Toshiro Isu

Distributions of growth rates on patterned surfaces measured by scanning microprobe reflection high‐energy electron diffraction

Microscopic distributions of growth rates on GaAs(001) layers next to (111)A and (111)B surfaces were measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high‐energy electron diffraction. The increase in the growth rate on the GaAs(001) surface near the edge of (111)A surfaces is observed and the decrease in the growth rate on the GaAs(001) surface near the edge of the (111)B surfaces is found out. The exponential variation of the growth rate as a function of the distance from the edge, reflects surface diffusion of Ga atoms. The diffusion lengths along the [110] and [110] directions are estimated to be about 1 and 8 μm at 560 °C, respectively.

Surface diffusion length observed by in situ scanning microprobe reflection high-energy electron diffraction

Abstract The dependence of surface diffusion phenomena of Ga adatoms on As flux during molecular beam epitaxy is investigated. Variations of growth rates of GaAs layers grown on the (001) surfaces adjacent to the (111) surfaces were measured by scanning microprobe reflection high-energy electron diffraction. The surface diffusion length is derived from the variations of the growth rates. It is found that the surface diffusion length of the Ga adatoms becomes larger under lower As flux.

GaAs/AlAs coupled multilayer cavity structures for terahertz emission devices

GaAs/AlAs coupled multilayer cavity structures are proposed as terahertz emission devices. Two cavity modes with an optical frequency difference in the terahertz region can be realized when two cavity layers are coupled by an intermediate distributed Bragg reflector multilayer. Interference between the enhanced light fields of the cavity modes has been demonstrated by the simultaneous injection of two cavity-mode lights using an ultrashort pulse laser. Such coupled multilayer cavity structures are promising for use as compact and room temperature operable terahertz emission devices based on difference frequency generation by the cavity-mode lights.

A GaAs/AlAs Multilayer Cavity with Self-Assembled InAs Quantum Dots Embedded in Strain-Relaxed Barriers for Ultrafast All-Optical Switching Applications

An optical Kerr signal has been simulated for GaAs/AlAs multilayer cavity structures by using the self-consistent transfer matrix method. Enhancement of the Kerr signal intensity was clearly demonstrated for the cavity mode (λ~1504 nm) owing to the strong optical field in the multilayer cavity. The Kerr signal intensity can be further enhanced with the use of a higher nonlinear refractive index only for the half-wavelength (λ/2) cavity layer. We propose a GaAs/AlAs multilayer structure with the λ/2 cavity layer containing InAs quantum dots embedded in strain-relaxed barriers for an ultrafast optical Kerr gate switch with extremely low switching energy.

Optical Matrix Elements in (110)-Oriented Quantum Wells

Optical matrix elements at the Brillouion zone center in (110)-oriented quantum wells (QWs) for linearly polarized light are theoretically investigated on the basis of an envelope-function approximation. For linear polarization parallel to the QW planes, the optical matrix elements in (110) QWs depend on the polarization direction, which is in contrast with polarization-independent character in (001)- or (111)-oriented QWs.

The Mechanism of Self-Limiting Growth of Atomic Layer Epitaxy of GaAs by Metalorganic Molecular Beam Epitaxy Using Trimethylgallium and Arsine

The self-limiting growth of atomic layer epitaxy by metalorganic molecular beam epitaxy using trimethylgallium (TMGa) and arsine (AsH3) was studied as a function of substrate surface condition. It was observed that GaAs growth from TMGa and AsH3 is limited to 1 monolayer (ML) on an As-stabilized surface and that TMGa can adsorb on a Gacovered surface. In-situ observation of RHEED intensity showed that the TMGa in excess of 1 ML on the GaAs substrate desorbed after TMGa supply was stopped. It was found that self-limiting monolayer growth resulted from desorption of the excess TMGa.

Real-Time Observation of GaAs(001) Surfaces During Molecular Beam Epitaxy by Scanning Microprobe Reflection High Energy Electron Diffraction

GaAs(001) surfaces during MBE growth was observed by scanning microprobe RHEED. A microprobe electron gun for RHEED was installed in an MBE chamber. Using intensity variation of RHEED patterns produced by the scanning of an incident beam, we obtained a microscopic image highly sensitive to the surface structure. Scanning reflection electron images revealed granular features over the whole surface, which came from small undulations on the surface of the MBE grown layer. The undulation became larger along [10] direction as growth proceeded. No contrast was observed in the secondary electron images.

Real-time μ-RHEED observations of GaAs surfaces during growth with alternating source supply

Abstract The microscopic surface features of GaAs and AlAs during growth with an alternating source supply were observed by scanning microprobe reflection high-energy electron diffraction in real time. Scanning reflection electron microscope images were obtained in the TV scan mode (50 frames per second). The creation of Ga droplets was observed when Ga atoms over the critical amount depending on the substrate temperature were supplied. They disappeared after a sufficient amount of As was supplied. When 5 monolayers of Ga atoms were supplied per cycle, small islands with bright contrast appeared due to three-dimensional nucleation after As was supplied.

Enhanced Optical Kerr Signal of GaAs/AlAs Multilayer Cavity with InAs Quantum Dots Embedded in Strain-Relaxed Barriers

Using time-resolved optical measurements, a strong, ultrafast optical Kerr signal was demonstrated in a GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed InGaAs barrier layers. The large optical nonlinearity of the resonant InAs QDs in the half wavelength (λ/2) cavity layer is further enhanced by the strong cavity effect. Although only two layers of InAs QDs were inserted in the λ/2 layer, the optical Kerr signal intensity was 60 times larger than that of a GaAs λ/2 cavity. The response time of the optical Kerr signal was less than 1 ps, which was much faster than the decay time (15 ps) of optically generated carriers in the InAs QDs.

Optical Anisotropy of Strongly Enhanced Sum Frequency Generation in (113)B GaAs/AlAs Coupled Multilayer Cavity

We have investigated optical anisotropy of sum frequency generation (SFG) of two cavity modes realized in a (113)B GaAs/AlAs coupled multilayer cavity. The SFG signals were measured by simultaneous excitation of two cavity modes with an optical frequency difference of 3.3 THz using 100-fs laser pulses. The peak intensity of the SFG signal was more than 400 times larger than that of the second-harmonic generation signal from the (113)B GaAs bulk substrate. Large optical anisotropy depending on the polarization angle of the excitation laser was observed in the strongly enhanced SFG in the (113)B coupled cavity sample. The observed anisotropic behavior was well explained by the in-plane anisotropy of the second-order nonlinear polarization on the (113)B GaAs substrate.