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. 2018 Aug 3;3(8):8677–8682. doi: 10.1021/acsomega.8b01078

Post-thermal-Induced Recrystallization in GaAs/Al0.3Ga0.7As Quantum Dots Grown by Droplet Epitaxy with Near-Unity Stoichiometry

Inah Yeo †, Kyung Soo Yi §, Eun Hye Lee †, Jin Dong Song †,*, Jong Su Kim ∥,*, Il Ki Han ‡,*
PMCID: PMC6644783  PMID: 31458998

Abstract

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Here, we investigate the stoichiometry control of GaAs/Al0.3Ga0.7As droplet epitaxy (DE) quantum dots (QDs). Few tens of core nonstoichiometries in the Ga(As) atomic percent are revealed in as-grown “strain-free” QDs using state-of-the-art atomic-scale energy-dispersive X-ray spectroscopy based on transmission electron microscopy. Precise systematic analyses demonstrate a successful quenching of the nonstoichiometry below 2%. The control of the chemical reactions with well-controlled ex situ annealing sheds light on the engineering of a novel single-photon source of strain-free DE QDs free of defects.

Introduction

Self-assembled semiconductor quantum dots (QDs) have gained much attention due to their rich variety of nanotechnological applications in photonic quantum technologies (e.g., information processing, cryptography, and quantum teleportation),1−6 quantum simulators of chemical and physical systems,7 optoelectronics,8−14 and chemical15−17 or biosensing.18,19 Conventionally, three-dimensional islands of Stranski–Krastanov (SK)-type QDs are formed on lattice-mismatched heteroepitaxial systems driven by the elastic relaxation of strain. Experiencing a substantial (intrinsic) strain field, the chemical composition profiles of SK QDs unfortunately significantly modify their optoelectronic properties, which is an obstacle to the integration of QD single-photon sources in on-chip optical circuits. For decades, droplet epitaxy (DE) has attracted attention as an alternative technique for fabricating deterministic QD structural morphologies that are free of strain.20−30 In the DE mode, liquid droplets of a metallic material (Ga) are formed on a group-III-terminated surface, followed by crystallization under the flux of a group-V element (As) (Figure 1a). While the structural morphology of QDs can be precisely controlled by growth parameters, such as the substrate temperature, the flux of As4, and the surface stoichiometry, the DE technique enables the fabrication of “strain-free” GaAs QDs on a lattice-matched AlGaAs matrix. Such controllability (solely via the crystallization conditions) and the precedent characteristics of DE QDs hold substantial promise for on-chip implementations of QD single-photon sources.

Figure 1.

Figure 1

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(a) Schematic overview of the growth mechanism of GaAs/AlGaAs QDs in the DE mode. (i) Surface reconstruction of a Ga-rich (4 × 6) surface via the injection of As4 flux and (ii) capping of subsequent crystallized QDs are depicted along with the growth sequence for a DE QD on the left-hand side. (b) A high-angle annular resolution dark-field transmission electron microscopy (TEM) image of an as-grown QD. (c) Energy-dispersive X-ray spectroscopy (EDS) element maps of Ga (green), As (red), and Al (yellow) of an as-grown GaAs DE QD. The white arrow drawn in (b) indicates the elemental line scan (Figure 2) across the center of the QD.

The presence of a small amount of defects in DE QDs, however, can strongly modify their optoelectronic properties and their chemi-physical nature, drastically decreasing the efficiency of photon emitters. Suffering from the low efficiency of optical emission in QD photoluminescence (PL), many studies have reported improving the quality of DE QDs by performing ex situ thermal treatments.22−24,26,27,29,31−33 The optimization of the post-thermal process (relative to improving the crystallinity of QDs) relies on phenomenological enhancements in the intensity of the QD PL. Such analyses need to be put in structural and chemical contexts that nanoscopic studies have been lacking.31−33 Atomic-resolution systematic analyses of such DE QDs are therefore in high demand to acquire a precise chemi-physical understanding to engineer defect-free and strain-free QDs.

In this study, we thoroughly investigate the nonstoichiometry in GaAs DE QDs with and without post-thermal-induced recrystallization processes. Precise structural and chemical investigations are systematically performed for 40 individual QDs using atomically resolved transmission electron microscopy (TEM) energy-dispersive X-ray spectroscopy (EDS). Different degrees of nonstoichiometric cores are revealed in strain-free QDs as a function of the post-thermal-induced recrystallization temperature. While resolving a few to a few tens of compositional changes in the atomic percent, near-unity stoichiometric QDs are acquired via a careful post-thermal-induced recrystallization process. In addition, we examine the Tex-an-dependent effect of Al interdiffusion on the electronic structures of DE QDs via an envelope function model. Defect-free and strain-free DE QDs of high optical quality would ultimately lead to the on-chip implementation of QD single-photon sources.

