Shinjiro Hara

Growth of Core–Shell InP Nanowires for Photovoltaic Application by Selective-Area Metal Organic Vapor Phase Epitaxy

We report on the formation of core–shell pn junction InP nanowires using a catalyst-free selective-area metalorganic vapor-phase epitaxy (SA-MOVPE) method. A periodically aligned dense core–shell InP nanowire array was fabricated and used in photovoltaic device applications. The device exhibited open-circuit voltage (VOC), short-circuit current (ISC) and fill factor (FF) levels of 0.43 V, 13.72 mA/cm2 and 0.57, respectively, which indicated a solar power conversion efficiency of 3.37% under AM1.5G illumination. This study demonstrates that high quality core–shell structure nanowire fabrication is possible by SA-MOVPE and that the nanowire arrays can be used in integrated nanowire photovoltaic devices.

Fabrication and characterization of freestanding GaAs/AlGaAs core-shell nanowires and AlGaAs nanotubes by using selective-area metalorganic vapor phase epitaxy

We fabricated GaAs∕AlGaAs core-shell nanowires by using selective-area metalorganic vapor phase epitaxy. First, GaAs nanowires were selectively grown on partially masked GaAs (111)B substrates; then AlGaAs was grown to form freestanding heterostructured nanowires. Investigation of nanowire diameter as a function of AlGaAs growth time suggested that the AlGaAs was grown on the sidewalls of the GaAs nanowires, forming GaAs∕AlGaAs core-shell structures. Microphotoluminescence measurements of GaAs and GaAs∕AlGaAs core-shell nanowires reveal an enhancement of photoluminescence intensity in GaAs∕AlGaAs core-shell structures. Based on these core-shell nanowires, AlGaAs nanotubes were formed by using anisotropic dry etching and wet chemical preferential etching to confirm the formation of a core-shell structure and to explore a new class of materials.

III–V Nanowires on Si Substrate: Selective-Area Growth and Device Applications

III-V nanowires (NWs) on Si are promising building blocks for future nanoscale electrical and optical devices on Si platforms. We present position-controlled and orientation-controlled growth of InAs, GaAs, and InGaAs NWs on Si by selective-area growth, and discuss how to control growth directions of III-V NW on Si. Basic studies on III-V/Si interface showing heteroepitaxial growth with misfit dislocations and coherent growth without misfit dislocations are presented. Finally, we demonstrate the integrations of a III-V NW-based vertical surrounding-gate field-effect transistor and light-emitting diodes array on Si. These demonstrations could have broad applications in high-electron-mobility transistors, laser diodes, and photodiodes with a functionality not enabled by conventional NW devices.

Structural transition in indium phosphide nanowires.

We study the catalyst-free growth of InP nanowires using selective-area metalorganic vapor phase epitaxy (SA-MOVPE) and show that they undergo transition of crystal structures depending on the growth conditions. InP nanowires were grown on InP substrates where the mask for the template of the growth was defined. The nanowires were grown only in the opening region of the mask. It was found that uniform array of InP nanowires with hexagonal cross section and with negligible tapering were grown under two distinctive growth conditions. The nanowires grown in two different growth conditions were found to exhibit different crystal structures. It was also found that the orientation and size of hexagon were different, suggesting that the difference of the growth behavior. A model for the transition of crystal structure is presented based on the atomic arrangements and termination of InP surfaces. Photoluminescence measurement revealed that the transition took place for nanowires with diameters up to 1 microm.

Electrical Characterizations of InGaAs Nanowire-Top-Gate Field-Effect Transistors by Selective-Area Metal Organic Vapor Phase Epitaxy

Single InGaAs nanowire-top-gate metal–semiconductor field-effect transistors (MESFETs) were fabricated and characterized. Silicon-doped n-InGaAs nanowires (with a typical diameter of 100 nm) were grown by catalyst-free selective-area metal–organic vapor-phase epitaxy (SA-MOVPE). The FETs of single nanowires on SiO2-coated Si substrates were fabricated by defining metal contacts at both ends of the nanowires and the metal top gate between contacts. According to the measurements of drain current–voltage and gate transfer characteristics, the top-gate MESFETs exhibited significant enhancements in device performance characteristics compared with FETs under back-gate operation; that is, a peak transconductance of 33 mS/mm and a current on–off ratio of 103 were obtained. A possibility for further improvements in FET characteristics was also considered.

