Hiroshi Imamoto

Migration‐enhanced epitaxy on a (111)B oriented GaAs substrate

AlGaAs layers with a featureless specular surface morphology were grown successfully on an exactly (111)B oriented GaAs substrate by migration‐enhanced epitaxy (MEE) even at growth temperatures below 500 °C. We have also observed reflection high‐energy electron diffraction (RHEED) intensity oscillation of AlGaAs on a (111)B oriented substrate by MEE. The single quantum well (SQW) is prepared by MEE on a (111)B and a (100) substrate simultaneously, and the photoluminescence intensity from (111) SQW is shown to be about 50 times higher than that from (100) SQW.

Fabrication and characterization of silicon antireflection structures for infrared rays using a femtosecond laser

We focus on IR sensors with lower reflection for the wavelength around 10 μm, strongly awaited for detecting human bodies. A concave structure was designed as a more suitable reflection-free structure for IR light, and an optical system with a femtosecond laser was employed for verification of the effectiveness of the structure. The microstructures prepared through this process were fabricated and optically measured using SEM, FT-IR, and Raman spectroscopy. The measurement revealed that good reflection-free structures were realized for IR sensors with lower reflection for the wavelength of around 10 μm.

Low‐temperature (350 °C) growth of AlGaAs/GaAs laser diode by migration enhanced epitaxy

Migration enhanced epitaxy, in which group III and group V elements are deposited onto the substrate alternately under ultrahigh vacuum, has been employed to reduce AlGaAs growth temperature. By optimizing growth conditions of AlGaAs, molecular‐beam intensities, and substrate temperatures between 600 and 250 °C, an AlGaAs/GaAs single‐quantum‐well laser diode has been fabricated successfully at very low temperature of 350 °C for the first time. A broad area laser diode emitting at 780 nm shows the threshold current density of 2.5 kA/cm2 at room temperature under the pulsed operation.

Low thermal expansion polymide buried ridge waveguide AlGaAs/GaAs single‐quantum‐well laser diode

A novel ridge waveguide laser diode has been developed in which the ridge is buried in a newly developed polyimide with a low thermal expansion coefficient close to that of AlGaAs which reduces the thermal stress to the junction and simplifies the wafer processing. A planar configuration, which is suitable for optoelectronic integration and an episide down mount for high‐power operation, has been achieved by an etch‐back process. Under cw operation, a low threshold current of 15 mA at 25 °C, a characteristic temperature T0 of 145 K, and maximum power output higher than 30 mW/facet have been obtained in a molecular‐beam‐epitaxial‐grown graded‐index separate confinement heterostructure single‐quantum‐well laser emitting at 780 nm.

GaAs double quantum well laser diode with short-period (AlGaAs)m(GaAs)n superlattice barriers

A short‐period (AlGaAs)m(GaAs)n superlattice has been applied to the barrier layers in a single and a multiple quantum well structure prepared by molecular beam epitaxy in order to improve the interface quality. With a 38 A thin GaAs quantum well without employing aluminum, a low threshold current density of 260 A/cm2, a high characteristic temperature (T0) of 205 K, and a high differential quantum efficiency of 75% have been achieved in a double quantum well ridge waveguide laser diode emitting at 780 nm.

Method to Evaluate the Influence of Etching Damage on Microcantilever Surface on Its Mechanical Properties

We propose a method to evaluate the effect of process damage on microcantilever surfaces, introduced by processes such as plasma etching, on their mechanical properties. Using this method, we can compare the mechanical properties before and after etching even if the process changes the microcantilever thickness. Defects at the microcantilever surface affect the quality (Q) factor of the microcantilever, but the Q factor cannot be used as an indicator to evaluate process damage because it also depends on the microcantilever thickness. On the basis of theoretical considerations, we propose using Q/f (f: resonance frequency) as an indicator because both Q and f are proportional to the thickness for very thin microcantilevers. We verified our method experimentally by etching microcantilever surfaces using conventional plasma etching and neutral beam etching, which can etch silicon without damage. As a result, the Q/f value markedly decreased after plasma etching but stayed nearly the same after neutral beam etching.

High-efficiency AlGaAs/GaAs single-quantum-well semiconductor laser with strained superlattice buffer layer

The use of a strained superlattice buffer (SSLB) layer composed of a short-period (InGaAs)(GaAs) superlattice in a lattice-matched AlGaAs/GaAs system in order to reduce the internal stress is discussed. A five-times-higher photoluminescence peak intensity has been observed from a single quantum well (SQW) with the SSLB than without the SSLB. A high-quantum efficiency, a small cavity loss, and high-output power operation have been achieved in a narrow ridge-waveguide 770-nm graded-index-separate confinement heterostructure SQW laser diode with the SSLB.<<ETX>>

Low-power-consumption CO 2 gas sensor using ionic liquids for green energy management

We propose a low-power-consumption CO2 gas sensor using ionic liquids We use ionic liquids of imidazolium salts EMIMBF4 EMIMTFSI and BMIMHFA The impedance of the ionic liquids decreases along with increase of the CO2 gas concentration By measuring the impedance of the ionic liquids we could estimate the CO2 gas concentration of the atmosphere This experiment showed that measurement resolution of our proposed sensor was 100 ppm and the power consumption was several tens of microwatts and this value is 1/1000 smaller than the conventional CO2 gas sensors (NDIR and Solid state electrolyte cell) We expect as a candidate of very low power consumption CO2 sensor suitable for green energy management.

Modeling of Vibrating-Body Field-Effect Transistors Based on the Electromechanical Interactions Between the Gate and the Channel

A coupled analysis method for the mechanical and electrical systems of vibrating-body field-effect transistors (VB-FETs) is described. To accommodate energy transfer between the gate and the FET channel, we represent the FET with a resistance–capacitance ladder circuit and use the Lagrange function to derive motion equations. By solving the equations, we derive the typical electrical characteristics of VB-FETs, namely, the transconductance and the current gain. The results show that the current gain is obtained even above the cutoff frequency of the FET at the antiresonance frequency of the mechanical vibrator. These characteristics strongly depend on the device dimensions and operating conditions. This means that a coupled analysis is helpful for determining an appropriate design of VB-FETs.

High-power broad mesa structure AlGaAs/GaAs single-quantum-well edge-emitting LED

An edge-emitting light-emitting diode (LED) with a wide emitting region for optical sensing and information processing which requires slit-shaped light sources is discussed. Through the use of a high-quantum efficiency single-quantum-well structure grown by molecular beam epitaxy an output power as high as 3 mW at an injected current of 100 mA with a 50- mu m-wide mesa structure is achieved at the 780-nm emission wavelength. A uniform and rectangular intensity profile, which is suitable for practical use, is obtained with a mesa-restricted structure.<<ETX>>