Results and Discussion

Control of Nonstoichiometry in Strain-Free DE QDs

Because the GaAs/AlGaAs DE QD system is known to be strain-free, the elemental distribution of GaAs QDs and any chemical changes in a QD are primarily of concern. To atomically resolve the structural and chemical characteristics of strain-free GaAs DE QDs, TEM EDS is performed on cross-sectional specimens with embedded GaAs islands (cf. Figure 1). For the pyramidal/conical shapes of the GaAs DE QDs, atomic-scale analyses of the in-depth profiles reveal a remarkable inhomogeneity in their chemical distributions (cf. Figure 1b,c). In particular, an As-deficient (Ga-rich) QD is resolved in the EDS maps. Here, the control parameters of the structural morphology include the surface stoichiometry, the crystallization temperature, and the As4 flux.22 To resolve the two-dimensional (2D) elemental distributions more precisely (with a high signal-to-noise ratio), we also perform line scans of the chemical constituents across the center of each QD along the growth axis (cf. Figure 2). A significant nonstoichiometry (∼7:3 in the Ga/As atomic ratio) is revealed to be present at the core of as-grown DE QDs. The primary mechanism responsible for metal-rich nonstoichiometric QDs is the insufficient diffusion of As atoms into the Ga liquid droplets (i.e., the Ga reservoirs) under the surface impinging density of As.22,27 The incomplete crystallization of GaAs DE QDs results in a gradient of the As composition in the droplet leaving the nonstoichiometric core. The As-deficient (Ga-rich) droplets are seen to turn into stoichiometric QDs at an enhanced temperature in the post-thermal-induced recrystallization process. The exemplary Ga depth profiles in Figure 2 show a remarkable improvement in the crystallinity by a few tens of atomic percent for four distinct temperatures of ex situ annealing.

Figure 2.

Figure 2

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Representative line composition profiles of Ga (green open circles), As (red circles), and Al (yellow diamonds) for specimens of as-grown QDs and QDs with ex situ annealing at 750, 800, and 850 °C using TEM EDS line scans. The atomic percentage of Al corresponds to the top (yellow) axis, and those of Ga and As correspond to the bottom axis.

To gain more systematic and profound insights into this process, we perform a statistical EDS of 40 individual QDs, 10 QDs for each of the cases with and without post-thermal-induced recrystallization treatments at various Tex-an. The maximum deviations between Ga and As in the atomic percent (CGa–As) are displayed in Figure 3 for the four distinct treatments. With no deliberate ex situ annealing treatment, the GaAs DE QDs present a significant mean nonstoichiometry of 37% (CGa–As) with an atomic ratio of Ga/As ranging from 1.5 to 3.7 (CGa–As = 10.1–28.5). Conversely, Keizer et al.34 reported an X-ray scanning tunneling microscopy (X-STM) analysis of as-grown DE QDs with no recognizable nonstoichiometry. While the X-STM probes the surface characteristics at the atomic scale, the additional capability of TEM EDS identifies the elemental compositions by measuring the energies of the characteristic X-ray peaks. At the same time, CGa–As is measured as varying from ∼20 to ∼58% in a given specimen (cf. Figure 3). The measured PL signals of as-grown QDs31,32 seem to arise relative to QDs with a low degree of nonstoichiometry. A nearly identical trend is seen in the elemental profiles of GaAs DE QDs with thermal treatment at 750 °C, even though the observed nonstoichiometry ratios fall in a smaller variance from 1.9 to 3.1. CGa–As is seen to diminish by more than 16% with an increase of 50 °C in Tex-an from 750 °C. Encouragingly, a few tens of the still existing core nonstoichiometry are dramatically quenched below ∼4% for GaAs DE QDs post-thermal recrystallized at 850 °C. The average standard deviation of CGa–As is extracted to 1.6% for DE QDs annealed at 850 °C. Here, the overall analytical accuracy of EDS analysis is approximately 2% (see details in Experimental Section and Computational Methods). Further reaction of the As atoms, therefore, eliminates the As composition gradient within the dot. Such recrystallization processes can be understood as reordering the As and Ga atoms dissolved within the GaAs DE QD. The diffusion coefficient of As (e.g., 5.8 × 10–14 at 850 °C) has been estimated to be too low to permit bulk equilibrium via vacancy diffusion from the surface at the annealing temperatures.35 Accordingly, the local rearrangement of defects has been identified as a viable mechanism for producing uniformity during post-growth annealing.35,36 The localized dissolution of As atoms from the substrate would create nonstoichiometric defects, whereas near-stoichiometry in GaAs is observed at the bottom of the QDs (cf. Figure 1). Remnants of metallic Ga within the GaAs DE QDs have also been reported by Mano et al.37 We note that individual phases of Ga and GaAs have been resolved to coexist within the GaAs nanostructures with no post-thermal treatment. The in situ annealing process seems to induce the nonstoichiometric phases of Ga and As for our as-grown DE QDs. Compared to the ambiguous optimization processes of post-thermal annealing (relying on the phenomenological enhancements in the QD PL intensity), the EDS statistics reveal a dramatic recovery of the crystallinity in 50 °C from 800 °C (cf. Figure 3). We note that the operating temperature of rapid thermal annealing (RTA) is measurement-based (i.e., based on the position of the temperature sensor and the Ar environment), and therefore no absolute value is given. For example, Bocquel et al.27 performed post-thermal annealing at 750 °C to complete the crystallization. Three-dimensional composition profiles of DE QDs have also been retrieved using atomic probe tomography (APT) across a volume of several tens of nanometers. The TEM EDS employed in this study facilitates the comparative statistical analysis by scanning a sample cross section of 2 orders of magnitude larger than that of APT. Here, our methodology shows how strain-free but nonstoichiometric QDs are converted into stoichiometric DE QDs at well-controlled temperatures of ex situ annealing. This provides powerful new insights into the engineering of strain-free QDs with unity stoichiometry.

