Planar Quantum Dots Research Articles

Demonstration of gating action in atomically controlled Si:P nanodots defined by scanning probe microscopy

We study the low temperature electrical characteristics of planar, highly phosphorus-doped nanodots. The dots are defined by lithographically patterning an atomically flat, hydrogenated Si(1 0 0):H surface using a scanning-tunneling-microscope (STM), phosphorus δ -doping and low temperature molecular beam epitaxy in an ultra-high vacuum environment. Ohmic contacts and a surface gate structure are aligned ex-situ using electron beam lithography. We present electrical transport measurements of a 25 × 21 nm 2 Si:P nanodot at 4 K containing about 1000 P atoms. We find significant gating action within a gate range of - 2 to 7 V. From the stability diagram, we observe a large conductance gap and the existence of electron resonances near threshold which can be modulated with the top gate. Our results show promise for the fabrication of planar quantum dots using this technique.

Current–voltage and noise characteristics of reverse-biased Au/n-GaAs Schottky diodes with embedded InAs quantum dots

Schottky contacts on n-type GaAs with embedded InAs quantum dots (QDs) were studied by current–voltage (I–V) and low-frequency noise measurements. For comparison, diodes not containing QDs were investigated as reference devices. A wide distribution of the ideality factor was observed, correlated with the level of the leakage current. Reverse I–V characteristics on the logarithmic scale indicate that the space-charge limited current dominates the carrier transport in these diodes. In all diodes, the reverse current noise spectra show 1/f behaviour, attributed to traps uniformly distributed in energy within the band-gap of the GaAs capping layer. Depth profiling measurements of the 1/f noise power spectral density demonstrate the impact of the QDs on these traps. In diodes containing QDs, in addition to the 1/f noise, a generation–recombination noise is found originating from a deep trap level localized in the vicinity of the QD plane.

Magnetic field induced two-channel Kondo effect in multiple quantum dots

We study the possibility of observing the two-channel Kondo physics in multiple planar quantum dot heterostructures in the presence of a strong in-plane magnetic field. We show that a fine tuning of the coupling parameters of the system and an external magnetic field may stabilize the two-channel Kondo critical point. We make predictions for the behavior of the scaling of the differential conductance in the vicinity of the quantum critical point, as a function of magnetic field, temperature, and source-drain potential.

Influence of lateral electric fields on multiexcitonic transitions and fine structure of single quantum dots

The quantum-confined Stark effect of excitonic states in self-assembled (In,Ga)As∕GaAs quantum dots was studied by microphotoluminescence spectroscopy. A similar Stark-shift behavior for excitons, biexcitons, and a charged state was observed. Investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane. Values of the polarizability could be derived for all the investigated states. Furthermore, high-resolution Fabry-Pérot interferometry was applied to resolve the excitonic fine structure splitting and to investigate the influence of a lateral electric field. For a single dot, the splitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs.

Optical properties of stacked InAs self-organized quantum dots on InP (3 1 1)B

We have investigated the structural and optical properties of multi-stacked self-organized InAs quantum dot (QD) structures on InP (3 1 1)B substrates grown by molecular beam epitaxy. The reciprocal space mapping measured for asymmetric (4 0 0) reflections in high resolution X-ray diffraction revealed that satellite peaks originating from a periodic structure were not only observed along the growth direction, but also along [ 2 ¯ 3 3 ] direction within each QD plane. The mean in-plane spacing of neighboring QDs was 97 nm, which was in good agreement with direct measurement of 100 nm by using atomic force microscope. A superior three-dimensional QD superlattice with a total density of 10 12 cm −2 was achieved by our growth technique. We also observed a strong photoluminescence emission at 1.55 μm with a narrow linewidth of 56.07 meV at room temperature.

Increase of charge-carrier redistribution efficiency in a laterally organized superlattice of coupled quantum dots

We report the observation of enhanced charge-carrier redistribution in laterally organized and coupled InAs/InP quantum dots (QDs). We show that a periodic organization appears in the QD plane for a high in-plane QD density (QDD). This organization enhances the lateral coupling between the dots, which is evidenced by photoluminescence and magnetophotoluminescence experiments. Electronic inter-QD lateral coupling results in an improved charge-carrier distribution at low temperature, as shown by electroluminescence on high QDD QD lasers. We conclude that the inter-QD tunneling occurs via the tunneling of excited states through the wetting layer, and discuss the prospects of using coupled QDs for improving the quantum efficiency and dynamical properties of QD lasers.

