Self-organized Quantum Dots Research Articles

2D Material-based Quantum Logic Gate Operating Via Self-Organization of Quantum Dots

In the paper, nanotrigger-based electronic device, capable of performing the quantum computation procedure, is described. The device represents a quantum logic gate formed from a two-dimensional material and controlled by a quantum dot. The operation of the quantum dot is analyzed. In the model representation, the transition between two states of the quantum dot, each of which controls the flow of the nanotransistor current (one of the shoulders of the nanotrigger), is equivalent to tunneling through an energy barrier separating the states. Fundamentally important is the fact that in one of these states the quantum dot is diamagnetic, and in the other it is paramagnetic. The paramagnetism of the quantum dot is due to the electronic exchange interaction, characteristic of the systems with unpaired electrons. Thus, the elementary self-organized 2D-material-derived logic gate disclosed in the present work can be employed for design of an electronic reversible unit.In other words, such a unit is able to prepare and to trigger the computation procedure.

InGaN/GaN Quantum Dots on Silicon With Coalesced Nanowire Buffer Layers: A Potential Technology for Visible Silicon Photonics

We have investigated the structural and optical characteristics of planar GaN epitaxial layers on silicon substrates, formed by coalescing an array of GaN nanowires, and of In0.32Ga0.68N/GaN self-organized quantum dots grown on the GaN layer. The as-grown coalesced GaN layer is of the cubic crystalline form, with a large density of stacking faults, that transforms to the wurtzite form, devoid of most of the stacking faults, upon post-growth annealing. Multiple layers of the quantum dots emit at 550 nm. Temperature-dependent and time-resolved photoluminescence measurements have been made to determine the temperature dependent radiative and non-radiative lifetimes of the quantum dots.

Features of spin dynamics of magnetic ions and charge carriers in self-organized quantum dots CdSe/ZnMnSe

Self-organized disk-shaped quantum dots of CdSe embedded in diluted magnetic ZnMnSe barrier were studied by means of pump-probe time-resolved Kerr rotation (TRKR) technique at low temperature T = 7 K. In absence of the external magnetic field TRKR signal exhibits long-living spin dynamics with the decay time exceeding the period between laser pulses. Such spin dynamics is not typical for diluted magnetic semiconductors and nano-structures based on them and could be a trace of a bound magnetic polaron. Resonant spin amplification measured in transversal magnetic field up to 1 T shows the only one peak near B = 0. In B = 1 T the long-living non-precessing signal practically vanishes, while the precessing one appears with the Larmor frequency corresponding to the Mn2+ ions’ net spin precession around the magnetic field. It was found that the signal consists of three components with slightly different precession frequencies that could be due to the fine structure of the manganese spin sublevels occurring because of a stress in quantum dots.

Photoluminescence of InGaAs/GaAs Quantum Nanodisk in Pillar Fabricated by Biotemplate, Dry Etching, and MOVPE Regrowth

The III–V compound semiconductor quantum dot (QD) photonic devices have attracted considerable attention due to their stable operation at high temperature, low threshold current, and high-speed modulation. The photoluminescence (PL) energy of QDs depends on the dot sizes and the energy height of barriers. It is difficult to encourage the growth of low indium content QDs because of small lattice distortions between the InGaAs well regions and GaAs barrier regions. In conventional self-organized QD devices produced by Stranski–Krastanow growth methods, the injected carriers are first captured by the wetting layers and then dropped to the QDs with relaxation to the ground state for QD devices. Our proposed unique quantum disk structures have no wetting layers because the quantum well (QW) region is fully etched by using a low-damage dry etching process. We produced 30 nm diameter and 9 nm thick InGaAs nanodisks using a biotemplate via neutral beam etching and metal–organic vapor phase epitaxy (MOVPE). We observed the InGaAs nanodisks via scanning electron microscopy and measured their photoluminescence (PL). To confirm the quantum confinement effects of InGaAs nanodisks, their energies and transient PL behaviors were measured as a function of temperature. The combined low-damage dry etching and MOVPE regrowth processes form an important technique, as they promise a low-damage interface and regrowth ability. This unique technology is attractive for carrier transportation dynamics and spintronics of hybrid QDs and QW nanostructures, and it is easy to adapt for industrial production systems.