Figure 3.

Figure 3

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Chemically and spatially resolved ratios of the nonstoichiometry are summarized as a function of the ex situ annealing temperature. CGa–As denotes the maximum deviation between Ga and As in the atomic percent, and Tex-an the temperature of the ex situ annealing. The individual solid dots represent the mean values of the corresponding CGa–As of 10 QDs under a given ex situ annealing temperature.

Interdiffusion of Detrimental Al Atoms in Strain-Free QDs

Along with the control of the nonstoichiometry, the interdiffusion of detrimental Al is also a critical issue in DE QDs,27,29,31,32 and disturbs the optoelectronic properties of single-photon emitters. We carefully examine whether the ex situ annealing process degrades the purity of the strain-free DE QDs. Figure 2 confirms a low level of Al intermixing for most of the QDs. Limited by the inevitable position drift and damage of the thin samples, the average Al elemental profiles of the EDS line scans level off at 4% in the central regions of QDs with widths of 2 nm. We note that ∼11.4% of the average Al composition was retrieved with an average deviation of ∼0.4% on the host material of Al0.3Ga0.7As for a scanning strip with a width of ∼20 nm. The Al defects underneath the QDs originate from the recrystallization of Al atoms, which dissolve into the Ga droplet.27,29 Meanwhile, the local enrichment of Al at the top facet is ascribed to the different mobilities of Ga and Al atoms. We also examine the influence of Al interdiffusion in the electronic structures of DE QDs employing the envelope function scheme within the k·p theory (see details in Experimental Section and Computational Methods). The sub-band energies EsubC/V in the conduction/valence band and the direct Coulomb interaction energy J of an electron–hole pair are evaluated for lens-shaped QDs. The average radius of the DE QDs is taken to be 15 nm, and the average heights are found to be 5.5, 7.7, 7.5, and 8.9 nm for the as-grown QD and the QDs post-annealed at 750, 800, and 850 °C, respectively. The results of Esub and J for the ground sub-bands are summarized in Table 1 for various temperatures of post-thermal-induced recrystallization. EsubC/V(□) and J(□) denote the results of an ideal confinement potential in the absence of Al interdiffusion. To configure a more realistic profile of band gap, the experimental mole fraction x(zQD) of the Al acquired in EDS is applied to an empirical band gap38 of EAlxGa(1–x)As(zQD) = 1.519 + 1.155x + 0.37x2. Here, zQD denotes the position measured from the bottom to the top facet along the growth axis of a QD. Our analysis shows that due to the interdiffusion of Al within the QDs, the QD recombination energy is modified by ∼1.7% for the as-grown sample. The QD excitonic energy is reduced by ∼2.7% via the post-thermal-induced recrystallization treatment up to 850 °C (see theoretical details in Experimental Section and Computational Methods). The interdiffusion effect on the band gap via x(zQD) yields ∼46.4% of the quantum confinement correction (i.e., the sum of the excitonic sub-band energy and the Coulomb interaction of Esub + J) for the annealed samples, while it yields ∼32% of the quantum confinement correction for as-grown QDs. In our calculation, a finite Tex-an treatment gives rise to a minor change of ±0.2% in Esub + J for the specimens treated at the three distinct temperatures. Therefore, we demonstrate that the optimal post-thermal-induced recrystallization process quenches the detrimental core nonstoichiometry of strain-free DE QDs. The Tex-an-dependent degradation of the QD purity by Al is believed to be minor.

