Damage protection from focused ion beam process toward nanocavity-implemented compound semiconductor nanowire lasers.

A focused ion beam (FIB) can precisely mill samples and freely form any nanostructure even on surfaces with curvature, like a nanowire surface, which are difficult to implement by using conventional fabrication techniques, e.g. electron beam lithography. Thus, this tool is promising for nanofabrication; however, fabrication damage and contamination are critical issues, which deteriorate optical properties. In this work, we investigated the protective performance of Al2O3against the FIB process (especially by a gallium ion). Nanowires were coated with Al2O3as a hard mask to protect them from damage during FIB nanofabrication. To estimate the protective performance, their emission properties by photoluminescence measurement and time-resolved spectroscopy were compared with and without Al2O3coating conditions. From the results, we confirmed that the Al2O3coating protects the nanowires. In addition, the nanowires also showed lasing behavior even after FIB processing had been carried out to implement nanostructures. This indicates that their optical properties are well maintained. Thus, our study proves the usefulness of FIBs for future nanofabrication.

Telecom-band lasing in single InP/InAs heterostructure nanowires at room temperature.

Telecom-band single nanowire lasers made by the bottom-up vapor-liquid-solid approach, which is technologically important in optical fiber communication systems, still remain challenging. Here, we report telecom-band single nanowire lasers operating at room temperature based on multi-quantum-disk InP/InAs heterostructure nanowires. Transmission electron microscopy studies show that highly uniform multi-quantum-disk InP/InAs structure is grown in InP nanowires by self-catalyzed vapor-liquid-solid mode using indium particle catalysts. Optical excitation of individual nanowires yielded lasing in telecom band operating at room temperature. We show the tunability of laser wavelength range in telecom band by modulating the thickness of single InAs quantum disks through quantum confinement along the axial direction. The demonstration of telecom-band single nanowire lasers operating at room temperature is a major step forward in providing practical integrable coherent light sources for optoelectronics and data communication.

Diameter-tailored telecom-band luminescence in InP/InAs heterostructure nanowires grown on InP (111)B substrate with continuously-modulated diameter from microscale to nanoscale.

We report diameter-tailored luminescence in telecom band of InP/InAs multi-heterostructure nanowires with continuously-modulated diameter from microscale to nanoscale. By using the self-catalyzed vapor-solid-liquid approach, we tune the indium particle size, and consequently the InP/InAs nanowire diameter, during growth by modulating the flow rate of the indium source material. This technique allows a high degree of continuous tuning in a wide scale from microscale to nanoscale. Hence it offers an original way to bridge the gap between microscale-featured photolithographic and nanoscale-featured nanolithographic processes and to incorporate InAs quantum disks with tunable diameters into a single InP/InAs quantum heterostructure nanowire. We realized site-defined nanowires with nanoscale diameters initiated from site-defined microscale-diameter particles made with a conventional photolithographic process. The luminescence wavelength from InAs quantum disks is directly connected to the nanowire diameter, by which the strain in the InAs quantum disks is tailored. This work provides new opportunities in the fabrication and design of nanowire devices that extends beyond what is achievable with the current technologies and enables the nanowire shape to be engineered thus offering the potential to broaden the application range of nanowire devices.

Bridging the Gap between the Nanometer-Scale Bottom-Up and Micrometer-Scale Top-Down Approaches for Site-Defined InP/InAs Nanowires.

This work presents a method that bridges the gap between the nanometer-scale bottom-up and micrometer-scale top-down approaches for site-defined nanostructures, which has long been a significant challenge for applications that require low-cost and high-throughput manufacturing processes. We realized the bridging by controlling the seed indium nanoparticle position through a self-assembly process. Site-defined InP nanowires were then grown from the indium-nanoparticle array in the vapor-liquid-solid mode through a "seed and grow" process. The nanometer-scale indium particles do not always occupy the same locations within the micrometer-scale open window of an InP exposed substrate due to the scale difference. We developed a technique for aligning the nanometer-scale indium particles on the same side of the micrometer-scale window by structuring the surface of a misoriented InP (111)B substrate. Finally, we demonstrated that the developed method can be used to grow a uniform InP/InAs axial-heterostructure nanowire array. The ability to form a heterostructure nanowire array with this method makes it possible to tune the emission wavelength over a wide range by employing the quantum confinement effect and thus expand the application of this technology to optoelectronic devices. Successfully pairing a controllable bottom-up growth technique with a top-down substrate preparation technique greatly improves the potential for the mass-production and widespread adoption of this technology.

Controlled 1.1-1.6 µm luminescence in gold-free multi-stacked InAs/InP heterostructure nanowires.

