Broadband Quantum-Junction Photodiode Achieving Ultralow-Noise Light Sensing

Low-noise photodetectors hold immense promise for advancing weak-light sensing in cutting-edge applications such as health monitoring, intelligent driving, and military surveillance. Quantum-junction photodiode (QJPD), composed solely of light-absorbing quantum dots, are highly expected to overcome the noise limitation of conventional narrow-bandgap semiconductor photodetectors due to their inherent advantages of low hot carrier density as well as weak inter-dot electronic coupling. However, the targeted device engineering of QJPD remains a critical exploration area to fully unlock their low-noise potential. Here, a low-noise, broadband QJPD is proposed through the strategic incorporation of an ultrathin ALD SnO_x layer. This SnO_x modification efficiently suppresses the interfacial barrier, reduces the interface resistance and minimizes dark current of QJPDs. Consequently, SnO_x-modified QJPDs achieve an ultralow noise current of 4.85 × 10⁻¹⁴ A Hz⁻^1/2 and a specific detectivity exceeding 10¹² Jones across a broad spectral range (350−1050 nm), significantly outperforming unmodified QJPDs (≈10¹⁰ Jones) and other n-i-p quantum dot photodetectors (≈10⁹ Jones). SnO_x-modified QJPDs enable high-accuracy blood oxygen saturation measurements using near-infrared light and real-time heartbeat monitoring under weak ambient light conditions. This work establishes a foundation for the development of low-noise QJPDs, underscoring their potential for weak-light detection in challenging environments with strong hindering and ambient light.