Fumiya Ishizaka

Integration of III-V nanowires on Si: From high-performance vertical FET to steep-slope switch

In this paper, we present recent progress in the integration of vertical III-V nanowire-channels on Si by selective-area epitaxy and demonstrations of high-performance III-V vertical surrounding-gate transistors with high-k dielectrics with an EOT of less than 1 nm, modulation doping technique, and challenges in steep subthreshold-slope switching using III-V nanowire/Si heterojunctions as building blocks for low power circuits.

GaAs nanowire growth on polycrystalline silicon thin films using selective-area MOVPE.

The growth mechanism of GaAs nanowires (NWs) grown on polycrystalline silicon (poly-Si) thin films using selective-area metalorganic vapor-phase epitaxy was investigated. Wire structures were selectively grown in the mask openings on a poly-Si substrate. The appearance ratio of wire structures strongly depended on the growth conditions and deposition temperature of the poly-Si substrate. Evaluation of the grown shapes and growth characteristics revealed that GaAs NWs grown on a poly-Si substrate have the same growth mechanism as conventional GaAs NWs grown on a single-crystalline GaAs or Si substrate. Experiments showed that the wire structure yield can be improved by increasing the Si grain size and/or increasing the Si deposition temperature. The growth model proposed for understanding NW growth on poly-Si is based on the mask opening size, the Si grain size, and the growth conditions. The ability to control the growth mode is promising for the formation of NWs with complex structures on poly-Si thin layers.

Bidirectional Growth of Indium Phosphide Nanowires

We present a bidirectional growth mode of InP nanowires grown by selective-area metalorganic vapor-phase epitaxy (SA-MOVPE). We studied the effect of the supply ratio of DEZn ([DEZn]) on InP grown structure morphology and crystal structures during the SA-MOVPE. Two growth regimes were observed in the investigated range of the [DEZn] on an InP(111)B substrate. At low [DEZn], grown structures formed tripod structures featuring three nanowires branched toward the [111]A directions. At high [DEZn], we obtained hexagonal pillar-type structures vertically grown on the (111)B substrate. These results show that the growth direction changes from [111]A to [111]B as [DEZn] is increased. We propose a growth mechanism based on the correlation between the incident facet of rotational twins and the shapes of the grown structures. Our results bring us one step closer to controlling the direction of nanowires on a Si substrate that has a nonpolar nature. They can also be applied to the development of InP nanowire devices.

Indium-Rich InGaP Nanowires Formed on InP (111)A Substrates by Selective-Area Metal Organic Vapor Phase Epitaxy

We studied the growth of indium-rich InGaP nanowires (NWs) on InP (111)A substrates by selective-area metal organic vapor phase epitaxy (SA-MOVPE). We obtained vertically aligned InGaP NWs by optimizing growth conditions, such as group III supply ratio and V/III ratio. We found that the height, diameter, shape, and composition of InGaP NWs depended significantly on the supply ratios of trimethylgallium (TMGa) and trimethylindium (TMIn). As the supply ratio of TMGa was increased, the lateral growth was drastically enhanced, and the uniformity of NWs deteriorated. Furthermore, the sidewall facets of NWs changed from {211} to {110} as the supply ratio of TMGa was increased, indicating the possibility of structural transition from wurtzite (WZ) to zinc blende (ZB). We propose a possible growth model for such lateral growth, uniformity, and structural transition. Photoluminescence (PL) measurements revealed that the Ga compositions ranged approximately from 0 to 15%. Our results show that highly uniform InGaP NWs can be grown by controlling the growth conditions.

Crystal phase transition to green emission wurtzite AlInP by crystal structure transfer

We grew AlInP on two types of GaN substrate in order to transfer the wurtzite (WZ) structure to grown layers. An AlInP epitaxial layer grown on GaN with high-density stacking faults was obtained. X-ray diffraction and Raman scattering analyses indicate that the dominant crystal structure of the AlInP layer grown on GaN was WZ. Cathode luminescence measurements at 35 K revealed strong green emissions from the WZ AlInP layer, suggesting an energy band gap change from indirect to direct. These results demonstrate the potential of WZ AlInP as a new candidate for high-efficiency green emission material.

Growth and characterization of wurtzite InP/AlGaP core–multishell nanowires with AlGaP quantum well structures

We report on the selective-area growth and characterization of wurtzite (WZ) InP/AlGaP core–multishell nanowires. Quantum well (QW) structures were fabricated in AlGaP multishells by changing the alloy composition. Transmission electron microscopy revealed that the AlGaP multishells were grown with a WZ structure on the side of the WZ InP core. The lattice constants of the WZ InP core and WZ AlGaP shell were determined by X-ray diffraction. Cathodoluminescence studies showed that the WZ AlGaP QW with an Al composition of 20% exhibited green emissions at 2.37 eV. These results open the possibility of fabricating green light-emitting diodes using WZ AlGaP-based materials.

Growth of All-Wurtzite InP/AlInP Core–Multishell Nanowire Array

We demonstrated the formation of all-wurtzite (WZ) InP/AlInP core-multishell (CMS) nanowires (NWs) by selective-area growth with the crystal structure transfer method. The CMS NWs consisting of an AlInP-based double heterostructure showed that the crystal structure of the multishell succeeded to the WZ phase from the WZ InP NW by the crystal structure transfer method. Transmission electron microscopy revealed that the core-shell interface had a few stacking faults due to lattice mismatch. In addition, lattice constants of WZ AlInP with a variation of Al content were determined by X-ray diffraction reciprocal space mappings, and the WZ AlInP shell had tensile strain along the c-axis. The WZ AlInP shells (Al content: 25-54%) showed cathode luminescence emissions at 1.6-2.1 eV, possibly related to In-rich domains due to composition fluctuation in the WZ AlInP shell.

Selective-area growth of InGaAs/InP/InAlAs/InP core-multishell nanowires on Si and tunneling transistor application

Tunnel transistors are promising building-blocks for future low-power integrated circuits because steeper subthreshold-slope of the device can effectively reduce the turn-on voltage of electronic switches. Here, we investigate selective-area growth of InGaAs/InP/InAlAs/InP core-multishell (CMS) nanowire with modulation-doping and demonstrate vertical tunnel transistor using the CMS nanowire/Si heterojunction. The device indicated rapid current-enhancement due to the modulation-doped nanowire while keeping steeper subthreshold slope.

Integration of Vertical InAs Nanowires on Ge(111) by Selective-Area MOVPE

We report on selective-area growth of InAs nanowires (NWs) on Ge(111) substrates by using selective-area metal-organic vapor phase epitaxy (MOVPE) and discuss how to control the growth direction of InAs NWs on the Ge(111). Then, we characterize the electrical properties of the InAs nanowire/Ge heterojunctions for the electrical device application.