Bahram Ganjipour

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-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.

High Current Density Esaki Tunnel Diodes Based on GaSb-InAsSb Heterostructure Nanowires

We present electrical characterization of broken gap GaSb-InAsSb nanowire heterojunctions. Esaki diode characteristics with maximum reverse current of 1750 kA/cm(2) at 0.50 V, maximum peak current of 67 kA/cm(2) at 0.11 V, and peak-to-valley ratio (PVR) of 2.1 are obtained at room temperature. The reverse current density is comparable to that of state-of-the-art tunnel diodes based on heavily doped p-n junctions. However, the GaSb-InAsSb diodes investigated in this work do not rely on heavy doping, which permits studies of transport mechanisms in simple transistor structures processed with high-κ gate dielectrics and top-gates. Such processing results in devices with improved PVR (3.5) and stability of the electrical properties.

Tunnel Field-Effect Transistors Based on InP-GaAs Heterostructure Nanowires.

We present tunneling field-effect transistors fabricated from InP-GaAs heterostructure nanowires with an n-i-p doping profile, where the intrinsic InP region is modulated by a top gate. The devices show an inverse subthreshold slope down to 50 mV/dec averaged over two decades with an on/off current ratio of approximately 10(7) for a gate voltage swing (V(GS)) of 1 V and an on-current of 2.2 μA/μm. Low-temperature measurements suggest a mechanism of trap-assisted tunneling, possibly explained by a narrow band gap segment of InGaAsP.

Demonstration of Defect-Free and Composition Tunable Ga(x)In(1-x)Sb Nanowires.

The Ga(x)In(1-x)Sb ternary system has many interesting material properties, such as high carrier mobilities and a tunable range of bandgaps in the infrared. Here we present the first report on the growth and compositional control of Ga(x)In(1-x)Sb material grown in the form of nanowires from Au seeded nanoparticles by metalorganic vapor phase epitaxy. The composition of the grown Ga(x)In(1-x)Sb nanowires is precisely controlled by tuning the growth parameters where x varies from 1 to ∼0.3. Interestingly, the growth rate of the Ga(x)In(1-x)Sb nanowires increases with diameter, which we model based on the Gibbs-Thomson effect. Nanowire morphology can be tuned from high to very low aspect ratios, with perfect zinc blende crystal structure regardless of composition. Finally, electrical characterization on nanowire material with a composition of Ga(0.6)In(0.4)Sb showed clear p-type behavior.

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).

Transport studies of electron-hole and spin-orbit interaction in GaSb/InAsSb core-shell nanowire quantum dots

We report low-temperature transport studies of parallel double quantum dots formed in GaSb/InAsSb core-shell nanowires. At negative gate voltages, regular patterns of Coulomb diamonds are observed in the charge stability diagrams, which we ascribe to single-hole tunneling through a quantum dot in the GaSb core. As the gate voltage increases, the measured charge stability diagram indicates the appearance of an additional quantum dot, which we suggest is an electron quantum dot formed in the InAsSb shell. We find that an electron-hole interaction induces shifts of transport resonances in the source-drain voltage from which an average electron-hole interaction strength of 2.9 +/- 0.3 meV is extracted. We also carry out magnetotransport measurements of a hole quantum dot in the GaSb core and extract level-dependent g factors and a spin-orbit interaction. (Less)