B. Mattias Borg

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.

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.

Synthesis and properties of antimonide nanowires

Antimonide semiconductors are suitable for low-power electronics and long-wavelength optoelectronic applications. In recent years research on antimonide nanowires has become a rapidly growing field, and nano-materials have promising applications in fundamental physics research, for tunnel field-effect transistors, and long-wavelength detectors. In this review, we give an overview of the field of antimonide nanowires, beginning with a description of the synthesis of these nano-materials. Here we summarize numerous reports on antimonide nanowire growth, with the aim to give an overall picture of the distinctive properties of antimonide nanowire synthesis. Secondly, we review the data on the physical properties and emerging applications for antimonide nanowires, focusing on applications in electronics and optics.

Diameter-Dependent Photocurrent in InAsSb Nanowire Infrared Photodetectors

Photoconductors using vertical arrays of InAs/InAs(1-x)Sb(x) nanowires with varying Sb composition x have been fabricated and characterized. The spectrally resolved photocurrents are strongly diameter dependent with peaks, which are red-shifted with diameter, appearing for thicker wires. Results from numerical simulations are in good agreement with the experimental data and reveal that the peaks are due to resonant modes that enhance the coupling of light into the wires. Through proper selection of wire diameter, the absorptance can be increased by more than 1 order of magnitude at a specific wavelength compared to a thin planar film with the same amount of material. A maximum 20% cutoff wavelength of 5.7 μm is obtained at 5 K for a wire diameter of 717 nm at a Sb content of x = 0.62, but simulations predict that detection at longer wavelengths can be achieved by increasing the diameter. Furthermore, photodetection in InAsSb nanowire arrays integrated on Si substrates is also demonstrated.

Single InAs/GaSb Nanowire Low-Power CMOS Inverter

III-V semiconductors have so far predominately been employed for n-type transistors in high-frequency applications. This development is based on the advantageous transport properties and the large variety of heterostructure combinations in the family of III-V semiconductors. In contrast, reports on p-type devices with high hole mobility suitable for complementary metal-oxide-semiconductor (CMOS) circuits for low-power operation are scarce. In addition, the difficulty to integrate both n- and p-type devices on the same substrate without the use of complex buffer layers has hampered the development of III-V based digital logic. Here, inverters fabricated from single n-InAs/p-GaSb heterostructure nanowires are demonstrated in a simple processing scheme. Using undoped segments and aggressively scaled high-κ dielectric, enhancement mode operation suitable for digital logic is obtained for both types of transistors. State-of-the-art on- and off-state characteristics are obtained and the individual long-channel n- and p-type transistors exhibit minimum subthreshold swings of SS = 98 mV/dec and SS = 400 mV/dec, respectively, at V(ds) = 0.5 V. Inverter characteristics display a full signal swing and maximum gain of 10.5 with a small device-to-device variability. Complete inversion is measured at low frequencies although large parasitic capacitances deform the waveform at higher frequencies.

Enhanced Sb incorporation in InAsSb nanowires grown by metalorganic vapor phase epitaxy

We demonstrate metalorganic vapor phase epitaxy of InAs1−xSbx nanowires (x=0.08–0.77) for applications in high-speed electronics and long-wavelength optical devices. The composition of the InAsSb nanowires and InAsSb epilayers on the same sample is independently determined using lab-setup high resolution x-ray diffraction, by making use of the size-dependent in-plane broadening of the nanowire Bragg peak. We find that the incorporation of Sb into the nanowires is significantly higher than for planar epitaxy under the same growth conditions. Thermodynamic calculations indicate that this is due to a dramatically decreased effective V/III ratio at the particle/nanowire interface.

Carrier control and transport modulation in GaSb/InAsSb core/shell nanowires

We report transport studies of GaSb/InAs core/shell nanowires. It is shown that with increasing InAs shell thickness, it is possible to tune the carrier concentrations and transport in the structures from p-type (core-dominated) to n-type (shell dominated). For nanowires with an intermediate shell thickness (5–7 nm), we show that the transport is ambipolar, such that an applied top-gate potential can provide further control of carrier type and transport path. In this range, the nature of the GaSb-InAs junction also changes from broken gap (semimetal) to staggered (narrow bandgap) with a small decrease in shell thickness. From a device point of view, we demonstrate that the presence of a thin (<3 nm) InAs shell improves p-type GaSb nanowire transistor characteristics.

