Isamu Matsuyama

Nanometer‐scale pattern formation of GaAs by in situ electron‐beam lithography using surface oxide layer as a resist film

We have studied the effects of the microscopic roughness of a GaAs epitaxial surface on the oxide layer, which is used as a resist‐mask material in in situ electron‐beam lithography. When electron beam patterning, followed by Cl2 gas etching, was carried out for an oxide‐mask layer formed on a rough surface, an indented pattern edge elongated along the [110] direction, typically having a size of several tens of nm, was formed. On the other hand, when we used a misoriented substrate in order to obtain a smooth epitaxial surface by introducing the so‐called step‐flow growth mode, the resulting pattern exhibited a sufficiently sharp edge. Based on this improvement, together with the optimized electron‐beam patterning system, we successfully fabricated an ultrafine trench structure with a width as small as 20 nm.

Characterization of In Situ Cl2-Etched GaAs Buffer Layers and Regrown GaAs/AlGaAs Quantum Wells

We studied the interface between an in situ Cl2-gas-etched GaAs buffer layer and a regrown AlGaAs layer, as a function of the etching temperature, together with the optical properties of etched buffer layers and GaAs/AlGaAs quantum wells (QWs) which were regrown on the buffer layers. In the case of etching at 70° C, degradation of the photoluminescence (PL) was observed for the etched GaAs buffer layer and the regrown QWs, which extended 100 nm from the interface. With increasing etching temperature up to 200° C, on the other hand, the PL was greatly improved for both the etched GaAs buffer layer and the regrown QWs. This was due to the reduced C and O impurity accumulation at the etched/regrown interface, which was confirmed by secondary ion mass spectroscopy measurements.

In Situ Fabrication of Buried GaAs/AlGaAs Quantum-Well Mesa-Stripe Structures with Improved Regrown Interfaces.

The mesa-stripe structures of GaAs/AlGaAs multi-quantum-wells (MQWs) were buried by AlGaAs overgrown layers using an in situ fabrication technique, including Cl2 gas etching and molecular-beam epitaxial regrowth without air exposure. In these structures, having stripe widths of less than several microns, the photoluminescence intensities from the MQWs were considerably improved, compared to conventionally fabricated buried structures as well as to as-etched open-sidewall structures. Correspondingly, it was observed by transmission-electron-microscope that the densities of the crystal defects at the in situ regrown interfaces were greatly reduced, indicating great usefulness of in situ techniques in fabricating high-quality microstructures.

Fabrication of GaAs quantum wires by metalorganic molecular beam epitaxy and their optical properties

Abstract We have grown GaAs quantum wires having nominal cross-sectional dimensions of 20 × 20 nm 2 buried in AlGaAs layers, by lateral metalorganic molecular beam epitaxy on the terraced sidewalls of mesa-grooved ( 111 ) substrates. In the photoluminescence spectrum of this sample at 77 K, a dominant emission has been observed at a peak wavelength of 780 nm which corresponds to a blue shift of ∼ 80 meV from the GaAs bulk transition. Emission spectroscopy from different positions and imaging by cathodoluminescence have demonstrated that this emission was generated from the sidewalls, indicating that it originates from the quantum wire.

In Situ GaAs Patterning and Subsequent Molecular-Beam Epitaxial Regrowth of AlGaAs/GaAs Wire Structures

We report on the use of in situ electron-beam (EB) lithography, a processing technique which is conducted entirely in an ultrahigh vacuum, to pattern GaAs substrates with mesa stripes on which AlGaAs/GaAs wire structures are subsequently regrown by molecular-beam epitaxy. First, a thin GaAs oxide layer is selectively formed by EB-stimulated oxidation in a controlled oxygen atmosphere, and then used as a mask material to define mesa stripes by Cl2 gas etching. Subsequently, ridge structures are formed on the mesa stripes by the regrowth of a GaAs layer. Wire structures are fabricated on the top of the ridges by the growth of an AlGaAs/GaAs quantum well. The high quality of the resulting structures has been confirmed by cathodoluminescence measurements at 77 K.

Improvement in Patterning Characteristics of GaAs Oxide Mask Used in In Situ Electron-Beam Lithography

In situ electron-beam (EB) lithography is a technique used to process GaAs/AlGaAs semiconductor structures in an ultrahigh vacuum environment. In this technique a thin GaAs oxide layer is selectively formed on a clean GaAs surface by EB-stimulated oxidation under a controlled oxygen atmosphere, and it is then used as a mask material to pattern the underlying layers by Cl2 gas etching. We have found that by using slightly misoriented GaAs substrates both the EB dose and the oxygen pressure necessary to form a resistive oxide mask can be markedly reduced.

Electron beam lithography using GaAs oxidized resist for GaAs/AlGaAs ultrafine structure fabrication.

A novel in situ electron-beam (EB) lithography, aiming at nanometer-scale ultrafine structure fabrication, in which a few monolayer thick surface oxide of GaAs is used both as the resist material for EB-patterning and as the etching mask, is described.