Table 1. Calculated Quantum Confinement Energies of GaAs/Al0.3Ga0.7As DE QDs for Distinct Tex-an.

Tex-an [°C] as-grown 750 800 850
EsubC [meV] 81.8 62.8 66.1 52.6
EsubV [meV] 30.7 24.0 25.0 18.7
J [meV] –24.6 –22.2 –23.3 –21.7
EsubC(□) [meV] 64.1 44.3 46.2 37.6
EsubV(□) [meV] 20.0 12.4 13.0 10.1
J(□) [meV] –24.6 –22.3 –22.6 –21.2
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Conclusions

We have systematically demonstrated full control of the detrimental nonstoichiometric defects in strain-free DE QDs by employing a post-thermal-induced recrystallization process. Harnessing unambiguous high-resolution TEM EDS, the nonstoichiometries present in the core of strain-free DE QDs have been quantitatively revealed. Chemically and spatially resolved complexes of as-grown DE QDs have an average nonstoichiometry of 6.85:3.15 (Ga/As) in the atomic percent. Using statistical analyses of TEM EDS for 40 individual DE QDs, we have demonstrated a successful approach to eliminate the core nonstoichiometry via a careful ex situ annealing process. At the threshold temperature, a near-unity stoichiometry has been attained in our strain-free QDs within the analytical accuracy of EDS. The effect of Al interdiffusion (versus the treatment temperature) on the quantum confinement characteristics is believed to be minor. In addition to shedding light on the Tex-an-dependent nanoscopic machinery, this novel engineering of strain-free QDs with unity stoichiometry is critical for the practical on-chip implementation of QD single-photon sources.

Experimental Section and Computational Methods

Fabrication of GaAs DE QDs

The GaAs QDs under investigation were grown by DE using molecular beam epitaxy.21−23 As illustrated in Figure 1, the epitaxial growth of GaAs QDs in the DE mode is based on the crystallization of Ga droplets via the injection of an As4 flux. The crystallized GaAs QDs are then capped with an AlGaAs protective layer. To form the Ga droplet, the partial pressure of As (PAs) in the chamber was maintained below ∼2 × 10–12 Torr prior to the injection of the Ga flux. We fabricated the Ga liquid droplets on an Al0.3Ga0.7As/GaAs substrate by supplying two monolayers of Ga at T ∼ 220 °C. The growth chamber background pressure was below 10–10 Torr during the injection of the Ga atoms. As depicted in Figure 1a, the As atoms were subsequently supplied to impinge on a Ga-terminated (Ga-rich) surface with the reconstruction of (4 × 6)22 under a moderate As4 beam equivalent pressure (BEP) of 1.0 × 10–5 Torr. The morphological reconstruction of the DE droplet involves the migration, accumulation, and desorption of Ga on the (4 × 6) Ga-rich surface (diffusion-stopping processes).22 After the crystallization time of 60 s, the GaAs DE QDs were thermally annealed at an increased substrate temperature of ∼560 °C (high temperature) with a reduced As4 BEP of 6.0 × 10–6 Torr. While controlling the shapes of nanostructures with higher thermal stability, we maintained the crystallization process of GaAs droplets for 720 s. We note that the migration of the Ga droplet is regulated with the control of the substrate temperature and the surface impinging density. The GaAs DE QDs were then protected with a 20 nm-thick Al0.3Ga0.7As layer at 560 °C. The average QD density of ∼3/μm2 was obtained by measuring the images of a charge-coupled device for optically excited samples. A detailed description of the growth mechanism and parameters can be found in our previous works.22,23 We note that the structural morphology and optical characteristics have also been reported for the similar type of DE QDs.24

Ex Situ Thermal Annealing Process

For systematic control of the stoichiometry of DE QDs, post-growth RTA processes were performed on as-grown QDs as a function of temperature in a standard RTA equipment. Each specimen was heated up to 750, 800, and 850 °C within 60 s and maintained for 240 s in ambient Ar. Then, the sample was cooled below 100 °C over 15 min. The cycling of ex situ RTA is optimized based on the phenomenological enhancement of PL intensity.23,31 We note that previous studies have shown a decrease of PL intensity for DE QDs post-annealed above 850 °C.

