Single InAs Quantum Research Articles

Edge transport in InAs and InAs/GaSb quantum wells

We investigate low-temperature transport through single InAs quantum wells and broken-gap InAs/GaSb double quantum wells. Nonlocal measurements in the regime beyond bulk pinch-off confirm the presence of edge conduction in InAs quantum wells. The edge resistivity of $1--2\mathrm{k}\mathrm{\ensuremath{\Omega}}/\ensuremath{\mu}\mathrm{m}$ is of the same order of magnitude as edge resistivities measured in the InAs/GaSb double quantum well system. Measurements in a tilted magnetic field suggests an anisotropy of the conducting regions at the edges with a larger extent in the plane of the sample than normal to it. Finger-gate samples on both material systems shine light on the length dependence of the edge resistance with the intent to unravel the nature of edge conduction in InAs/GaSb coupled quantum wells.

Electrical switching of photoluminescence of single site‐controlled InAs quantum dots

Voltage-controlled photoluminescence (PL) switching is demonstrated for single site-controlled InAs quantum dots (QDs) embedded in Schottky–i–n diodes grown by molecular beam epitaxy on nanoimprint lithography patterned GaAs templates. The PL emission was quenched by applying a voltage over the diode structure due to the increased tunnelling rate of charge carriers out of the QDs.

Coherent photocurrent spectroscopy of single InP-based quantum dots in the telecom band at 1.5 µm

In this work we study the resonant and coherent properties of single InP-based InAs quantum dots, which show an optical emission in the telecom C-band and L-band. High-resolution resonant photocurrent spectroscopy on p–i–n devices reveals narrow linewidths and fully resolved fine structure splittings. We observe Lorentzian line shapes, which allow for the extraction of dephasing times as a function of the applied bias voltage. Coherent ps laser excitation results in pronounced Rabi rotations with increasing pulse area. For π-pulse excitation, we obtain more than 93 % of the theoretically expected photocurrent amplitude. Our results also demonstrate that such state-of-the-art InP-based quantum dots for the telecom band exhibit promising key parameters comparable to well-established InAs/GaAs counterparts.

Superconducting transport in single and parallel double InAs quantum dot Josephson junctions with Nb-based superconducting electrodes

We report conductance and supercurrent measurements for InAs single and parallel double quantum dot Josephson junctions contacted with Nb or NbTiN superconducting electrodes. Large superconducting gap energy, high critical field, and large switching current are observed, all reflecting the features of Nb-based electrodes. For the parallel double dots, we observe an enhanced supercurrent when both dots are on resonance, which may reflect split Cooper pair tunneling.

Spin injection in indium arsenide

In a two dimensional electron system (2DES), coherent spin precession of a ballistic spin polarized current, controlled by the Rashba spin orbit interaction, is a remarkable phenomenon that’s been observed only recently. Datta and Das predicted this precession would manifest as an oscillation in the source-drain conductance of the channel in a spin-injected field effect transistor (Spin FET). The indium arsenide single quantum well materials system has proven to be ideal for experimental confirmation. The 2DES carriers have high mobility, low sheet resistance, and high spin orbit interaction. Techniques for electrical injection and detection of spin polarized carriers were developed over the last two decades. Adapting the proposed Spin FET to the Johnson-Silsbee nonlocal geometry was a key to the first experimental demonstration of gate voltage controlled coherent spin precession. More recently, a new technique measured the oscillation as a function of channel length. This article gives an overview of the experimental phenomenology of the spin injection technique. We then review details of the application of the technique to InAs single quantum well (SQW) devices. The effective magnetic field associated with Rashba spin-orbit coupling is described, and a heuristic model of coherent spin precession is presented. The two successful empirical demonstrations of the Datta Das conductance oscillation are then described and discussed.

Estimate of the plasmonic enhancement for the erbium arsenide (metallic nanoparticle): indium arsenide (quantum dot) system

The epitaxially grown system comprising semimetallic ErAs (erbium arsenide) nanoparticles (MNP) and InAs (indium arsenide) single quantum dots (QDs) hold great promise for plasmonic applications. The distinguishing feature of the ErAs-InAs hybrid is that the hybrid separation can be varied and accurately controlled in the molecular-beam epitaxial growth. In order to assess the potential of this system for plasmonic applications, this paper aims to estimate and optimize the expected magnitude of the plasmonic enhancement. We use the Sun–Khurghin theory to estimate the expected absorption and photoluminescence (PL) enhancement. Using a carefully selected set of materials’ parameters as input, we predict about 500-fold plasmonic PL enhancement for this system under resonant conditions.

