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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.

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.

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.

In order to clarify the electron-beam (EB) patterning mechanism of GaAs photo-oxide mask layers used in “ in situ EB lithography”, the effects of surface stoichiometry and heat treatment of the photo-oxide mask layers on the patterning characteristics were studied. The threshold dose for EB patterning was shown to depend on the amounts of both oxygen and arsenic in the photo-oxide mask layers. For photo-oxide mask layers containing larger amounts of As and O, the threshold dose for EB patterning increased. Moreover, for photo-oxide mask layers heated at temperatures above 400° C, where arsenic was mostly desorbed and stable Ga oxides were thought to be dominant, the mask durability was enhanced and, as a result, EB/Cl2 patterning became impossible. These results strongly indicate that As oxides play an essential role in EB/Cl2 patterning of “ in situ EB lithography”.

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.

Patterning of GaAs at the sub-100 nm size has been demonstrated using in situ electron beam (EB) lithography processes. Patterning is carried out by EB exposure of an ultrathin oxide layer on GaAs which is used as a mask material. The patterns are transferred into GaAs by Cl2 gas etching. A high-brightness Schottky electron gun is used in the exposure process. The size of the etched feature is as small as 50 nm, which is equal to the diameter of the electron beam. The results show that in situ EB lithography processes using an ultrathin oxide mask are very promising for fabricating nanometer-scale structures.

We examined electron-beam-(EB) induced damage in GaAs/AlGaAs quantum-well (QW) structures as a function of EB energy and dose by Photoluminescence (PL) measurements. Upon 5 to 25 keV EB irradiation with doses less than 1×1018 cm-2, no effect on the photoluminescence properties was found. Even for a high electron dose of 1×1019 cm-2, only a slight reduction of the PL intensity was observed. The damage profile was found to exhibit an EB energy dependence. The most remarkable damage was induced at EB energies from 10 to 15 keV, extending to 120 nm from the surface. On the contrary, no damage was detected at a low energy of 5 keV and a high energy of 25 keV. This is because the low-energy electrons do not penetrate into the QW regions and the energy-loss rate of the high-energy electrons is too low to cause any damage.

The electron beam (EB)-induced modification of GaAs oxide for the in situ lithography process has been investigated. The oxide formed on an as-grown GaAs surface is irradiated with an electron beam at doses in the range of 1.5×1017 to 4×1019 electrons/cm2 at beam energies of 10-25 keV. Subsequent Cl2 gas exposure results in selective etching only in the irradiated area. EB-induced modification of the GaAs oxide is found to involve a partial removal of oxygen by in situ Auger measurements. The threshold dose for the etching depends on electron energy, whereas it is independent of the wafer temperature between 25 and 100°C during EB-irradiation. These results indicate that the modification is not a thermally activated process, but a direct interaction of the electron beam with the oxide.

Memory phenomena in the ordered state of triangular ferromagnets CoCl2- and NiCl2 graphite intercalation compounds (GIC) are investigated by the measurement of thermoremanent and field induced magnetizations. The results are compared with the theoretically expected one for spin glass. Measurement for a typical spin glass CuMn alloy is undertaken for a comparison. An anomalous memory is revealed in the intermediate state, which shows apparently characteristics of both ordered and disordered states.