Multiatomic step formation mechanism of metalorganic vapor phase epitaxial grown GaAs vicinal surfaces and its application to quantum well wires

The multiatomic steps formed on GaAs vicinal surfaces by metalorganic vapor phase epitaxy (MOVPE) are studied by atomic force microscopy (AFM). An AFM image of an epitaxially grown GaAs surface showed coherent multiatomic steps with extremely straight edges over a wide area. The average height and spacing of the multiatomic steps are 1.2-8 and 30-110 nm, respectively. These terrace widths change with the growth conditions. Narrower terrace widths are obtained at higher growth rates, and under higher AsH 3 partial pressures and higher impurity doping conditions. The results suggest that the migration distance of a Ga atom on the terrace and the sticking coefficient at the step sites depend on these growth conditions. Using multiatomic steps, GaAs/AlGaAs quantum well wires (QWWs) were grown on a GaAs vicinal surface. Cross-sectional transmission electron microscopy and photoluminescence show the successful fabrication of QWWs

Growth and Characterization of InGaAs Nanowires Formed on GaAs(111)B by Selective-Area Metal Organic Vapor Phase Epitaxy

We fabricated InGaAs nanowires (NWs) in SiO2 mask openings on a GaAs(111)B substrate at growth temperatures of 600–700 °C using catalyst-free selective-area metal organic vapor phase epitaxy. At a growth temperature of 600 °C, particle-like depositions occurred, but they decreased in number and density when the growth temperature was increased to 650 °C and disappeared above 675 °C. The heights and growth rates of the NWs increased when the growth temperature was increased and the mask opening diameter was decreased from 300 to 50 nm. Photoluminescence (PL) spectra measured for the NWs indicated a blue shift in the peak from 0.95 to 1.3 eV as the growth temperature was increased from 600 to 700 °C, indicating an increase in the Ga composition from 62 to 88% in the InGaAs NWs.

Multiatomic step formation on GaAs(001) vicinal surfaces during thermal treatment

Abstract We observed the surface morphology of vicinal GaAs(001) after thermal treatment in AsH 3 H 2 atmosphere by atomic force microscopy (AFM). Clear multiatomic steps were formed under the high temperature thermal treatment. Next, we investigated the mechanism of step bunching during thermal treatment by two experiments from the view point of Ga atom evaporation. One is the selective thermal treatment using a partially masked GaAs wafer, and the evaporation amount of Ga atoms was estimated by AFM. The other is the investigation of photoluminescence (PL) peak energy shifts for AlGaAs GaAs single quantum wells with a thermal treatment process at the top of the GaAs quantum well layer, compared to those without thermal treatment. These results indicate that the evaporation hardly occurs during the thermal treatment process. Therefore, step bunching phenomena on GaAs(001) vicinal surfaces during thermal treatment are probably caused by migration of the atoms detached from upside steps and their re-incorporation to downside steps.

Comparison of the magnetic properties of GaInAs/MnAs and GaAs/MnAs hybrids with random and ordered arrangements of MnAs nanoclusters

Random arrangements of ferromagnetic MnAs nanoclusters were deposited on (111)B-GaInAs surfaces by standard metal-organic vapor-phase epitaxy (MOVPE). Ordered arrangements of MnAs nanoclusters and cluster chains were obtained by selective-area MOVPE on prepatterned (111)B-GaAs substrates. This new method enables one to control the arrangement of nanoclusters in the growth process offering interesting opportunities to tune the properties of individual MnAs clusters as well as the interaction between the carriers in the surrounding semiconductor matrix and the clusters. The magnetic anisotropy of the MnAs clusters was investigated by magnetic force microscopy and ferromagnetic resonance measurements. The in-plane magnetic anisotropy is mainly determined by the interplay of cluster shape and magnetocrystalline anisotropy while the hard magnetic axis of the clusters is perpendicular to the sample plane independent of cluster shape. The magnetotransport measurements demonstrate that the cluster arrangements st...

Fabrication of Axial and Radial Heterostructures for Semiconductor Nanowires by Using Selective-Area Metal-Organic Vapor-Phase Epitaxy

The fabrication of GaAs- and InP-based III-V semiconductor nanowires with axial/radial heterostructures by using selective-area metal-organic vapor-phase epitaxy is reviewed. Nanowires, with a diameter of 50–300 nm and with a length of up to 10 μm, have been grown along the 〈111〉B or 〈111〉A crystallographic orientation from lithography-defined SiO2 mask openings on a group III-V semiconductor substrate surface. An InGaAs quantum well (QW) in GaAs/InGaAs nanowires and a GaAs QW in GaAs/AlGaAs or GaAs/GaAsP nanowires have been fabricated for the axial heterostructures to investigate photoluminescence spectra from QWs with various thicknesses. Transmission electron microscopy combined with energy dispersive X-ray spectroscopy measurements have been used to analyze the crystal structure and the atomic composition profile for the nanowires. GaAs/AlGaAs, InP/InAs/InP, and GaAs/GaAsP core-shell structures have been found to be effective for the radial heterostructures to increase photoluminescence intensity and have enabled laser emissions from a single GaAs/GaAsP nanowire waveguide. The results have indicated that the core-shell structure is indispensable for surface passivation and practical use of nanowire optoelectronics devices.