Multiple spin-flip Raman scattering in CdSe/ZnMnSe quantum dots

Multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering (SFRS) in Voigt geometry was observed in self-organized disk-shaped quantum dots (QDs) of $\mathrm{CdSe}/{\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Se}$, where magnetic ions and QD carriers are spatially separated and therefore the exchange interaction between them is expected to be weak. Many lines (about ten) were observed in SFRS spectra, yet the overlapping of the hole wave function with ${\mathrm{Mn}}^{2+}$ ions is very small, in agreement with both the absence of observable Zeeman splitting of the photoluminescence line and the calculation. Interesting polarization properties of SFRS spectra were observed which could be affected by tilting the sample out of normal alignment and changing the temperature. These polarization properties were attributed to the selection rules in SFRS in Voigt geometry. It has been found that the theoretical model suggested by St\"uhler et al. [J. Cryst. Growth 159, 1001 (1996)] does not describe the SFRS spectra in systems with weak exchange interaction between charge carriers and magnetic ions. A qualitative model is suggested here for description of SFRS in such systems.

Study of p-type contact topography influence on characteristics of microdisk and microring lasers

The results are presented for injection microlasers with InAs/InGaAs self-organized quantum dot active region grown on a GaAs substrate. Lasing wavelength is around 1.26…1.28 µm. It is demonstrated that the use of ring-shaped p-contact for microdisk quantum dot lasers provides lowering the threshold current as compared to that of conventional microdisk lasers with round-shaped contact topology. Both spectral characteristics and thermal resistance are preserved, whereas the series electrical resistance slightly increases.

Red and Near-Infrared III-Nitride Quantum Dot Lasers

InGaN/GaN self-organized quantum dots and similar dots in GaN nanowires are formed by strain relaxation and the luminescence from these nanostructures extends to longer wavelengths than generally obtained with quantum wells. We have exploited this advantage by incorporating these nanostructures in the gain region of edge-emitting diode lasers. Here, we describe the characteristics of 650 nm self-organized quantum dot lasers epitaxially grown on GaN and 1.3 μm dot-in-nanowire array lasers grown on (001) Si by molecular beam epitaxy. The devices are characterized by relatively low threshold currents, excellent temperature stability (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> > 200 K), and differential gain ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The highest measured small-signal modulation bandwidth of the dot-in-nanowire laser is 3.3 GHz. The experiments have been complemented by theoretical modeling of the self-organized quantum dots and lasers made with them. These new classes of devices open up new opportunities in applications such as displays, optical data storage, heads-up displays in automobiles, plastic fiber communication, and silicon photonics.

On the high characteristic temperature of an InAs/GaAs/InGaAsP QD laser with an emission wavelength of ~1.5 μm on an InP substrate

We report on a study of lasers with an emission wavelength of about 1.5 μm and high temperature stability, synthesized on an InP (001) substrate. Self-organized InAs quantum dots capped with a thin GaAs layer are used as the active region of the laser. A quaternary InGaAsP solid solution with a band-gap width of 1.15 eV serves as the waveguide/matrix layer. A high characteristic temperature of the threshold current, T 0 = 205 K, is reached in the temperature range 20–50°C in ridge-waveguide laser diodes. A correlation between the values of T 0 and the band-gap width of the waveguide layers is found.

Elongated Quantum Dots of Ge on Si Growth Kinetics Modeling with Respect to the Additional Energy of Edges

In this paper refining of mathematical model for calculation of parameters of selforganised quantum dots (QDs) of Ge on Si grown by the method of molecular beam epitaxy (MBE) is done. Calculations of pyramidal and wedge-like clusters formation energy were conducted with respect to contributions of surface energy, additional edge energy, elastic strain relaxation, and decrease in the atoms attraction to substrate. With the help of well-known model based on the generalization of classical nucleation theory it was shown that elongated islands emerge later than pyramidal clusters. Calculations of QDs surface density and size distribution function for wedge-like clusters with different length to width ratio were performed. The absence of special geometry of islands for which surface density and average size of islands reach points of extremum that was predicted earlier by the model not taking into account energy of edges was revealed when considering the additional contribution of edge formation energy.

