Shyun Koshiba

Formation of GaAs ridge quantum wire structures by molecular beam epitaxy on patterned substrates

A ridge quantum wire structure has been successfully fabricated on a patterned (001) GaAs substrate by first growing a (111)B facet structure with a very sharp ridge and then depositing a thin GaAs quantum well on its top. Electron microscope study has shown that a GaAs wire with the effective lateral width of 17–18 nm is formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that one of the luminescence lines comes from the wire region at the ridge and its blue shift (∼60 meV) agrees with the quantum confined energy calculated for the observed wire structure.

Surface diffusion processes in molecular beam epitaxial growth of GaAs and AlAs as studied on GaAs (001)‐(111)B facet structures

Mechanisms of molecular beam epitaxy have been investigated for GaAs and AlAs by growing and analyzing the shapes of facet structures consisting of an (001) top surface and two (111)B side surfaces. It is found that all of the Ga flux on the three facet planes is incorporated into the film, but the growth rates on (111)B and (001) depend strongly on the As flux and are mainly determined by the diffusion of Ga ad‐atoms between the two planes. In contrast, the diffusion of Al is found to be almost negligible, irrespective of the As flux. By analyzing the shape of the facet, the diffusion length, λ, of Ga on a (001) surface is estimated to be about 1 μm at 580 °C, while that of Al is about 0.02 μm. On (111)B, λ of Ga is found to be several μms. The reflectivity of diffusing Ga atoms is found to be far less than 1 for the (001)‐(111)B boundary, and almost unity at facet boundaries where the (111)B side surfaces are bound by the (110) side walls.

Enhanced crystallographic selectivity in molecular beam epitaxial growth of GaAs on mesas and formation of (001)‐(111)B facet structures for edge quantum wires

The crystallographic selectivity of molecular beam epitaxial growth of GaAs on mesas consisting of a (001) surface and (111)B facets is studied systematically. It was found that the growth rate on (111)B facets can be drastically reduced to ∼1/30 of the growth rate on (001) surface by the reduction of As flux on the (111)B facets. This enhanced selectivity results from the enhanced intersurface migration, and strongly indicates a feasibility of forming microheterostructures needed for the fabrication of edge quantum wires on (001)‐(111)B mesas.

MODULATION OF ONE-DIMENSIONAL ELECTRON DENSITY IN N-ALGAAS/GAAS EDGE QUANTUM WIRE TRANSISTOR

An array of AlGaAs/GaAs edge quantum wires (EQWIs) with an effective width of 80 nm was successfully prepared on a (111)B microfacet structure on a patterned substrate by molecular beam epitaxy. By forming a gate electrode on the wires, field effect transistor action has been successfully demonstrated. The conductance of the wire measured in magnetic fields has exhibited a clear Shubnikov–de Haas (SdH) oscillation, and its Landau plot shows a characteristic nonlinearity caused by the magnetic depopulation of one‐dimensional (1D) subbands. It has been found that as the gate voltage decreases, the SdH peaks shift systematically toward lower magnetic fields, indicating a successful modulation of 1D electron density in the EQWI.

Large conductance anisotropy in a novel two‐dimensional electron system grown on vicinal (111)B GaAs with multiatomic steps

A novel selectively doped AlGaAs/GaAs heterojunction was successfully formed on a vicinal (111)B substrate, in which both quasiperiodic and aperiodic multiatomic steps with the average spacings of ∼20 nm are introduced. It is found that the electrical conductance G⊥ of two‐dimensional electrons across the steps is far lower than that of G∥ along the steps and the ratio G∥/G⊥ exceeds 100 at 18 K. While G∥ is almost independent of temperature T below ∼70 K, G⊥ increases exponentially with 1/T with the activation energy of ∼5 meV, indicating the presence of potential barriers for the electron motion across the aperiodic steps.

Solid-immersion photoluminescence microscopy of carrier diffusion and drift in facet-growth GaAs quantum wells

Carrier diffusion and drift in facet-growth quantum wells (QWs) on mesa-patterned substrates by molecular beam epitaxy was studied by high-resolution microscopic photoluminescence spectroscopy and imaging using a solid immersion lens at low temperatures. Under point excitation, excitation-position-dependent anisotropic carrier migration was observed, which was explained by carrier diffusion and drift due to spatial change in the quantization energy in QWs.

MBE growth of GaAs nanometer-scale ridge quantum wire structures and their structural and optical characterizations

Abstract A novel method for fabrication of the quantum wire structures has been investigated by which a quantum wire has been successfully fabricated on top of a (111)B facet structure with a very sharp ridge. Electron microscope study has shown that GaAs wires with the effective lateral width of 16–18 nm and with the thickness of 6–9 nm are formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that ID quantum confinement of electrons is realized at the ridge top and its blue shift agrees with the quantum confined energy calculated for the observed wire structure.

Control of ridge shape for the formation of nanometer-scale GaAs ridge quantum wires by molecular beam epitaxy

Abstract We have investigated the molecular beam epitaxial (MBE) growth mechanisms of nanometer scale GaAs ridge structures formed on patterned substrates and studied the way to control the widths of ridges and those of quantum wires grown on them. It is found that the width of the ridge structure decreases, as the growth temperature is reduced, reaching about 20 nm when grown below 580°C. The width of an AlAs ridge (10 nm at 570°C) is always found to be narrower than that of GaAs. A Monte Carlo simulation is performed to investigate the diffusion process of atoms in these ridge structures and indicates the important role of thermodynamical stability on the shape of a nanometer structure.

Stimulated emission in ridge quantum wire laser structures measured with optical pumping and microscopic imaging methods

We report the observation of stimulated emission in ridge quantum wire (QWR) structures at temperatures from 4.7 to 290 K. To examine the origin of the stimulated emission, the spatially and spectrally resolved microscopic images of the emission were measured. It was most likely attributed to the optical transition between the excited states in QWRs.

Formation of N-AlGaAs/GaAs edge quantum wire on (111)B micro facet by MBE and magnetic depopulation of quasi-one-dimensional electron gas

Abstract An edge quantum wire (EQWI) structure with a feature width of 120 nm was successfully prepared on (111)B micro facets; the structure was fabricated by an ensemble of several growth modes in molecular beam epitaxy (MBE) on a patterned (001) substrate without resorting to any advanced lithographic technique. A clear deviation from the linear relationship is observed in a Landau plot of magnetoresistance at low magnetic fields, providing the first evidence of magnetic depopulation of one-dimensional subbands in a facet EQWI. The sheet electron concentration measured is 5.4 × 10 11 cm −2 which corresponds to the linear concentration of 4.8 × 10 6 cm −1 , and the mobility is 3 × 10 4 cm 2 V −1 s −1 or higher. These values indicate a high crystal quality of the facet EQWI thus prepared.