Abstract
Abstract The rapidly developing quantum communication technology requires deterministic quantum emitters that can generate single photons and entangled photon pairs in the third telecom window, in order to be compatible with existing optical fiber networks and on-chip silicon photonic processors. InAs/InP quantum dots (QDs) are among the leading candidates for this purpose, due to their high emission efficiency in the required spectral range. However, fabricating versatile InAs/InP QD-based quantum emitters is challenging, especially as these QDs typically have asymmetric profiles in the growth plane, resulting in a substantial bright-exciton fine structure splitting (FSS). This hinders the generation of entangled photon pairs and thus, compromises the versatility of InAs/InP QDs. We overcome this by implementing droplet epitaxy (DE) synthesis of low surface density (2.8 × 108 cm−2) InAs x P1−x QDs with x = (80 ± 15)% on an (001)-oriented InP substrate. The resulting QDs are located in etched pits, have concave bases, and most importantly, have symmetric in-plane profiles. We provide an analytical model to explain the kinetics of pit formation and QD base shape modification. Our theoretical calculations of electronic states reveal the properties of neutral and charged excitons and biexcitons confined in such QDs, which agree with the optical investigations of individual QDs. The optical response of QDs' ensemble suggests that FSS may indeed be negligible, as reflected in the vanishing degree of linear polarization. However, single QD spectrum gathered from an etched mesa shows moderate FSS of (50 ± 5) µeV that we link to destructive changes made in the QD environment during the post-growth processing. Finally, we show that the studied DE QDs provide a close-to-ideal single-photon emission purity of (92.5 ± 7.5)% in the third telecom window.
Highlights
Epitaxially-grown semiconductor quantum dots (QDs), among other applications [1], are considered as nearly perfect quantum emitters [2, 3] for applications in quantum communication [4] and quantum computation [1, 5] technologies
We present a detailed investigation of the morphology, chemical composition, electronic structure and optical properties of ensemble, and individual droplet epitaxy (DE) InAs(P)/InP QDs grown in metalorganic vapor-phase epitaxy (MOVPE)
We have examined in detail the morphology and chemical composition of surface and buried QD-in-pit structures using atomic force microscopy (AFM) imaging, scanning transmission microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX)
Summary
Epitaxially-grown semiconductor quantum dots (QDs), among other applications [1], are considered as nearly perfect quantum emitters [2, 3] for applications in quantum communication [4] and quantum computation [1, 5] technologies. InAs/InP QDs naturally emit at longer wavelengths, while offering high photon emission purity [18, 26] and indistinguishability [27]. Controlling their growth kinetics and the resulting morphology with respect to size, shape anisotropy, and surface density is complicated. The obtained DE QDs have been shown to possess high inplane symmetry and exhibit very low FSS values [39, 40], allowing them to be employed in quantum communication schemes utilizing entangled photon pairs [4]. We measure an exciton FSS value on the level of (50 ± 5) μeV, this is likely related to the post-growth processing of the QD structure and requires further optimization
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