Lapas attēli
PDF
ePub

QUANTUM MAGNETO FLUX LOGIC PROJECT: SUPERCOMPUTER THROUGH SUPERCONDUCTOR

Project Director: Eiichi Goto

The advancements of supercomputers have made it possible to simulate many complicated natural and artificial phenomena, such as meteorology, large-scale integration (LSI) design, and even protein structures. However, more than several hundreds of times faster computers should be required to accurately analyze these large and complicated systems, including long-term weather forecast, accurate ultra LSI (ULSI) design, and DNA simulations. The silicon LSIS, which are mainly utilized in today's supercomputers, should face such limitations as power consumption and propagation delay in these very large and fast computer systems. Therefore, very fast operation and low power consumption devices should be necessary for the future high performance computer systems.

A quantum flux parametron (QFP) device was first reported in 1984 (Ref 1) and was named after the parametron device (Ref 2) invented about 30 years before. QFP is a very promising candidate for future supercomputers because of its very low power dissipation (109 W/ gate) and fast switching speed (10-12 s/ gate), with its capability of threedimensional (3D) packaging through magnetic coupling. These characteristics should make it possible to integrate the whole system in a very small volume. Thus, a TFLOPS (1012 floating operations per second) computer should be made possible by QFP circuits. The Quantum Magneto Flux Logic (QMFL) project aims to demonstrate (1) multi GHz operation of QFP circuits; (2) highly functional QFP logic circuits; (3) a suitable operation system for fast QFP computers; (4) a reliable refrigerator; and (5) removal of

trapped magnetic flux from superconductors, as the first step toward a TFLOPS QFP computer. The QMFL project consists of three groups, Fundamental Property, Computer Architecture, and Magnetic Shielding. The former two were at Hitachi's Central Research Laboratory and the third one was with ULVAC Corp.

The detailed achievements of this project are listed in References 3 and 4 and a brief summary is stated here. The Fundamental Property Group aimed to clarify the physics of QFP devices to investigate the QFP logic circuit design, to demonstrate very fast operation of QFP circuits, and to verify 3D integration. A highly functional logic gate, D-gate, was proposed and was shown to be twice as powerful as compared with semiconductor logic gates. The operation of D-gate was successfully demonstrated by experiment. A shift register circuit, fabricated by 5-μm Pb alloy technology, operated at 16 GHz and was simulated to operate at 100 GHz if 2.5-μm Nb technology was utilized. Other circuits, such as analog-to-digital converters, were also demonstrated to operate at more than 18 GHz. Threedimensional signal transfer via magnetic coupling was also successfully demonstrated. These results suggest that QFP is a very promising candidate for the future high performance computers.

while conventional pipeline accelerates only a factor of 3 using the same hardware.

The Magnetic Shielding Group aimed for the verification of a high performance refrigerator and establishment of trapped flux detection and removal technology. The characteristics of a bellows type refrigerator were examined first, and it was found that multilayering eases the maximum stress. The fatigue strength of bellows was also made clear. The new pulse-tube type refrigerator, which has no moving parts in the low temperature region, was investigated and high efficiency and reliability were ascertained. The trapped flux in the superconductor film was detected by superconducting quantum interference device (SQUID), with a spatial resolution of 100 μm and a magnetic field resolution of less than 0.1 flux quanta. The trapped flux was successfully removed by irradiating a laser beam to the superconductor film and heating it up to the normal state (micro heat flushing).

These achievements of the QMFL project, which came to an end in September 1991 and has already reached the applied research level, will be succeeded by a high-tech consortium, with Hitachi and ULVAC, supported by JRDC. The other achievements, in a relatively basic research level, will be further pursued by the Goto Research Laboratory at RIKEN.

KUNITAKE MOLECULAR ARCHITECTURE PROJECT: NOVEL FUNCTIONS THROUGH SELFORGANIZATION OF MOLECULAR MATERIALS

The Computer Architecture Group looked into the cyclic pipeline architecture (CPC), which is suitable for the very fast latching circuits, such as QFPS. Compilers and several application softCompilers and several application software were designed, and the performance of CPC was verified. Along with these simulation studies, a CPC computer, made of silicon emitter-controlled logic (ECL) integrated circuits, was designed and fabricated, and the per- Project Director: Toyoki Kunitake formance was evaluated using standard supercomputer benchmarks. It was made clear that CPC architecture accelerates the performance by a factor of 10, if 12 jobs are processed in parallel,

The biomembrane is formed by self-organization of component molecules (lipids and proteins) that are derived from their unique steric

structures. The preparation of synthetic bilayer membranes has been successfully achieved in Japan and other countries by using a large variety of novel organic compounds. These compounds are composed of hydrophilic head groups and hydrophobic hydrocarbon chains in an analogy to biolipid molecules. This new molecular system provides an exciting possibility to produce artificial organization and to lead to a new field of chemical science.

