Haruhiro Hasegawa

0.1- mu m gate-length superconducting FET

A superconducting field-effect transistors (FET) with a 0.1- mu m-length gate electrode was fabricated and tested at liquid-helium temperature. Two superconducting electrodes (source and drain) were formed on the same Si substrate surface with an oxide-insulated gate electrode by a self-aligned fabrication process. Superconducting current flowing through the semiconductor (Si) between the two superconducting electrodes (Nb) was controlled by a gate-bias voltage.<<ETX>>

A disturbance-free read scheme and a compact stochastic-spin-dynamics-based MTJ circuit model for Gb-scale SPRAM

A magnetic-tunnel-junction (MTJ) circuit model, which considers spin dynamics under finite temperature, electrical bias, a stochastic process, and spin-transfer torque, was developed. Switching behaviors simulated by this model were verified by experimental measurements. Moreover, a disturbance-free read scheme for Gbit-scale spin-transfer torque RAM (SPRAM) was also developed. The feasibility of this scheme was confirmed by circuit simulation using the model and on-chip measurement of switching probability.

Electrical properties of Au/Nb‐doped‐SrTiO3 contact

Current‐voltage measurements for Au/Nb‐doped‐SrTiO3 contacts have been performed over a temperature range from 4.2 to 277 K. The forward characteristics above 46 K are well described by the Schottky diode model. The temperature dependence of the parameter E{=[∂log I/∂(qV)]−1} and the saturation current IS show that a thermionic‐field emission is dominant in the carrier transport mechanism across contacts above 101 K. It is found that the parameter E is too large compared with an estimated value from the Schottky diode model using a permittivity obtained from the inverse characteristics of capacitance‐voltage measurements. We discuss the barrier properties and suggest the possibility of imperfect ionization of the impurity Nb at the surface of the SrTiO3:Nb substrate.

Fabrication of Pb(Zr, Ti)O3 Microscopic Capacitors by Electron Beam Lithography

An epitaxial microscopic capacitor array of Pb(Zr, Ti)O3/SrRuO3 with 0.5-to 2-µ m squares was fabricated by electron-beam lithography and Ar-ion etching on a SrTiO3(001) substrate. Atomic force microscopic observation revealed that line-and-spacing larger than 1 µ m assures adequate separation between capacitors. A thin barrier is formed on the terrace edge of the capacitors due to re-deposition of Ar-ion etched oxide.

YBa2Cu3O7 step‐edge dc SQUID with coplanar control lines

A dc superconducting quantum interference device (SQUID) with coplanar control lines has been fabricated from a single layer of YBa2Cu3O7 thin film. Step‐edge junctions were formed using resist/Si double‐mask layers. Current–voltage characteristics of this SQUID at 77 K show Shapiro steps under microwave irradiation and are qualitatively consistent with the resistively shunted junction (RSJ) model. The spacing between the closed loop and the coplanar control lines in the tested SQUIDs ranged from 0.2 to 1.5 μm. The measured output voltage‐control current characteristics showed that the experimentally derived mutual inductances were a function of the spacing, in agreement with the calculated results. The typical mutual inductance for a spacing of 1.0 μm was ∼0.5 pH, which is adequate for detecting signals of flux‐transfer logic gates in which the typical logic amplitude is ∼0.1 mA. A coplanar structure is thus useful for logic gate applications.

Optical input/output interface system for Josephson junction integrated circuits

An optical input/output interface system for a Josephson junction integrated circuit is fabricated and tested. The system consists of a superconducting optical detector, a dc powered Josephson circuit, a dc powered Josephson high voltage circuit, a liquid-He-cooled semiconductor amplifier, and a liquid-He-cooled semiconductor laser. Features of the system are use of an ultrathin NbN film for the optical detector and adoption of the dc powered Josephson circuits for logic operation circuits. Correct optical output signal is detected by a liquid-He-cooled semiconductor photodiode. The optical input/output interface has the advantage of low heat penetration and low crosstalk compared to the interface using conventional coaxial lines. Moreover, dc powered Josephson circuits have an advantage of low crosstalk from power supply lines compared to conventional Josephson circuits, which are driven by ac supply current.<<ETX>>

