Kiyoshi Asakawa

Reactive ion beam etching of Y‐Ba‐Cu‐O superconductors

It has been found that a reactive ion beam etching (RIBE) using Cl2 gas is useful for microfabrication of Y‐Ba‐Cu‐O superconductors. The etching yield enhancement of Y‐Ba‐Cu‐O is observed for Cl2 RIBE. The etching yield of Y‐Ba‐Cu‐O at 400 V accelerating voltage, 2×10−3 Torr Cl2 pressure for Cl2 RIBE is 2, which is twice that for Ar ion beam etching. Y‐Ba‐Cu‐O submicron patterns have been fabricated by focused ion beam lithography and Cl2 RIBE. Moreover, a Y‐Ba‐Cu‐O superconducting line with a 4‐μm linewidth has been fabricated by annealing an as‐sputtered Y‐Ba‐Cu‐O line pattern.

Sequential Operations of Quantum Dot/Photonic Crystal All-Optical Switch With High Repetitive Frequency Pumping

A photonic crystal (PC)-based symmetric Mach-Zehnder type all-optical switch (PC-SMZ), previously operated by single pump pulse alone, has been operated newly by a multiple-pump pulse train corresponding to a repetition frequency of 40 GHz at pulse energy as low as 10 fJ. The device involves quantum dots (QDs) in two parallel PC arms as optical nonlinear media and functions as a time-differential phase modulator caused by the pump pulse inducing carriers in the QD. Prior to the switch operation, sequential time response of the phase shift for a probe pulse was investigated in detail by changing the power and repetition rate of the pump pulse in the straight PC waveguide configuration. Besides, PC and QD parameters were explored for possibility of 100% on-off switching ratio. As a result, five QD layers, 40-ps QD relaxation-time, 500-mum PC-length and use of as low as 0.05 c PC group-velocity (c; light velocity in vacuum) were found to implement the 100% switching ratio. Since each of these parameters has ever been achieved experimentally, the result will pave a promising way to an ultra-small and ultra-fast integrated all-optical switch.

An AlGaAs laser with high-quality dry etched mirrors fabricated using an ultrahigh vacuum in situ dry etching and deposition processing system

Highly reliable AlGaAs lasers with dry etched mirrors have been successfully fabricated with an ultrahigh-vacuum in situ processing system, equipped with reactive ion-beam etching (RIBE) and dielectric film deposition chambers. Etched mirror surfaces are protected against air-exposure contamination and nonvolatile-reaction-products adsorption with in situ Al/sub 2/O/sub 3/ passivation subsequent to the CI/sub 2/ RIBE mirror formation. Ion-bombardment-induced damage is repaired by thermal annealing. The annealing effect is enhanced by a contamination-free interface between the etched mirror surface and Al/sub 2/O/sub 3/ passivation film. The lasers exhibit an increase in catastrophic optical damage (COD) level and long-life operation. Their COD levels are twice as high as that for as-etched lasers and are almost the same as those for conventional cleaved lasers. >

Chlorine-Based Smooth Reactive Ion Beam Etching of Indium-Containing III-V Compound Semiconductor

Very smooth and vertical etching of InP by Cl2 reactive ion beam etching has been achieved under high temperature (≈200°C), high ion energy (≈1 keV) and low Cl2 pressure (~10-5 Torr). The roughness is estimated to be a few nm by scanning tunneling microscopy and no contamination except for Cl was observed by in situ Auger electron spectroscopy. Under these etching conditions, the etched depth is precisely controlled (σ=22 nm) by simply monitoring the electrode curtent of the ion accelerating grid. Other III-V compound semiconductors, such as GaAs, InGaAs, AlGaInP and InAlAs have also been etched smoothly and vertically. Multilayers of these materials, such as InP/InGaAsP, AlGaInP/GaInP, and InAlAs/InGaAs/InP have been etched without steps between the layers on the sidewalls.