In our ERATO project, the major emphasis is placed on self-organization of the above-mentioned types of organic molecules in the form of surface monolayers, Langmuir-Blodgett (LB) films, aqueous bilayers, and cast films. We aim at construction of molecular organizations that would possess particular electronic, magnetic, and chemical functions. Improvements of these functions will be attained by means of chemical modifications of simple assemblage and formation of multiply composed organizations. These studies should provide clues for the development of

novel industrial materials that are equipped with sophisticated functions analogous to those of the biological organization and yet are characterized by stability and processability of synthetic materials.

Our laboratories are set up at Kurume Research Park on Kyushu Island. The total staff is 21, as of 1 October 1991, including project director, administrative staff, researchers, and technicians. The whole research team is divided into three groups. The current activities of each group are described below.

Fundamental Design Group. We have prepared a series of surface monolayers that possess hydrogen bonding functionalities such as phenolic hydroxyl, carboxylic acid, 2,6-diaminotriazine, diphenyl-urea, and guanidinium. These monolayers bind biologically important compounds (sugars, amino acids,

nucleotides, ATP) in specific manners. The strong binding behavior observed in exposure to bulk water at the airwater surface is surprising.

Surface force measurements provided the evidence for a strong attractive force between two hydrophobic surfaces at an unprecedented distance of 300 nm. Any theory to explain this effect has not been proposed yet. The subsequent measurement of repulsion between two polyelectrolyte-modified surfaces produced data that could be discussed in the conventional theoretical framework.

Scanning tunneling microscopy (STM) showed that azobenzenecarboxylic acids were regularly aligned on highly oriented poly graphite (HOPG). The liquid crystalline nature of the azobenzene derivatives was closely related to complete wetting of graphite.

Functional Architecture Group. The self-assembling property of synthetic bilayer membranes was used to produce regular organization of metal ions (transition metals and lanthanides) on

a two-dimensional molecular surface. Multilayered films of ultrathin silicate and aluminosilicate layers were prepared by using cast films of synthetic bilayer membranes as molecular templates. The morphology of the highly porous silicate was replicas of aqueous bilayer dispersions and showed surprising variations depending on the composition and the preparative conditions. An ion exchange technique applied to cast multilayer films provided an additional template procedure. These methods were also used to obtain metal oxide multilayers from ultra-fine particles of metal oxides.

A series of novel oligo (phenylenevinylene) amphiphiles formed stable monolayers on water. Some of these wholly pi-conjugated monolayers produced Z-type LB films with nonlinear optical properties.

Composite Architecture Group. Regular multilayer films were obtainable from aqueous mixtures of bilayers and polar bifunctional monomers. Photoirradiation and removal of the template produced multilayered twodimensional (2D) polymer networks. When allylammonium monomers were used, multilayered ultrathin ion exchange films were formed. A similar technique was realized in nonaqueous media by the use of polyfluorinated amphiphiles, which formed ordered dispersions in some organic solvents. These fluorocarbon amphiphiles also gave uniform, stable monolayers.

Molecularly thin 2D networks were prepared by covalent bonding of ion complexes of oppositely charged polymers formed at the air-water interface. Defect-free films of a few molecular layers were thus formed by proper combination of the starting polymers.

Direct Observation of Molecular Assembly by Scanning Tunneling Microscopy

Masahito Sano, Fundamental Design Group

Azobenzene derivatives, selfassembled on the graphite surface, were imaged directly by STM in air (Ref 5). The images confirmed the model previously proposed for layered aggregates and compared well with the bulk correlation length. The methyl terminated compounds frequently showed bimolecular packings, while the brominated derivatives indicated a monomolecular structure. The difference in the distance between the carbon-hydrogen and the carbon-bromine atoms is an important factor contributing to these different packing styles.

In the course of sample preparation, it has been found that the liquid crystalline molecules with a sufficiently long alkyl chain in the nematic phase

wet the basal plane of graphite completely. Also, alkylated compounds that are not liquid crystalline can be made to wet the graphite completely by forming a mixture with other compounds. These observations, together with STM images of these compounds, allow us to argue a mechanism of wetting on a molecular scale.

