Temperature dependent electrical characterization of monolayer MoSe₂ with ionic liquid gating

A CVD grown MoSe₂ monolayer was electrically characterized in a field effect transistor configuration using an ionic liquid (IL) as the gate dielectric in the temperature range 200K < T < 350K. The high specific capacitance of the IL permitted efficient coupling between gate and the semiconducting channel. As a result, ambipolar charge transport was observed in the device at low drain(D)/source(S) and gate(G) operating voltages. The n-type operation of the device was present at all recorded temperatures, while the onset of p-type operation appeared to shift toward larger negative gate voltages beyond -2V for temperatures below 220K. The device on/off ratio was ~10⁵ for electron and hole transport at 350K and decreased with decreasing temperature. IL gating thins the Schottky barrier between the metal electrode contact and the MoSe₂ monolayer, permitting efficient charge injection of both carrier types into the MoSe₂ channel. A non-monotonic behavior in the temperature dependence of the channel current (I_DS), similar to a metal-insulator transition, was observed during n-type operation above 315K and which was absent for p-type operation. The transition temperature was seen to increase in a stepwise manner with increasing gate voltage. Defects and Se vacancies in the atomic layer unintentionally result in n-doping of MoSe₂. Gating the device with a positive voltage also increases the electron charge concentration in the channel. These could be some of the reasons for the observed non-monotonicity in I_DS(T) under n-type operation.