We report controlled 1.1-1.6 µm luminescence in gold-free multi-stacked InAs/InP heterostructure nanowires (NWs). We realized the NWs by using an indium-particle-assisted vapor-liquid-solid synthesis approach. The growth temperature, as low as 320 °C, enables the formation of an atomically abrupt InP/InAs interface by supressing the diffusion and weakening the reservoir effect in the indium droplet. The low growth temperature also enables us to grow multi-stacked InAs/InP NWs in the axial direction without any growth on the NW side face. The high controllability of the growth technology ensures that the luminescence can be tailored by the thickness of InAs segment in InP NWs and cover the 1.3-1.5 µm telecommunication window range. By using the nanoscale-spatial-resolution technology combing cathodoluminescence with scanning electron microscopy, we directly correlated the site of different-thickness InAs segments with its luminescence property in a single NW and demonstrate the InAs-thickness-controlled energy of optical emission in 1.1-1.6 µm.

Movable high-Q nanoresonators realized by semiconductor nanowires on a Si photonic crystal platform.

Subwavelength semiconductor nanowires have recently attracted interest for photonic applications because they possess various unique optical properties and offer great potential for miniaturizing devices. However, realizing tight light confinement or efficient coupling with photonic circuits is not straightforward and remains a challenge. Here we show that a high-Q nanocavity can be created by placing a single III­V semiconductor nanowire with a diameter of under 100 nm in a grooved waveguide in a Si photonic crystal, by means of nanoprobe manipulation. We observe very fast spontaneous emission (91 ps) from nanowires accelerated by the strong Purcell enhancement in nanocavities, which proves that very strong light confinement can be achieved. Furthermore, this system enables us to move the nanocavity anywhere along the waveguide. This configuration provides a significant degree of flexibility in integrated photonics and permits the addition and displacement of various functionalities of III­V nanocavity devices in Si photonic circuits.

Topological Raman band in the carbon nanohorn.

Raman spectroscopy has been used in chemistry and physics to investigate the fundamental process involving light and phonons. The carbon nanohorn introduces a new subject to Raman spectroscopy, namely topology. We show theoretically that a photoexcited carrier with a nonzero winding number activates a topological D Raman band through the Aharonov-Bohm effect. The topology-induced D Raman band can be distinguished from the ordinary D Raman band for a graphene edge by its peak position.

VLS growth of alternating InAsP/InP heterostructure nanowires for multiple-quantum-dot structures.

We investigated the Au-assisted growth of alternating InAsP/InP heterostructures in wurtzite InP nanowires on InP(111)B substrates for constructing multiple-quantum-dot structures. Vertical InP nanowires without stacking faults were obtained at a high PH(3)/TMIn mole flow ratio of 300-1000. We found that the growth rate changed largely when approximately 40 min passed. Ten InAsP layers were inserted in the InP nanowire, and it was found that both the InP growth rate and the background As level increased after the As supply. We also grew the same structure using TBAs/TBP and could reduce the As level in the InP segments. A simulation using a finite-difference time-domain method suggests that the nanowire growth was dominated by the diffusion of the reaction species with long residence time on the surface. For TBAs/TBP, when the source gases were changed, the formed surface species showed a short diffusion length so as to reduce the As background after the InAsP growth.

Supercurrent through InAs nanowires with highly transparent superconducting contacts.

One-dimensional superconducting transistors have been fabricated with individual InAs nanowires (NWs) using radio-frequency sputter cleaning followed by in situ metal deposition. Because of the highly transparent contacts formed in between the InAs NWs and the metals, supercurrent, multiple Andreev reflections and Shapiro steps under microwave radiation have been observed. Near pinch-off gate regions, Fabry-Perot interference and a normal conductance quantization with resonant features have been observed, which were found to be correlated with a supercurrent flow.

Parallel-aligned GaAs nanowires with 110 orientation laterally grown on [311]B substrates via the gold-catalyzed vapor-liquid-solid mode.

We report parallel aligned GaAs nanowires (NWs) with 110 orientation laterally grown on [311]B substrates via the vapor-liquid-solid mode and demonstrate their controllability and growth mechanism. We control the size, density, and site of the lateral NWs by using size- and density-selective Au colloidal particles and Au dot arrays defined by electron-beam lithography. The lateral NWs grow only along the [110] and [Formula: see text] directions and formation of the stable facets of (111)B and (001) on the sides of the lateral NWs is crucial for lateral NW growth. We clarify the growth mechanism by comparing the growth results on [311]B, (311)A, and (001) substrates and the surface energy change of lateral and freestanding NWs.

Growth of GaInAs/AlInAs heterostructure nanowires for long-wavelength photon emission.

We investigated the growth of GaInAs/AlInAs heterostructure nanowires on InP(111)B and Si(111) substrates in a metalorganic vapor phase epitaxy reactor. Au colloids were used to deposit Au catalysts 20 and 40 nm in diameter on the substrate surfaces. We obtained vertical GaInAs and AlInAs nanowires on InP(111)B surfaces. The GaInAs nanowires capped with GaAs/AlInAs layers show room-temperature photoluminescence. The peak exhibits a blue-shift when the Ga content in the core GaInAs nanowire is increased. For the GaInAs/AlInAs heterostructure growth, it is possible to change the Ga content sharply but Al also exists in the GaInAs layer regions. We also found that the ratios of Ga and Al contents to In content tend to increase and the axial growth rate to decrease along the nanowire toward the top. We were also able to make vertical GaInAs nanowires on Si(111) surfaces after a short growth of GaP and InP.