Synthesis of Multilayered
Inorganic Ultrathin Films by
Using Cast Multi-Bilayer Films
as Molecular Templates

Munetoshi Isayamagi, Functional Architecture Group

can form self-supporting, multi-bilayer
films by casting from water or from
appropriate organic solvents. Especially,
those amphiphiles that have olefinic or
ether units in the tail exhibit higher
solubilities (dispersibilities) in many
organic solvents and produce well-
ordered films rather readily.

These fluorocarbon amphiphiles can
be used as molecular templates to pre-
pare multilayered poly(stearyl acrylate).
The thickness of the individual layer is
determined to be 500 Å or less by scan-
ning electron microscopy (SEM). The
formation of multi-bilayer cast films
even from organic solvents enlarges
the usefulness of the casting technique.

NISHIZAWA TERAHERTZ
PROJECT: EXPLORATION
FOR TERAHERTZ
SEMICONDUCTOR DEVICES

Cu2+ ion was incorporated into a multi-bilayer cast film of a phosphate amphiphile by an ion exchange process. The Cu2+ ion showed different electron spin resonance (ESR) patterns depending on the angle of the cast film Project Director: Jun'ichi Nishizawa surface against the magnetic field.

Silicate and silica-alumina ions can be also inserted into the cast film of an ammonium amphiphile in stoichiometric quantities by the same technique. Ultrathin films of silicate and silicaalumina could be obtained after extraction of the amphiphile.

The high regularity and the ion exchange ability of a multi-bilayer cast film that was prepared from a selfassembling amphiphile enable orientation control of inorganic structure units and synthesis of molecularly thin inorganic materials.

Preparation of Ultrathin
Fluorocarbon Film and Its
Application

Kenji Fukuta, Composite Architecture
Group

Novel double-chain ammonium amphiphiles, whose alkyl tails are composed of fluorocarbon and hydrocarbon segments, are synthesized. They

Background

The last several decades has witnessed a tremendous growth in the understanding and development of solidstate materials, resulting in a proliferation of semiconductor devices. These can be applied to ever-increasing regions of the electromagnetic spectrum.

This research will hopefully lead to the fabrication of semiconductor elements at the level of molecular layers, giving birth to a new engineering field that might well be called Molecular Electronics. It should also contribute to a sharp increase in not only the measuring techniques of lattice and molecular oscillations but also the future speed and volume of communication devices.

Research Strategy

This project is investigating compound semiconductors of the size of molecular ultrathin layers fabricated by carefully controlled photo-stimulated molecular layer epitaxial growth. Since it is not certain whether knowledge concerning electron behavior in conventional element structures is applicable to devices operating in the terahertz frequency region, crystal technology at the molecular-layer level is being studied.

Circuits that can operate as a mixer, detector, harmonic generator, and traveling-wave type optical modulator are also being investigated. New devices that are very thin, less than several hundred molecular layers, must be achieved not only for devices used to amplify signals but also for detectors and receivers that can also convert optical waves into lower frequencies.

In the electromagnetic wave region
the usable frequency band has been
extended to 10 Hz (3-mm wavelength)
owing to technological advancements Research Progress
of microscopic semiconductor elements
(less than 0.1 micron). Further, the
near-infrared region (near 1-micron
wavelength) is now usable owing to
progress in quantum electronics. Jun'ichi
Nishizawa is presently following up his
very successful Perfect Crystal project
(described in post-project-phase
ERATO projects) with a study of the
manipulation and use of the hitherto
unexplored terahertz region (1012 Hz
or 300-micron wavelength).

A buried heterostructure Raman laser has been investigated in which an intermediate layer is formed for pump power introduction, while Stokes radiation is confined within the active region without any disturbance from the entrance window. Lasing has been demonstrated and the pump power improved to as low as 0.5 W. The fabrication method is based on liquid-phase epitaxy by a temperature-difference

method under controlled vapor pressure, which can maintain a very low nonstoichiometric defect concentration. For the first time very perfect control of the crystal thickness in single crystals has been achieved.

In molecular-layer epitaxy the carrier concentration in a film strongly depends on the injection period of impurity gas, as well as substrate surface orientation. From studying the doping efficiency of Si using SiH, it could be doped on a Ga-compound substrate surface. Sites may be influenced by the AsH, pressure. Moreover, Si can be incorporated as a donor on the (111)B or (100) surfaces and as an acceptor on the (111)A surface.

