Deterministic nanofabrication of quantum dot-circular Bragg grating resonators with high process yield using in-situ electron beam lithography

TL;DR

This work demonstrates a scalable, marker-free approach for deterministic integration of InGaAs quantum dots into circular Bragg grating resonators using cryogenic in-situ electron-beam lithography (iEBL). By combining CL-based emitter identification with direct patterning in a single cryogenic session, the authors achieve high alignment accuracy and a process yield >$90\%$ across 103 devices, with 95 optically active. They show that 3–4-ring CBGs deliver near-maximal brightness while reducing fabrication time and footprint, supported by FEM-guided design and experimental validation. Quantum optical measurements on 41 devices reveal bright, spectrally clean emission with $g^{(2)}(0)=0.011\pm0.002$ and moderate HOM visibility, indicating high-quality single-photon sources suitable for scalable on-chip quantum photonics. Overall, iEBL provides a robust, high-yield route toward wafer-scale deterministic integration of QD-based single-photon sources for photonic quantum technologies.

Abstract

The controlled integration of quantum dots (QDs) as single-photon emitters into quantum light sources is essential for the implementation of large-scale quantum networks. In this study, we employ the deterministic in-situ electron-beam lithography (iEBL) nanotechnology platform to integrate individual QDs with high accuracy and process yield into circular Bragg grating (CBG) resonators. Notably, CBG devices comprising just 3 to 4 rings exhibit photon extraction efficiencies comparable to those of structures with more rings. This facilitates faster fabrication, reduces the device footprint, and enables compatibility with electrical contacting. To demonstrate the scalability of this process, we present results of 95 optically active QD-CBG devices fabricated across two lithography sessions. These devices exhibit bright, narrow-linewidth single-photon emission with excellent optical quality. To evaluate QD placement accuracy, we apply a powerful characterization technique that combines cathodoluminescence (CL) mapping and scanning electron microscopy. Statistical analysis of these devices reveals that our iEBL approach enables high alignment accuracy and a process yield of over >90% across various CBG geometries. Our findings highlight a reliable route toward the scalable fabrication of high-performance QD-based single-photon sources for use in photonic quantum technology applications.

Figures (5)

• Figure 1: (a) Final hybrid flip-chip bonded device architecture with SiO2/Au mirror stack. Conceptual schematic diagram of the single-photon emission process from a QD embedded in a CBG resonator. (b) Layer structure of the epitaxially grown heterostructure comprising a GaAs membrane and sacrificial Al0.9Ga0.1As layers. QDs are indicated by red triangles. (c) FEM simulation results showing photon extraction efficiency and Purcell factor for CBGs with 1 to 5 rings, highlighting the $(895 \pm 3)$ nm integration window.
• Figure 2: Structural and optical overview of a representative 20 × 20 µm² device field fabricated via the iEBL process. (a) High-resolution SEM image displays three exemplary QD-CBG structures with increasing ring numbers (2, 3, and 5 rings) in a single mapping field. (b) The CL intensity map, acquired from the corresponding field at 20 K, with consistent photon emission from the integrated QD. (c) Normalized CL spectra from three devices to confirm spectral reproducibility and successful spectral targeting during iEBL integration.
• Figure 3: (a) A representative example of a 2D Gaussian function fitted to the integrated emission intensity (in the indicated spectral range of 895.4 to 895.9 nm) of the QD CL spectra. This fitting is used to extract the emission center of the deterministically integrated QD with an offset of $(66 \pm 34)$ nm. (b) Spatial distribution of determined QD positions of a total of 60 deterministically fabricated QD devices in the 2D plane accompanied by error bars derived from fit residuals and SEM scan resolutions, defined as the vectorial distance between the QD emission center (from the Gaussian fit of CL emission) and the geometric center of the mesa structure (from SEM analysis), The gray area indicates the size of the mesa. Dashed circles with a diameter of 100 nm and 200 nm contain approximately 20% and 55% of the devices, respectively. (c) The offset histograms along the x and y axes were then fitted with Gaussian functions, yielding a mean offset of $\mu_x = (-59 \pm 117)$ nm and $\mu_y = (-20 \pm 161)$ nm. These values represent the upper-bound alignment precision and accuracy due to extended and asymmetric CL emission profiles.
• Figure 4: (a) Photon extraction efficiency versus $\Delta r_{QD}$. The simulated data (lines) for 1-ring (black) to 5-ring (cyan) devices are combined with experimentally measured PEE values (bars). (b) Statistical distribution of devices exhibiting over 30%, 40%, and 50% is grouped by CBG geometry and evaluated within a 200 nm integration offset. The employment of 3-4 ring designs has been demonstrated to consistently yield high-performing devices, thereby highlighting their suitability for scalable quantum photonic applications.
• Figure 5: Time-resolved and quantum optical characterization of a representative 3-ring QD–CBG device. (a) Time-resolved PL decay under 881 nm pulsed excitation, fitted linearly, yielding a spontaneous emission lifetime of $\tau_{\mathrm{QD}}=(0.70 \pm 0.04)$ ns (inset: spectrum shows the charged exciton ($X^{+}$) emission). (b) Second-order autocorrelation histogram showing $g^{(2)}(0)=0.011 \pm 0.002$, indicating high single-photon purity, and (c) HOM interference histogram with a 4 ns pulse delay, revealing raw visibility of 53% and fitted visibility of 63% (inset: shows a close-up of the central dip near zero delay).