Electron Transport of Germanium Sulfide Field- Effect Transistor Using Layered Black Phosphorus

Layered 2D materials have recently attracted attention due to their unique electrical and optical properties. In particular, Germanium Sulfide(GeS) which has been predicted to exhibit high mobility. However, few research work reported in the literature on GeS devices and they exhibit high contact resistance. The origin of this contact resistance is still not clear. Here, we investigate the electron transport of pristine GeS field effect transistors. A high contact resistance was observed. Various methods were used to overcome this issue including thermal, electrical annealing, and 2D heterojunction contact. Both annealing methods did not yield any improvement in the measured IV_ds. However, we observe an anomalous negative differential resistance(NDR) with low contact resistance and n-type conductivity when black phosphorus(BP) are deposited between GeS and S/D electrodes. This NDR is uncontrollable in ambient and insensitive to gate voltage. We believe this NDR is caused by chemical reaction between GeS and degraded BP where the measured electrical conductivity leads to chemical doping, causing the observation of conductivity switch from p-type for pristine GeS, to n-type for GeS with degraded BP contacts. Further investigations are needed to characterize the origin of this NDR.