Nanomaterials

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19 pages, 5572 KiB

Open AccessArticle

Charge Tunable GaAs Quantum Dots in a Photonic n-i-p Diode

by Hans Georg Babin, Julian Ritzmann, Nikolai Bart, Marcel Schmidt, Timo Kruck, Liang Zhai, Matthias C. Löbl, Giang N. Nguyen, Clemens Spinnler, Leonardo Ranasinghe, Richard J. Warburton, Christian Heyn, Andreas D. Wieck and Arne Ludwig

Nanomaterials 2021, 11(10), 2703; https://doi.org/10.3390/nano11102703 - 13 Oct 2021

Cited by 7 | Viewed by 3010

Abstract

In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to [...] Read more.

In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n- and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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12 pages, 1558 KiB

Open AccessArticle

Independent Control Over Size and Surface Density of Droplet Epitaxial Nanostructures Using Ultra-Low Arsenic Fluxes

by Sergey V. Balakirev, Natalia E. Chernenko, Mikhail M. Eremenko, Oleg A. Ageev and Maxim S. Solodovnik

Nanomaterials 2021, 11(5), 1184; https://doi.org/10.3390/nano11051184 - 30 Apr 2021

Cited by 8 | Viewed by 2074

Abstract

Modern and future nanoelectronic and nanophotonic applications require precise control of the size, shape and density of III-V quantum dots in order to predefine the characteristics of devices based on them. In this paper, we propose a new approach to control the size [...] Read more.

Modern and future nanoelectronic and nanophotonic applications require precise control of the size, shape and density of III-V quantum dots in order to predefine the characteristics of devices based on them. In this paper, we propose a new approach to control the size of nanostructures formed by droplet epitaxy. We reveal that it is possible to reduce the droplet volume independently of the growth temperature and deposition amount by exposing droplets to ultra-low group-V flux. We carry out a thorough study of the effect of arsenic pressure on the droplet characteristics and demonstrate that indium droplets with a large initial size (>100 nm) and a low surface density (<10⁸ cm⁻²) are able to shrink to dimensions appropriate for quantum dot applications. Small droplets are found to be unstable and difficult to control, while larger droplets are more resistive to arsenic flux and can be reduced to stable, small-sized nanostructures (~30 nm). We demonstrate the growth conditions under which droplets transform into dots, ring and holes and describe a mechanism of this transformation depending on the ultra-low arsenic flux. Thus, we observe phenomena which significantly expand the capabilities of droplet epitaxy. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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15 pages, 2050 KiB

Open AccessArticle

Luminescence from Droplet-Etched GaAs Quantum Dots at and Close to Room Temperature

by Leonardo Ranasinghe, Christian Heyn, Kristian Deneke, Michael Zocher, Roman Korneev and Wolfgang Hansen

Nanomaterials 2021, 11(3), 690; https://doi.org/10.3390/nano11030690 - 10 Mar 2021

Cited by 3 | Viewed by 2360

Abstract

Epitaxially grown quantum dots (QDs) are established as quantum emitters for quantum information technology, but their operation under ambient conditions remains a challenge. Therefore, we study photoluminescence (PL) emission at and close to room temperature from self-assembled strain-free GaAs quantum dots (QDs) in [...] Read more.

Epitaxially grown quantum dots (QDs) are established as quantum emitters for quantum information technology, but their operation under ambient conditions remains a challenge. Therefore, we study photoluminescence (PL) emission at and close to room temperature from self-assembled strain-free GaAs quantum dots (QDs) in refilled AlGaAs nanoholes on (001)GaAs substrate. Two major obstacles for room temperature operation are observed. The first is a strong radiative background from the GaAs substrate and the second a significant loss of intensity by more than four orders of magnitude between liquid helium and room temperature. We discuss results obtained on three different sample designs and two excitation wavelengths. The PL measurements are performed at room temperature and at T = 200 K, which is obtained using an inexpensive thermoelectric cooler. An optimized sample with an AlGaAs barrier layer thicker than the penetration depth of the exciting green laser light (532 nm) demonstrates clear QD peaks already at room temperature. Samples with thin AlGaAs layers show room temperature emission from the QDs when a blue laser (405 nm) with a reduced optical penetration depth is used for excitation. A model and a fit to the experimental behavior identify dissociation of excitons in the barrier below T = 100 K and thermal escape of excitons from QDs above T = 160 K as the central processes causing PL-intensity loss. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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13 pages, 2176 KiB

