Holes In Quantum Dots Research Articles

High-performance quantum dot light-emitting diodes with hybrid hole transport layer via doping engineering.

Here, we report on the hybrid hole transport materials 4,4'-bis-(carbazole-9-yl)biphenyl (CBP) or poly-N-vinylcarbazole (PVK) doped into poly(4-butyl-phenyl-diphenyl-amine) (Poly-TPD) as the hybrid hole transport layer (HTL) to tailor the energy band alignment between hole injection layer (HIL) and quantum dot (QD) light emitting layer in order to realize efficient quantum dot light emitting diodes (QLEDs) in all solution-processed fabrication. Compared to the pristine Poly-TPD based device, it is found that the electroluminescence (EL) performance of QLEDs can be significantly improved by 1.5 fold via addition of CBP into Poly-TPD, which can be attributed to the lowered highest occupied molecular orbital (HOMO) level of Poly-TPD to reduce the energy barrier between HTL and valance band (VB) of QDs. Thus, after doping small molecules into polymer under optimized proportion (Poly-TPD:CBP = 2:1 by weight), the hole transport rate can be balanced, facilitating the carrier injection from HTL to QDs and enhancing the efficiency of QLEDs. As a result, a maximum luminance, a maximum current efficiency and a maximum power efficiency of 7600 cd/m2, 5.41 cd/A and 4.25 lm/W can be obtained based on this variety of hybrid HTL employed QLEDs.

Highly tuneable hole quantum dots in Ge-Si core-shell nanowires

We define single quantum dots of lengths varying from 60 nm up to nearly half a micron in Ge-Si core-shell nanowires. The charging energies scale inversely with the quantum dot length between 18 and 4 meV. Subsequently, we split up a long dot into a double quantum dot with a separate control over the tunnel couplings and the electrochemical potential of each dot. Both single and double quantum dot configurations prove to be very stable and show excellent control over the electrostatic environment of the dots, making this system a highly versatile platform for spin-based quantum computing.

Double-layer-gate architecture for few-hole GaAs quantum dots

We report the fabrication of single and double hole quantum dots using a double-layer-gate design on an undoped accumulation mode /GaAs heterostructure. Electrical transport measurements of a single quantum dot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantum dot with tunable inter-dot coupling.

Magic numbers and persistent current oscillations in electron–hole quantum dots

The system of Kohn–Sham equations is solved self-consistently for the two-dimensional, spatially separated electrons and holes. We find the series of magic numbers for the total angular momentum of the electrons and holes in a strong magnetic field. The change of the angular momentum of the charge carriers is shown to lead to the persistent current oscillations.

Electrical control of quantum-dot fine-structure splitting for high-fidelity hole spin initialization

We demonstrate electrical control of the neutral exciton fine-structure splitting in a single InAs/GaAs self-assembled quantum dot by significantly reducing the splitting to near zero through the application of a vertical electric field in the fast electron tunneling regime. This is verified by performing high-resolution photocurrent spectroscopy of the two fine-structure split exciton eigenstates as a function of reverse bias voltage. Using the qubit initialization scheme for a quantum-dot hole spin based on rapid electric-field ionization of a spin-polarized exciton, our results suggest a practical approach towards achieving qubit initialization with near-unity fidelity in the absence of magnetic fields.

High-fidelity initialization of long-lived quantum dot hole spin qubits by reduced fine-structure splitting

We demonstrate an on-demand hole spin qubit initialization scheme meeting four key requirements of quantum information processing: fast initialization (1/e ~ 100 ps), high fidelity (F > 99%), long qubit lifetime $(2T_{1}>T_{2}^{*}\simeq10\:\mathrm{ns})$, and compatibility with optical coherent control schemes. This is achieved by rapidly ionizing an exciton in an InGaAs quantum dot with very low fine-structure splitting at zero magnetic field. Furthermore, we show that the hole spin fidelity of an arbitrary quantum dot can be increased by optical Stark effect tuning of the fine-structure splitting close to zero.

Electron-Hole Confinement Symmetry in Silicon Quantum Dots.

We report electrical transport measurements on a gate-defined ambipolar quantum dot in intrinsic silicon. The ambipolarity allows its operation as either an electron or a hole quantum dot of which we change the dot occupancy by 20 charge carriers in each regime. Electron-hole confinement symmetry is evidenced by the extracted gate capacitances and charging energies. The results demonstrate that ambipolar quantum dots offer great potential for spin-based quantum information processing, since confined electrons and holes can be compared and manipulated in the same crystalline environment.

