Size control of InP nanowires byin situannealing and its application to the formation of InAsP quantum dots.

We carried outin situannealing of InP nanowires (NWs) in a metal-organic vapor phase epitaxial (MOVPE) growth reactor to control and reduce the tip size of InP NWs. InP NWs were grown by selective-area (SA) MOVPE on partially masked (111)A InP substrates, and annealing was successively applied in tertiarybutylphosphine (TBP) ambient. Initially, the InP NWs had a hexagonal cross-section with{112¯}facets vertical to the substrates; they became tapered, and the edges were rounded by annealing. By appropriately selecting the annealing temperature and initial NW diameter, the tip size of the NW was reduced and NWs with a tip size of 20 nm were successfully formed. Subsequently, a thin InAsP layer was grown on the annealed NWs and their photoluminescence was investigated at low temperatures. The characterization results indicated the formation of InAsP quantum dots (QDs) emitting in the telecom band. Our approach is useful for reducing the size of the NWs and for the controlled formation of InAsP QDs embedded in InP NWs in photonic devices compatible with telecom bands.

Demonstration of InP/InAsP/InP axial heterostructure nanowire array vertical LEDs.

Semiconductor nanowires (NWs), which have nanoscale footprints, enable us to realize various quantum structures with excellent position and size controllability, utilizing a wide range of materials for heterostructures. In addition, enhancing light extraction and controlling spontaneous emission by modifying their size and shape are possible. Thus, NWs are promising materials for nanoscale light sources applicable from visible to telecommunication bands. In this study, we grew InP/InAsP/InP axial heterostructure NWs, where the InAsP layer was embedded to serve as an active layer, by selective-area growth and demonstrated vertical NW array light-emitting diodes (LEDs) as a step towards realizing on-demand single photon sources. The NW array LEDs showed rectifying characteristics and electroluminescence originating from the embedded InAsP layer in the near-infrared region.

Characterization of nanowire light-emitting diodes grown by selective-area metal-organic vapor-phase epitaxy.

We report a systematic study on the current injection and radiative carrier recombination in InP nanowire (NW) light-emitting diodes (LEDs). The InP NWs with axial p-n structures, grown by selective-area metal organic vapor-phase epitaxy, had mixed crystal structures between those of zincblende and wurtzite, mainly in the p-regions. The temperature dependence of the current-voltage (I-V), electroluminescence (EL), and current-light output (I-L) characteristics was investigated. The temperature dependence of the I-V characteristics revealed that tunneling was the main mechanism of carrier transport through the p-n junction in the present NW-LEDs. The temperature and bias voltage dependences of EL showed a complex but systematic behavior, where peaks exhibiting bias-dependent and independent energy positions coexisted and the relative intensity showed a transition with increasing temperature. The external quantum efficiency showed a droop at low temperatures, indicating a reduced injection efficiency at low temperatures. These observations were explained by the radiative and nonradiative tunneling, and suggested a strong effect of the nonradiative tunneling at low temperatures.

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.

Far-field emission patterns of nanowire light-emitting diodes.

We investigated far-field (FF) emission patterns of nanowire light-emitting diodes (NW-LEDs). NW-LEDs were fabricated using vertical InP-NW arrays with axial pn-junctions grown on InP (111)A substrates, and the emission intensity of NW-LEDs was measured as a function of view angle θ, where θ = 0° indicates the direction normal to the substrate or that along the NWs. For NW arrays with pitch a of around 1 µm, we found a clear dip in the emission intensity at θ = 0°, which was explained by an analogy with dipole antenna, or a smaller contribution of the lowest order guided modes for emission as compared with higher order guided and free-space radiation modes. Results of the simulation of radiation patterns by the finite-difference time-domain method and near-field to far-field transformation are also described. They also confirm that the dip at θ = 0° is specific to light emission from NWs. We also investigated the dependence of the FF pattern on the pitch of the NW array, and the observation was qualitatively explained by the relative contribution of the guided and free-space radiation modes.

InP Crystal Phase Heterojunction Transistor with a Vertical Gate-All-Around Structure.

Crystal phase transitions can form a new type of heterojunction with different atomic arrangements in the same material: crystal phase heterojunction (CPHJ). The CPHJ has an inherently strong impact on band engineering without concerns over critical thicknesses with misfit dislocations and a semiconductor-metal transition. In-plane CPHJ was recently demonstrated in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials and utilized for conventional planar field-effect transistor applications. However, scalability such as gate electrostatic control, miniaturization, and multigate structure have been limited because of the geometrical issue. Here, we demonstrated a transistor using the CPHJ with a vertical gate-all-around structure by forming a CPHJ in conventional III-V semiconductors. The CPHJ, composed of wurtzite InP nanowires with zincblende InP substrates, showed an atomically flat heterojunction without dislocations and indicated a Type-II band discontinuity across the junction. The CPHJ transistor had moderate to good gate electrostatic controllability with high on-state currents and transconductance. The CPHJ offer will provide a new switching mechanism and add a new junction and device design choice to the long history of transistors.