TEM and EDS Analyses

The high-resolution structural analysis was performed by employing a Titan 80-300 TEM operating at 300 kV. We used focused ion beam (FIB) techniques for the preparation of specimens, in which QD droplets are randomly embedded. To resolve structural profiles of all constituents in DE QDs precisely, chemical characterization with compositional mapping was carried out using a Talos F200X of 200 kV. The EDS elemental maps and line scans were measured in FIB-prepared cross-sectional specimens with a thickness of less than 50 nm. While 50% of the As concentration is theoretically estimated in the matrix of Al0.3Ga0.7As, the average concentration profiles of As were measured to be 52.3% with a standard deviation error of 1.8% for a scanning strip with a width of 20 nm. The 2D mapping could include an incorrect distribution of elements due to the unavoidable spatial drift and damage of thin samples during several minutes of data collection time. For statistical analyses of 40 individual QDs, we performed the EDS line scans for a faster elemental mapping to increase the contrast of elemental distributions. Given the point resolution of 1.6 Å in scanning transmission electron microscope mode, the in-depth concentration profiles of DE QDs were seen by scanning across the center of the QDs in the growth direction [001]. The TEM EDS maps and line scan profiles in Figures 1 and 2 probe small compositional changes of Ga, As, and Al in the DE QDs post-annealed at distinct temperatures.

Calculation of QD Quantum Confinement Energies

We consider a lens-shaped DE QD of GaAs/Al0.3Ga0.7As with in-plane cylindrical symmetry, whose base length and height are extracted from the cross-sectional TEM images. We employ the envelope function scheme based on the k·p theory to calculate the quantum confinement characteristics of DE QDs.39 The excitonic recombination energy EX of QD is given by the sum of the fundamental band gap Egap of QD, the sub-band confinement energies Esub of carriers, and the electron–hole Coulomb interaction energy J: EX = Egap + Esub + J. The latter two terms (i.e., Esub and J) in EX describe the quantum confinement characteristics of an electron–hole system. The QD sub-band energies and wavefunctions are calculated by numerically solving a Schrödinger equation written as

graphic file with name ao-2018-01078f_m001.jpg

To conform realistic quantum confinement potentials inside the DE QDs, the empirical distributions of Al in the atomic percent are least-squares-fitted with a polynomial function of degrees up to seven and converted into the position-dependent mole fraction of x(zQD). We apply the experimental x(zQD) to an empirical gap formula38 of EAlxGa(1–x)As(zQD) = 1.519 + 1.155x + 0.37x2. The conduction band offset is taken to be 60% of the band gap difference.40,41 The direct Coulomb energy J projected onto an exciton state |ψeψh⟩ is written as42Inline graphic, where the Coulomb interaction is screened by the local dielectric function ϵ(|r – r′|) for a given position r with spin σ. The material parameters used in the calculation can be found in refs (38−40).

Acknowledgments

The authors thank Yeong Woo Jung and Min Kyung Cho at Korea Institute of Science and Technology (KIST) for assistance with TEM analysis, and Hye Jung Kim at University of Ulsan and Dokyun Kim at Pusan National University for fruitful discussions and help in numerical calculation. This research was supported by Basic Science Research Program (Nos 2017R1A6A3A04001981 and 2013R1A1A4A01004433) and Priority Research Centers Program (No. 2014R1A6A1031189) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, and by the Pioneer Research Center Program through the NRF funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3065033). The authors at KIST also acknowledge financial support from the KIST Internal Program of flagship (2E26420).

The authors declare no competing financial interest.

