Heidi Potts

From Twinning to Pure Zincblende Catalyst-Free InAs(Sb) Nanowires

III-V nanowires are candidate building blocks for next generation electronic and optoelectronic platforms. Low bandgap semiconductors such as InAs and InSb are interesting because of their high electron mobility. Fine control of the structure, morphology, and composition are key to the control of their physical properties. In this work, we present how to grow catalyst-free InAs1-xSbx nanowires, which are stacking fault and twin defect-free over several hundreds of nanometers. We evaluate the impact of their crystal phase purity by probing their electrical properties in a transistor-like configuration and by measuring the phonon-plasmon interaction by Raman spectroscopy. We also highlight the importance of high-quality dielectric coating for the reduction of hysteresis in the electrical characteristics of the nanowire transistors. High channel carrier mobilities and reduced hysteresis open the path for high-frequency devices fabricated using InAs1-xSbx nanowires.

High Yield of GaAs Nanowire Arrays on Si Mediated by the Pinning and Contact Angle of Ga

GaAs nanowire arrays on silicon offer great perspectives in the optoelectronics and solar cell industry. To fulfill this potential, gold-free growth in predetermined positions should be achieved. Ga-assisted growth of GaAs nanowires in the form of array has been shown to be challenging and difficult to reproduce. In this work, we provide some of the key elements for obtaining a high yield of GaAs nanowires on patterned Si in a reproducible way: contact angle and pinning of the Ga droplet inside the apertures achieved by the modification of the surface properties of the nanoscale areas exposed to growth. As an example, an amorphous silicon layer between the crystalline substrate and the oxide mask results in a contact angle around 90°, leading to a high yield of vertical nanowires. Another example for tuning the contact angle is anticipated, native oxide with controlled thickness. This work opens new perspectives for the rational and reproducible growth of GaAs nanowire arrays on silicon.

Tuning growth direction of catalyst-free InAs(Sb) nanowires with indium droplets

The need for indium droplets to initiate self-catalyzed growth of InAs nanowires has been highly debated in the last few years. Here, we report on the use of indium droplets to tune the growth direction of self-catalyzed InAs nanowires. The indium droplets are formed in situ on InAs(Sb) stems. Their position is modified to promote growth in the 〈11-2〉 or equivalent directions. We also show that indium droplets can be used for the fabrication of InSb insertions in InAsSb nanowires. Our results demonstrate that indium droplets can initiate growth of InAs nanostructures as well as provide added flexibility to nanowire growth, enabling the formation of kinks and heterostructures, and offer a new approach in the growth of defect-free crystals.

Nonstoichiometric Low-Temperature Grown GaAs Nanowires

The structural and electronic properties of nonstoichiometric low-temperature grown GaAs nanowire shells have been investigated with scanning tunneling microscopy and spectroscopy, pump-probe reflectivity, and cathodoluminescence measurements. The growth of nonstoichiometric GaAs shells is achieved through the formation of As antisite defects, and to a lower extent, after annealing, As precipitates. Because of the high density of atomic steps on the nanowire sidewalls, the Fermi level is pinned midgap, causing the ionization of the subsurface antisites and the formation of depleted regions around the As precipitates. Controlling their incorporation offers a way to obtain unique electronic and optical properties that depart from the ones found in conventional GaAs nanowires.

Synthesis, Morphological, and Electro-optical Characterizations of Metal/Semiconductor Nanowire Heterostructures

In this letter, we demonstrate the formation of unique Ga/GaAs/Si nanowire heterostructures, which were successfully implemented in nanoscale light-emitting devices with visible room temperature electroluminescence. Based on our recent approach for the integration of InAs/Si heterostructures into Si nanowires by ion implantation and flash lamp annealing, we developed a routine that has proven to be suitable for the monolithic integration of GaAs nanocrystallite segments into the core of silicon nanowires. The formation of a Ga segment adjacent to longer GaAs nanocrystallites resulted in Schottky-diode-like I/V characteristics with distinct electroluminescence originating from the GaAs nanocrystallite for the nanowire device operated in the reverse breakdown regime. The observed electroluminescence was ascribed to radiative band-to-band recombinations resulting in distinct emission peaks and a low contribution due to intraband transition, which were also observed under forward bias. Simulations of the obtained nanowire heterostructure confirmed the proposed impact ionization process responsible for hot carrier luminescence. This approach may enable a new route for on-chip photonic devices used for light emission or detection purposes.

