van der Waals Doping and Room Temperature Resonant Tunneling Observed in Black Phosphorus/Germanium Sulfide Transistors

van der Waals semiconductors have proven to be exceptional for electronic and photonic applications. Although most research is extensively focused on some transition metal dichalcogenides materials (MX2) such as MoS2, WS2, WSe2, and MoSe2, studies on 2D metal monochalcogenides such as germanium sulfide (GeS) has been widely under investigated, mainly due to the high contact resistance GeS devices exhibit. Here, a van der Waals field-effect transistor (VdW-FET) based on GeS is investigated and resonant tunneling behavior is shown at room temperature due to VdW doping via black phosphorus (BP), evident by the observation of multiple decades of negative differential resistance (NDR) during doping transient state. These NDR decades are caused by confinement of carriers inside the double barrier quantum well, which allows tunneling to occur for discrete energy levels. Moreover, a noticeable conductivity switch from a low p-type to a high n-type is observed, with a conductivity enhancement of 2 orders of magnitude compared to pristine GeS devices. The underlying mechanism behind the observed NDR and the conductivity switch is discussed and it is shown that these phenomena are likely caused by phosphorus doping due to BP sublimation, evident by the detected P-Ge Raman peak in the measured Raman spectra. The results can open doors for electrical oscillators and switching devices for the next generation of nanoelectronics.