Evolution of InGaAs quantum dot molecules
TL;DR: In this paper, the formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) is studied in terms of configuration, volume, and types of QDMs.
Abstract: The formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) are studied in terms of configuration, volume, and types of QDMs. QDMs are formed around self-assembled GaAs nanoscale island induced by adapting a hybrid growth approach combining droplet homoepitaxy and Stranski–Krastanov mode. In distinction from our previous results [Lee et al., Appl. Phys. Lett. 89, 202101 (2006)], hexa-QDMs are fabricated without the formation of background QDs, which can be due to a combinational effects of enhanced intermixing of Ga and In atoms, enhanced surface diffusion (high mobility) of adatoms, and higher In desorption rate due to the higher thermal energy provided during the fabrication of QDMs. In addition, a detailed evolution mechanism from bi-QDMs (two QDs per each GaAs island) to hexa-QDMs (six QDs per island) is proposed based on atom diffusion, material transfer, and equilibrium dimension (saturation) of QDs. Under a fixed InAs coverage, depending on postannealing process after ...
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Journal Article•
TL;DR: In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and the phonon broadening of these lines is considered.
Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.
788 citations
Journal Article•
TL;DR: In this paper, Zhou et al. investigated the simulation of vacancy defects in graphene layers and showed that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and four single vacancies reconstruct into two collective 555-777 defects at higher temperatures.
Abstract: Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles calculations confirm that the 555-777 defect is energetically much more stable than two separated single vacancies, and the energy of the 555-777 defect is also slightly lower than that of the 5-8-5 double vacancy. In TBMD simulation, it is also found that the four single vacancies reconstruct into two collective 555-777 defects which is the unit for the hexagonal haeckelite structure proposed by Terrones et al. [Phys. Rev. Lett. 84, 1716 (2000)].
380 citations
TL;DR: In this paper, a review of recent developments in droplet epitaxy and its application in the optoelectronic field is presented, focusing on the challenges and promise of its application.
Abstract: Droplet epitaxy was proposed to fabricate quantum dots in the early 1990s. Even though many research efforts have been devoted to droplet epitaxy since then, it is only until recently that droplet epitaxy has received worldwide attention. Compared with the well-known Stranski–Krastanow (S–K) growth mode, droplet epitaxy consists of the formation and crystallization of droplets, which enables fabrication of three-dimensional nanostructures in both lattice-mismatched and lattice-matched material systems. The flexibility of the droplet epitaxy growth method has brought to light the great potential of droplet epitaxy in optoelectronic applications. However, most works on droplet epitaxy focus on fabrication, optical properties and understanding the growth mechanisms of various nanostructures. In terms of device applications, droplet epitaxy has fallen behind conventional nanostructure self-assembly using the S–K mode. One of the major reasons is the relative low optical quality of the nanostructures obtained at low growth temperatures, and so careful attention has to be given to growth conditions to create device-grade materials. Through the developments in droplet epitaxy in the last decade, the issues can be overcome and more importantly a rich spectrum of nanostructures can be obtained, which enables the development of novel devices. This review focuses on recent developments in droplet epitaxy and presents the challenge and promise of its application in the optoelectronic field.
36 citations
TL;DR: This study can provide an aid point for the fabrication of nanowires on Si (111) and while Au droplets become much larger with increased deposition amount, the extended annealing duration only mildly affects droplet size and density.
Abstract: We investigate the effect of annealing temperature on self-assembled Au droplets on Si (111). The annealing temperature is systematically varied while fixing other growth parameters such as deposition amount and annealing duration clearly to observe the annealing temperature effect. Self-assembled Au droplets are fabricated by annealing from 50°C to 850°C with 2-nm Au deposition for 30 s. With increased annealing temperatures, Au droplets show gradually increased height and diameter while the density of droplets progressively decreases. Self-assembled Au droplets with fine uniformity can be fabricated between 550°C and 800°C. While Au droplets become much larger with increased deposition amount, the extended annealing duration only mildly affects droplet size and density. The results are systematically analyzed with cross-sectional line profiles, Fourier filter transform power spectra, height histogram, surface area ratio, and size and density plots. This study can provide an aid point for the fabrication of nanowires on Si (111).
30 citations
Cites background from "Evolution of InGaAs quantum dot mol..."