Films have been made that are thinner than the carrier mean free path, resulting in the absence of both collisions and scattering by lattice vibration, enabling a mesoscopic static induction transistor (SIT) to be fabricated with an operation speed greater than 1 THz.

Progress has also been made towards an SIT device with a very thin base layer, thus limiting the frequency. Eventually, both Tunnett and SIT prep

aration will be combined.

A fabrication procedure for Pt/GaAs diodes (used for low-noise mixers with low conversion loss in the THz region) has been developed. Reactive ion etching and surface treatment of GaAs wafer surfaces have been improved and the diode diameter reduced to obtain a higher cutoff frequency: 0.8-microndiameter diodes have been fabricated with a 11,600-K mixer noise temperature and a 19.1-dB conversion loss at a signal frequency of 1.4 THz. Further, an antenna pattern suitable for imaging optics and antenna impedance of a good Schottky diode by tuning the director element of a Yagi-Uda antenna has been achieved. A new-type travelingwave optical modulator is being developed and analyzed in which the optical

wave and modulating wave phase velocity are matched. Three-dimensional fine processing is necessary to fabricate steps processing is necessary to fabricate steps of a few microns height and vertical, smooth sidewalls. The etching of the dielectrics and semiconductors used for optical waveguides and metals for electrodes has progressed.

There are three subgroups in our project located in Sendai: Basic Analysis, Functional Device, and Circuit Configuration. In the Basic Analysis Group, the photo-stimulated molecuGroup, the photo-stimulated molecular layer epitaxy (MLE) technique is used to obtain the very thin epitaxial layers for the very high speed transistors and diodes. Several kinds of devices such as the SIT, the Tunnett diode, the semiconductor Raman laser, etc. have been developed. The circuits that operate as the mixer, detector, harmonic generator, travelling-wave-type optical modulator, etc. are also being investigated.

Ideal Static Induction Transistor (ISIT)

Piotr Plotka, Functional Device Research Laboratory

Yutaka Oyama, Basic Analysis Research Laboratory

The ISIT transistor may be looked at as composed of some "building blocks": n*n'p*n'n* sandwich, extrinsic gate, and nonalloyed contacts. To develop a fabrication process, methods for fabrication of these blocks should be developed first.

Nearly ballistic operation of the ISIT can be obtained if the distance between highly doped n* regions of drain and source is on the order of 100 nm (Ref 6). To induce a potential barrier, we applied a very thin, fully depleted p* layer inside an n layer, an idea taken from high power SIT devices and planar doped barrier devices.

Such n*n*p*n'n* sandwiches have bean fabricated with MLE on GaAs(100) n wafers. The fabrication process started with a 20-nm-thick n* buffer followed by subsequent growths of a 200-nm-thick n' layer, thin p* layer, 50-nm-thick n ́ layer, 20-nm-thick n' source layer, and finally a 10-nm-thick n** layer for contact. The potential barrier value is a very sensitive function of the Zn doping of the p* layer.

So far, selective fabrication of semiconductor regions of one conduction type, surrounded by regions of opposite type, was characteristic for silicon rather than for III-V device technology. We applied MLE for selective growth of GaAs pn structures in grooves, similar to those required for gates of ISIT devices. A GaAs substrate with an n' Si doped metal organic chemical vapor deposited (MOCVD) layer was covered by photoexcited chemical vapor deposited (PECVD) silicon nitride. The SiN was patterned to form square openings. This served as a mask for subsequent wet etching of grooves in GaAs and as a mask for epitaxy. Two different MLE processes were checked: one doped with Zn and the second doped with carbon.

Current-voltage measurements were performed to evaluate quality of the regrown pn junctions. Characteristics of diodes containing MLE layers on (100) bottom and sidewalls parallel to the {01-1) direction of the groove and terminated at overhanging SiN are not worse than characteristics of diodes with MLE layers on (100) bottoms only. However, the performance of regrown pn junctions fabricated with Zn as dopant is better than for C as dopant. The ideality factor of forward characteristics for Zn-doped junctions is n=1.3, whereas in the case of C doping n=1.6.

The technique developed for selective growth of pn junctions with MLE can find application not only for fabrication of gates on ISITS but also

for many other devices. It is attractive especially because of the low temperature required to grow MLE layers.

