Universal Growth Scheme for Quantum Dots with Low Fine-Structure Splitting at Various Emission Wavelengths

Joanna Skiba-Szymanska, R. Mark Stevenson, Christiana Varnava, Martin Felle, Jan Huwer, Tina Müller, Anthony J. Bennett, James P. Lee, Ian Farrer, Andrey B. Krysa, Peter Spencer, Lucy E. Goff, David A. Ritchie, Jon Heffernan, and Andrew J. Shields

Phys. Rev. Applied 8, 014013 – Published 14 July 2017

Abstract

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘‘droplet-epitaxy’’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at $\sim 900 nm$ , and metal-organic vapor-phase epitaxy on InP emitting at $\sim 1550 nm$ , with mean FSS $4 \times$ smaller than for Stranski-Krastanow QDs.

Received 5 September 2016

DOI:https://doi.org/10.1103/PhysRevApplied.8.014013

Physics Subject Headings (PhySH)

Entanglement production Growth Optical quantum information processing Optoelectronics Photonics

III-V semiconductors Quantum dots

Epitaxy

Quantum Information, Science & TechnologyCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Joanna Skiba-Szymanska^1,*, R. Mark Stevenson¹, Christiana Varnava^1,2, Martin Felle^1,2, Jan Huwer¹, Tina Müller¹, Anthony J. Bennett¹, James P. Lee^1,2, Ian Farrer^3,4, Andrey B. Krysa⁴, Peter Spencer³, Lucy E. Goff³, David A. Ritchie³, Jon Heffernan⁴, and Andrew J. Shields¹

¹Toshiba Research Europe Limited, 208 Science Park, Milton Road, Cambridge CB4 0GZ, United Kingdom
²Cambridge University Engineering Department, 9 J. J. Thomson Avenue, Cambridge CB3 0FA, United Kingdom
³Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
⁴EPSRC National Centre for III-V Technologies, Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom

^*To whom correspondence should be addressed. joanna.skiba@crl.toshiba.co.uk

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Vol. 8, Iss. 1 — July 2017

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Images

Figure 1
Illustration of droplet formation. Scanning electron micrographs of uncrystallized indium droplets deposited on GaAs (001) (a) and InP (001) surface (c). (b) and (d) are cross-section illustrations of the DE QDs grown in GaAs and InP, respectively. The droplet-epitaxy growth scheme is illustrated in (e): a substrate is deposited with group-III material followed by crystallization with a group-V flux leading to the formation of quantum dots.
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Figure 2
Comparison of uncapped SK and DE dots grown on InP. Atomic force microscope (afm) images of metalorganic vapor-phase epitaxy (MOVPE) crystallized DE QDs on InP (001) surface (a) and SK QDs on InP (001) surface (b). The scan area in both cases is $2 \times 2 μ m^{2}$ . The insets are enlarged images of individual dots. Line scans over a typical DE QD and SK QD on InP are shown on (c) and (d), respectively. (e) and (f) are 3D representations of afm scans of DE QD and SK QD on InP, respectively.
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Figure 3
(a) Spectrum taken on an optical cavity showing an ensemble of DE QDs at 20 K. The cavity mode is present at the central wavelength of $1551 \pm 0.35 nm$ and $FWHM = 30 nm$ . The inset shows a spectrum taken on an individual dot with a fine grating of the spectrometer. (b) RT reflectance spectrum taken on the same optical cavity showing the cavity peak centered at 1567 nm.
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Figure 4
(a)–(c) Spectra of individual InAs quantum dots for (a) SK QDs on InP emitting at $\sim 1550 nm$ , (b) DE QDs on InP emitting at $\sim 1550 nm$ , and (c) DE QDs on GaAs emitting at $\sim 900 nm$ . $X$ and $X X$ transitions are identified. (d)–(f) Corresponding polarized $X$ emission spectra as a function of the angle of a quarter-wave plate (d),(e) or half-wave plate (f). (g),(h) Example FSS measurements of (g) DE QDs on InP emitting at $\sim 1550 nm$ , and (h) DE QDs on GaAs emitting at $\sim 900 nm$ . Each panel comprises two example measurements for $X$ lines with small and large FSS. For details, see text.
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Figure 5
Exciton fine-structure splitting of quantum dots emitting at around 900 and 1550 nm (telecom $C$ band). (a) FSS histogram for SK QDs grown on InP emitting at 1550 nm. (b) FSS histogram for DE QDs grown on InP emitting around 1550 nm. (c) FSS histogram for DE QDs grown on GaAs emitting around 900 nm. (d) Corresponding FSS dependence on the emission wavelength measured for DE (green) and SK (blue) QDs grown on InP, and DE QDs grown on GaAs (red). The insets in (b) and (c) show polar plots for FSS values measured for DE QDs grown on InP and GaAs, respectively.
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