Kimberly A. Dick

InAs/GaSb Heterostructure Nanowires for Tunnel Field-Effect Transistors

InAs/GaSb nanowire heterostructures with thin GaInAs inserts were grown by MOVPE and characterized by electrical measurements and transmission electron microscopy. Down-scaling of the insert thickness was limited because of an observed sensitivity of GaSb nanowire growth to the presence of In. By employing growth interrupts in between the InAs and GaInAs growth steps it was possible to reach an insert thickness down to 25 nm. Two-terminal devices show a diode behavior, where temperature-dependent measurements indicate a heterostructure barrier height of 0.5 eV, which is identified as the valence band offset between the InAs and GaSb. Three-terminal transistor structures with a top-gate positioned at the heterointerface show clear indications of band-to-band tunnelling.

High-Quality InAs/InSb Nanowire Heterostructures Grown by Metal-Organic Vapor-Phase Epitaxy.

Recently, semiconducting nanowires have attracted much interest due to their advantageous physical properties and a wide range of potential applications in electronics, optoelectronics, and biosensors. High-quality compound nanowire heterostructures have been grown using various techniques, including chemical-beam epitaxy (CBE) and metal–organic vapor-phase epitaxy (MOVPE), with both methods producing sharp interfaces. To date, most studies have concentrated on ‘‘classic’’ III–V semiconductors, such as InAs, InP, GaAs, GaP, and nitride nanowires. Antimonycontaining nanowires would extend the range of usable materials in these heterostructures and devices. InSb is the III–V semiconductor with the narrowest bandgap (0.17 eV) and the largest bulk electron mobility ( 7.7 10 cm V 1 s ); it also has a high thermoelectric figure of merit (0.6). This material is therefore extremely interesting for applications in high-speed and low-power electronics, infrared optoelectronics, quantum-transport studies, and thermoelectric power generation. Despite all of these promising advantages, very few studies on InSb nanowires have been reported. There have so far been no reports on InSb-containing nanowire heterostructures. Moreover, due to the large lattice mismatch between InAs and InSb (7%), no combination of these materials has been reported using planar growth. In this Communication, we demonstrate for the first time growth of high-quality InAs/InSb heterostructure nanowires.

InSb heterostructure nanowires: MOVPE growth under extreme lattice mismatch.

We demonstrate the growth of InSb-based nanowire heterostructures by metalorganic vapour phase epitaxy and use it to integrate InSb on extremely lattice-mismatched III-V nanowire templates made of InAs, InP, and GaAs. Influence of temperature, V/III ratio, and diameter are investigated in order to investigate the growth rate and morphology. The range of growth temperatures used for InSb nanowire growth is very similar to that used for planar growth due to the nature of the precursor decomposition. This makes optimization of growth parameters very important, and more difficult than for most other nanowire III-V materials. Analysis of the InSb nanowire epitaxial quality when grown on InAs, InP, and GaAs, along with InSb segment and particle compositions are reported. This successful direct integration of InSb nanowires, on nanowire templates with unprecedented strain levels show great promise for fabrication of vertical InSb devices.

Growth related aspects of epitaxial nanowires

We use metal–organic vapour phase epitaxy for growth investigations of epitaxial nanowires in III–V materials, such as GaAs, GaP, InAs, and InP. In this paper we focus on gold assisted growth of nanowires. The nature of the metal particle—whether it is in the solid or liquid state—is discussed. For InAs and InP we have demonstrated that gold assisted wires can only grow at temperatures where the particle is solid. We continue with a discussion concerning the kinetic aspects of nanowire growth. Under common growth conditions one observes that thinner wires grow faster than thicker wires, contrary to what was described in the early days of whisker growth. We address and resolve this discrepancy by discussing a simple transport model and comparing the supersaturations of different systems. Finally, we describe the morphology of epitaxial III–V nanowires with emphasis on the crystal structure.

A general approach for sharp crystal phase switching in InAs, GaAs, InP, and GaP nanowires using only group V flow.

III-V-based nanowires usually exhibit random mixtures of wurtzite (WZ) and zinc blende (ZB) crystal structure, and pure crystal phase wires represent the exception rather than the rule. In this work, the effective group V hydride flow was the only growth parameter which was changed during MOVPE growth to promote transitions from WZ to ZB and from ZB to WZ. Our technique works in the same way for all investigated III-Vs (GaP, GaAs, InP, and InAs), with low group V flow for WZ and high group V flow for ZB conditions. This strongly suggests a common underlying mechanism. It displays to our best knowledge the simplest changes of the growth condition to control the nanowire crystal structure. The inherent reduction of growth variables is a crucial requirement for the interpretation in the frame of existing understanding of polytypism in III-V nanowires. We show that the change in surface energetics of the vapor-liquid-solid system at the vapor-liquid and liquid-solid interface is likely to control the crystal structure in our nanowires.

High-Current GaSb/InAs(Sb) Nanowire Tunnel Field-Effect Transistors

We present electrical characterization of GaSb/InAs(Sb) nanowire tunnel field-effect transistors. The broken band alignment of the GaSb/InAs(Sb) heterostructure is exploited to allow for interband tunneling without a barrier, leading to high on-current levels. We report a maximum drive current of 310 μA/μm at VDS = 0.5 V. Devices with scaled gate oxides display transconductances up to gm = 250 mS/mm at VDS = 300 mV, which are normalized to the nanowire circumference at the axial heterojunction.

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.

Interface dynamics and crystal phase switching in GaAs nanowires

Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically grown nanowires are ideal systems for studying the fundamental physics of phase selection, and could lead to new electronic applications based on the engineering of crystal phases. Here we image gallium arsenide (GaAs) nanowires during growth as they switch between phases as a result of varying growth conditions. We find clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry. We explain these differences, and the phase selection, using a model that relates the catalyst volume, the contact angle at the trijunction (the point at which solid, liquid and vapour meet) and the nucleation site of each new layer of GaAs. This model allows us to predict the conditions under which each phase should be observed, and use these predictions to design GaAs heterostructures. These results could apply to phase selection in other nanowire systems.

Unit Cell Structure of Crystal Polytypes in InAs and InSb Nanowires

The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.

Large Thermoelectric Power Factor Enhancement Observed in InAs Nanowires.

We report the observation of a thermoelectric power factor in InAs nanowires that exceeds that predicted by a single-band bulk model by up to an order of magnitude at temperatures below about 20 K. We attribute this enhancement effect not to the long-predicted 1D subband effects but to quantum-dot-like states that form in electrostatically nonuniform nanowires as a result of interference between propagating states and 0D resonances.