Template-Assisted Scalable Nanowire Networks

Topological qubits based on Majorana Fermions have the potential to revolutionize the emerging field of quantum computing by making information processing significantly more robust to decoherence. Nanowires are a promising medium for hosting these kinds of qubits, though branched nanowires are needed to perform qubit manipulations. Here we report a gold-free templated growth of III-V nanowires by molecular beam epitaxy using an approach that enables patternable and highly regular branched nanowire arrays on a far greater scale than what has been reported thus far. Our approach relies on the lattice-mismatched growth of InAs on top of defect-free GaAs nanomembranes yielding laterally oriented, low-defect InAs and InGaAs nanowires whose shapes are determined by surface and strain energy minimization. By controlling nanomembrane width and growth time, we demonstrate the formation of compositionally graded nanowires with cross-sections less than 50 nm. Scaling the nanowires below 20 nm leads to the formation of homogeneous InGaAs nanowires, which exhibit phase-coherent, quasi-1D quantum transport as shown by magnetoconductance measurements. These results are an important advance toward scalable topological quantum computing.

Molecular beam epitaxy of InAs nanowires in SiO2 nanotube templates: challenges and prospects for integration of III-Vs on Si.

Guided growth of semiconductor nanowires in nanotube templates has been considered as a potential platform for reproducible integration of III-Vs on silicon or other mismatched substrates. Herein, we report on the challenges and prospects of molecular beam epitaxy of InAs nanowires in SiO2/Si nanotube templates. We show how and under which conditions the nanowire growth is initiated by In-assisted vapor-liquid-solid growth enabled by the local conditions inside the nanotube template. The conditions for high yield of vertical nanowires are investigated in terms of the nanotube depth, diameter and V/III flux ratios. We present a model that further substantiates our findings. This work opens new perspectives for monolithic integration of III-Vs on the silicon platform enabling new applications in the electronics, optoelectronics and energy harvesting arena.

Towards defect-free 1-D GaAs/AlGaAs heterostructures based on GaAs nanomembranes

We demonstrate the growth of defect-free zinc-blende GaAs nanomembranes by molecular beam epitaxy. Our growth studies indicate a strong impact of As4 re-emission and shadowing in the growth rate of the structures. The highest aspect ratio structures are obtained for pitches around 0.7-1 μm and a gallium rate of 1 Å s(-1). The functionality of the membranes is further illustrated by the growth of quantum heterostructures (such as quantum wells) and the characterization of their optical properties at the nanoscale. This proves the potential of nanoscale membranes for optoelectronic applications.

Bistability of Contact Angle and Its Role in Achieving Quantum-Thin Self-Assisted GaAs nanowires

Achieving quantum confinement by bottom-up growth of nanowires has so far been limited to the ability of obtaining stable metal droplets of radii around 10 nm or less. This is within reach for gold-assisted growth. Because of the necessity to maintain the group III droplets during growth, direct synthesis of quantum sized structures becomes much more challenging for self-assisted III-V nanowires. In this work, we elucidate and solve the challenges that involve the synthesis of gallium-assisted quantum-sized GaAs nanowires. We demonstrate the existence of two stable contact angles for the gallium droplet on top of GaAs nanowires. Contact angle around 130° fosters a continuous increase in the nanowire radius, while 90° allows for the stable growth of ultrathin tops. The experimental results are fully consistent with our model that explains the observed morphological evolution under the two different scenarios. We provide a generalized theory of self-assisted III-V nanowires that describes simultaneously the droplet shape relaxation and the NW radius evolution. Bistability of the contact angle described here should be the general phenomenon that pertains for any vapor-liquid-solid nanowires and significantly refines our picture of how nanowires grow. Overall, our results suggest a new path for obtaining ultrathin one-dimensional III-V nanostructures for studying lateral confinement of carriers.

Engineering the Size Distributions of Ordered GaAs Nanowires on Silicon

Reproducible integration of III-V semiconductors on silicon can open new path toward CMOS compatible optoelectronics and novel design schemes in next generation solar cells. Ordered arrays of nanowires could accomplish this task, provided they are obtained in high yield and uniformity. In this work, we provide understanding on the physical factors affecting size uniformity in ordered GaAs arrays grown on silicon. We show that the length and diameter distributions in the initial stage of growth are not much influenced by the Poissonian fluctuation-induced broadening, but rather are determined by the long incubation stage. We also show that the size distributions are consistent with the double exponential shapes typical for macroscopic nucleation with a large critical length after which the nanowires grow irreversibly. The size uniformity is dramatically improved by increasing the As4 flux, suggesting a new path for obtaining highly uniform arrays of GaAs nanowires on silicon.