Spectral Responsivity and Photoconductive Gain in Thin Film Black Phosphorus Photodetectors

We have fabricated black phosphorus photodetectors and characterized their full spectral responsivity. These devices, which are effectively in the bulk thin film limit, show broadband responsivity ranging from <400 rim to the similar to 3.8 mu m bandgap. In the visible, an intrinsic responsivity >6 A/W can be obtained due to internal gain mechanisms. By examining the full spectral response, we identify a sharp contrast between the visible and the infrared behavior. In particular, the visible responsivity shows a large photoconductive gain and gate-voltage dependence, while the infrared responsivity is nearly independent of gate voltage and incident light intensity under most conditions. This is attributed to a contribution from the surface oxide. In addition, we find that the polarization anisotropy in responsivity along armchair and zigzag directions can be as large as 10(3) and extends from the band edge to 500 nm. The devices were fabricated in an inert atmosphere and encapsulated by Al2O3, providing stable operation for more than 6 months.