Synthesis of a Selectively Nb-Doped WS 2 -MoS 2 Lateral Heterostructure for a High-Detectivity PN Photodiode.

In this study, selective Nb doping ( P -type) at the WS 2 layer in a WS 2 -MoS 2 lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO 3 > WO 3 > Nb 2 O 5 ) enable the separation of the growth temperature of intrinsic MoS 2 to 700 °C (first grown inner layer) and Nb-doped WS 2 to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p -type WS 2 layer is selectively controlled from approximately 1.87 × 10 7 /cm 2 at 1.5 at.% Nb to approximately 1.16 × 10 13 /cm 2 at 8.1 at.% Nb, while the electron carrier density in n -type MoS 2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS 2 -MoS 2 heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (10 5 ) and a minimum channel resistance (10 8 Ω). With this optimized photodiode (8.1 at.% Nb at V g = -30 V), an I photo / I dark ratio of 6000 and a detectivity of 1.1 × 10 14 Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.