Terahertz Photon-Assisted Tunneling in InAs Quantum Dots

We have investigated electron transport in a single self-assembled InAs quantum dot (QD) coupled to nanogap metal electrodes under terahertz (THz) radiation. The fabricated QD samples operated as single electron transistors in a few electron regime, exhibiting clear shell structures. Under the THz radiation, in addition to the original Coulomb oscillation peaks, new side-peaks showed up. The dependence of the new side-peak current on the THz power follows the prediction of the photon-assisted tunneling (PAT) theory. Moreover, two types of PAT processes were observed in the THz range; the ground state resonance and the photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, a very high coupling efficiency between the THz waves and the QDs was realized in our system and we observed multi-photon absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade. This high coupling efficiency between THz wave and electrons in QDs opens a way to the manipulation of single electron charge/spin states in the THz frequency range.

Optical transition of the charged excitons in InAs single quantum dots

Quantum dot (QD) samples studied in the experiment are grown by molecular-beam epitaxy on semi-insulating GaAs substrates. The photoluminescences (PLs) of the excitons in a single QD are measured at 5 K. The PL spectra of the excitons, biexcitons and charged excitons are identified by measuring and analyzing both PL peaks of the circular and linear polarization and power-dependent PL properties. The charged exciton emissions can be tuned by applying a bias voltage, i.e., negatively charged excitons are changed into positively charged excitons by changing the voltage from 1.0 to -1.0 V. It is shown that the electron-spin will slowly relax compared with that of the hole when they relax from wetting layer into the QD.

Electrical and structural characterization of a single GaSb∕InAs∕GaSb quantum well grown on GaAs using interface misfit dislocations

Interface misfit formation has been used for the growth of high mobility GaSb∕InAs single quantum wells (SQW) formed on GaAs substrates. The SQW structure was topped with 800Å GaSb, followed by 100Å GaSb:Si (5×108cm−3), 10nm GaSb, 10nm InAs, and finally 250nm GaSb on a GaAs substrate. The structural quality was examined using high resolution x-ray diffraction and transmission electron microscopy. Reciprocal space mapping indicated that the GaSb was completely relaxed. A high resolution x-ray rocking curve showed good agreement between the proposed structure and the simulation, assuming that all layers were relaxed to the GaSb lattice, and clearly showed interference fringing from individual layers. Atomic force microscopy showed the film appeared textured, and that the final growth occurred by step flow growth. The observed peak-to-peak roughness was 7nm over a 100×100μm2 square area. Plane view transmission electron microscopy analysis showed a nearly regular array of Lomer dislocations responsible for the relaxation of the strain in the two films. The mobility of the SQW was measured as a function of temperature. The room temperature mobility was 13900cm2∕Vs and a peak mobility of 25200cm2∕Vs was observed at ∼60K, and remained nearly constant at lower temperatures. The sheet concentration was 2.3×1012∕cm2 at room temperature, and dropped to 7.35×1011∕cm2 at 60K. Magnetoresistance measurements revealed the presence of Shubnikov–de Haas oscillations at 1.2K; indicating the presence of a good quality two-dimensional electron gas (2DEG). Further support for the high quality of the 2DEG is provided by the observation of sharply defined quantum Hall plateaus.

DESIGN OF SEMICONDUCTOR HETEROSTRUCTURES VIA INVERSE QUANTUM SCATTERING

The use of one-dimensional inverse quantum scattering theory for the design of semiconductor heterostructures is reviewed within the contexts of (i) the single-band constant effective mass model (ii) the single-band variable effective mass model and (iii) the multi-band pseudopotential model. As a fourth application a different inverse method is developed for the multi-band k · p model. This method is based on the one-dimensional coupled-channel Marchenko method. Preliminary numerical results are presented for a two-band k · p model of an InAs / GaSb / InAs single quantum well in which the strength of the interband coupling is approximately constant.

Quantitative scanning capacitance microscopy on single subsurface InAs quantum dots

Quantitative scanning capacitance microscopy on InAs quantum dots requires low modulation frequencies and complete darkness. Using a modified feedback method on our atomic force microscope, which allows to switch off the laser temporarily while the feedback loop keeps running, images of buried InAs self assembled quantum dots were acquired. The dots are clearly visible as bright areas in a contrast-rich capacitance landscape, which we attribute to thickness variations of the InAs wetting layer. Even at room temperature, spectroscopic data recorded at on-dot and off-dot positions exhibit a completely different behavior and show evidence of quantized states inside the dots.

Near-field optical study of highly dense laterally coupled InAs single quantum dots

The authors report on high-resolution photoluminescence (PL) spectroscopy and microscopy study of laterally coupled, self-assembled InAs∕GaAs quantum dots (QDs) by using the low-temperature near-field scanning optical microscope. In particular, they have observed characteristic doublet PL spectra, which can be indirectly but unambiguously attributed to two neighboring coupled QDs, as indicated by the identical spatial images of the PL intensity. This work may open a unique and systematic way to study the lateral coupling of single QDs for qualitative nano-optical analysis and characterization.

