Telecom Photon Emitters Based on Isolated Group-IV Quantum Dots Deterministically Coupled to High-Q Photonic Crystal Cavities
Authors: Poempool Thanavorn , Aberl Johannes , Clementi Marco , Spindlberger Lukas , Vukusic Lada , Galli Matteo , Gerace Dario , Brehm Moritz , Fromherz Thomas 
Published: August 2024

The realization of scalable telecom light emitters that can be monolithically implemented into large-scale Si integration technology has been a driving factor in the field of Si photonics for the last decades. Applications are envisioned in the short and medium-range data transfer and for realizing scalable, Si-based quantum information technology. For the latter, CMOS-compatible sources that can be controlled on the single photon level are needed. In solid-state materials, group-III-V quantum dots (QDs) are one of the leading platforms for realizing single photon emitters of excellent optical quality [1,2]. However, their emission outside the telecom band and the difficulty of combining them with the mature Si photonics components such as waveguides remain an ongoing problem.

In the group-IV (SiGe) system, the bandgap of the bulk materials is indirect, and electron and hole states in QDs are spatially separated due to the type-II band alignment. These material properties lead to relatively long radiative lifetimes and small emission efficiencies. However, for Si/SiGe QDs grown on silicon on insulator substrates (SOI), these efficiencies can be strongly and deterministically Purcell-enhanced by aligning photonic crystal cavities with single QDs [3]. One necessary prerequisite is, therefore, to achieve perfect nucleation site control of the QDs on the substrates [4,5]. This nucleation control can be achieved by deterministic substrate patterning combined with Ge growth in the supersaturation regime of the two-dimensional wetting layer [6], utilizing Ge's extensive surface diffusion lengths at growth temperatures above 600°C [7]. In such a way, it is possible to grow isolated QDs with virtually arbitrary inter-QD distance [4,5].

Here, one isolated QD was grown on a substrate area of the size of 100´100 µm2 [8]. We demonstrate the accurate QD positioning in PhC cavities [3,8], particularly in bichromatic cavities. Bichromatic cavities, a relatively novel design of photonic crystal cavity, offer ultra-high quality factor, and therefore Purcell enhancement, that is achievable with simple design rules [9]. Unlike in standard line defect cavities, in the present bichromatic layout, the line defect consists of a periodic sequence of holes, leaving only narrow Si regions.

For this reason, an extremely high accuracy for QD positioning is required, which can be provided by interlacing site control of QD growth and PhC cavity formation. In micro photoluminescence spectra, the modes of single cavities are clearly observed. From the dependence of the mode intensities on the temperature and the excitation intensity, modes fed by the single QD in the cavity can be identified. We observe ultra-high quality factors of up to 100,000 for the modes coupled to QDs [8]. For resonators loaded with a QD, such high Q-factor values are record-high for the silicon-on-insulator integrated optics platform. Results on the excited state lifetime will be discussed.

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[9] A. Simbula et al., APL Photonics 2, 056102 (2017).