Masanari Koguchi

GROWTH AND OPTICAL PROPERTIES OF NANOMETER-SCALE GAAS AND INAS WHISKERS

The growth process, crystal structure, and optical properties of ultrathin GaAs and InAs wires (whiskers) as thin as 15–40 nm and about 2 μm long are reviewed and discussed. Experimental results for growing whiskers using Au as a growth catalyst during metalorganic vapor phase epitaxy (MOVPE) and the shape and growth direction of whiskers provide new insight into growth control of GaAs and InAs whiskers. The crystal structure of whiskers, Au behavior during MOVPE, and their growth mechanism are reviewed and discussed on the basis of transmission electron microscopic analysis. The photoluminescence spectra of GaAs wires are compared with those of a GaAs epitaxial layer, and the effect of surface treatment on the luminescence peak energy shift is discussed. The time dependent photoluminescence of GaAs wires is also discussed. The application of GaAs whiskers to light emitting devices is reviewed because a semiconductor wire structure employing quantum size effects is a very important element of electronic a...

Effect of one monolayer of surface gold atoms on the epitaxial growth of InAs nanowhiskers

This letter shows that selective heteroepitaxy of nanometer‐scale InAs whiskers on SiO2‐patterned GaAs substrates [Yazawa, Koguchi, and Hiruma, Appl. Phys. Lett. 58, 1080 (1991)] is induced by surface contamination with Au resulting from the fluorocarbon plasma etching process used to etch the SiO2 mask. We demonstrate that high densities (≂1010/cm2) of InAs nanowhiskers 20–30 nm in diameter can be epitaxially grown on InAs(111)B substrates onto which 1 monolayer of Au atoms had been deposited. This wirelike growth appears to be induced by ultrafine alloy droplets generated by the reactions between Au‐clusters and InAs substrates.

GaAs free‐standing quantum‐size wires

Ultrathin GaAs wires as thin as 15–40 nm and about 2 μm long have been grown on a GaAs substrate by metal‐organic vapor‐phase epitaxy. The wires, which consist of whiskers, are grown between 380 and 550 °C using trimethylgallium and arsine (AsH3) as source materials. It is found that the wire growth direction is parallel to the [111] arsenic dangling‐bond direction and can be perfectly controlled by the crystallographic orientation of the GaAs substrate surface. From transmission electron microscopic analysis it is revealed that the crystal structure of the wire coincides with the zinc‐blende type for the growth temperature range of 460–500 °C, but it changes to the wurtzite type at 420 °C and temperatures higher than 500 °C. It is also found that the wires have a twin‐type structure around the [111] growth axis for zinc blende and [0001] growth axis for wurtzite. Photoluminescence study of these wires shows that the luminescence peak energy shifts to a higher energy as the wire width decreases from 100 t...

Crystal structure change of GaAs and InAs whiskers from zinc-blende to wurtzite type

Crystal structures of GaAs and InAs whiskers grown by metalorganic vapor phase epitaxy are evaluated by means of a transmission electron microscope. The whiskers are grown epitaxially on GaAs substrates with diameters of 20-100 nm and lengths of 1-5 µm. They have the following characteristics. 1) GaAs whiskers have layered structures with 2-30 nm period, that are the 111 rotating twins of the zinc-blende type. 2) InAs whiskers also have layered structures which consist of wurtzite and zinc-blende type crystals. The wurtzite type InAs is observed for the first time in this study. The volume ratio of these two types strongly depends on the growth conditions, such as substrate temperature and material gas pressure. This suggests that defect-free whiskers with a single phase that are useful for quantum wire devices can be grown by controlling the growth conditions.

Quantum size microcrystals grown using organometallic vapor phase epitaxy

Needle‐shaped quantum size microcrystals as thin as 10 nm have been selectively grown by employing reduced pressure organometallic vapor phase epitaxy using trimethylgallium and arsine as source materials. The microcrystals grown within a SiO2 window area have their growth axes along the [111] direction. Transmission electron diffraction analysis shows that the crystal structure of microcrystals is consistent with the zinc‐blende structure of GaAs. The mechanism for growing the needle‐shaped crystals is similar to a vapor‐liquid‐solid (VLS) equilibrium phase growth model. From photoluminescence measurements at 4.2 K, it is found that the microcrystals show a very distinct spectra for free exciton and neutral acceptor‐bound exciton recombinations, meaning good crystal quality.

Heteroepitaxial ultrafine wire‐like growth of InAs on GaAs substrates

We demonstrate heteroepitaxial ultrafine wire‐like growth of InAs. Ultrafine InAs whiskers with diameters less than 20 nm are grown selectively on SiO2‐patterned GaAs substrates using metalorganic vapor phase epitaxy. These InAs nanowhiskers grow epitaxially with a growth axis parallel to the 〈111〉As dangling bond direction of the GaAs substrate surface irrespective of substrate orientation.

Microstructures of Co/Cr Bilayer Films Epitaxially Grown on MgO Single-Crystal Substrates

Microstructures of Co/Cr bilayer films epitaxially grown on MgO (100) and (110) single-crystal substrates have been studied by high-resolution transmission electron microscopy. The bicrystalline Co layer formed on the MgO (100) substrate contains a number of (0001) stacking faults. The single-crystal Co layer formed on the MgO (110) substrate consists of slightly misoriented subgrains, grown on (211)-oriented Cr domains. Dislocations and lattice strain are observed at the Cr/MgO (110) interface. The misorientations and the defects are thought to be introduced to accommodate the large lattice mismatch of about -16% in the MgO [10] direction at the Cr/MgO (110) interface.

Nanocolumns composed of GaAs‐InAs jointed whiskers and SiO2 covers

A dense array of nanocolumns composed of GaAs‐InAs jointed whiskers and SiO2 covers has been fabricated on InAs substrates. The cylindrical GaAs whisker with a 20 nm diameter and 1.5 μm long is jointed on top of 0.3‐μm‐long InAs whisker by vapor‐liquid‐solid epitaxy. The nanocolumns array exhibited photoluminescence at 14 K.

Ferroelectric 90° domain structure in a thin film of BaTiO3 fine ceramics observed by 300kV electron holography

We observed 90° ferroelectric domain structure in a thin film of BaTiO3 by 300kV electron holography, especially paying attention to beam-induced influence on the polarization and diffraction effect. The beam-induced influence was minimized by recording holograms on high-resolution films at low electron-optical magnification. As for the diffraction effect, we chose such an orientation of the specimen as to minimize the corresponding amplitude contrast. Furthermore, to minimize any phase shifts that are not intrinsic to ferroelectricity, such as the mean-inner potential of the material or the electron-beam-induced charging under the illumination condition, we calculated the difference between phase images taken below and above the Curie temperature of the material to obtain a consistent value for the spontaneous polarization.

Observation of Fe-Mn oxidation process using specimen transfer chamber and ultrahigh-vacuum transmission electron microscope

A specimen transfer chamber that enables a specimen to be transferred in vacuum from a thin-film growth apparatus to an ultrahigh-vacuum transmission electron microscope (UHV-TEM) was developed. This chamber was applied to observe, for the first time, the as-grown state of Fe-Mn thin films and the changes occurring during their oxidation. The oxidation process of 12-nm-thick Fe-Mn films on a carbon support film was observed by controlling the vacuum in the transfer chamber. Just after a film specimen was transferred, it had a polycrystalline single-phase structure. When the film was exposed to pressures from 3.8×10-6 Pa to 1 atm, (Fe-Mn)2O3 gradually appeared around the Fe-Mn grain boundaries. The Fe-Mn grains changed to (Fe-Mn)2O3 grains after 47 h of exposure to air.