InAs Two-dimensional Electron Gas Research Articles

Spin-filter device using the Zeeman effect with realistic channel and structure parameters

We have theoretically calculated the performance of a Zeeman-type spin polarizer, which consists of ferromagnetic (FM) and metal–insulator–semiconductor (MIS) gate nanostructures on top of an InAs two-dimensional electron gas (2DEG) channel. For the calculations, we have taken a realistic electron concentration, electron effective mass and the effective g-factor of the InAs 2DEG into account. In addition, we have assumed realistic FM and MIS structure sizes by conventional electron beam lithography. In the calculations, we have demonstrated clear oscillation of spin polarization over ±85%. Furthermore, we have shown that it works not only as a spin-polarized current generator but also a detector. In addition, we have proposed a novel spin-filter device utilizing the Zeeman-type spin polarizers. We have found that its conductance characterization allows us to evaluate the operation of Zeeman-type spin polarizers. We expect the Zeeman-type spin polarizers and spin-filter devices to open up a new field of spintronics.

Near-surface InAs two-dimensional electron gas on a GaAs substrate: Characterization and superconducting proximity effect

We have studied a near-surface two-dimensional electron gas based on an InAs quantum well on a GaAs substrate. In devices without a dielectric layer we estimated large electron mobilities on the order of ${10}^{5}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}$. We have observed quantized conductance in a quantum point contact, and determined the $g$ factor. Using samples with an epitaxial Al layer, we defined multiple Josephson junctions and found the critical current to be gate tunable. Based on multiple Andreev reflections the semiconductor-superconductor interface is transparent, with an induced gap of $125\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{eV}$. Our results suggest that this InAs system is a viable platform for use in hybrid topological superconductor devices.

Clean quantum point contacts in an InAs quantum well grown on a lattice-mismatched InP substrate

Indium arsenide (InAs) has a small effective mass, strong spin-orbit coupling, and surface Fermi level pinning. Together with improvements in the epitaxial growth of quantum well heterostructures, InAs presents an enticing platform for studying mesoscopic physics. Here, the authors investigate quantized conductance and magnetotransport through quantum point contacts (QPCs) fabricated on a deep InAs two-dimensional electron gas. These QPCs can serve as a robust building block for further experiments in quantum circuits and electron quantum optics.

Gate tuning of fractional quantum Hall states in an InAs two-dimensional electron gas

We report the observation of fractional quantum Hall (FQH) effects in a two-dimensional electron gas (2DEG) confined to an InAs/AlGaSb quantum well, using a dual-gated Hall-bar device allowing for the independent control of the vertical electric field and electron density. At a magnetic field of 24 T, we observe FQH states at several filling factors, namely $\ensuremath{\nu}=5/3, 2/3$, and $1/3$, in addition to the $\ensuremath{\nu}=4/3$ previously reported for an InAs 2DEG. The $\ensuremath{\nu}=4/3$ and $5/3$ states, which are absent at zero back-gate voltage, emerge as the quantum well is made more symmetric by applying a positive back-gate voltage. The dependence of zero-field electron mobility on the quantum-well asymmetry reveals a significant contribution of interface-roughness scattering, with much stronger scattering at the lower InAs/AlGaSb interface. However, the dependence of the visibility of the FQH effects on the quantum-well asymmetry is not entirely consistent with that of mobility, suggesting that a different source of disorder is also at work.

Transport studies in a gate-tunable three-terminal Josephson junction

Josephson junctions with three or more superconducting leads have been predicted to exhibit topological effects in the presence of few conducting modes within the interstitial normal material. Such behavior, of relevance for topologically-protected quantum bits, would lead to specific transport features measured between terminals, with topological phase transitions occurring as a function of phase and voltage bias. Although conventional, two-terminal Josephson junctions have been studied extensively, multi-terminal devices have received relatively little attention to date. Motivated in part by the possibility to ultimately observe topological phenomena in multi-terminal Josephson devices, as well as their potential for coupling gatemon qubits, here we describe the superconducting features of a top-gated mesoscopic three-terminal Josephson device. The device is based on an InAs two-dimensional electron gas (2DEG) proximitized by epitaxial aluminum. We map out the transport properties of the device as a function of bias currents, top gate voltage and magnetic field. We find a very good agreement between the zero-field experimental phase diagram and a resistively and capacitively shunted junction (RCSJ) computational model.

Gate-defined quantum point contact in an InAs two-dimensional electron gas

We experimentally study quantized conductance in an electrostatically defined constriction in a high-mobility InAs two-dimensional electron gas. A parallel magnetic field lifts the spin degeneracy and allows for the observation of plateaus in integer multiples of $e^2/h$. Upon the application of a perpendicular magnetic field, spin-resolved magnetoelectric subbands are visible. Through finite bias spectroscopy we measure the subband spacings in both parallel and perpendicular direction of the magnetic field and determine the $g$-factor.