Radical beam/ion beam etching of GaAs

A novel combined radical beam and ion beam etching (RBIBE) technique that uses a microwave‐excited radical beam combined with an Ar+ ion beam has been developed for smooth, low damage, and rapid etching of GaAs structures. Separate control of both argon ion (Ar+) energy and/or current and chlorine radical (Cl*) beam flux density has enabled us to realize highly chemically enhanced GaAs etching. This RBIBE technique differs qualitatively from and offers greater flexibility than reactive ion etching (RIE), ion beam assisted etching (IBAE), where the reactive etch gas is not plasma excited, or reactive ion beam etching (RIBE), where independent control of radical and ion beam flux is difficult. Our work examines etch rate as a function of flow rate, temperature, microwave power, and ion beam current. The etch rate and reactive sputter yield are typically eight times greater with the microwave plasma on (RBIBE) than with the plasma off (IBAE). With RBIBE, high etch rates (2.5 μm/min) have been realized at roo...

Photonic crystal all-optical switches

A photonic crystal all-optical switch is an all-optical switch (photonic crystal structure), in particular a migrating nonlinear photonic crystal all-optical switch of a defect mode. The photonic crystal all-optical switch comprises triangular lattice photonic crystals (1), a waveguide area (2), a traction area (3) and a nonlinear resonant cavity (4), wherein the triangular lattice photonic crystals (1) are large air hole type photonic crystals in triangular periodic distribution along X and Z axes, symmetric linear defects are introduced at two ends of the triangular lattice photonic crystalsto form the waveguide area (2), a central air hole of the triangular lattice photonic crystals (1) is provided with the nonlinear resonant cavity (4), the traction area (3) is arranged between the waveguide area (2) and the nonlinear resonant cavity (4), and signal light is input from the left end of the waveguide area (2) and output from the right end of the waveguide area (2); and control lightvertical to the X and Z axes is incident to the central nonlinear resonant cavity (4) along a medium column.

Acceleration and suppression of photoemission of GaAs quantum dots embedded in photonic crystal microcavities

We fabricated optical microcavities in a photonic crystal slab embedded with GaAs quantum dots by electron beam lithography and droplet epitaxy. The Purcell effect of exciton emission of GaAs quantum dots was confirmed for the first time by microphotoluminescence and lifetime measurements.

Smooth and vertical InP reactive ion beam etching with Cl2 ECR plasma

Smooth and vertical InP reactive ion beam etching has been achieved with electron cyclotron resonance Cl2 plasma at high ion energy (≥900 eV), high temperature (230°C) and relatively low Cl2 pressure (~10-4 Torr). Smooth etching of an InP system by Cl2 plasma has often been reported as difficult compared to that of the GaAs system due to low volatility of reactive products such as InClx. In the present work, precise control of incident ion energy and Cl2 pressure contributed to the improvement of both the vertical profile and bottom smooth surface under high substrate temperature (~200°C). Vertical profiles were easily achieved even at high temperatures by varying the Cl2 pressure. While etching conditions suitable for vertical wall-formation were maintained, surface morphology was drastically improved by increasing ion energy above 900 eV and the bottom roughness became less than 100 nm at 1450 eV.

Surface‐emitting GaAs/AlGaAs lasers with dry‐etched 45° total reflection mirrors

Surface‐emitting GaAs/AlGaAs lasers with 45° total reflection mirrors have been successfully produced using a 45° tilted reactive ion beam etching technique. The ratio of surface‐emitted light output power to edge‐emitted light output power was obtained at values as high as 77%. The total reflection mirror was formed within 1° of the desired precise 45° angle. This type of surface‐emitting laser is promising for optoelectronic integrated circuits because of the simplicity of its structure and fabrication.

Electrical characterization of very-low energy (0–30 eV) Cl-radical/ion-beam-etching induced damage using two-dimensional electron gas heterostructures

Damage induced by very low energy (30 eV) Cl reactive ion beam etching (RIBE) and radical etching (RE) has been electrically characterized. GaAs/n-AlGaAs two-dimensional electron gas heterostructures were used as damage sensitive probes. Sheet carrier concentrations and Hall mobilities were measured at 77 K, under dark as well as illuminated conditions. By carefully designing the sample structure, the damaged layer thickness could be estimated by comparing dark and illuminated data. In case of RE, no degradation was detected at depths as shallow as 25 nm from the etched surface. For 30 eV-RIBE, the damage was detected but found to be reasonably small (38% decrease in electron mobility) and shallow (<50 nm). Electrical and optical damage are compared briefly.