H. Noge

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.

Transport properties of two-dimensional electron gas in AlGaAs/GaAs selectively doped heterojunctions with embedded InAs quantum dots

Transport properties of two‐dimensional electron gas (2DEG) are studied in selectively doped GaAs/n‐AlGaAs heterojunctions, in which nanometer‐scale InAs dots are embedded in the vicinity of the GaAs channel. When the distance Wd between the InAs dot layer and the channel is reduced from 80 to 15 nm, the mobility μ of electrons at 77 K decreases drastically from 1.1×105 to 1.1 ×103 cm2/V s, while the carrier concentration increases from 1.1×1011 to 5.3×1011 cm−2. Such a reduction of mobility is found only when the average thickness of InAs layer is above the onset level (∼1.5 monolayer) for the dot formation. Origins of these changes in μ and Ns are discussed in connection with dot‐induced modulations of the electronic potential V(r) in the channel.

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.

Single electron transport and current quantization in a novel quantum dot structure

We report on single electron transport via a novel quantum dot structure fabricated by a combination of mesa etching and gate formation. In this device electrons are confined in an etched submicron wire and squeezed further by two barrier gates. The resulting dot is of a very small size, and the number of confined electrons can be tuned down to the few electron limit. This novel structure has a large charging energy and an improved current quantization during turnstile operation. In small dots, containing only a few electrons, we found Coulomb oscillations with an unexplained multiple peak structure.

Structure of pentacene/tetracene superlattices deposited on glass substrate

Pentacene/tetracene organic superlattices were formed by deposition on a glass substrate, and their structure was investigated by x‐ray diffraction and photoluminescence. For samples in which the number (n) of constituent pentacene (or tetracene) molecular layers within a period is such that n≥3, the first‐order x‐ray diffraction peak was found to consist of several equally spaced subpeaks. Comparison of the spectra with calculated structure factors established that a well‐defined superlattice structure was achieved in these samples. In contrast, samples with n<3 showed no evidence of superlattice formation. Photoluminescence spectra show clear correlation with the x‐ray results.

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.

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.

Molecular‐beam‐epitaxial growth of n‐AlGaAs on clean Cl2‐gas etched GaAs surfaces and the formation of high mobility two‐dimensional electron gas at the etch‐regrown interfaces

By using an ultrahigh vacuum multichamber process system, very clean GaAs surface is successfully prepared by chlorine‐gas etching and AlGaAs is subsequently grown by molecular beam epitaxy to show that two‐dimensional electron gas is successfully formed at etch‐regrown AlGaAs/GaAs interface. Mobility as high as 114 000 cm2/V s at 9.8 K is achieved for the carrier concentration NS=4.5×1011 cm−2. From the secondary‐ion‐mass‐spectroscopy measurement, the carbon concentration at the interface is estimated to be 2×1010 cm−2, and is found to be a dominant scatterer for the two‐dimensional electrons. A transmission‐electron‐microscope image has evidenced a very flat feature of etch‐regrown interface.

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.