Investigating the transient response of Schottky barrier back-gated MoS2 transistors

Molybdenum disulfide (MoS2) MOSFETs have been widely reported to exhibit hysteresis behavior, which is usually attributed to charge trapping effects due to defective/sub-stoichiometric compositions in the material, or defects near, or at, the oxide/channel interfaces. It is also suggested that defective MoS2 transistors show current limitations caused by the Schottky barrier junctions formed at the contacts. Here, we report on the static and dynamic device response of back-gated MoS2 transistors directly fabricated on a SiO2/Si substrate using chemical vapor deposition synthesis, without film transfer, and standard CMOS optical lithography. The devices exhibit an atypical hysteresis in the transfer characteristics, as well as a delayed response in the formation of the conducting channel in response to voltage pulses applied to the back gate. Analysis of the output characteristic is consistent with two back-to-back Schottky diodes, allowing the Fermi level pinning position at the Ni/MoS2 source and drain contacts and blocking the MoS2 hole channel. Capacitance-voltage characterization demonstrates that the grown MoS2 thin film is p-type, resulting in a nominally-off, inversion mode, n-channel device. Analysis of the transient response and hysteresis as a function of device temperature, illumination and ambient conditions indicates that the dynamic response of the device is determined by the net charge in the MoS2 film combined with the minority carrier generation lifetime in the underlying silicon substrate. The work demonstrates the strong dependence of the device response time on substrate, temperature, illumination, and net charge in the MoS2 layer opening the possibility of applications in photo-detectors and sensors.