Fig. 1: Concept of the high-speed vertical WSe2 p-i-n photodiode.
a Development target for self-powered photodetectors. Existing photodetectors are invariably constrained by a speed-responsivity trade-off (blue solid line), manifesting as either rapid response with limited responsivity, high responsivity with sluggish response, or intermediate performance in both metrics. The ultimate target (red five-pointed star) of self-powered photodetector is the concurrent realization of enhanced responsivity and accelerated response speed. b Schematic diagram of the vertical WSe2 p-i-n photodiode. Niobium (Nb) doped WSe2 (Nb-WSe2, blue dash line region), pristine WSe2 (i-WSe2) and Argon (Ar) plasma treated WSe2 (Ar-WSe2, red dash line region) are used as p type/intrinsic/n type layer, respectively. P-type doping is achieved through niobium substitutional doping, while n-type doping is realized by generating selenium vacancies via Ar plasma bombardment of the WSe₂ surface. c Band diagrams of metal contact with lightly p-doped (upper) and degenerately p-doped (bottom) semiconductor, respectively. Light doping creates a wide and high potential barrier, resulting in large contact resistance and consequently increased RC time (tRC). In contrast, degenerate doping produces a narrow barrier and direct tunneling transport of carriers, yielding lower contact resistance and reduced RC time. d Band diagrams of the p-i-n photodiode with lightly doped (upper) and degenerately doped (bottom) p-type layer under light illumination, respectively. Light doping creates a narrow depletion region in the intrinsic layer with relatively weak electric field, resulting in low drift velocity of photo-generated carriers under illumination. In contrast, degenerate doping enables complete depletion of the intrinsic layer, where the strong electric field facilitates high-speed carrier drift. e Schematic diagram of the photodiode response time with lightly doped (upper) and degenerately doped (bottom) p-type layer, respectively. Light doping results in a high RC time and slow carrier drift, leading to a sluggish detector response speed, as evidenced by the gradual transition in the current-time (I-t) curve. In contrast, degenerate doping reduces the RC time and accelerates carrier drift, enabling fast detector response with sharp transitions in the I-t curve.
