures suitable for Abrikosov vortex flux flow transistors, we propose a model to describe the current–voltage characteristics measured on microbridges operating in the flux creep regime and we show how the effective superconducting thickness of the samples, as well as the pinning potential range, the maximum velocity, the pinning energy, and the depinning current of the vortices can be determined. This model is applied to microbridges with structures containing one or several YBa2Cu3O7−δ layers which were deposited either on LaAlO3 or on SrTiO3 substrates. The results demonstrate that most YBa2Cu3O7−δ microbridges exhibit very similar properties and are not suitable for flux flow devices. However, bilayer structures comprised of a YBa2Cu3O7−δ film and a conductive capping layer display interesting properties.

Characterization of the vortex dynamics in Abrikosov superconductive flux flow transistors

Abstract In order to develop superconducting thin films or multilayer structures suitable for Superconducting Flux Flow Transistors (SFFTs), we propose a model to describe the current–voltage characteristics measured on microbridges in the flux creep regime and we show how the effective superconducting thickness of the samples, as well as the pinning potential range, the maximum velocity, the pinning energy, and the depinning current of the vortices can be determined. This model is applied to YBCO microbridges in order to compare samples and to develop films which may easily be driven into the flux flow regime.

Effect of a uniform magnetic field on the I-V curves of SFFTs

The connection between the effect of a uniform magnetic field on the I-V curves measured on the microbridges of superconducting flux flow transistors and the vortex dynamics is studied with the help of a model assuming that the motion of the vortices is due to a diffusion process. It is shown that the application of the field results in a critical current reduction which depends on the amplitude of the field and the thickness of the film only. As a consequence the effective thickness of the superconducting layer of the studied microbridges can be estimated from I-V measurements carried out with and without applied field.