Stark spectroscopy of Coulomb interactions in individual InAs/GaAs self‐assembled quantum dots

AbstractWe present results of polarization‐resolved micro‐photoluminescence (μ‐PL) experiments on self‐organized InAs/GaAs (QDs). We have investigated how Coulomb interactions are modified by application of an electric field. We observed systematic variation of bright exciton splitting for different electric field configurations (applied in the quantum dot plane or along the sample growth axis). The results show also dependence of direct Coulomb interactions of carriers inside the dots, manifested by changes of PL line position of charged excitons (X+ and X‐) with respect to the neutral excitonic line. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room-temperature InAs/InP Quantum Dots laser operation based on heterogeneous “25 D” Photonic Crystal

The authors report on the design, fabrication and operation of heterogeneous and compact "2.5 D" Photonic Crystal microlaser with a single plane of InAs quantum dots as gain medium. The high quality factor photonic structures are tailored for vertical emission. The devices consist of a top two-dimensional InP Photonic Crystal Slab, a SiO(2) bonding layer, and a bottom high index contrast Si/SiO(2) Bragg mirror deposited on a Si wafer. Despite the fact that no more than about 5% of the quantum dots distribution effectively contribute to the modal gain, room-temperature lasing operation, around 1.5 microm, was achieved by photopumping. A low effective threshold, on the order of 350 microW, and a spontaneous emission factor, over 0.13, could be deduced from experiments.

Intra-valence band transitions in self-assembled InAs∕GaAs quantum dots studied using photocurrent spectroscopy

We present a polarization-dependent study of the intra-valence band photocurrent signal in p-type self-assembled InAs∕GaAs quantum dots (QDs). The observed transitions are found to be strongly polarized in the quantum dot plane and associated with hole transitions from the dot states to the wetting layer states. Photocurrent spectra from p-doped QD samples are compared with photocurrent spectra from identically grown n-type QD samples. At 10K, the magnitude of the photocurrent signal is found to be smaller for p-type QD samples, compared with n-type QD samples. However, the rapid growth of the signal with increasing temperature due to thermal activation of holes from the light hole wetting layer states results in a comparable photoresponse to the n-type QDs at 50K.

Intersublevel polaron laser with InAs∕GaAs self-assembled quantum dots

We propose a three-level scheme to achieve intersublevel population inversion, optical gain, and intersublevel lasing effect in n-doped InAs∕GaAs self-assembled quantum dots under optical pumping. The proposed Ruby-type laser scheme uses the natural splitting of the s-p polaron intersublevel transitions around 25μm wavelength. The relaxation time engineering, which leads to optical gain in the system, relies (i) on the slow polaron relaxation from the P− state down to the S ground state, governed by the specific strong coupling regime for the electron-phonon Fröhlich interaction and (ii) on the fast nonradiative relaxation of the polaron between the P+ and P− levels through the irreversible emission of acoustic phonons. TE-polarized optical gain up to 330cm−1 is calculated for 80 quantum dot planes in an in-plane monomode waveguide geometry and a laser pump intensity threshold as low as 930W∕cm2, two orders of magnitude smaller than for quantum wells, is predicted.

Photocurrent in self-organized InAs quantum dots in 1.3 μm InAs/InGaAs/GaAs semiconductor laser heterostructures

The photocurrent spectra of InAs/InGaAs/GaAs laser heterostructures with self-organized InAs quantum dots (QDs) are studied. The study has been performed with a sample illuminated perpendicularly or in parallel to the QD plane. The optical density of QDs, maximum gain in the laser structure, radiative recombination time, and polarization characteristics of absorption have been determined from the experiment. The photocurrent spectra were correlated with specific features of lasing. The absorption spectrum is interpreted as a superposition of optical transitions between states of the discrete energy spectrum, those between the states of the continuous spectrum, and “mixed” transitions.

Capacitance–voltage profile characteristics of Schottky barrier structure with InAs quantum dots grown on InAlAs/InP(001)

Capacitance–voltage, C( V) studies have been carried out on Schottky barrier structure containing a sheet of self-organized InAs quantum dots (QDs) grown on InAlAs lattice matched to InP in order to deduce the electrical properties of the QDs. Three electron levels have been detected in n-type material, and were attributed to the s ground, the p excited, and the d excited states. Some parameters of the structure, such as the position of the InAs QD plane, the electron concentration in the QDs and an approximate QD height were deduced from the C( V) profile analysis. These results are in good agreement with the transmission electron microscopy (TEM) study realized on the structure.

Nonlinear Elasticity in Wurtzite GaN/AlN Planar Superlattices and Quantum Dots

The elastic stiffness tensors for wurtzite GaN and AlN show a significant hydrostatic pressure dependence, which is the evidence of nonlinear elasticity of these compounds. We have examined how pressure dependence of elastic constants for wurtzite nitrides influences elastic and piezoelectric properties of GaN/AlN planar superlattices and quantum dots. Particularly, we show that built-in hydrostatic pressure, present in both quantum wells of the GaN/AlN superlattices and GaN/AlN quantum dots, increases significantly by 0.3–0.7 GPa when nonlinear elasticity is used. Consequently, the compressive volumetric strain in quantum wells and quantum dots decreases in comparison to the case of the linear elastic theory. However, the z-component of the built-in electric field in the quantum wells and quantum dots increases considerably when nonlinear elasticity is taken into account. Both effects, i.e., a decrease in the compressive volumetric strain as well as an increase in the built-in electric field, decrease the band-to-band transition energies in the quantum wells and quantum dots.