GaSb nanowire single-hole transistor

We present an experimental study of single hole transistors (SHTs) made from p-type GaSb nanowires. Closely spaced source-drain electrodes are fabricated onto GaSb nanowires to define a SHT within a GaSb nanowire. Room temperature back-gate transfer characteristics show typical hole transport behavior. The fabricated devices are characterized by transport measurements at 1.5 K, where periodic conductance oscillations due to Coulomb blockade are observed and a charging energy of 5 meV is determined.

Influence of doping on the electronic transport in GaSb/InAs(Sb) nanowire tunnel devices

The GaSb/InAs heterostructure has an uncommon broken type II band alignment, for which the bottom of the conduction band in InAs lies below the top of the GaSb valence band. This broken gap leads to very efficient band-to-band tunneling, with the possibility of high current densities. With the recent focus on developing low-power devices, and in particular tunnel-field-effect transistors (TFETs), there is a renewed interest in staggered and broken-gap heterojunctions as they may enable high on-currents and steep subthreshold swing. 1‐4 For transistor applications, it is also important to efficiently control the electric potential of the channel region, and the gate-all-around geometry made possible by the small diameter of nanowires enables gate modulation near the quantum capacitance limit. 5 It is thus highly relevant to realize broken-gap tunnel devices in nanowires. Recently, we demonstrated GaSb/InAs(Sb) nanowire Esaki diodes with high peak current levels and peak-to-valley current ratio (PVCR). 6 By optimizing the doping profiles of these heterostructures, it is hoped that even better performance can be achieved. Doping control may also be crucial for TFET applications, to enable potential modulation in selected parts of the device, as well as to control the threshold voltage and minimize access resistance. In this letter, we thus investigate the influence of p-doping (Zn) and n-doping (Se) on the performance of GaSb/InAs(Sb) nanowire tunnel diodes and present significantly improved device performance over earlier work. In addition, we explore the effects of various doping profiles on the temperature dependent I–V and the threshold voltage of top gated devices. GaSb/InAs nanowires are grown from Au seed particles deposited on GaAs(111)B substrates in a manner similar to that which has been reported previously. 7 The GaSb/InAs heterojunction is characterized as being almost atomically abrupt, but with an Sb transient (InAs1� xSbx) reaching into the InAs segment and decaying exponentially from x ¼0.3‐0.4 down to 0.03‐0.07 over 5‐13nm. This segment will thus be denoted InAs(Sb) in the following. After growth, the samples are annealed in H2 at 490 � C for 10min to form a narrow constriction (neck) at the heterojunction. 8 This removes the short-circuit between the InAs(Sb) segment and an unintentionally formed InAs(Sb) shell (� 3‐5nm thick) surrounding the GaSb nanowire. Doping of the nanowire heterostructures is performed in-situ by adding controlled amounts of diethylzinc (DEZn) or ditertiarybutylselenium (DTBSe) during the growth. Three different doping profiles were investigated: (A) Zn-doped GaSb and unintentionally doped InAs(Sb), (B) Zn-doped GaSb and Zn-doped InAs(Sb), and (C) Zn-doped GaSb and Se-doped InAs(Sb). For type A, several different Zn doping levels were investigated. The ratios between the doping precursor and group-III precursor flows are labelled Zn/Ga, Zn/In, or Se/In in the text and used as qualitative measure of the doping level. The crystal structure of nanowires from all three types of samples was characterized using transmission electron microscopy. In all cases, the GaSb segment and the first 200‐500nm of the InAs(Sb) segment were pure zincblende, with the remainder containing a few (C) or many (A and B) twins but no wurtzite. Previous studies on InAs nanowires show that the electrical properties are not strongly affected unless inclusions of wurtzite are present. 9

Diameter reduction of nanowire tunnel heterojunctions using in situ annealing

We selectively etch axial GaSb/InAsSb nanowires locally at the heterojunction using in situ thermal annealing. This results in broken-gap tunnel diodes with a significantly reduced diameter only in the tunnel region. The etching mechanism proceeds by material removal from unstabilized {111}A facets which may form due to a reduced thermal stability at the heterointerface of GaSb/InAsSb nanowires. By removing the parallel conduction path between the InAsSb shell and nanowire the selective etching strongly improves the device performance. This is demonstrated in fabricated tunnel diodes that exhibit a peak-to-valley ratio of 1.3 and high peak current densities (8.1 kA/cm2).