Spin injection into a single self-assembled quantum dot in a p-i-n II-VI/III-V structure

The authors report on efficient spin injection from a n-Zn0.96Mn0.04Se layer into an InAs single quantum dot (SQD) in a p-i-n diode structure using magnetomicrophotoluminescence experiments. A circular polarization degree of up to 60% at B=5T is obtained from the InAs single dot emission while no polarized light is observed in a structure with no electrical doping of any of its layers. A pronounced dependence of the spin injection efficiency on the external bias is found resulting in a strong decrease of the SQD circular polarization degree when the biasing is applied in the forward direction. The experiments emphasize the impact of excess energy and current flow on the spin injection efficiency in a SQD.

Imaging of subbands in InAs/GaSb double quantum wells by low-temperature scanning tunneling spectroscopy

The spatial distribution of the electron local density of states (LDOS) in InAs/GaSb double quantum wells (DQWs) was investigated by low-temperature scanning tunneling spectroscopy on cleaved surfaces. For DQW with a thick central barrier, clear standing wave patterns corresponding to subbands confined to each InAs single quantum well appeared in the spatial variation of LDOS spectra. In contrast, for the DQW with a thin central barrier, the standing wave patterns extended over both quantum wells. The deviation of the pattern arising from the asymmetry due to a slight difference of the well thickness appeared clearly. The observed spectra are well explained by the calculated LDOS taken to be the sum of LDOS contributed from all energetically accessible subbands.

Mid-infrared luminescence of an InNAsSb∕InAs single quantum well grown by molecular beam epitaxy

The authors report the growth and characterization of a dilute nitride, InNAsSb∕InAs, by solid source molecular beam epitaxy. Optimizing growth conditions for nitrogen incorporation has resulted in high-quality InNAsSb epilayers without any structural degradation, as confirmed by high-resolution x-ray diffraction. Optical properties were investigated by temperature dependent and excitation power dependent photoluminescence. The authors obtained mid-infrared luminescence around 4μm at low temperature, which reveals strong carrier localization behavior at low temperature, induced by nitrogen and antimony interaction. The band alignment of InNAsSb∕InAs can be type I instead of the conventional type II found for InAsSb∕InAs. A conduction band offset, Ec, of ∼102meV was obtained.

Optical study of single InAs on In0.12Ga0.88As self-assembled quantum dots: biexciton binding energy dependence on the dots size

Single self-assembled InAs quantum dots embedded in a In0.12Ga0.88As quantum well and emitting in the near infrared have been optically investigated. The dependence on the excitation power of the single quantum dot photoluminescence has been used to identify the emission of the biexciton complex. The biexciton binding energy, which has been measured for a dozen dots, increases with increasing exciton transition energy for the dot sizes investigated in the present work, as a consequence of stronger confinement in a smaller quantum dot. The obtained data is compared with experimental results available in the literature for InAs quantum dots.

High-resolution near-field spectroscopy of InAs single quantum dots at 70 K

We have performed high-resolution near-field laser spectroscopy and microscopy of single InAs/GaAs quantum dots near liquid nitrogen temperature. By simultaneous use of 100-nm shadow mask apertures and a 30-nm-aperture fiber probe, we have achieved photoluminescence (PL) resolution comparable to that available at liquid helium temperature. In particular, we have obtained a PL linewidth of 0.6 meV for single exciton states, and observed biexciton states with quadratic power dependence.

A novel InAs quantum wire system

We report here a new technique in patterning high quality low-dimensional electrons in single InAs quantum wells. Grown by molecular beam epitaxy, the single InAs quantum well is sandwiched between AlSb barriers and capped by a thin layer of InAs. When the InAs cap layer is patterned by electron beam lithography and selectively removed, electrons are induced in the InAs quantum well below due to the different surface Fermi level pinning voltages on the exposed AlSb layer from the InAs cap. One-dimensional quantum wires can thus be conveniently defined by lithography and nm-shallow etching. We demonstrate that these one-dimensional electrons posses a long elastic mean free path (>1.4 μm) and a long coherence length (>3 μm) at 2 K .

Acceptor-induced threshold energy for the optical charging of InAs single quantum dots

We study the photoluminescence of single InAs/GaAs self-assembled quantum dots for a range of excitation powers, excitation energies and sample temperatures 4 K30 K), this effect vanishes due to the essential decrease of the steady-state free electron concentration in the GaAs barrier as a result of thermally excited free holes appearing in the GaAs barrier valence band which provides an effective recombination channel for the free electrons. These experimental observations could be used as an effective tool to create and study charged excitons in quantum dots.

Correlation spectroscopy of excitons and biexcitons on a single quantum dot

We report the observation of antibunched emission from single self-assembled InAs quantum dots under various conditions. We have measured the correlation function of the photon emission pertaining to single emission lines from single quantum dots under continuous and pulsed laser excitation, as well as under continuous white light excitation. The measurements were performed under different excitation intensities at liquid helium temperatures on two samples with distinct structures. At higher temperatures (30 K), an antibunching dip was still observed. We have also observed antibunching on a second emission line in the quantum dot spectrum, attributed to the biexciton, demonstrating the possibility of generating photon pairs with a single quantum dot. Polarization correlations on the biexciton and exciton line were also measured in an attempt to generate entangled photon pairs.