Open AccessArticle

Modeling of Al and Ga Droplet Nucleation during Droplet Epitaxy or Droplet Etching

by Christian Heyn and Stefan Feddersen

Nanomaterials 2021, 11(2), 468; https://doi.org/10.3390/nano11020468 - 12 Feb 2021

Cited by 4 | Viewed by 1952

Abstract

The temperature dependent density of Al and Ga droplets deposited on AlGaAs with molecular beam epitaxy is studied theoretically. Such droplets are important for applications in quantum information technology and can be functionalized e.g., by droplet epitaxy or droplet etching for the self-assembled [...] Read more.

The temperature dependent density of Al and Ga droplets deposited on AlGaAs with molecular beam epitaxy is studied theoretically. Such droplets are important for applications in quantum information technology and can be functionalized e.g., by droplet epitaxy or droplet etching for the self-assembled generation of quantum emitters. After an estimation based on a scaling analysis, the droplet densities are simulated using first a mean-field rate model and second a kinetic Monte Carlo (KMC) simulation basing on an atomistic representation of the mobile adatoms. The modeling of droplet nucleation with a very high surface activity of the adatoms and ultra-low droplet densities down to 5 × 10

^{6}

cm

^{- 2}

is highly demanding in particular for the KMC simulation. Both models consider two material related model parameters, the energy barrier

E_{S}

for surface diffusion of free adatoms and the energy barrier

E_{E}

for escape of atoms from droplets. The rate model quantitatively reproduces the droplet densities with

E_{S}

= 0.19 eV,

E_{E}

= 1.71 eV for Al droplets and

E_{S}

= 0.115 eV for Ga droplets. For Ga, the values of

E_{E}

are temperature dependent indicating the relevance of additional processes. Interestingly, the critical nucleus size depends on deposition time, which conflicts with the assumptions of the scaling model. Using a multiscale KMC algorithm to substantially shorten the computation times, Al droplets up to 460 °C on a 7500 × 7500 simulation field and Ga droplets up to 550 °C are simulated. The results show a very good agreement with the experiments using

E_{S}

= 0.19 eV,

E_{E}

= 1.44 eV for Al, and

E_{S}

= 0.115 eV,

E_{E}

= 1.24 eV (

T \leq

300 °C) or

E_{E}

= 1.24 + 0.06 (T[°C] − 300)/100 eV (

T > 300

°C) for Ga. The deviating

E_{E}

is attributed to a re-nucleation effect that is not considered in the mean-field assumption of the rate model. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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16 pages, 1146 KiB

Open AccessEditor’s ChoiceArticle

Polarization Anisotropies in Strain-Free, Asymmetric, and Symmetric Quantum Dots Grown by Droplet Epitaxy

by Marco Abbarchi, Takaaki Mano, Takashi Kuroda, Akihiro Ohtake and Kazuaki Sakoda

Nanomaterials 2021, 11(2), 443; https://doi.org/10.3390/nano11020443 - 10 Feb 2021

Cited by 4 | Viewed by 2488

Abstract

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001), and (111)A surfaces. Despite a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A [...] Read more.

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001), and (111)A surfaces. Despite a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A and (111)A samples leads to a larger carrier confinement compared to (001), where a wetting layer is present. This leads to a more pronounced dependence of the binding energies of s-shell excitons on the quantum dot size and to the strong anti-binding character of the positive-charged exciton for smaller quantum dots. In-plane geometrical anisotropies of (311)A and (001) quantum dots lead to a large electron-hole fine interaction (fine structure splitting (FSS) ∼100 μeV), whereas for the three-fold symmetric (111)A counterpart, this figure of merit is reduced by about one order of magnitude. In all these cases, we do not observe any size dependence of the fine structure splitting. Heavy-hole/light-hole mixing is present in all the studied cases, leading to a broad spread of linear polarization anisotropy (from 0 up to about 50%) irrespective of surface orientation (symmetry of the confinement), fine structure splitting, and nanostructure size. These results are important for the further development of ideal single and entangled photon sources based on semiconductor quantum dots. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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15 pages, 819 KiB

Open AccessArticle

Exciton Dynamics in Droplet Epitaxial Quantum Dots Grown on (311)A-Oriented Substrates

by Marco Abbarchi, Takaaki Mano, Takashi Kuroda and Kazuaki Sakoda

Nanomaterials 2020, 10(9), 1833; https://doi.org/10.3390/nano10091833 - 14 Sep 2020

Cited by 5 | Viewed by 2519

Abstract

Droplet epitaxy allows the efficient fabrication of a plethora of 3D, III–V-based nanostructures on different crystalline orientations. Quantum dots grown on a (311)A-oriented surface are obtained with record surface density, with or without a wetting layer. These are appealing features for quantum dot [...] Read more.