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage ( C– V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.

Deep levels induced by InAs/GaAs quantum dots

Self-organised InAs/GaAs quantum dots (QDs) were formed by molecular beam epitaxy using the Stranski–Krastanov growth mode. Deep-level transient spectroscopy as well as secondary ion mass spectrometry have been used to characterise structures containing the QDs. DLTS depth profiling procedures indicate that deep level-related defects are localised in GaAs in the vicinity of the QD plane. For the first time, we report the presence of a deep level-related trap with an extremely high thermal activation energy of E c − 1.03 eV. An electron trap at E c − 0.78 eV can be identified as the well-known level related to the EL2 family. We conclude that a third trap revealed at E c −0.57 eV is the familiar PL killer related to the intrinsic point defect-oxygen complex. The latter is confirmed by results of the SIMS study, which indicates that the amount of oxygen accumulated at the InAs/GaAs heterointerface is increased. This paper demonstrates that the EL2 and oxygen-related deep-level centers occur by the presence of InAs/GaAs QDs. We present the hypothesis that deep states could be a factor limiting the efficiency of QD-based devices.

Spin relaxation in quantum dots with random spin-orbit coupling

We investigate the longitudinal spin relaxation arising due to spin-flip transitions accompanied by phonon emission in quantum dots where the strength of the Rashba spin-orbit coupling is a random function of the lateral (in-plane) coordinate on the spatial nanoscale. In this case the Rashba contribution to the spin-orbit coupling cannot be completely removed by applying a uniform external bias across the quantum dot plane. Due to the remnant random contribution, the spin relaxation rate cannot be decreased by more than two orders of magnitude even when the external bias fully compensates the regular part of the spin-orbit coupling.

Magneto-optical response of layers of semiconductor quantum dots and nanorings

In this paper a comparative theoretical study was made of the magneto-optical response of square lattices of nanoobjects (dots and rings). Expressions for both the polarizability of the individual objects as their mutual electromagnetic interactions (for a lattice in vacuum) was derived. The quantum-mechanical part of the derivation is based upon the commonly used envelope function approximation. The description is suited to investigate the optical response of these layers in a narrow region near the interband transitions onset, particularly when the contribution of individual level pairs can be separately observed. A remarkable distinction between clearly quantum-mechanical and classical electromagnetic behavior was found in the shape and volume dependence of the polarizability of the dots and rings. This optical response of a single plane of quantum dots and nanorings was explored as a function of frequency, magnetic field, and angle of incidence. Although the reflectance of these layer systems is not very strong, the ellipsometric angles are large. For these isolated dot-ring systems they are of the order of magnitude of degrees. For the ring systems a full oscillation of the optical Bohm-Ahronov effect could be isolated. Layers of dots do not display any remarkable magnetic field dependence. Both type of systems, dots and rings, exhibit an outspoken angular-dependent dichroism of quantum-mechanical origin.

Spin and charge polarization in quantum dot arrays

AbstractThe role of spin and charge distribution in a planar semiconductor quantum dot array (square geometry) with two electrons is studied in the presence of a driver cell and magnetic nanoparticles located near two quantum dots providing a local magnetic field. We use an extended Hubbard model to describe the electrons in the cell, taking into account intra and intercell Coulomb interaction, intracell tunneling, Rashba spin‐orbit interaction while hopping, as well as the local magnetic interaction with the magnetic nanoparticles. We find that under physically suitable conditions, controlled by the driver, spin and charge cell polarization (spin‐switching effect) can be attained, which is fully tunable by an electrical mechanism. This effect can be useful for spintronics and/or quantum computation applications. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Rare‐earth doped GaN and InGaN quantum dots grown by plasma assisted MBE

We report on the MBE growth of GaN and InGaN quantum dots (QDs) doped with rare earth ions, namely Eu, Tm and Tb exhibiting red, blue and green luminescence, respectively. Intense photoluminescence/cathodoluminescence is observed, resulting from the spatial localization of rare earth ions in dots combined with the confinement properties of the carriers. White light emission has been produced by combining the three rare earths in a multilayer sample of stacked GaN QD planes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Interdot carrier transfer in asymmetric bilayer InAs∕GaAs quantum dot structures

Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel–Kramers–Brillouin approximation, the observed “transfer time-barrier thickness-relation” supports nonresonant tunneling as the microscopic carrier transfer mechanism.