Modified dislocation filter method: toward growth of GaAs on Si by metal organic chemical vapor deposition

In this paper, metamorphic growth of GaAs on (001) oriented Si substrate, with a combination method of applying dislocation filter layer (DFL) and three-step growth process, was conducted by metal organic chemical vapor deposition. The effectiveness of the multiple InAs/GaAs self-organized quantum dot (QD) layers acting as a dislocation filter was researched in detail. And the growth conditions of the InAs QDs were optimized by theoretical calculations and experiments. A 2-μm-thick buffer layer was grown on the Si substrate with the three-step growth method according to the optimized growth conditions. Then, a 114-nm-thick DFL and a 1-μm-thick GaAs epilayer were grown. The results we obtained demonstrated that the DFL can effectively bend dislocation direction via the strain field around the QDs. The optimal structure of the DFL is composed of three-layer InAs QDs with a growth time of 55 s. The method could reduce the etch pit density from about 3 × 106 cm−2 to 9 × 105 cm−2 and improve the crystalline quality of the GaAs epilayers on Si.

An electrically driven cavity-enhanced source of indistinguishable photons with 61% overall efficiency

We report on an electrically driven efficient source of indistinguishable photons operated at pulse-repetition rates f up to 1.2 GHz. The quantum light source is based on a p-i-n-doped micropillar cavity with integrated self-organized quantum dots, which exploits cavity quantum electrodynamics effects in the weak coupling regime to enhance the emission of a single quantum emitter coupled to the cavity mode. We achieve an overall single-photon extraction efficiency of (61 ± 11) % for a device triggered electrically at f = 625 MHz. Analyzing the suppression of multi-photon emission events as a function of excitation repetition rate, we observe single-photon emission associated with g(2)HBT(0) values between 0.076 and 0.227 for f ranging from 373 MHz to 1.2 GHz. Hong-Ou-Mandel-type two-photon interference experiments under pulsed current injection at 487 MHz reveal a photon-indistinguishability of (41.1 ± 9.5) % at a single-photon emission rate of (92 ± 23) MHz.

High performance red-emitting multiple layer InGaN/GaN quantum dot lasers

InGaN/GaN self-organized quantum dots can provide useful advantages over quantum wells for the realization of long-wavelength visible light sources because the dots are formed by strain relaxation. A III–nitride based laser emitting in the red (λ ∼ 630 nm), which has not been demonstrated with quantum wells, would be useful for a host of applications. We have investigated the epitaxy and characteristics of self-organized InGaN/GaN multiple layer quantum dots grown by plasma-assisted molecular beam epitaxy and have optimized their properties by tuning the growth parameters. Red-emitting (λ ∼ 630 nm) quantum dots have radiative lifetime ∼2.5 ns and internal quantum efficiency greater than 50%. Edge-emitting red-lasers with multi-dot layers in the active region exhibit an extremely low threshold current density of 1.6 kA/cm2, a high temperature coefficient T0 = 240 K, and a large differential gain dg/dn = 9 × 10−17 cm2.

Room temperature continuous wave operation of injection quantum dot microdisk lasers

We present results of characterization of injection microdisk lasers with InAs/InGaAs self-organized quantum dot active region grown on a GaAs substrate. The minimal microdisk diameter is 15 μm. The microlasers operate in continuous wave regime at room temperature without external cooling. Lasing wavelength is around 1.26... 1.27 μm, minimal threshold current is 1.6 mA (threshold current density 900 A/cm2). Specific thermal impedance is estimated to be 5·0-3 °C·cm2/W.

SnSe2 island films investigation

In this paper, heterostructures with self-organized quantum dots have been produced using incongruent evaporation. Island films on the basis of the intermediate phases forming in the Sn–Se have been grown. The surface morphology of the structures has been studied using atomic force microscopy (AFM). A certain change of the band gap confirms the presence of quantum states in the electron spectrum of the structures. We have found that for obtaining structures with a homogeneous distribution of islands the process of incongruent evaporation should be carried out at high condensate selection rates. By varying the rate of incongruent evaporation of the film material one can achieve directed growth of islet films with a preset islet size distribution.