Selective-Area Growth of Vertical InGaAs/GaSb Core-Shell Nanowires on Silicon and Dual Switching Properties.

The epitaxy of the Sb-related quantum well structure has been extensively investigated. However, the GaSb facet growth in selective-area growth (SAG) and GaSb nanostructures has not been investigated because of the surface diffusion complexity and surfactant effect of Sb adatoms. Here, the growth morphology of GaSb structures in SAG was characterized via InGaAs nanowires (NWs) monolithically grown on a Si template. SAG of GaSb using NWs included four growth processes: lateral-over growth along the ⟨1̅10⟩ directions, axial growth along the vertical ⟨111⟩ B direction, downward step-flow growth, and desorption of Sb adatoms from the NW sidewalls. The dominant processes could be controlled by the GaSb growth temperature and could form smooth GaSb shell layers. The vertical diode of InGaAs/GaSb core-shell NWs on Si exhibited moderate rectifying properties because of the InGaAs/GaSb heterojunction band alignment. In the vertical transistor application, specific dual-carrier modulation behaviors, such as p-channel field-effect transistor and n-channel tunnel field-effect transistor modes, occurred in the same transistor architecture. This was because the carrier transport changed with respect to the bias polarity. This specific transistor behavior in the InGaAs/GaSb core-shell NW on Si would expand possibilities for integrated circuit technologies using only a single transistor structure.

Zinc blende and wurtzite crystal phase mixing and transition in indium phosphide nanowires.

Indium phosphide (InP) nanowires, which have crystal phase mixing and transition from zinc blende (ZB) to wurtzite (WZ), are grown in intermediate growth conditions between ZB and WZ by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE). The shape of InP nanowires is tapered unlike ZB or WZ nanowires. A growth model has been developed for the tapered nanowires, which is simply described as the relationship between tapered angle and the ratio of ZB and WZ segments. In addition, the peak energy shift in photoluminescence measurement was attributed to the quantum confinement effect of the quantum well of the ZB region located in the polytypic structure of ZB and WZ in nanowires.

Creation of unexplored tunnel junction by heterogeneous integration of InGaAs nanowires on germanium.

Heteroepitaxy has inherent concerns regarding crystal defects originated from differences in lattice constant, thermal expansion coefficient, and crystal structure. The selection of III-V materials on group IV materials that can avoid these issues has however been limited for applications such as photonics, electronics, and photovoltaics. Here, we studied nanometer-scale direct integration of InGaAs nanowires (NWs) on Ge in terms of heterogenous integration and creation of functional materials with an as yet unexplored heterostructure. We revealed that changing the initial Ge into a (111)B-polar surce anabled vertical InGaAs NWs to be integrated for all In compositions examined. Moreover, the growth naturally formed a tunnel junction across the InGaAs/Ge interface that showed a rectification property with a huge current density of several kAcm-2 and negative differential resistance with a peak-to-valley current ratio of 2.8. The described approach expands the range of material combinations for high-performance transistors, tandem solar cells, and three-dimensional integrations.

GaAs/AlGaAs core multishell nanowire-based light-emitting diodes on Si.

We report on integration of GaAs nanowire-based light-emitting-diodes (NW-LEDs) on Si substrate by selective-area metalorganic vapor phase epitaxy. The vertically aligned GaAs/AlGaAs core-multishell nanowires with radial p-n junction and NW-LED array were directly fabricated on Si. The threshold current for electroluminescence (EL) was 0.5 mA (current density was approximately 0.4 A/cm(2)), and the EL intensity superlinearly increased with increasing current injections indicating superluminescence behavior. The technology described in this letter could help open new possibilities for monolithic- and on-chip integration of III-V NWs on Si.

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.

Rational synthesis of atomically thin quantum structures in nanowires based on nucleation processes.

Excitonic properties in quantum dot (QD) structure are essential properties for applications in quantum computing, cryptography, and photonics. Top-down fabrication and bottom-up growth by self-assembling for forming the QDs have shown their usefulness. These methods, however, still inherent issues in precision integrating the regimes with high reproducibility and positioning to realize the applications with on-demand quantum properties on Si platforms. Here, we report on a rational synthesis of embedding atomically thin InAs in nanowire materials on Si by selective-area regrowth. An extremely slow growth rate specified for the synthesis demonstrated to form smallest quantum structures reaching nuclear size, and provided good controllability for the excitonic states on Si platforms. The system exhibited sharp photoluminescence spectra originating from exciton and bi-exciton suggesting the carriers were confined inside the nuclei. The selective-area regrowth would open new approach to integrate the exciton states with Si platforms as building-blocks for versatile quantum systems.

