Toshiyuki Usagawa

A New Two-Dimensional Electron Gas Field-Effect Transistor Fabricated on Undoped AlGaAs–GaAs Heterostructure

A new two-dimensional electron gas (2DEG) field-effect transistor (FET) which operates in an MOS transistor-like mode is fabricated on undoped AlGaAs–GaAs heterostructures grown by both molecular beam epitaxy and organometallic vapour phase epitaxy. This device with very simple structure is expected to have performance comparable to those of selectively doped heterostructure FETs and can easily be integrated.

Air-Annealing Effects for Pt/Ti Gate Si-Metal--Oxide--Semiconductor Field-Effect Transistors Hydrogen Gas Sensor

Air-annealing Pt (15 nm)/Ti (5 nm) gate Si-metal–oxide–semiconductor field-effect transistors (Si-MOSFETs) at 400 °C for 2 h revives and enhances the hydrogen sensing activity of nearly all the inert sensor characteristics of the Pt (15 nm)/Ti (5 nm) gate Si-MOSFETs. The sensing amplitudes are changed from 0.0–0.16 V to 0.82–0.96 V under the air-diluted 1000 ppm-hydrogen gas exposure before and after the air annealing. A transmission electron microscope (TEM) and Auger analysis clearly show that one of the causes is the novel mixing of nanocrystalline TiOx and super heavily oxygen-doped amorphous Ti in the Ti-layer formed on the gate oxide SiO2.

A Pt–Ti–O gate Si-metal-insulator-semiconductor field-effect transistor hydrogen gas sensor

A hydrogen gas sensor based on platinum–titanium–oxygen (Pt–Ti–O) gate silicon-metal-insulator-semiconductor field-effect transistors (Si-MISFETs) was developed. The sensor has a unique gate structure composed of titanium and oxygen accumulated around platinum grains on top of a novel mixed layer of nanocrystalline TiOx and superheavily oxygen-doped amorphous titanium formed on SiO2/Si substrates. The FET hydrogen sensor shows high reliability and high sensing amplitude (ΔVg) defined by the magnitude of the threshold voltage shift. ΔVg is well fitted by a linear function of the logarithm of air-diluted hydrogen concentration C (ppm), i.e., ΔVg(V)=0.355 log C(ppm)−0.610, between 100 ppm and 1%. This high gradient coefficient of ΔVg for the wide sensing range demonstrates that the sensor is suitable for most hydrogen-safety-monitoring sensor systems. The Pt–Ti–O structures of the sensor are typically realized by annealing Pt (15 nm)/Ti (5 nm)-gate Si-metal-oxide-semiconductor structures in air at 400 °C for...

(110) YBa2Cu3O7-δ Homoepitaxy on (110) YBa2Cu3O7-δ Single Crystal Substrates

We report on high-quality (110) oriented YBa2Cu3O7-δ films grown on mechanically polished (110) YBa2Cu3O7-δ single crystal substrates by off-axis RF magnetron sputtering method. The homoepitaxial growth keeps the original surface flatness of substrates. Atomic Force Microscopy (AFM) measurements of the films show a surface roughness of less than 1 nm over 1×1 µm2 area range. Furthermore, cross sectional Transmission Electron Microscopy (TEM) and Reflection High Energy Electron Diffraction (RHEED) observations confirm the surface flatness of our films. The homoepitaxial films show a high T c (zero resistivity) of 90–92 K.

Threshold voltage control of Pt-Ti-O gate Si-metal-insulator semiconductor field-effect transistors hydrogen gas sensors by using oxygen invasion into Ti layers

Although the threshold voltages (Vth) of the as-processed Pt(15 nm)/Ti(5 nm)-gate Si-MOSFETs under same channel ion dose conditions show a large variation such as 0.846 V among several wafers, the air-annealing and succeeding hydrogen post-annealing procedure for the FETs hydrogen gas sensors leads to excellent uniform Vth distributions and large sensing amplitude ΔVg. The oxygen invasion process through Pt grain boundaries to amorphous Ti layers at 400 °C air-annealing for two hours is not a simple dopant diffusion process but super-heavily oxygen-doped process partly to grow nano-crystalline TiOx. The oxygen-invaded Ti layers change to a kind of new materials; novel mixing layers of nano-crystalline TiOx and super-heavily oxygen-doped amorphous Ti formed on SiO2/Si substrates. The Ti mixing layers change from metals to semiconductors or insulators. As the Ti layers are so thin like 5 nm, the total amount of oxygen invaded into Ti layers will be saturated and stabilized. From the device operation point of view, it is crucial to control the Vth precisely that the Ti novel mixing layers are thin and fully depleted. This is supported by the fact that the Vth change before and after air-annealing procedures can be well explained by the difference of vacuum work function between Pt and Ti.