Single-charge transport in ambipolar silicon nanoscale field-effect transistors

We report single-charge transport in ambipolar nanoscale MOSFETs, electrostatically defined in near-intrinsic silicon. We use the ambipolarity to demonstrate the confinement of either a few electrons or a few holes in exactly the same crystalline environment underneath a gate electrode. We find similar electron and hole quantum dot properties while the mobilities differ quantitatively like in microscale devices. The understanding and control of individual electrons and holes are essential for spin-based quantum information processing.

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 $2.9\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ 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.

Auger recombination in In(Ga)Sb/InAs quantum dots

We report on the epitaxial formation of type II In0.5Ga0.5Sb/InAs and InSb/InAs quantum dot ensembles using metal organic vapor phase epitaxy. Employing scanning tunneling spectroscopy, we determine spatial quantum dot dimensions smaller than the de Broglie wavelength of InGaSb, which strongly indicates a three dimensional hole confinement. Photoluminescence spectroscopy at low temperatures yields an enhanced radiative recombination in the mid-infrared regime at energies of 170–200 meV. This luminescence displays a strong excitation power dependence with a blueshift indicating a filling of excited quantum dot hole states. Furthermore, a rate equation model is used to extract the Auger recombination coefficient from the power dependent intensity at 77 K yielding values of 1.35 × 10−28 cm6/s for In0.5Ga0.5Sb/InAs quantum dots and 1.47 × 10−27 cm6/s for InSb/InAs quantum dots, which is about one order of magnitude lower as previously obtained values for InGaSb superlattices.

Effect of geometry and composition on the intraband transitions of holes in quantum dots

The effect of shape and size anisotropy on unipolar intraband transitions of holes in quantum dots (QDs) is studied. The optical matrix elements are calculated for transitions of holes in valence band. To get the optical matrix elements, energy eigenvalues and eigenvectors are calculated using 4 × 4 Luttinger Hamiltonian in the effective mass approximation. The formulation is applied to InGaAs/GaAs QD with parabolic confinement potential in xy-plane. The optical matrix elements for intraband hole transitions are calculated for x and y polarised light. The transitions are considered from ground state to other excited states. The effect of In concentration on optical matrix elements is also investigated. It is important to note that the transitions of holes are governed by the character of initial and final states for different light polarisations that give specific transition selection rules. It is found that the polarisation is strongly dependent on the in-plane anisotropy of the QDs.

Ultrafast high-fidelity initialization of a quantum-dot spin qubit without magnetic fields

We demonstrate the initialization of a single quantum-dot hole spin with high fidelity (lower bound $g97%$), on picosecond time scales, and without the need for magnetic fields. Using the initialization scheme based on rapid electric-field ionization of a resonantly excited exciton, this is achieved by employing a self-assembled quantum dot with a low conduction-to-valence band offset ratio, allowing control of the relative electron and hole tunneling rates over three orders of magnitude. This large difference in tunneling rates could permit spin-storage efficiencies $g99.5%$ by fast-switching to a low electric-field condition. Our results may provide a practical route towards ultrafast high-fidelity initialization of individual quantum-dot hole spins for the implementation of quantum error correction in a scalable spin-based quantum computer.

All-optical implementation of a dynamic decoupling protocol for hole spins in (In,Ga)As quantum dots

The first experiment to show dynamic decoupling of quantum dot hole spins by ultrashort laser pulses in the optical rather than microwave regime. By shortening the time delay between rotating pulses in spin echo experiments by a factor of two, the hole spin coherence times can be doubled. The technique used may lead to more complex dynamic decoupling schemes that are optimized for keeping the coherence of a quantum bit alive.

Effects of the thickness of NiO hole transport layer on the performance of all-inorganic quantum dot light emitting diode

All-inorganic quantum dot light emitting diodes (QLEDs) have recently gained great attention owing to their high stability under oxygenic, humid environment and higher operating currents. In this work, we fabricated all-inorganic CdSe/ZnS core-shell QLEDs composed of ITO/NiO/QDs/ZnO/Al, in which NiO and ZnO thin film deposited via all-solution method were employed as hole and electron transport layer, respectively. To achieve high light emitting efficiency, the balance transport between electrons and holes play a key role. In this work, the effects of the thickness of NiO film on the performance of QLEDs were explored experimentally in details. NiO layers with various thicknesses were prepared with different rotation speeds. Experimental results showed that thinner NiO layer deposited at higher rotation speed had higher transmittance and larger band gap. Four typical NiO thickness based QLEDs were fabricated to optimize the hole transport layer. Thinner NiO layer based device performs bright emission with high current injection, which is ascribed to the reduced barrier height between hole transport layer and quantum dot.