A DC-powered Josephson logic family that uses hybrid unlatching flip-flop logic elements (Huffles)

This paper describes the development of a dc-powered Josephson logic family that uses hybrid unlatching flip-flop logic elements (Huffles). The Huffle circuit used in this study is modified by adding a parallel resistor to the original Hebard-type Huffle circuit. Analysis of the circuits operation shows that the undesirable hung-up phenomena are prevented by this modification. Based on the result of the analysis, the circuits parameters are derived and a typical operating margin of /spl plusmn/26% is obtained. Besides AND/OR operations using a threshold logic operation, two-input exclusive OR (XOR), two-input multiplexor (MUX), and three-input majority (MAJ) operations are realized using a Huffle gate in which 2-Josephson-interferometers (2JI) in the standard Huffle gate are replaced by stacked-2JIs. Thus, a Huffle logic family, formed from NOT, AND, OR, XOR, MUX, MAJ, and flip-flop (FF), are constructed. By using this Huffle logic family, a 6-b arithmetic logic operating unit (ALU), a 6-b analog-to-digital converter (ADC), and a 6-b gray-to-binary converter (GBC) have been successfully operated. During high-speed testing, a 1-b comparator was operated up to an input bandwidth of 6 GHz.

Electric field effect on the transport properties of ultrathin Y0.9Pr0.1Ba2Cu3Oy channels in the insulating phase of superconductor-insulator transition

The electric field effect on the transport properties of ultrathin Y0.9Pr0.1Ba2Cu3Oy channels was examined as a function of channel thickness and channel length. Samples having a planar YBa2Cu3Ox ultrathin Y0.9Pr0.1Ba2Cu3Oy- YBa2Cu3Ox structure with a gate electrode on the backside of their SrTiO3 substrate were used. The channel layer exhibited superconducting properties when its thickness was more than 9 nm, whereas channels less than 7.5 nm thick were in the insulating phase of the superconductor-insulator transition. The modulation ratio of the conductance of the insulating (I)-phase channels was a few orders of magnitude higher than the modulation ratio of the carrier density. The field-effect mobility increased with increasing channel thickness when the film was in the insulating phase. The sheet conductance of channels shorter than 10 μm was higher than that of a 100-μm-long channel of the same thickness due to a size effect on the transport of the I-phase Y0.9Pr0.1Ba2Cu3Oy channel. The field-effec...

Measurement of the critical current distribution for the resistively shunted junctions

A measurement method for the critical-current (Ic) distribution of the resistively shunted junction (RSJ) is discussed. According to numerical calculations, the Ic steps in the current–voltage (I–V) curve can be detected for the series array of the RSJ-type junctions when the I–V curve is changed from nonhysteretic to hysteretic. Bicrystal Josephson junctions in which YBa2Cu3O7−x film was used as electrodes were shunted by planar capacitors and the ratio of the Imin to Ic was decreased to less than 0.8. The Ic distribution of junctions numbering more than 400 could be measured by connecting them in series. A typical value of the standard deviation 1σ in Ic was 26%.

Design and fabrication of QFP logic gates based on YBa/sub 2/Cu/sub 3/O/sub 7/ step-edge junctions

Quantum Flux Parametron (QFP) logic gates were designed and fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7/ thin film based on step-edge Josephson junctions. Self-inductance (L) of the QFP loop, an important circuit parameter, as is critical current (J/sub c/) of the Josephson junction, was determined by using directly coupled dc superconducting quantum interference devices (SQUIDs). Magnetically coupled dc SQUIDs were used to detect the output signal from the QFP. Simulated waveforms were used to identify the operation modes of the QFP as a function of the circuit parameters. From examination of the output waveforms of the QFP, operation in agreement with simulations was confirmed.