...This trend, increased droplet dimensions associated with decreased density along with increased fabrication temperature, is a conventional behavior of metal droplets [30-32] and even of quantum structures and nanostructures [33-35] on various semiconductor surfaces....
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TL;DR: In this article, a molecular beam epitaxial (MBE) growth technique called "Droplet Epitaxy" was developed for several quantum nanostructures such as quantum rings (QRs), quantum dot rings and QDRs.
19 citations
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TL;DR: In this paper, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed, where the effective long-range interaction is mediated by the vacuum field of a high finesse microcavity.
Abstract: The electronic spin degrees of freedom in semiconductors typically have decoherence times that are several orders of magnitude longer than other relevant time scales. A solid-state quantum computer based on localized electron spins as qubits is therefore of potential interest. Here, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed. The effective long-range interaction is mediated by the vacuum field of a high finesse microcavity. By using conduction-band-hole Raman transitions induced by classical laser fields and the cavity-mode, parallel controlled-not operations, and arbitrary single qubit rotations can be realized.
1,702 citations
TL;DR: In this paper, a quantum-gate mechanism based on electron spins in coupled semiconductor quantum dots is considered and the magnetization and the spin susceptibilities of the coupled dots are calculated.
Abstract: We consider a quantum-gate mechanism based on electron spins in coupled semiconductor quantum dots. Such gates provide a general source of spin entanglement and can be used for quantum computers. We determine the exchange coupling $J$ in the effective Heisenberg model as a function of magnetic $(B)$ and electric fields, and of the interdot distance $a$ within the Heitler-London approximation of molecular physics. This result is refined by using $\mathrm{sp}$ hybridization, and by the Hund-Mulliken molecular-orbit approach, which leads to an extended Hubbard description for the two-dot system that shows a remarkable dependence on $B$ and $a$ due to the long-range Coulomb interaction. We find that the exchange $J$ changes sign at a finite field (leading to a pronounced jump in the magnetization) and then decays exponentially. The magnetization and the spin susceptibilities of the coupled dots are calculated. We show that the dephasing due to nuclear spins in GaAs can be strongly suppressed by dynamical nuclear-spin polarization and/or by magnetic fields.
1,178 citations
TL;DR: In this paper, the strain distribution in and around pyramidal InAs/GaAs quantum dots (QD's) on a thin wetting layer fabricated recently with molecular-beam epitaxy, is simulated numerically.
Abstract: The strain distribution in and around pyramidal InAs/GaAs quantum dots (QD's) on a thin wetting layer fabricated recently with molecular-beam epitaxy, is simulated numerically. For comparison analytical solutions for the strain distribution in and around a pseudomorphic slab, cylinder, and sphere are given for isotropic materials, representing a guideline for the understanding of strain distribution in two-, one-, and zero-dimensional pseudomorphic nanostructures. For the pyramidal dots we find that the hydrostatic strain is mostly confined in the QD; in contrast part of the anisotropic strain is transferred from the QD into the barrier. The optical-phonon energies in the QD are estimated and agree perfectly with recent experimental findings. From the variation of the strain tensor the local band-gap modification is calculated. Piezoelectric effects are additionally taken into account. The three-dimensional effective-mass single-particle Schr\"odinger equation is solved for electrons and holes using the realistic confinement potentials. Since the QD's are in the strong confinement regime, the Coulomb interaction can be treated as a perturbation. The thus obtained electronic structure agrees with luminescence data. Additionally AlAs barriers are considered.
1,056 citations
TL;DR: The growth of multilayer arrays of coherently strained islands self-organizes into a more regular three-dimensional arrangement, providing a possible route to obtain the size uniformity needed for electronic applications of quantum dot arrays.
Abstract: We investigate the growth of multilayer arrays of coherently strained islands, which may serve as ``quantum dots'' in electronic devices. A simple model reproduces the observed vertical correlation between islands in successive layers. However, the arrangement of islands is not simply repeated from layer to layer. Instead, the island size and spacing grow progressively more uniform. In effect, the structure ``self-organizes'' into a more regular three-dimensional arrangement, providing a possible route to obtain the size uniformity needed for electronic applications of quantum dot arrays.
1,040 citations
TL;DR: In this article, the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)] were analyzed theoretically, and the phonon broadening of these lines was considered.
Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.
843 citations