Taking aim – accurate integration of quantum dots and bullseye cavities

C.R. Copeland, A.L. Pintar, R.G. Dixson, A. Chanana, K. Srinivasan, D.A. Westly, B.R. Ilic, M.I. Davanco, S.M. Stavis
National Institute of Standards and Technology,
United States

Keywords: localization microscopy, photonic integration, traceability


Self-assembled quantum dots are promising light sources for quantum networks and sensors. These emerging technologies require the accurate integration of quantum dots and photonic structures, but epitaxial growth forms quantum dots at random positions in semiconductor substrates. Optical localization of these random positions can guide the placement of photonic structures by electron-beam lithography. This integration process requires the reliable registration of position data across microscopy and lithography systems. However, large errors can result from multiple sources, including lithographic and cryogenic variation of reference dimensions for microscope calibration, as well as localization errors from optical distortion. Such errors tend to increase across an imaging field, presenting a critical impediment to exploiting the throughput and scalability of widefield microscopy. In this study, we take aim at this problem and show how our solution enables accurate integration to improve device performance and process yield. We develop our methods of traceable localization to calibrate a cryogenic localization microscope. This instrument is an optical microscope with the sample and objective lens inside of a cryostat, and custom optics outside of the cryostat. We fabricate and characterize arrays of submicrometer pillars in silicon (100), creating microscopy standards with both traceable reference positions at approximately 293 K and traceable reference data for thermal expansion coefficient. We image these arrays with the cryogenic microscope at approximately 1.8 K, localize the pillar positions, and use the reference data to calibrate the microscope. Our calibration determines the scale factor of the imaging system and corrects position errors due to complex distortion, among other aberration effects. We combine the results of this cryogenic calibration with our previous assessment of fabrication accuracy by electron-beam lithography, introducing a comprehensive model of the effects of registration errors on Purcell factor. This performance metric quantifies the radiative enhancement that occurs upon placing quantum dot into a bullseye cavity. For an exemplary system, the Purcell factor reaches a maximum value of approximately 11 for error-free registration of the quantum dot and cavity center. Our model determines distributions of Purcell factor for an array of positions spanning the field, combining errors and uncertainties from throughout the processes of fabrication, calibration, and measurement, and demonstrating the possibility of significantly improving Purcell factor across a wide field. Depending on the Purcell factor threshold, accurate integration can increase yield by one to two orders of magnitude. This foundation of accuracy will enable a transition from demonstration devices to efficient processes, leading to the reliable production and statistical characterization of quantum information systems.