Abstract
The detection of few photons in single-photon detectors is of interest in various applications. For instance, if we focus on Near-Infrared (NIR) single photons: light detection and ranging (LiDAR), quantum key distribution (QKD), and time-correlated single photon counting (TCSPC) are the main application őelds calling for suitable detectors. Indeed, the lack of optical detectors yielding high quantum efficiency and low time resolution, these measurements would be not only difficult but also inaccurate. Among the technologies available for NIR single-photon detection, single-photon avalanche diodes (SPADs) based on silicon (Si) are the most technologically mature single-photon detector available on the market. A lot of work has been recently done on Si-based SPAD, with the main purpose of overcoming the limited absorption of silicon in the NIR region. The most promising solution to overcome the technology limits of thick SPAD and simultaneously the low Si absorption appears to be the coupling of thin SPAD with light-trapping nanostructures . In this work, we integrate CMOS-compatible grating nanostructures on top of the SPAD active region to conőne the incident NIR light. Speciőcally, plasmonic nanostructures supporting SPPs were proposed due to manifold attractive optical properties. Several challenges linked to the production of plasmonic nanostructures directly on an active device were faced: surface topography, the device’s composition, thermal stability, as well as side damage induced by the integration. Among the nanofabrication techniques compatible with CMOS technology, electron beam lithography (EBL) and focused ion beam (FIB) were utilized due to their capability to deőne high-resolution nanostructures. The integrated light-conőning plasmonic structures delivered consist of an array of metallic squares with a őxed periodicity, geometrically őne-tuned using time domain simulation software, to achieve maximum detector performance at 950 nm, wavelength of interest for may above-mentioned ap plications. Then, to guarantee the plasmonic grating’s best geometry and resolution and to avoid irreversible damage in the detector, the nanofabrication processes both via EBL+lift-off and FIB have been őnely-tuned by atomic force microscope (AFM), secondary ion Mass spectroscopy (SIMS) and scanning electrons microscope (SEM) characterization. Finally, the integrated detectors were electro-optically characterized by measuring their quantum efficiency as a function of the wavelength. Despite the presence of some non-idealities in the nanofabrication geometry, the maximum performance at 950 nm of 13% was reached with a 3 µm thick detector, showing a gain of 2.2 times more than the reference detector without grating. This result suggested that is possible to produce a device capable of detecting single NIR photons, at a moderate cost and compatible with CMOS, thus integrable on existing technology platforms. Nevertheless, further investigation and improvement of the structures are required, suggesting that better and better performance could be achieved in the next future.