Droplet epitaxy allows the efficient fabrication of a plethora of 3D, III–V-based nanostructures on different crystalline orientations. Quantum dots grown on a (311)A-oriented surface are obtained with record surface density, with or without a wetting layer. These are appealing features for quantum dot lasing, thanks to the large density of quantum emitters and a truly 3D lateral confinement. However, the intimate photophysics of this class of nanostructures has not yet been investigated. Here, we address the main optical and electronic properties of s-shell excitons in individual quantum dots grown on (311)A substrates with photoluminescence spectroscopy experiments. We show the presence of neutral exciton and biexciton as well as positive and negative charged excitons. We investigate the origins of spectral broadening, identifying them in spectral diffusion at low temperature and phonon interaction at higher temperature, the presence of fine interactions between electron and hole spin, and a relevant heavy-hole/light-hole mixing. We interpret the level filling with a simple Poissonian model reproducing the power excitation dependence of the s-shell excitons. These results are relevant for the further improvement of this class of quantum emitters and their exploitation as single-photon sources for low-density samples as well as for efficient lasers for high-density samples. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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9 pages, 8005 KiB

Open AccessFeature PaperArticle

Reentrant Behavior of the Density vs. Temperature of Indium Islands on GaAs(111)A

by Artur Tuktamyshev, Alexey Fedorov, Sergio Bietti, Shiro Tsukamoto, Roberto Bergamaschini, Francesco Montalenti and Stefano Sanguinetti

Nanomaterials 2020, 10(8), 1512; https://doi.org/10.3390/nano10081512 - 31 Jul 2020

Cited by 3 | Viewed by 2637

Abstract

We show that the density of indium islands on GaAs(111)A substrates have a non-monotonic, reentrant behavior as a function of the indium deposition temperature. The expected increase in the density with decreasing temperature, indeed, is observed only down to 160

°

C, where [...] Read more.

We show that the density of indium islands on GaAs(111)A substrates have a non-monotonic, reentrant behavior as a function of the indium deposition temperature. The expected increase in the density with decreasing temperature, indeed, is observed only down to 160

°

C, where the indium islands undertake the expected liquid-to-solid phase transition. Further decreasing the temperature causes a sizable reduction of the island density. An additional reentrant increasing behavior is observed below 80

°

C. We attribute the above complex behavior to the liquid–solid phase transition and to the complex island–island interaction which takes place between crystalline islands in the presence of strain. Indium solid islands grown at temperatures below 160

°

C have a face-centered cubic crystal structure. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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Review

Jump to: Research

24 pages, 52248 KiB

Open AccessReview

Atomic-Scale Characterization of Droplet Epitaxy Quantum Dots

by Raja S. R. Gajjela and Paul M. Koenraad

Nanomaterials 2021, 11(1), 85; https://doi.org/10.3390/nano11010085 - 3 Jan 2021

Cited by 21 | Viewed by 4737

Abstract

The fundamental understanding of quantum dot (QD) growth mechanism is essential to improve QD based optoelectronic devices. The size, shape, composition, and density of the QDs strongly influence the optoelectronic properties of the QDs. In this article, we present a detailed review on [...] Read more.

The fundamental understanding of quantum dot (QD) growth mechanism is essential to improve QD based optoelectronic devices. The size, shape, composition, and density of the QDs strongly influence the optoelectronic properties of the QDs. In this article, we present a detailed review on atomic-scale characterization of droplet epitaxy quantum dots by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). We will discuss both strain-free GaAs/AlGaAs QDs and strained InAs/InP QDs grown by droplet epitaxy. The effects of various growth conditions on morphology and composition are presented. The efficiency of methods such as flushing technique is shown by comparing with conventional droplet epitaxy QDs to further gain control over QD height. A detailed characterization of etch pits in both QD systems is provided by X-STM and APT. This review presents an overview of detailed structural and compositional analysis that have assisted in improving the fabrication of QD based optoelectronic devices grown by droplet epitaxy. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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