Nonlinear I–V characteristics and proximity effects for PrBa2Cu3O7−δ/YBa2Cu3O7−δ bilayered structures grown on (001) YBa2Cu3O7−δ single crystal substrates

We have observed superconductivity induced by proximity effects for 100 nm thick (001) PrBa2Cu3O7−δ films pseudomorphically grown on (001) YBa2Cu3O7−δ single crystal substrates by vertical four-probe measurements. The induced superconductivity disappeared around 28 K and was recovered at around 15 K. The 220 nm thick (001) PrBa2Cu3O7−δ/YBa2Cu3O7−δ bilayers show nonlinear I–V behavior with 0.2 mV built-in potential at low temperature below 50 K. The local maximum of the vertical resistance appears around 25 K.

Stability of ultrasmooth surface morphology of (110) YBa2Cu3O7−δ homoepitaxial films and Nb/Au/(110) YBa2Cu3O7−δ junctions

We report on ultrasmooth surface morphology and chemical activeness of (110)-oriented YBa2Cu3O7−δ (YBCO) films grown on (110) YBCO single-crystal substrates by scanning electron microscopy, reflection high-energy electron diffraction, and transmission electron microscopy observations. The stable (110) YBCO interface can be formed by in situ deposited thin Au film against the chemically active elements like Nb. The in situ formed Nb/Au/(110)YBCO junction shows clear two superconducting transitions Tc1=91.8 K and Tc2=8.20 K associated with YBCO and Nb films and large supercurrent (at least larger than 2.0×102 A/cm2) much more than expected phenomenological arguments of s wave and dx2−y2 wave.

Extremely low non‐alloyed specific contact resistance ρc (10−8 Ω cm2) to metalorganic molecular beam epitaxy grown super heavily C‐doped (1021 cm−3) p++GaAs

The specific contact resistance ρc of non‐alloyed ohmic metals (Mo/Au and WSix) to p‐type GaAs is evaluated by transmission line method (TLM) patterns for various doping concentrations from 8×1018 to 1021 cm−3. The obtained specific contact resistance ρc, less than 10−8 Ω cm2, to metalorganic molecular beam epitaxy grown C‐doped ( 1021 cm−3) GaAs is the lowest reported value. From the application point of view for the emitter electrodes of PNp type (two‐dimensional electron gas base) heterojunction bipolar transistors, the specific contact resistance of a refractory metal, or the sputter‐deposited WSix (x=0.45), is also studied. By reducing the growth temperature of molecular beam epitaxy from 600 to 450 °C and enhancing the As4/Ga flux ratio to 15, a nondiffusive heavily Be doped (1020 cm−3) GaAs is obtained. The specific contact resistance of the WSix to this sample is less than 10−6 Ω cm2. The obtained specific contact resistance can be well explained by a simple modeling based on the tunneling transpo...

Photo-CVD SiN Assisted Self-Aligned Metal Contact Technology for High-Speed GaAs Devices–Recess Gate Structure with High Breakdown Voltage for GaAs FETs–

A new self-aligned recess metal contact process utilizing photo-CVD SiN as a side-wall between the gate metal and the n+ source region has been accomplished for recessed gate GaAs MESFETs with a single lithographic procedure. The process achieves excellent transconductance, Gm, of 190~240 mS/mm for a gate length, Lg, of 1.2 µm with keeping the source-gate breakdown voltage as high as 7-14 V.

Homoepitaxial (110) YBa2Cu3O7-δ/YBa2(Cu1-xFex)3O7-δ/YBa2Cu3O7-δ SNS Junctions

We report on the first successful single crystal SNS junction based on (110) YBa2Cu3O7-δ (YBCO)/YBa2(Cu1-xFex)3O7-δ (YBCFeO: 150 nm)/YBa2Cu3O7-δ films grown on (110) YBCO single crystal substrates. The surface flatness is nearly the same level as good as (110) YBCO homoepitaxial films, as confirmed by Scanning Electron Microscopy, Reflection High Energy Electron Diffraction and Transmission Electron Microscopy observations. The single crystal SNS junction shows an anisotropic field dependence in large-junction behavior at 77 K. The field supression of critical supercurrent Ic(T) is greater than 95% at 77 K and 9.0 K. The temperature dependence of Ic(T) is well described by proximity SNS theory near the junction transition temperature Tcn, but exibits nearly T-linear behavior at low temperature.