Masahiro Asada

Metal (CoSi2)/insulator(CaF2) resonant tunneling diode

Negative differential resistance (NDR) of nanometer-thick triple-barrier metal( CoSi2)/insulator( CaF2) resonant tunneling diode (RTD) and the structure dependence of its characteristics are demonstrated. The device consists of metal-insulator (M-I) heterostructures with two metallic ( CoSi2) quantum wells and three insulator ( CaF2) barriers grown on an n-Si(111) substrate. A typical peak-to-valley current ratio (P/V ratio) obtained at 77 K was 2–3 and the largest P/V ratio was 25. A P/V ratio as high as 2 was obtained at 300 K. M-I RTDs with two quantum wells of various thicknesses were fabricated in order to investigate the dependence of resonance voltage on the thickness of the two quantum wells. Reasonable agreement was obtained between theory and experiment for this dependence.

Theoretical analysis and fabrication of small area metal/insulator resonant tunneling diode integrated with patch antenna for terahertz photon assisted tunneling

Alternative voltage induced across a resonant tunneling diode under the THz radiation, which is an important factor for photon assisted tunneling, is theoretically analyzed in general form. As an application, integration of a small-area resonant tunneling diode with a circular patch antenna is proposed. A patch antenna has high radiation directivity and suitability for a diode with vertical layer structure on a high-doped substrate. Size of the antenna and diode necessary to reduce the loss and capacitance was discussed taking into account fabrication limit. The estimated voltage in this integrated device together with an appropriate lens system is comparable to the THz photon energy under relatively low power radiation (∼100 mW). To achieve requirement of the device size, a fabrication process is investigated for a 1 mm-diameter metal/insulator (CoSi2/CaF2) resonant tunneling diode with a patch antenna. Fabricated diodes show negative differential resistance in static characteristics with the maximum peak-to-valley ratio of 2.8 at room temperature.

Shortening of Detection Time for Observation of Hot Electron Spatial Distribution by Scanning Hot Electron Microscopy

The scanning hot electron microscope (SHEM) is a tool to observe non-thermal- equilibrium electrons under the surface of a solid, and it enables us to study the hot electron diffraction pattern caused by a small structure in the propagation layer. To observe the hot electron spatial distribution by SHEM, the detection time must be shortened. The reduction of the noise current including vibrational noise is investigated comprehensively. By the reduction of noise, in particular, that of nonstationary noise with digital measurements, the hot electron current was detected in 30 s, 1/20 of the measurement time reported previously.

A Possible Three-Terminal Amplifier Device in the Terahertz Frequency Range Using Photon-Assisted Tunneling

A new electron device with nanometer-thick multilayer heterostructure is proposed for amplification of an electromagnetic wave. The device is composed of the input part utilizing photon-assisted tunneling and the output part utilizing radiation from a charge density wave modulated at the input. A simple analysis shows that amplification up to the terahertz frequency range is possible in the device.

Detection of hot electron current with scanning hot electron microscopy

Scanning hot electron microscopy (SHEM) has been proposed as an experimental technique which allows for detection of hot electrons emitted from a subsurface semiconductor structure, thus making it possible to obtain the spatial distribution of hot electrons in a device. Here we present the experimental evidence of SHEM operation. Hot electrons with energies of 3 eV are injected by means of a Si/CaF2/Au heterostructure and subsequently detected at the tip of a scanning tunneling microscope in the SHEM configuration. The measured hot electron current was approximately 4 pA for a tunnel current of 5 nA. These results, although still of a preliminary nature, show the potential of SHEM as a technique suitable for the visualization of electron wave effects in semiconductor structures.

Proposal and Analysis of Very Short Channel Field Effect Transistor Using Vertical Tunneling with New Heterostructures on Silicon

We propose and analyze a very short channel tunneling field effect transistor using new heterostructures (CoSi2/Si/CdF2/CaF2) lattice-matched to Si substrate. In device operation, drain current from source (CoSi2) to drain (CoSi2) through tunneling barriers (Si) and channel (CdF2) is controlled by gate electric field applied to the barrier between source and channel through gate insulator (CaF2). Theoretical analysis shows that this transistor has characteristics similar to those of conventional metal- oxide-semiconductor field effect transistors even with the channel lengths of 20nm and 5nm.

TERAHERTZ RESPONSE WITH GRADUAL CHANGE FROM SQUARE-LAW DETECTION TO PHOTON-ASSISTED TUNNELING IN TRIPLE-BARRIER RESONANT TUNNELING DIODES

Current change under irradiation of a terahertz electromagnetic wave was measured for a triple-barrier resonant tunneling diode integrated with a planar patch antenna. Gradual change from the classical square-law detection to photon-assisted tunneling with increasing photon energy was observed.

Formation of Silicon and Cobalt Silicide Nanoparticles in CaF2.

We have demonstrated a method of forming silicon and cobalt silicide nanoparticles (< 10 nm) in CaF 2 grown on a Si(111) substrate using codeposition of Si, Co and CaF 2 . It is shown that the size and density of silicon and silicide particles can be controlled by the substrate temperature and the evaporation rate of CaF 2 and Si. At a growth temperature of around 200°C and a flux ratio of Co :Si :CaF 2 of about 1 :2 :2, an average diameter of 7 nm was obtained.

Metal (CoSi2)/Insulator (CaF2) Hot Electron Transistor Fabricated by Electron-Beam Lithography on a Si Substrate.

We fabricated a small-area metal (CoSi2)/insulator (CaF2) hot electron transistor using electron-beam lithography. The transistor is composed of a CoSi2/CaF2 (1.9 nm)/CoSi2 (1.9 nm) tunnel emitter and a CaF2 (5 nm) collector barrier on an n-Si(111) substrate. The emitter mesa area is 0.9 × 0.9 µm2. Although the measured characteristics show, for the first time, clear transistor action with a curve similar to those of semiconductor HETs, the collector current increases without saturation due to leakage current through the SiO2 film under the external electrode pads. The intrinsic device characteristics (zero leakage current) exhibited saturation, and a current gain β ≥ 36 was obtained at 77 K.

Hole capture rate of GaInAs/InP strained quantum-well lasers

The property of hole capture of quantum wells is important in the static properties of lasers above threshold, such as the differential efficiency and light output power. We investigate experimentally the hole capture rate and its influence on the carrier overflow in the optical confinement layers for compressive-strained, tensile-strained and unstrained GaInAs/GaInAsP/InP quantum-well lasers emitting at 1.5 μm by measuring the spontaneous emission from the optical confinement layers above threshold. The carrier density in the optical confinement layers increases with current owing to finite hole capture rates. This increase is dependent on well thickness and barrier height determined by the strain. This increase is comparable in the tensile-strained and unstrained lasers with relatively low threshold, while in the compressive-strained laser it is about double that in the other two types. The dependence of this increase on threshold carrier density is also observed, that is the carrier density in the optical confinement layers increases rapidly in high-threshold samples, in particular, in the tensile-strained laser with large hole barrier height. From these results, laser operation with high output power and high efficiency is expected by reducing threshold carrier density in the tensile-strained laser and by increasing well numbers in the compressive-strained laser as long as the inhomogeneous injection between wells is not severe. By fitting measurements with theory, the hole capture time is estimated as 0.1 to 0.25 ps in these strained and unstrained lasers.