NAMBE 2025 Session WME2-SuM: Quantum-Dot based Single Photon Emitters II

Entanglement is one of the most peculiar phenomena in quantum science and a key resource for quantum technologies. More than two decades after the initial proposal [1], semiconductor quantum dots (QDs) are now beginning to outperform other light sources for the generation of entangled photon pairs.
Among different material systems, QDs in the (Al)GaAs material platform have demonstrated the highest degree of polarization entanglement to date together with other appealing features for quantum science and technology [2–4]. These QDs are obtained by GaAs overgrowth of an AlGaAs surface with nanoholes and are characterized by small inhomogeneous broadening, high oscillator strengths, shape with high in-plane symmetry, and high optical quality, especially when embedded in charge-tunable diode structures. In this talk, we will discuss the properties of GaAs QDs obtained by the droplet etching method [5] and present recent results relevant to their application in quantum communication, such as entanglement-based quantum key distribution [6], as well as open challenges [7].

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[2] S. F. C. da Silva, G. Undeutsch, B. Lehner, S. Manna, T. M. Krieger, M. Reindl, C. Schimpf, R. Trotta and A. Rastelli, Appl. Phys. Lett. 119, 120502 (2021).

[3] L. Zhai, G. N. Nguyen, C. Spinnler, J. Ritzmann, M. C. Löbl, A. D. Wieck, A. Ludwig, A. Javadi and R. J. Warburton, Nat. Nanotechnol. 17, 829–833 (2022).

[4] L. Zaporski, N. Shofer, J. H. Bodey, S. Manna, G. Gillard, M. H. Appel, C. Schimpf, S. F. Covre da Silva, J. Jarman, G. Delamare, G. Park, U. Haeusler, E. A. Chekhovich, A. Rastelli, D. A. Gangloff, M. Atatüre and C. Le Gall, Nat. Nanotechnol. 18, 257–263 (2023).

[5] C. Heyn, A. Stemmann, T. Köppen, C. Strelow, T. Kipp, M. Grave, S. Mendach and W. Hansen, Appl. Phys. Lett. 94, 183113 (2009).

[6] C. Schimpf, M. Reindl, D. Huber, B. Lehner, S. F. Covre Da Silva, S. Manna, M. Vyvlecka, P. Walther and A. Rastelli, Sci. Adv. 7, eabe8905 (2021).

[7] B. U. Lehner, T. Seidelmann, G. Undeutsch, C. Schimpf, S. Manna, M. Gawełczyk, S. F. Covre da Silva, X. Yuan, S. Stroj, D. E. Reiter, V. M. Axt and A. Rastelli, Nano Lett. 23, 1409–1415 (2023).

The lack of scalable photon sources has been a major roadblock for quantum photonics to realize their full potential. Self-assembled InAs QDs currently hold the best all-around single photon emitter performance as a solid-state source, offering advantages of CMOS-compatible fabrication, highly tunable optical properties, and deterministic emission. The key challenge for deploying the InAs QD single photon source at large scale is the spatial and spectral randomness of each dot due to the self-assembling process on planar substrates. The prevalent method to combat this involves manipulating substrates to create preferential nucleation sites, either grooves or mesas. However, these “site-controlled” QDs typically exhibit inferior optical qualities and less repeatable charge tunability compared to their randomly situated counterparts on planar substrates. Such substrate alternations also limit the integrability with other devices. In this work, we utilize the intrinsic material properties, especially the coefficient of thermal expansion (CTE) mismatch between the GaAs substrate and the oxide layers and the asymmetric surface diffusion of indium adatoms, to develop site-controlled InAs QD single photon emitters nucleated on compartmentalized finite surfaces that will solve both issues simultaneously at wafer scale.

The growth template was fabricated first by oxide deposition on (001) GaAs. To ensure the “epi-ready” surface quality, the hexagonal pockets in two different orientations with respect to the III-V crystal were finished HF wet etching to remove the remaining post-dry etching oxide. The InAs QD material was then deposited in a Veeco Gen II molecular beam epitaxy chamber at elevated temperatures. Due to the CTE mismatch between the GaAs substate and the oxide layers, the substrate was under a global biaxial compression at the QD deposition temperature of 500 °C. The oxide patterns introduce local non-uniform profile with higher strain at the vertices of the hexagon and the strain level lowers toward the center of the pocket. The slow diffusion axis in the [1 1 0] orientation shows as “ridges” on the calculated potential energy profile. As the vertices are being filled, the energy penalty for adding more atoms increases and would generate new local and central potential energy minimums on either side of the slow diffusion axis. Thus, additional indium atoms are funneled toward the newly defined energy minimums. Hyperspectral images were taken under cryogenic temperatures of the as-grown InAs QDs embedded in GaAs.Emission from a single QD within one of the central minimums was observed in the pocket in the preferred orientation.

To fully leverage single quantum dots for quantum photonics requires spatial and spectral alignment of photonic nanostructures to individual quantum dots. Limited control of both spatial location and spectral properties of individual quantum dots during growth significantly hampers scalability and even initial development of single quantum dot devices.We have developed a cryogenic micro-photoluminescence setup for automatic position finding and spectroscopy of individual quantum dots on a massive scale. Our system employs two-color, wide-field illumination photoluminescence imaging to obtain positions of individual quantum dots relative to fiducial markers over 50-micron square fields of view with high throughput and nanometer scale uncertainties. The system then automatically performs confocal illumination and fluorescence collection sequentially over single quantum dots, repeating over a number of fields covering millimeter scale areas. Thousands of quantum dots can be located and spectrally characterized in tens of hours. Images of a calibration array of fiducial markers covering the field of view provide information about image distortion aberrations to correct quantum dot locations. The estimated RMS position accuracy is 29 nm after correction.We applied our system to the characterization of wafers containing GaAs liquid droplet etching and epitaxy quantum dots, obtaining photoluminescence spectra of thousands of individual quantum dots separated into three 10-nm spectral ranges, including excitation power-dependent spectra and polarization dependence. Spectra exhibiting sharp emission lines are almost exclusively found at the short-wavelength end of the quantum dot distribution. Efforts to fabricate spectrally tailored photonic nanostructures around targeted quantum dots are ongoing.