Fabry-Pérot microcavity modes observed in the micro-photoluminescence spectra of the single nanowire with InGaAs/GaAs heterostructure.

We report on the fabrication of the nanowires with InGaAs/GaAs heterostructures on the GaAs(111)B substrate using selective-area metal organic vapor phase epitaxy. Fabry-Pérot microcavity modes were observed in the nanowires with perfect end facets dispersed onto the silicon substrate and not observed in the free-standing nanowires. We find that the calculated group refractive indices only considering the material dispersion do not agree with the experimentally determined values although this method was used by some researchers. The calculated group refractive indices considering both the material dispersion and the waveguide dispersion agree with the experimentally determined values well. We also find that Fabry-Pérot microcavity modes are not observable in the nanowires with the width less than about 180 nm, which is mainly caused by their poor reflectivity at the end facets due to their weak confinement to the optical field.

Selective-area growth of vertically aligned GaAs and GaAs/AlGaAs core-shell nanowires on Si(111) substrate.

We report on selective-area growth of vertically aligned GaAs nanowires on Si(111) substrate. Modification of the initial Si(111) surface by pretreatment under an AsH(3) atmosphere and low-temperature growth of GaAs were important for controlling the growth orientations of the GaAs nanowire on the Si(111) surface. We also found that the size of openings strongly affected the growth morphology of GaAs nanowires on Si(111). Small diameter openings reduced the antiphase defects and improved the optical properties in the GaAs nanowires. Moreover, we realized coherent growth without misfit dislocation at the GaAs/Si interface. Finally, we demonstrated fabrication of a GaAs/AlGaAs core-shell nanowire array on a Si surface and revealed that the luminescence intensity was markedly enhanced by passivation effects. These results are promising for future III-V nanowire-based optoelectronic integration on Si platforms.

Promising low-damage fabrication method for the photonic crystals with hexagonal or triangular air holes: selective area metal organic vapor phase epitaxy.

The photonic band diagrams of the photonic crystal slabs (PCSs) with various structural air holes were calculated by plane wave expansion method with super cell method. The calculated results indicate that the PCSs with hexagonal or triangular air holes have enough large photonic band gaps in the guided mode spectrum, hence they are good candidates to be used for the PC devices. The PCs with hexagonal or triangular air holes were fabricated successfully on n-type GaAs (111)B substrate by selective-area metal organic vapor phase epitaxy (SA-MOVPE). Vertical and smooth facets are formed and the uniformities are very good. The same process was also used to fabricate hexagonal air hole arrays with the width of 100 nm successfully. A procedure was proposed and utilized to fabricate the air-bridge PCS with normal hexagonal air holes. The fabricated hexagonal air holes are very uniform and the sidewalls are smooth and vertical. Our experimental results indicate that SA-MOVPE growth is a promising low-damage fabrication method for PC devices and photonic nano-strucutres.

Single GaAs/GaAsP coaxial core-shell nanowire lasers.

Highly uniform GaAs/GaAsP coaxial nanowires were prepared via selective-area metal organic vapor phase epitaxy. Photoluminescence spectra from a single nanowire indicate that the obtained heterostructures can produce near-infrared (NIR) lasing under pulsed light excitation. The end facets of a single nanowire form a natural mirror surface to create an axial cavity, which realizes resonance and give stimulated emission. This study is a considerable advance toward the realization of nanowire-based NIR light sources.

Control of InAs nanowire growth directions on Si.

We report on control of growth directions of InAs nanowires on Si substrate. We achieved to integrate vertical InAs nanowires on Si by modifying initial Si(111) surface in selective-area metal-organic vapor phase epitaxy with flow-rate modulation mode at low temperature. Cross-sectional transmission electron microscope and Raman scattering showed that misfit dislocation with local strains were accommodated in the interface.

Growth characteristics of GaAs nanowires obtained by selective area metal-organic vapour-phase epitaxy.

GaAs nanowires were selectively grown by metal-organic vapour-phase epitaxy within a SiO(2) mask window pattern fabricated on a GaAs(111)B substrate surface. The nanowires were 100-3000 nm in height and 50-300 nm in diameter. The height decreased as the mask window diameter was increased or the growth temperature was increased from 700 to 800 °C. The dependence of the nanowire height on the mask window diameter was compared with a calculation, which indicated that the height was inversely proportional to the mask window diameter. This suggests that the migration of growth species on the nanowire side surface plays a major role. Tetrahedral GaAs grew at an early stage of nanowire growth but became hexagonal as the growth process continued. The calculated change in Gibbs free energy for nucleation growth of the crystals indicated that tetrahedra were energetically more favourable than hexagons. Transmission and scanning electron microscopy analyses of a GaAs nanowire showed that many twins developed along the [Formula: see text] B direction, suggesting that twins had something to do with the evolution of the nanowire shape from tetrahedron to hexagon.