Growth and Electrical Characterization of Hybrid Core/Shell InAs/CdSe Nanowires

Core-only InAs nanowires (NWs) remain of continuing interest for application in modern optical and electrical devices. In this paper, we utilize the II-VI semiconductor CdSe as a shell for III-V InAs NWs to protect the electron transport channel in the InAs core from surface effects. This unique material configuration offers both a small lattice mismatch between InAs and CdSe and a pronounced electronic confinement in the core with type-I band alignment at the interface between both materials. Under optimized growth conditions, a smooth interface between the core and shell is obtained. Atom probe tomography (APT) measurements confirm substantial diffusion of In into the shell, forming a remote n-type doping of CdSe. Moreover, field-effect transistors (FETs) are fabricated, and the electron transport characteristics in these devices is investigated. Finally, band structure simulations are performed and confirm the presence of an electron transport channel in the InAs core that, at higher gate voltages, extends into the CdSe shell region. These results provide a promising basis toward the application of hybrid III-V/II-VI core/shell nanowires in modern electronics.

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.

Selective-Area Growth Mechanism of GaN Microrods on a Plateau Patterned Substrate

This study provides experimental evidence regarding the mechanism of gallium nitride (GaN) selective-area growth (SAG) on a polished plateau-patterned sapphire substrate (PP-PSS), on which aluminum nitride (AlN) buffer layers are deposited under the same deposition conditions. The SAG of GaN was only observed on the plateau region of the PP-PSS, irrespective of the number of growth cycles. Indirect samples deposited on the bare c-plane substrate were prepared to determine the difference between the AlN buffer layers in the plateau region and silicon oxide (SiO2). The AlN buffer layer in the plateau region exhibited a higher surface energy, and its crystal orientation is indicated by AlN [001]. In contrast, regions other than the plateau region did not exhibit crystallinity and presented lower surface energies. The direct analysis results of PP-PSS using transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) are similar to the results of the indirect samples. Therefore, under the same conditions, the GaN SAG of the deposited layer is related to crystallinity, crystal orientation, and surface energy.

Supercurrent in Bi4Te3 Topological Material-Based Three-Terminal Junctions

In this paper, in an in situ prepared three-terminal Josephson junction based on the topological insulator Bi4Te3 and the superconductor Nb the transport properties are studied. The differential resistance maps as a function of two bias currents reveal extended areas of Josephson supercurrent, including coupling effects between adjacent superconducting electrodes. The observed dynamics for the coupling of the junctions is interpreted using a numerical simulation of a similar geometry based on a resistively and capacitively shunted Josephson junction model. The temperature dependency indicates that the device behaves similar to prior experiments with single Josephson junctions comprising topological insulators' weak links. Irradiating radio frequencies to the junction, we find a spectrum of integer Shapiro steps and an additional fractional step, which is interpreted with a skewed current-phase relationship. In a perpendicular magnetic field, we observe Fraunhofer-like interference patterns in the switching currents.

Small Charging Energies and g-Factor Anisotropy in PbTe Quantum Dots

PbTe is a semiconductor with promising properties for topological quantum computing applications. Here, we characterize electron quantum dots in PbTe nanowires selectively grown on InP. Charge stability diagrams at zero magnetic field reveal large even-odd spacing between Coulomb blockade peaks, charging energies below 140 μeV and Kondo peaks in odd Coulomb diamonds. We attribute the large even-odd spacing to the large dielectric constant and small effective electron mass of PbTe. By studying the Zeeman-induced level and Kondo splitting in finite magnetic fields, we extract the electron g-factor as a function of magnetic field direction. We find the g-factor tensor to be highly anisotropic with principal g-factors ranging from 0.9 to 22.4 and to depend on the electronic configuration of the devices. These results indicate strong Rashba spin-orbit interaction in our PbTe quantum dots.

InP nanowire light-emitting diodes with different pn-junction structures

We report on the characterization of wurtzite (WZ) InP nanowire (NW) light-emitting diodes (LEDs) with different pn junctions (axial and radial). The series resistance tended to be smaller in the NW-LED using core-shell InP NWs with a radial pn junction than in the NW-LED using InP NWs with an axial pn junction, indicating that radial pn junctions are more suitable for current injection. The electroluminescence (EL) properties of both NW LEDs revealed that the EL had three peaks originating from the zinc-blende (ZB) phase, WZ phase, and ZB/WZ heterojunction. Transmission electron microscopy showed that the dominant EL in the radial pn junction originated from the ZB/WZ interface across the stacking faults.

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 MOCVD Growth and Carrier-Transport-Type Control of InAs(Sb)/GaSb Core-Shell Nanowires

We report the first selective-area growth of high quality InAs(Sb)/GaSb core-shell nanowires on Si substrates using metal-organic chemical vapor deposition (MOCVD) without foreign catalysts. Transmission electron microscopy (TEM) analysis reveals that the overgrowth of the GaSb shell is highly uniform and coherent with the InAs(Sb) core without any misfit dislocations. To control the structural properties and reduce the planar defect density in the self-catalyzed InAs core nanowires, a trace amount of Sb was introduced during their growth. As the Sb content increases from 0 to 9.4%, the crystal structure of the nanowires changes from a mixed wurtzite (WZ)/zinc-blende (ZB) structure to a perfect ZB phase. Electrical measurements reveal that both the n-type InAsSb core and p-type GaSb shell can work as active carrier transport channels, and the transport type of core-shell nanowires can be tuned by the GaSb shell thickness and back-gate voltage. This study furthers our understanding of the Sb-induced crystal-phase control of nanowires. Furthermore, the high quality InAs(Sb)/GaSb core-shell nanowire arrays obtained here pave the foundation for the fabrication of the vertical nanowire-based devices on a large scale and for the study of fundamental quantum physics.