Ultra-High Responsivity Black-Si/Graphene Heterojunction Photodetectors Enabled by Enhanced Light Absorption and Local Electric Fields

Photodetectors with high responsivity, fast response, and broad spectral response are of great importance for a wide range of applications in fundamental science and various industries. However, conventional photodiodes operating at low bias voltage do not provide any gain. The graphene (Gr)/Si van der Waals heterostructure, on the other hand, offers a potential gain due to the limited density of states near the Dirac point. In this work, a highly photoresponsive broadband pyramidal black-Si/Gr heterojunction photodetector is presented. The device, with an active area of 5x5 mm2 and a bias voltage of -5 V, exhibits an ultra-high responsivity of 4.1 A W-1. The photoresponsivity can be further increased to 1379 A W-1 by reducing the device area. Comparative experiments reveal that the pyramidal black-Si/Gr photodetectors exhibit the largest responsivity compared with pyramidal-Si/Gr and flat-Si/Gr photodetectors. The gain in pyramidal black-Si/Gr photodetectors is attributed to both the pyramidal nanoporous structures and the shift of the Fermi level of Gr under bias. Furthermore, the high responsivity and stable operation of the photodetectors enable the demonstration of imaging applications. The results provide a new strategy for enhancing the performance of photodetectors based on 2D materials. The authors report high photoresponsivity (1379 A W-1) and broadband (308-1300 nm) photodetectors based on pyramidal black-Si/graphene (Gr) heterojunctions. The incorporation of black-Si not only augments the absorption of incident light, but also intensifies the electrical fields near the apexes. This, together with the reduced Schottky barrier resulting from the upward shift of the Fermi level of Gr, contributes to a significant gain.image