Soliton nanoantennas in two-dimensional arrays of quantum dots

We consider two-dimensional (2D) arrays of self-organized semiconductor quantum dots (QDs) strongly interacting with electromagnetic field in the regime of Rabi oscillations. The QD array built of two-level states is modelled by two coupled systems of discrete nonlinear Schrödinger equations. Localized modes in the form of single-peaked fundamental and vortical stationary Rabi solitons and self-trapped breathers have been found. The results for the stability, mobility and radiative properties of the Rabi modes suggest a concept of a self-assembled 2D soliton-based nano-antenna, which is stable against imperfections In particular, we discuss the implementation of such a nano-antenna in the form of surface plasmon solitons in graphene, and illustrate possibilities to control their operation by means of optical tools.

Study of the self-organization processes in lead sulfide quantum dots

A procedure is described for the synthesis of nanoparticles based on lead chalcogenides. The procedure combines the synthesis of colloidal quantum dots (QDs) in aqueous solutions with simultaneous organization of the QDs into ordered arrays. The processes of the self-organization of QDs are analyzed at the nano- and microscopic levels by the photoluminescence method, atomic-force microscopy, and optical microscopy.

Microscopic model for intersubband gain from electrically pumped quantum-dot structures

We study theoretically the performance of electrically pumped self-organized quantum dots as a gain material in the mid-IR range at room temperature. We analyze an AlGaAs/InGaAs based structure composed of dots-in-a-well sandwiched between two quantum wells. We numerically analyze a comprehensive model by combining a many-particle approach for electronic dynamics with a realistic modeling of the electronic states in the whole structure. We investigate the gain both for quasi-equilibrium conditions and current injection. Comparing different structures we find that steady-state gain can only be realized by an efficient extraction process, which prevents an accumulation of electrons in continuum states, that make the available scattering pathways through the quantum dot active region too fast to sustain inversion. The tradeoff between different extraction/injection pathways is discussed. Comparing the modal gain to a standard quantum-well structure as used in quantum cascade lasers, our calculations predict reduced threshold current densities of the quantum dot structure for comparable modal gain. Such a comparable modal gain can, however, only be achieved for an inhomogeneous broadening of a quantum-dot ensemble that is close to the lower limit achievable today using self-organized growth.

Modeling the formation kinetics of wedge-shaped germanium quantum dots on silicon

A mathematical model for calculating the dependences of the parameters of self-organized germanium quantum dots (QDs) on silicon on the conditions of growth in molecular beam epitaxy is described. The energies of formation for pyramidal and wedge-shaped islands in the Ge/Si(001) system are calculated with allowance for the contributions from surface energy and elastic stress relaxation, and the drop in the energy of attraction between atoms and the substrate.

Accommodation of SiGe strain on a universally compliant porous silicon substrate

The growth of heteroepitaxial planar fully strained SiGe layers with high Ge concentration and large thickness enables tailoring electronic properties for enhanced transport properties and photoemission. We give here the first experimental and theoretical proof that high temperature flashed porous silicon layers (HT-PSi) perfectly accommodate the stress of SiGe layers and provide compliant substrates with unprecedented capabilities for the fabrication of planar SiGe nanomembranes. We show that the stress driven morphological evolution leading to self-organized quantum dots commonly observed on nominal Si (001) is fully inhibited when growing SiGe on such a HT-PSi substrate. The elastic behavior of HT-PSi results from two specific features: It is ten times softer than Si and tensily strained. Theoretical analysis proves that the compliant behavior of HT-PSi is due to the strain effect, while on the contrary its elastic softness favors the development of 3D growth. The inhibition due to the tensile strain produces atomically flat layers free of misfit dislocation.

Temperature-dependent measurement of Auger recombination in In0.40Ga0.60N/GaN red-emitting (λ = 630 nm) quantum dots

We have derived the Auger recombination coefficients, as a function of temperature, for In0.4Ga0.6N/GaN self-organized quantum dots from large-signal modulation measurements made on lasers in which the quantum dots form the gain media. The value of Ca = 1.3 ±0.2 × 10−31 cm6 s−1 at room temperature and the coefficient decreases with increase of temperature.