MoS2 and WSe2 in field effect transistors

Transition-metal dichalcogenides, such as MoS2 or WSe2, have recently become very popularfornext-generation electronic devices and sensors as alternative or complement to graphene. Such materials offer remarkable properties, which include layer-dependent bandgap, intrinsic n- or p-type conduction, strong light interaction, good mechanical strength, etc. Monolayer MoS2 and WSe2 have direct bandgap and enable field-effect transistor with high On/Off current ratio and strong photoresponse. A drawback is the low carrier mobility, in the order of few tens cm2V-1s-1 on substrate, and the sensitivity to oxygen, water or other adsorbates, which make unprotected devices rather unstable. Here, we discuss the current-voltage (I-V) characteristics at high drain bias of monolayer MoS2 transistors with Schottky contacts [1]. We show that oxidized Titanium contacts, due to a long air exposure, form rectifying junctions on MoS2 and cause asymmetric output characteristics, which we explain in terms of two slightly asymmetric back-to-back Schottky barriers. We show that, the highest current arises from image-force barrier lowering at the electrically forced junction, while the reverse current is due to Schottky-barrier limited injection at the grounded junction. We demonstrate that features commonly observed in MoS2 transistors, such as persistent photoconductivity and hysteresis in the transfer characteristic, are peculiarities of the MoS2 channel rather than effects of the contacts. We use transistors with ohmic contacts, at low drain bias, to deeply investigate the photoconductive and photogating effects [2]. We point out that the photoconductivity can persist with a decay time longer than 104 s, due to photo-charge trapping in extrinsic and intrinsic defects, which are also the cause of hysteresis. We highlight the important role of intrinsic donor-like defects.Finally, we demonstrate n-type conduction in WSe2 transistors that, combined with the low workfunction of WSe2, we exploit for field emission applications [4]. We show that the field emission current from a WSe2 flake can be modulated by a back gate, thus enabling a new field emission transistor.