References

  1. Jayakumar H.; Predojević A.; Kauten T.; Huber T.; Solomon G. S.; Weihs G. Time-bin Entangled Photons from a Quantum Dot. Nat. Commun. 2014, 5, 4251 10.1038/ncomms5251. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Kim H.; Bose R.; Shen T. C.; Solomon G. S.; Waks E. A Quantum Logic Gate between a Solid-state Quantum Bit and a Photon. Nat. Photonics 2013, 7, 373. 10.1038/nphoton.2013.48. [DOI] [Google Scholar]
  3. Gazzano O.; Almeida M.; Nowak A.; Portalupi S.; Lemaîe A.; Sagnes I.; White A.; Senellart P. Entangling Quantum-logic Gate Operated with an Ultrabright Semiconductor Single-photon Source. Phys. Rev. Lett. 2013, 110, 250501 10.1103/PhysRevLett.110.250501. [DOI] [PubMed] [Google Scholar]
  4. Sun S.; Kim H.; Solomon G. S.; Waks E. A Quantum Phase Switch between a Single Solid-state Spin and a Photon. Nat. Nanotechnol. 2016, 11, 539. 10.1038/nnano.2015.334. [DOI] [PubMed] [Google Scholar]
  5. Kimble H. J. The Quantum Internet. Nature 2008, 453, 1023. 10.1038/nature07127. [DOI] [PubMed] [Google Scholar]
  6. Bouwmeester D.; Pan J.-W.; Mattle K.; Eibl M.; Weinfurter H.; Zeilinger A. Experimental Quantum Teleportation. Nature 1997, 390, 575. 10.1038/37539. [DOI] [Google Scholar]
  7. Aspuru-Guzik A.; Walther P. Photonic Quantum Simulators. Nat. Phys. 2012, 8, 285. 10.1038/nphys2253. [DOI] [Google Scholar]
  8. Ghali M.; Ohtani K.; Ohno Y.; Ohno H. Generation and Control of Polarization-entangled Photons from GaAs Island Quantum Dots by an Electric Field. Nat. Commun. 2012, 3, 661. 10.1038/ncomms1657. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Salter C. L.; Stevenson R. M.; Farrer I.; Nicoll C. A.; Ritchie D. A.; Shields A. J. An Entangled-light-emitting Diode. Nature 2010, 465, 594. 10.1038/nature09078. [DOI] [PubMed] [Google Scholar]
  10. Zhang J.; Wildmann J. S.; Ding F.; Trotta R.; Huo Y.; Zallo E.; Huber D.; Rastelli A.; Schmidt O. G. High Yield and Ultrafast Sources of Electrically Triggered Entangled-photon Pairs based on Strain-tunable Quantum Dots. Nat. Commun. 2015, 6, 10067 10.1038/ncomms10067. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Wu J.; Wang Z. M. Droplet Epitaxy for Advanced Optoelectronic Materials and Devices. J. Phys. D: Appl. Phys. 2014, 47, 173001 10.1088/0022-3727/47/17/173001. [DOI] [Google Scholar]
  12. Chistyakov A. A.; Zvaigzne M. A.; Nikitenko V. R.; Tameev A. R.; Martynov I. L.; Prezhdo O. V. Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications. J. Phys. Chem. Lett. 2017, 8, 4129. 10.1021/acs.jpclett.7b00671. [DOI] [PubMed] [Google Scholar]
  13. Stokes E. B.; Stiff-Roberts D. A.; Dameron C. T. Quantum Dots in Semiconductor Optoelectronic Devices. Electrochem. Soc. Interface 2006, 15, 23. [Google Scholar]
  14. Ward M. B.; Farrow T.; See P.; Yuan Z. L.; Karimov O. Z.; Bennett A. J.; Shields A. J.; Atkinson P.; Cooper K.; Ritchie D. A. Electrically Driven Telecommunication Wavelength Single-photon Source. Appl. Phys. Lett. 2007, 90, 063512 10.1063/1.2472172. [DOI] [Google Scholar]
  15. Wu J.; Shao D.; Dorogan V. G.; Li A. Z.; Li S.; DeCuir E. A. Jr.; Manasreh M. O.; Wang Z. M.; Mazur Y. I.; Salamo G. J. Intersublevel Infrared Photodetector with Strain-Free GaAs Quantum Dot Pairs Grown by High-Temperature Droplet Epitaxy. Nano Lett. 2010, 10, 1512. 10.1021/nl100217k. [DOI] [PubMed] [Google Scholar]