To exploit fully the advantages of very thin layers that can be grown with MLE, we are working on development of nonalloyed contacts to both n and p type GaAs.

Contacts to n-type GaAs were formed by lift-off on MLE layers heavily doped with selenium. The specific contact resistance, evaluated with the transmission line method, was in the range of low 10-cm2. This is the best reported result for homogeneously doped n-type GaAs.

The specific contact resistance for p-type GaAs MLE layers heavily doped with Zn is 2 x 106 Q2-cm2. Although other methods enable obtaining even lower values of contact resistance, and are selective as well, the advantage of this method is the possibility of fabricating high quality pn junctions with excellent coverage of sidewalls of complicated shapes. This makes the method suitable for contacts to thin, buried layers or two-dimensional holegas layers.

Tunnett Diode

Toshifumi Suzuki, Functional Device
Research Laboratory

The tunneling-electron transit-time diode, the Tunnett diode, is considered the most important semiconductor device in the submillimeter-wave region. We have made Tunnett diodes using MLE, controlling layer thickness with an accuracy on the order of one molecular layer (Ref 7).

The Tunnett diodes we have made

utilize ap*n*i(n)n* structure. Tunnel current is injected from the p*n* region into the i(n) region. The oscillation frequency depends on the running time of these carriers in the i(n) running region. Therefore, to obtain a higher frequency, it is desirable to reduce the thickness of the running region.

In conventional impact avalanche transit-time (IMPATT) diodes, reductransit-time (IMPATT) diodes, reducing the device dimensions causes an increase of the electric field intensity in the injection region. Eventually, tunneling replaces avalanching as the high frequency carrier injection mechanism. Therefore, it can be said that the Tunnett diode is necessary to obtain higher frequencies.

we have the basic technology for frequency measurements from the far infrared to infrared.

The Tunnett diode oscillation system is very convenient because of its small size and power requirements. It is considered that submillimeter oscillation from the Tunnett diode will be very important for the development of devices in this frequency region. Further, it will make a significant contribution to the study of the interaction between submillimeter waves and matter.

Kenji Yamamoto, Functional Device
Research Laboratory

A field intensity in the injection region of about 106 V/cm is needed to get tunnel injection. On the other hand, in the running region a field intensity of about 104 V/cm is sufficient to obtain the saturation velocity of carriers. To Photoexcited Etching of GaAs achieve such field intensities in these two regions, it is necessary to control thickness and doping concentration of each layer precisely. MLE is a promising method to realize the necessary precision. It is considered that submillimeter oscillation of the Tunnett diode with optimized potential distribution will be realized with an applied voltage of about 1 V. We are now making Tunnett diodes to oscillate in the terahertz region.

We are also researching resonators for the submillimeter region. One type is a waveguide cavity resonator and another is an open resonator. We cannot analyze the behavior of resonators in the submillimeter region directly because no network analyzer for this region exists. So we made a W-band resonator (75 to 110 GHz) and measured network characteristics with good results. In the submillimeter region, results. In the submillimeter region, the electromagnetic wavelength is very small, so it is very difficult to make waveguide cavity resonators for this region. Therefore, it is expected that the larger dimensions of the open resonator will make it easier to fabricate.

We have used a Fourier-transform infrared (FTIR) system to measure the oscillation frequency of Tunnett diodes. The Michelson interferometer in this system has a Mylar film as a half mirror. We have measured a wavenumber of 4.5 cm1 (135 GHz). With this system,

To fabricate a superlattice structure or a quantum effect device, atomic scale controllability of processing is demanded. In recent years, there have been many reports on the use of the alternate operation of gas feeding, which has atomic scale controllability in thinfilm growth, but few such reports in etching. In conventional etching, the etchant and the energetic particles (ion, electron, photon) are introduced onto the substrate simultaneously. On the other hand, in digital photoexcited etching (DPE), the etchant and ultraviolet (UV) irradiation are applied alternately (Ref 8,9). The basic concept is as follows: (1) The etchant is introduced into the chamber and is adsorbed on the substrate (the etchant should not etch the substrate spontaneously at this moment). (2) Excess etchant is evacuated. (3) UV light irradiates the surface to initiate the photochemical reaction between adsorbed etchant and the surface atoms and to promote the photodesorption of products.

In the present study, DPE of GaAs (100) and (111) has been demonstrated with a Xe/Hg lamp/Cl2 system. Dependence of etching rate per cycle on UV irradiation time, Cl, evacuation time,

« iepriekšējāTurpināt »