Determining the stability and activation energy of Si acceptors in AlGaAs using quantum interference in an open hole quantum dot

We fabricated an etched hole quantum dot in a Si-doped (311)A AlGaAs/GaAs heterostructure to study disorder effects via magnetoconductance fluctuations (MCF) at millikelvin temperatures. Recent experiments in electron quantum dots have shown that the MCF are sensitive to the disorder potential created by remote ionized impurities. We utilize this to study the temporal/thermal stability of Si acceptors in $p$-type AlGaAs/GaAs heterostructures. In particular, we use a surface gate to cause charge migration between Si acceptor sites at $T=40$ mK, and detect the ensuing changes in the disorder potential using the MCF. We show that Si acceptors are metastable at $T=40$ mK and that raising the device to a temperature $T=4.2$ K and returning to $T=40$ mK is sufficient to produce complete decorrelation of the MCF. The same decorrelation occurs at $T\ensuremath{\sim}165$ K for electron quantum dots; by comparing with the known trap energy for Si DX centers, we estimate that the shallow acceptor traps in our heterostructures have an activation energy ${E}_{A}\ensuremath{\sim}3$ meV. Our method can be used to study charge noise and dopant stability towards optimization of semiconductor materials and devices.

Scaling of the Kondo zero-bias peak in a hole quantum dot at finite temperatures

We have measured the zero bias peak in differential conductance in a hole quantum dot. We have scaled the experimental data with applied bias and compared to real time renormalization group calculations of the differential conductance as a function of source-drain bias in the limit of zero temperature and at finite temperatures. The experimental data show deviations from the T=0 calculations at low bias, but are in very good agreement with the finite T calculations. The Kondo temperature T_K extracted from the data using T=0 calculations, and from the peak width at 2/3 maximum, is significantly higher than that obtained from finite T calculations.

Proposed Quenching of Phonon-Induced Processes in Photoexcited Quantum Dots due to Electron-Hole Asymmetries

Differences in the confinement of electrons and holes in quantum dots are shown to profoundly impact the magnitude of scattering with acoustic phonons. Using an extensive model that includes the non-Markovian nature of the phonon reservoir, we show how the effect may be addressed by photoluminescence excitation spectroscopy of a single quantum dot. We also investigate the implications for cavity QED, i.e., a coupled quantum dot-cavity system, and demonstrate that the phonon scattering may be strongly quenched. The quenching is explained by a balancing between the deformation potential interaction strengths and the carrier confinement and depends on the quantum dot shape. Numerical examples suggest a route towards engineering the phonon scattering.

Electrical control of single hole spins in nanowire quantum dots

The development of viable quantum computation devices will require the ability to preserve the coherence of quantum bits (qubits). Single electron spins in semiconductor quantum dots are a versatile platform for quantum information processing, but controlling decoherence remains a considerable challenge. Hole spins in III-V semiconductors have unique properties, such as a strong spin-orbit interaction and weak coupling to nuclear spins, and therefore, have the potential for enhanced spin control and longer coherence times. A weaker hyperfine interaction has previously been reported in self-assembled quantum dots using quantum optics techniques, but the development of hole-spin-based electronic devices in conventional III-V heterostructures has been limited by fabrication challenges. Here, we show that gate-tunable hole quantum dots can be formed in InSb nanowires and used to demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tunable between hole and electron quantum dots, which allows the hyperfine interaction strengths, g-factors and spin blockade anisotropies to be compared directly in the two regimes.

All-electric qubit control in heavy hole quantum dots via non-Abelian geometric phases

We demonstrate how non-Abelian geometric phases can be used to universally process a spin qubit in heavy hole quantum dots in the absence of magnetic fields. A time dependent electric quadrupole field is used to perform any desired single qubit operation by virtue of non-Abelian holonomy. During the proposed operations, the degeneracy of the time dependent two level system representing the qubit is not split. Since time reversal symmetry is preserved and hyperfine coupling is known to be weak in spin qubits based on heavy holes, we expect very long coherence times in the proposed setup.

Polarized light from excitonic recombination in selectively etched GaN/AlN quantum dot ensembles on Si(111)

Multiple layers of GaN/AlN quantum dot (QD) ensembles were grown by the Stranski–Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperature, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing the vertically stacked QDs leads to an additional biaxial tensile stress of 20–30 kbar in the III-nitride film. We have selectively modified the thermal stress in the QD layers by etching a cross-hatched pattern into the as-grown sample using inductively coupled Cl2/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width from ∼2 to 10 µm and orientation along [11−20] and [1−100] can create regions of in-plane uniaxial stress that enable a selective and local control of the polarized luminescence from ensembles of QDs which were probed with cathodoluminescence. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing e–h pair excitation for the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼46 K). A theoretical modelling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the and effective-mass methods for calculations of the e–h wavefunctions and electron and hole quasi-Fermi levels for varying levels of state filling. We attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during e–h pair excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing e–h pair excitation density at T = 300 K.