  16. Wu J.; Wang Z. M.; Dorogan V. G.; Li S.; Zhou Z.; Li H.; Lee J.; Kim E. S.; Mazur Y. I.; Salamo G. J. Strain-free Ring-shaped Nanostructures by Droplet Epitaxy for Photovoltaic Application. Appl. Phys. Lett. 2012, 101, 043904 10.1063/1.4738996. [DOI] [Google Scholar]
  17. Honeychurch K. C.Nanosensors for Chemical and Biological Applications: Sensing with Nanotubes, Nanowires and Nanoparticles, 2nd ed.; Academic Press: London, 2017. [Google Scholar]
  18. Martynenko I. V.; Litvin A. P.; P.-Milton F.; Baranov A. V.; Fedorov A. V.; Gun’ko Y. K. Application of Semiconductor Quantum Dots in Bioimaging and Biosensing. J. Mater. Chem. B 2017, 5, 6701. 10.1039/C7TB01425B. [DOI] [PubMed] [Google Scholar]
  19. Wang Y.; Hu R.; Lin G.; Roy I.; Yong K.-T. Functionalized Quantum Dots for Biosensing and Bioimaging and Concerns on Toxicity. ACS Appl. Mater. Interfaces 2013, 5, 2786. 10.1021/am302030a. [DOI] [PubMed] [Google Scholar]
  20. Koguchi N.; Takahashi S.; Chikyow T. New MBE Growth Method for InSb Quantum Well Boxes. J. Cryst. Growth 1991, 111, 688. 10.1016/0022-0248(91)91064-H. [DOI] [Google Scholar]
  21. Watanabe K.; Koguchi N.; Gotoh Y. Fabrication of GaAs Quantum Dots by Modified Droplet Epitaxy. Jpn. J. Appl. Phys. 2000, 39, L79 10.1143/JJAP.39.L79. [DOI] [Google Scholar]
  22. Kim J. S. references therein., Ga-migration on a Ga-rich and As-stabilized Surfaces: Ga-droplet and GaAs Nanostructure Formation. Materials Science in Semiconductor Processing 2017, 57, 70. 10.1016/j.mssp.2016.10.003. [DOI] [Google Scholar]
  23. Lee E. H.; Song J. D.; Han I. K.; Chang S. K.; Langer F.; Höling S.; Forchel A.; Kamp M.; Kim J. S. Structural and Optical Properties of Position-retrievable Low-density GaAs Droplet Epitaxial Quantum Dots for Application to Single Photon Sources with Plasmonic Optical Coupling. Nanoscale Res. Lett. 2015, 10, 1. 10.1186/s11671-015-0826-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Tighineanu P.; Daveau R.; Lee E. H.; Song J. D.; Stobbe S.; Lodahl P. Decay Dynamics and Exciton Localization in Large GaAs Quantum Dots Grown by Droplet Epitaxy. Phys. Rev. B 2013, 88, 155320 10.1103/PhysRevB.88.155320. [DOI] [Google Scholar]
  25. Belhadj T.; Kuroda T.; Simon C.-M.; Amand T.; Mano T.; Sakoda K.; Koguchi N.; Marie X.; Urbaszek B. Optically Monitored Nuclear Spin Dynamics in Individual GaAs Quantum Dots Grown by Droplet Epitaxy. Phys. Rev. B. 2008, 78, 205325 10.1103/PhysRevB.78.205325. [DOI] [Google Scholar]
  26. Heyn C.; Stemmann A.; Schramm A.; Welsch H.; Hansen W.; Nemcsics Á. Regimes of GaAs Quantum Dot Self-assembly by Droplet Epitaxy. Phys. Rev. B 2007, 76, 075317 10.1103/PhysRevB.76.075317. [DOI] [Google Scholar]
  27. Bocquel J.; Giddings A. D.; Mano T.; Prosa T. J.; Larson D. J.; Koenraad P. M. Composition Profiling of GaAs/AlGaAs Quantum Dots Grown by Droplet Epitaxy. Appl. Phys. Lett. 2014, 105, 153102 10.1063/1.4897006. [DOI] [Google Scholar]
  28. Huo Y. H.; Křàpek V.; Rastelli A.; Schmidt O. G. Volume Dependence of Excitonic Fine Structure Splitting in Geometrically Similar Quantum Dots. Phys. Rev. B 2014, 90, 041304(R) 10.1103/PhysRevB.90.041304. [DOI] [Google Scholar]
