Kazuyoshi Kubota

Strain-driven self-positioning of micromachined structures

We introduce a method to make self-positioned micromachined structures by using the strain in a pair of lattice-mismatched epitaxial layers. This method allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice-mismatched epitaxial layers, in semiconductors or metals. As an application example, a standing mirror was fabricated. A multilayer structure including an AlGaAs/GaAs dielectric mirror and an InGaAs strained layer was grown by molecular-beam epitaxy on a GaAs substrate. After releasing the multilayer structure from the substrate by selective etching, it moved to its final position powered by the strain release in the InGaAs layer.

Optical actuation of micromirrors fabricated by the micro-origami technique

Micromirrors were fabricated by the micro-origami technique. This technique allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice mismatched epitaxial layers, in semiconductors or metals. A multilayer structure, including AlGaAs/GaAs component layers and an InGaAs strained layer, was grown by molecular beam epitaxy on a GaAs substrate. After definition of the hinge and mirror’s shape by photolithography, the micromirrors were released from the substrate by selective etching. They moved to their final position powered by the strain release in the InGaAs layer. Optical actuation was achieved by irradiation with the 488 nm line of an argon laser, and the mirror’s position was measured by sensing the reflection of a He–Ne laser. Continuous wave irradiation with a power density of 450 mW/mm2 produced an angular deflection of the mirror of around 0.5°. The frequency response of the mirrors shows a resonance at 25 kHz.

Quantum-well microtube constructed from a freestanding thin quantum-well layer

We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission.

Photoluminescence of GaAs/AlGaAs micro-tubes containing uniaxially strained quantum wells

Abstract We fabricated micro-tubes containing two GaAs/AlGaAs quantum wells (QWs) in a section of the tube layer, and studied the optical properties of the embedded QWs. A multilayer structure composed of GaAs and AlGaAs layers and a lattice-mismatched InGaAs layer was epitaxially grown on a GaAs substrate by MBE. This multilayer structure rolled up by the built-in strain when it was freed from the substrate. By measuring the photoluminescence peak shift of the QWs caused by the uniaxial strain, we were able to determine the radial profile of the strain within the micro-tube wall.

Valley-fold and mountain-fold in the micro-origami technique

A method to make two types of hinges which bend toward opposite directions from the same epitaxial layers using the micro-origami technique is proposed and successfully applied to make standing micro-stages on a GaAs substrate. This method opens a path to fabricate more complex three-dimensional self-positioned micro-machined structures.

Photodoping of Ag into Single Crystal As2S3

Photodoping of silver into the cleaved surface of single crystal As 2 S 3 (natural orpiment) was observed by electrical resistance measurements and Rutherford backscattering spectrometry. The initial stage of the phenomenon was concluded to be photo-oxidation of silver at the interface between silver and As 2 S 3 , and was proved to be independent of the amorphous nature like photodarkening observed in amorphous chalcogenides.

Performance analysis of lateral p–n junction laser-transistor

We study the performance of a lateral p–n junction quantum-well edge-emitting laser-transistor with an extra gate contact. The incorporation of the gate contact provides an opportunity to control the threshold current and output optical power by the gate voltage. The application of negative gate voltages can lead to a substantial decrease in the threshold current. This is due to the confinement of the electrons injected into the p-type portion of the quantum well serving as the active region. Using the developed device model, we calculate the laser-transistor threshold and output characteristics. We also estimate the device cutoff modulation frequency associated with the gate recharging.

Characterization of Hydrogenated Amorphous Silicon: Some Behaviors of Hydrogen and Impurities Studied by Film Characterization Techniques

Rutherford backscattering spectrometry and infrared absorption measurement were applied to determine composition in hydrogenated amorphous silicon fabricated either by glow discharge in SiH4 plus H2 or by reactive sputtering in Ar containing H2 in a tetrode or diode sputtering apparatus. The atomic density of Si, the content and depth distribution of H, and the amount of impurities such as Ar were studied for the films deposited under several conditions of substrate temperature and gas pressure and constitution. Some difference was clarified between glowdischarge and sputter deposited films.

Lasing characteristics and modal gain of a lateral-junction InGaAs/GaAs edge-emitting laser diode grown on a patterned GaAs (311) A-oriented substrate

Edge-emitting InGaAs/GaAs laser diodes were grown on patterned GaAs (311)A-oriented substrates. Due to the amphoteric properties of Si as a dopant in high-index GaAs, a device with a lateral p–n junction was obtained. CW lasing was observed up to 200 K. The dependence of the threshold current with temperature was measured. The characteristic temperatures were T0=180 and T0=57 K for experimental temperatures below and above 100 K, respectively, meaning that the carrier confinement structure must be improved. Quantitative results of the cavity gain were experimentally obtained.

Self-Assembled GaAs Micromirrors Monolithically Integrated with LEDs

We present the results on the monolithic integration of self-assembled GaAs micromirrors with light emitting diodes. The micromirrors were self-assembled by the strain-driven mechanism, which control the micromirror standing position and flatness. The device epi-layer structure and the fabrication processes were optimized. Self-locking mechanism was also employed to precisely position and enhance the mechanical strength of the assembled micromirrors. Light emission was observed on the integrated devices. For the first time, the effectiveness of the self-assembled micromirrors was confirmed in the monolithic integration with LEDs on GaAs. This result shows the feasibility of GaAs-based micro-opto-electromechanical systems for photonics applications.