Synthesis of a Selectively Nb-Doped WS₂-MoS₂ Lateral Heterostructure for a High-Detectivity PN Photodiode.

Abstract

In this study, selective Nb doping (<i>P</i>-type) at the WS₂ layer in a WS₂-MoS₂ lateral heterostructure <i>via</i> 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 <i>p</i>-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 <i>n</i>-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 <i>N-N</i> junction (0 at.% Nb) to the <i>P-N</i> junction (4.5 at.% Nb) and the <i>P-N</i> tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the <i>P-N</i> 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 <i>V</i>_g = -30 V), an <i>I</i>_photo/<i>I</i>_dark 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 <i>P-N</i> junctions.