  29. Bietti S.; Bocquel J.; Adorno S.; Mano T.; Keizer J. G.; Koenraad P. M.; Sanguinetti S. Engineering of Epitaxial Quantum Dots by Grown Kinetics. Phys. Rev. B 2015, 92, 075425 10.1103/PhysRevB.92.075425. [DOI] [Google Scholar]
  30. Basset F. B.; Bietti S.; Reindl M.; Esposito L.; Fedorov A.; Huber D.; Rastelli A.; Bonera E.; Trotta R.; Sanguinetti S. High-Yield Fabrication of Entangled Photon Emitters for Hybrid Quantum Networking Using High-Temperature Droplet Epitaxy. Nano Lett. 2018, 18, 505. 10.1021/acs.nanolett.7b04472. [DOI] [PubMed] [Google Scholar]
  31. Moon P.; Lee J. D.; Ha S. K.; Lee E. H.; Choi W. J.; Song J. D.; Kim J. S.; Dang L. S. The Effect of Post-growth Thermal Annealing on the Emission Spectra of GaAs/AlGaAs Quantum Dots Grown by Droplet Epitaxy. Phys. Status Solidi RRL 2012, 6, 445. 10.1002/pssr.201206369. [DOI] [Google Scholar]
  32. Sanguinetti S.; Mano T.; Gerosa A.; Somaschini C.; Bietti S.; Koguchi N.; Grilli E.; Guzzi M.; Gurioli M.; Abbarchi M. Rapid Thermal Annealing Effects on Self-assembled Quantum Dot and Quantum Ring Structures. J. Appl. Phys. 2008, 104, 113519 10.1063/1.3039802. [DOI] [Google Scholar]
  33. Mano T.; Kuroda T.; Mitsuishi K.; Yamagiwa M.; Guo X.-J.; Furuya K.; Sakoda K.; Koguchi N. Ring-shaped GaAs Quantum Dot Laser Grown by Droplet Epitaxy: Effects of Post-growth Annealing on Structural and Optical Properties. J. Cryst. Growth 2007, 301, 740. 10.1016/j.jcrysgro.2006.11.216. [DOI] [Google Scholar]
  34. Keizer J. G.; Bocquel J.; Koenraad P. M.; Mano T.; Noda T.; Sakoda K. Atomic Scale Analysis of Self Assembled GaAs/AlGaAs Quantum Dots Grown by Droplet Epitaxy. Appl. Phys. Lett. 2010, 96, 062101 10.1063/1.3303979. [DOI] [Google Scholar]
  35. Swaminathan V.; Caruso R.; Pearton S. J. Photoluminescence from Annealed Semi-insulating GaAs Crystals: The 1.36-eV Band. J. Appl. Phys. 1988, 63, 2164. 10.1063/1.341078. [DOI] [Google Scholar]
  36. Fisher D. J.Diffusion in GaAs and Other III-V Semiconductors: 10 Years of Research, 1st ed.; Academic Press: Zurich, 1998. [Google Scholar]
  37. Mano T.; Mitsuishi K.; Nakayama Y.; Noda T.; Sakoda K. Structural Properties of GaAs Nanostructures Formed by a Supply of Intense As4 flux in Droplet Epitaxy. Appl. Surf. Sci. 2008, 254, 7770. 10.1016/j.apsusc.2008.02.025. [DOI] [Google Scholar]
  38. Aspnes D. E. GaAs Lower Conduction-band Minima: Ordering and Properties. Phys. Rev. B 1976, 14, 5331. 10.1103/PhysRevB.14.5331. [DOI] [Google Scholar]
  39. Yeo I.; Kim H. J.; Song J. D.; Yi K. S. Hybrid Quantum Optomechanics with Quantum-dot Single Photon Source. Phys. Rev. B 2016, 94, 165422 10.1103/PhysRevB.94.165422. [DOI] [Google Scholar]
  40. Missous M.Properties of Aluminum Gallium Arsenide, 1st ed.; Academic Press: London, 1993. [Google Scholar]
  41. Wang Y.; Zahid F.; Zhu Y.; Liu L.; Wang J.; Guo H. Band Offset of GaAs/AlxGa-xAs Heterojuctions from Atomistic First Principles. Appl. Phys. Lett. 2013, 102, 132109 10.1063/1.4800845. [DOI] [Google Scholar]
  42. Lee S.; Jonsson L.; Wilkins J. W.; Bryant G.; Klimeck G. Electron-hole Correlations in Semiconductor Quantum Dots with Tight-binding Wave Functions. Phys. Rev. B 2001, 63, 195318 10.1103/PhysRevB.63.195318. [DOI] [Google Scholar]

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