Assessing the alignment accuracy of state-of-the-art deterministic fabrication methods for single quantum dot devices

TL;DR

The paper benchmarks three deterministic QD placement strategies—marker-based PL imaging, marker-based CL imaging, and marker-free in-situ EBL—by fabricating circular mesas around preselected QDs and measuring the final QD-to-mesa offsets. Localization uncertainties estimated from fit procedures (∼nm scale) underrepresent the true final alignment accuracy, which is limited by marker localization and EBL fabrication errors. CL imaging generally yields smaller final offsets than PL due to sharper marker contrast, while in-situ EBL avoids marker-related errors but is susceptible to cryostat drift; compensating for such drift is essential for reliable, scalable device fabrication. Across methods, final alignment uncertainties remain ≲100 nm, indicating that current deterministic positioning approaches, while precise at localization, require further refinement to meet scalability targets for quantum photonic circuits. The work also shows that device performance metrics like Purcell factor and extraction efficiency deteriorate with QD displacement, underscoring the practical importance of achieving tighter alignment.

Abstract

The realization of efficient quantum light sources relies on the integration of self-assembled quantum dots (QDs) into photonic nanostructures with high spatial positioning accuracy. In this work, we present a comprehensive investigation of the QD position accuracy, obtained using two marker-based QD positioning techniques, photoluminescence (PL) and cathodoluminescence (CL) imaging, as well as using a marker-free in-situ electron beam lithography (in-situ EBL) technique. We employ four PL imaging configurations with three different image processing approaches and compare them with CL imaging. We fabricate circular mesa structures based on the obtained QD coordinates from both PL and CL image processing to evaluate the final positioning accuracy. This yields final position offset of the QD relative to the mesa center of $μ_x$ = (-40$\pm$58) nm and $μ_y$ = (-39$\pm$85) nm with PL imaging and $μ_x$ = (-39$\pm$30) nm and $μ_y$ = (25$\pm$77) nm with CL imaging, which are comparable to the offset $μ_x$ = (20$\pm$40) nm and $μ_y$ = (-14$\pm$39) nm obtained using the in-situ EBL method. We discuss the possible causes of the observed offsets, which are significantly larger than the QD localization uncertainty obtained from simply imaging the QD light emission from an unstructured wafer. Our study highlights the influences of the image processing technique and the subsequent fabrication process on the final positioning accuracy for a QD placed inside a photonic nanostructure.

Figures (5)

• Figure 1: Procedure for nanostructures positioning around pre-selected QDs. Images are taken using either PL or CL imaging systems and processed using rigorous image analysis algorithms to precisely extract the location of the target QDs relative to alignment markers. The determined coordinates are then used to fabricate nanostructures (here, circular mesas) around the QDs using EBL. The In-situ EBL combines CL imaging for marker-free QD localization with EBL structuring in one process setup.
• Figure 2: Determining the location of QDs with reference to alignment markers using QD imaging techniques. (a)-(d) displays the two-color PL method (Single-image 1) and (e)-(h) the CL method. Image obtained using (a) PL imaging setup and (e) CL mapping setup. (b) Intensity line cut profile (x-axis) of QD PL (blue dots) and its Gaussian fit (red line, with one standard deviation peak position error) and (c) intensity line cut profile (x-axis) of cross-correlated marker image along the located center, of the image in (a). (d) Histogram of the uncertainties in the QDs location, alignment markers location, and the combined uncertainties of the QD and marker (QD+marker) of the image in (a), measured from the line cuts from 15 images (taken at different field regions on the sample). (f) and (g) Intensity line cut (x-axis) of QD CL and marker image, respectively, of the image in (e). (h) Histogram of the uncertainties in the QD location, alignment marker location, and the combined uncertainties of the QD and marker (QD + marker) of the image in (e). The location uncertainties of the QDs were extracted from the 2D Gaussian fit of the QDs profiles for both techniques. The location uncertainties of the alignment markers in the PL technique were extracted from the polynomial fit of a cropped region of interest (20 x 20-pixel area) around the cross-correlation center (with a 68% confidence interval), while the CL marker uncertainties were determined from a straight line fit through the marker center. Combined QD+ alignment marker uncertainties are obtained by propagating the uncertainties of the QDs and alignment markers.
• Figure 3: Finding the accuracy of the determined QD location after nanostructure fabrication. (a) 3D sketch illustration of the QD misalignment relative to the center of the mesa structure. (b) SEM image of fabricated mesas around determined QDs coordinates. (c) CL map and SEM of QD in mesa structure, showing the QD emission profile within the mesa structure (QD center obtained from 2D Gaussian fit, and the mesa edges fitted with an ellipse). QD offset distribution around the mesa center of CL imaging method in (d), and PL imaging (Single-image 1) method (shaded region) using cross-correlation, edge detection marker, and auto-cross correlation localization approaches in (e), (f), and (g), respectively. (h) Histogram of the offsets' distribution of all the methods with their mean offsets and standard deviation (uncertainty). N10, O10, L11, and J11 are the fields where the QDs have been located.
• Figure 4: Comparing the accuracy of marker-based positioning technique with in-situ EBL technique. (a) Comparison plot of the QDs offset distribution around the mesa center for marker-based PL (edge detection) and CL imaging after EBL error is compensated (to remove user-dependent error). (b) QDs offset distribution around the mesa center for the in-situ EBL technique.
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