Synthesis of a Selectively Nb-Doped WS<sub>2</sub>–MoS<sub>2</sub> Lateral Heterostructure for a High-Detectivity PN Photodiode
Văn Tú Vũ, Thanh Luan Phan, Thi Thanh Huong Vu, Mi Hyang Park, Van Dam, Viet Q. Bui, Kunnyun Kim, Young Hee Lee, Woo Jong Yu
Abstract
In this study, selective Nb doping (P-type) at the WS2 layer in a WS2–MoS2 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 (MoO3 > WO3 > Nb2O5) enable the separation of the growth temperature of intrinsic MoS2 to 700 °C (first grown inner layer) and Nb-doped WS2 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 WS2 layer is selectively controlled from approximately 1.87 × 107/cm2 at 1.5 at.% Nb to approximately 1.16 × 1013/cm2 at 8.1 at.% Nb, while the electron carrier density in n-type MoS2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS2–MoS2 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 (105) and a minimum channel resistance (108 Ω). With this optimized photodiode (8.1 at.% Nb at Vg = −30 V), an Iphoto/Idark ratio of 6000 and a detectivity of 1.1 × 1014 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.