Hidenori Kawanishi

Sub-100 nm patterning of GaAs using in situ electron beam lithography

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.

AES and XPS studies of surface of AlxGa1-xAs (110) treated by ammonium sulfide

We applied the technique of ammonium sulfide treatment to AlxGa1-xAs (110) surfaces and investigated the atomic composition of a surface by Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was found that surface oxidation was significantly reduced by the sulfur treatment and that sulfur atoms were mainly bonded to Al atoms in an Al-enriched sample of AlxGa1-xAs (x=0.78). The surface compositions of the Al-enriched specimen (x=0.43, 0.78) were also found to be As-riched as a result of both sulfur treatment and the preferential etching of metallic atoms by an acidic solution, which prevents the initial oxidation of the surface of AlxGa1-xAs.

Photoluminescence Study of Electron-Beam-Induced Damage in GaAs/AlGaAs Quantum Well 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.

Role of an Electron Beam in the Modification of a GaAs Oxide Mask for in situ EB Lithography

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.

Pattern etching and selective growth of GaAs by in-situ electron-beam lithography using an oxidized thin layer

Pattern etching of GaAs by in situ electron-beam (EB) lithography using an oxidized thin ayer is performed in a multichamber system comprising seven chambers for loading sample exchange sample pre-heating molecular beam epitaxy (MBE) surface treatment etching and surface analysis. The in situ EB lithography process comprises the following steps using the multichamber system: (1 ) preparation of a clean GaAs surface by MBE (2) formation of GaAs oxide as a resist film by photo-oxidation in pure oxygen gas (3) direct patterning of the oxide resist film by EB-induced Cl2 etching (4) 012 gas etching of the GaAs surface as a pattern transfer and (5) removal of residual GaAs oxide layer by heating the wafer above about 600 00 in a vacuum under a partial pressure of arsenic. Overgrowth of GaAs and/or AIGaAs layers is also possible on a patterned GaAs wafer without exposing the surface to air. Damage to the patterned area was evaluated through photoluminescnece measurements and compared with the case of conventional dry etching of GaAs. The results showed that damage was extremely small for EB-induced Cl2 etching. The reason is considered to be due to a difference in momentum transfer from charged particles to the lattice. Selective-area epitaxy using this GaAs oxide layer as a mask film was carried out by combination of EB-induced Cl2 etching and metal organic molecular beam epitaxy (MOMBE) using a separate ultra-high

Improvement of high-power characteristics of 780-nm AlGaAs laser diode by (NH4)2S facet treatment

Coherent cw operation has been obtained with a 10x4 array of Grating Surface Emitting (GSE) lasers consisting of 40 lasers and 50 emitting sections. The array is a GaAs quantum well device, grown on an A1GaAs substrate, operating at 861 nm to which the substrate is transparent. It is mounted p-down to metallized traces on a BeO slab to provide isolated electrical contacts and thermal contact to a simple chilled-water cooler. A single spectral line 0.5 A wide indicates coupling of the 40 laser sections. More detailed measurements on a section of an array containing ten laterally coupled lasers, 20 outputs, show that it is operating at a junction temperature of 30°C, has a line width of 0.1 5A, and a measured coherence of >75%. The expected wavelength stabilization with temperature due to the DBR grating is found, with a value of =0.6A/°C. An array of 4 longitudinally coupled lasers produced a line width of 130MHz and evidence of high coherence.

High-Power CW Operation in V-Channeled Substrate Inner-Stripe Lasers with “Torch”-Shaped Waveguide

High-power AlGaAs lasers with a waveguide pattern like a torch have been grown by a two-step liquid-phase epitaxy process. The waveguide pattern consists of a 6 µm-wide region about 220 µm long and a 10 µ-wide region about 20 µm long connected by a tapered region about 10 µm long. A stable fundamental transverse mode has been obtained of up to more than 200 mW output power in a 4%–95% coated device in the wavelength range of 830 nm. The lasers have extremely high reliability, and stable continuous operation for over 4000 hours has been confirmed at 50°C, 50 mW with no obvious degradation.

Stable Single-Longitudinal-Mode Operation over Wide Temperature Range on Semiconductor Lasers with a Short External Cavity

V-channeled Substrate Inner Stripe lasers coupled with a Short External Cavity maintain one single longitudinal mode over wide temperature ranges without mode hopping. Temperature stability is obtained when the thermal shift of the modulated spectrum envelope coincides with that of the longitudinal modes. We enhanced the stability by using a Si submount in place of a Cu submount for the laser. The temperature dependence of the gain maximum is compensated by the small thermal expansion of Si. The laser shows a longitudinal mode locking range of more than 46°C. The data are consistent with theoretical calculations.

Electric Field Induced Carrier Sweep-Out in Tandem InGaN Multi-Quantum-Well Self-Pulsating Laser Diodes

Mechanisms of carrier sweep-out in tandem InGaN multiple-quantum-well self-pulsating laser diodes were investigated. Laser diodes showed self-pulsating characteristics without significant change in the light output–current (I–L) characteristics when an electric field high enough was established in the saturable absorber by the applied reverse bias. Improvements in the design of the band-energy profile allowed a substantial reduction in the bias required for self-pulsating operation. These results indicate that carrier lifetime can be controlled by the electric field in the saturable absorber and that band-energy profile engineering is effective for the reduction of carrier lifetime.

Chemically Assisted Ion Beam Etching of GaAs/AlGaAs Using Chlorine Ions

Chemically assisted ion beam etching of GaAs/AlGaAs has been studied using chlorine gas and chlorine/argon ions. The use of chlorine ions results in a very smooth surface after etching at an ion energy of 400 eV. By contrast, a porous surface appears when argon ions are used, unless the ion energy is increased to 700 eV. The smooth etching characteristics in the former case have been ascribed to the effective removal of Al2O3, which is formed on the AlGaAs surface and acts as a micromask, by chemically reactive chlorine ions.