Contribution of top barrier materials to high mobility in near-surface InAs quantum wells grown on GaSb(001)

Near-surface InAs two-dimensional electron gas (2DEG) systems have great potential for realizing networks of multiple Majorana zero modes towards a scalable topological quantum computer. Improving mobility in the near-surface 2DEGs is beneficial for stable topological superconducting states as well as for correlation of multiple Majorana zero modes in a complex network. Here, we investigate near-surface InAs 2DEGs (13 nm away from the surface) grown on GaSb(001) substrates, whose lattice constant is closely matched to InAs, by molecular beam epitaxy. The effect of 10-nm-thick top barrier to the mobility is studied by comparing Al$_{0.9}$Ga$_{0.1}$Sb and In$_{0.75}$Ga$_{0.25}$As as a top barrier on otherwise identical InAs quantum wells grown with identical bottom barrier and buffer layers. A 3-nm-thick capping layer on Al$_{0.9}$Ga$_{0.1}$Sb top barrier also affects the 2DEG electronic transport properties by modifying scattering from 2D remote ionized impurities at the surface. The highest transport mobility of 650,000 cm$^2$/Vs with an electron density of 3.81 $\times$ 10$^{11}$ cm$^{-2}$ was observed in an InAs 2DEG with an Al$_{0.9}$Ga$_{0.1}$Sb top barrier and an In$_{0.75}$Ga$_{0.25}$As capping layer. Analysis of Shubnikov-de Haas oscillations in the high mobility sample suggests that long-range scattering, such as remote ionized impurity scattering, is the dominant scattering mechanism in the InAs 2DEGs grown on GaSb(001) substrates. In comparison to InAs quantum wells grown on lattice-mismatched InP, the ones grown on GaSb show smoother surface morphology and higher quantum mobility. However, In$_{0.75}$Ga$_{0.25}$As top barrier in InAs quantum well grown on GaSb limits the transport mobility by charged dislocations formed in it, in addition to the major contribution to scattering from the alloy scattering.

Superconducting gatemon qubit based on a proximitized two-dimensional electron gas.

The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting circuits, from setting qubit transition frequencies1 and interqubit coupling strengths2 to the gain of parametric amplifiers3 for quantum-limited readout. The inductance is either set by tailoring the metal oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices with local current-biased flux lines. JJs based on superconductor-semiconductor hybrids represent a tantalizing all-electric alternative. The gatemon is a recently developed transmon variant that employs locally gated nanowire superconductor-semiconductor JJs for qubit control4,5. Here we go beyond proof-of-concept and demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a suitable platform for building a scalable gatemon-based quantum computer. We show that 2DEG gatemons meet the requirements6 by performing voltage-controlled single qubit rotations and two-qubit swap operations. We measure qubit coherence times up to ~2 μs, limited by dielectric loss in the 2DEG substrate.

Conductance fluctuations in InAs quantum wells possibly driven by Zitterbewegung

The highly successful Dirac equation predicts peculiar phenomena such as Klein tunnelling and Zitterbewegung (ZB) of electrons. From its conception by Erwin Schrödinger, ZB has been considered key in understanding relativistic quantum mechanics. However, observing the ZB of electrons has proved difficult, and instead various emulations of the phenomenon have been proposed producing several successes. Concerning charge transport in semiconductors and graphene, expectations were high but little has been reported. Here, we report a surprisingly large ZB effect on charge transport in a semiconductor nanostructure playing “flat pinball”. The setup is a narrow strip of InAs two-dimensional electron gas with strong Rashba spin–orbit coupling. Six quantum point contacts act as pinball pockets. In transiting between two contacts, ZB appears as a large reproducible conductance fluctuation that depends on the in-plane magnetic field. Numerical simulations successfully reproduced our experimental observations confirming that ZB causes this conductance fluctuation.

Dynamic conductivity of an InAs two-dimensional electron gas measured using terahertz time-domain spectroscopy

We utilized a coherent terahertz time-domain spectroscopy system to measure the dynamic conductivity of an InAs two-dimensional electron gas. Based on the theoretical transmission analysis, the real and imaginary parts of the complex conductivity of the sample were extracted from the transmitted terahertz time-domain waveforms. We also measured the sample at low temperatures and fitted the real and imaginary parts of the conductivity with the Drude model to extract its densities and mobilities. The dominant scattering mechanisms are discussed according to the temperature dependence of the extracted densities and mobilities.

Metamorphic growth of InAlAs/InGaAs MQW and InAs HEMT structures on GaAs

The large electron mobility at room temperature and the absence of Schottky barrier to metals make InAs two-dimensional electron gas (2DEG) a good candidate for SPIN-FET (FET—field-effect-transistor) applications. So far the growth was done either on the InAlAs epilayers compositionally matched to InP substrates, or on Sb-based compound semiconductors. Here we aim to grow InAs 2DEG on GaAs substrates by using a strain-relaxing buffer layer. We introduce In 0.4Al 0.6As glue layer to the metamorphic structure and investigate the physical properties of InAlAs/InGaAs multi-quantum-well (MQW) structures via X-ray diffraction, transmission electron microscopy, and photoluminescence. We find that the use of As dimer instead of tetramer and the choice of proper growth temperature are essential for successful growth. InAs-inserted-channel InAlAs/InGaAs inverted high-electron-mobility transistor (HEMT) structures show promising results for SPIN-FET application.

Local Hall effect in hybrid ferromagnetic/semiconductor devices

The authors have investigated the magnetoresistance of ferromagnet-semiconductor devices in an InAs two-dimensional electron gas system in which the magnetic field has a sinusoidal profile. The magnetoresistance of their device is large. The longitudinal resistance has an additional contribution which is odd in applied magnetic field. It becomes even negative at low temperature where the transport is ballistic. Based on the numerical analysis, they confirmed that their data can be explained in terms of the local Hall effect due to the profile of negative and positive field regions. This device may be useful for future spintronic applications.

Resistance modulation using amperian field in a two-dimensional electron gas system

In this study, we adopted a current line to produce a localized amperian field and demonstrate the resistance modulation of an InAs two-dimensional electron gas channel with the amperian field. The wafer structure is InAlAs(6 nm)/InGaAs(2.5 nm)/InAsSQW(2 nm)/InGaAs(13.5 nm)/InAlAs(20 nm). A 90-nm-thick and 3.2-μm-wide gold line, perpendicular to the mesa pattern, was adopted as a source of amperian field. Resistance changes of the mesa wherein the amperian gold line is positioned were measured with a sensing current of 20 μA. We obtained 10% of resistance change ( R( I Au=10 mA)− R( I Au=0))/ R( I Au=0) with the current density, J Au=3.47×10 6 A/cm 2, which is comparable with that of the MRAM digit line. The resistance change increases with the amperian current. This result seems to be related to weak localization in the InAs 2DEG system and shows the feasibility of an amperian device to control the resistance of the InAs 2DEG system.

Spin transport in an InAs based two-dimensional electron gas nanochannel

A spin device composed of two ferromagnetic electrodes and InAs two-dimensional electron gas was fabricated. Submicron spin channels were defined to enhance spin transport characteristics. Electrical transport measurement was performed to detect spin-polarized electrons. In potentiometric geometry a voltage change, ΔV=0.17mV, sensed by a ferromagnetic electrode was obtained at 5 and 77K. In the nonlocal method ΔV=0.057mV, which resulted from accumulated spin-polarized electrons, was obtained at 77K. The main reason for theses large signals is that the short and narrow spin channels increase the possibility for spin-polarized electrons to arrive at the spin detector.

Anticrossings of spin-split Landau levels in an InAs two-dimensional electron gas with spin-orbit coupling

We report tilted-field transport measurements in the quantum Hall regime in an $\mathrm{In}\mathrm{As}∕{\mathrm{In}}_{0.75}{\mathrm{Ga}}_{0.25}\mathrm{As}∕{\mathrm{In}}_{0.75}{\mathrm{Al}}_{0.25}\mathrm{As}$ quantum well. We observe anticrossings of spin-split Landau levels, which suggest a mixing of spin states at Landau level coincidence. We propose that the level repulsion is due to the presence of spin-orbit and of band-nonparabolicity terms which are relevant in narrow-gap systems. Furthermore, electron-electron interaction is significant in our structure, as demonstrated by the large values of the interaction-induced enhancement of the electronic $g$ factor.

Detection of spin-polarized electrons injected into a two-dimensional electron gas.

The spin-dependent mean-free path of electrons in a high-mobility InAs two-dimensional electron gas (2DEG) is measured. Ferromagnetic metal/insulator/2DEG junctions are fabricated on a common channel in a nonlocal geometry and used as spin injectors and detectors. For electrons in spin-orbit eigenstates at 4.5 K, lower bounds for the spin mean-free path and relaxation time are Lambda(S) > or = 4.6 microm and tau(s) > or = 3.8 ps, respectively. The temperature dependence is weak over the range 4.5<T<150 K.