Dual-gate MOSFETs on monolayer CVD MoS2 films
H. Liu, M. Si, S. Najmaei, A. T. Neal, Y. Du, P. M. Ajayan, J. Lou, P. D. Ye
71st Device Research Conference
Monolayer MoS2 with a direct band gap of 1.8 eV is a promising two-dimensional material with a potential to surpass graphene in next generation nanoelectronic applications.    We have synthesized monolayer MoS2 thin films via chemical vapor deposition (CVD) method on Si/SiO2 substrate and comprehensively study the device performance and variation on dual-gated MoS2 field-effect transistors. Over 100 devices are studied to obtain a statistical description of device performance on
... er CVD MoS2. We have achieved record high drain current of 62.5 mA/mm in CVD monolayer MoS2 films ever reported.  We further extract the intrinsic contact resistance of low work function metal Ti on monolayer CVD MoS2 with an expectation value of 175 Ω·mm, which can be significantly decreased to 10 Ω·mm by appropriate gating. By taking the impact of contact resistance into account, average and max intrinsic field-effect mobility is estimated to be 13.0 and 21.6 cm 2 /V·s in monolayer CVD MoS2 films. The MoS2 crystal growth was carried out in a furnace on 285 nm SiO2 capped p ++ Si substrate by sulfurization of MoO3 via CVD method shown in Fig. 1 (a). Optical and AFM images of the monolayer MoS2 crystals with triangle domains are shown in Fig. 1(b) and (c). MoS2 crystals are single layers with good uniformity.  After material synthesis, source/drain regions are defined with e-beam lithography followed by Ti/Au deposition as contacts. After a 1 nm Al deposition as the seeding layer, 15 nm Al2O3 was grown at 200ºC by ALD used as top gate dielectric. Ni/Au that covers the whole channel served as the top gate. Fig. 2(a) shows the schematic device structure of the dual-gate MoS2 MOSFET. Over 100 devices were fabricated with channel length varied between 100 nm to 1 μm. Fig. 2(b) shows the output curves of a 100 nm channel length back-gate devices with record-high maximum drain current of 62.5 mA/mm. Fig. 2(c) shows the output curves of a 1μm channel length top-gate devices showing the flexibility of the dual gate device to be modulated by either gates. Maximum drain current of this device is 2.71 mA/mm and 14.9 mA/mm for top-gate and back-gate modulation, respectively. The difference is attributed to the larger contact resistance on top-gate configuration. Average drain current at various channel length is 36.7±14.2, 27.1±12.2, 22.3±10.0 and 12.9±5.0 mA/mm for 100, 200, 500 nm and 1 μm channel lengths, as plotted with maximum values in Fig. 3 , showing the scaling properties of transistors. The total resistance for top-gated device is mostly contributed by contact resistance, which is plotted in Fig. 4 , with an expected value of Rc=175 Ω·mm without back-gate biasing. The large value of Rc explains the early saturation in transfer curves of top-gated device, as shown in inset of Fig. 4 . The contact resistance at 100 V back-gate voltage can be extracted to be Rc=10 Ω·mm in Fig. 5 by TLM. [6,7] The intrinsic field-effect mobility is estimated from peak transconductance after subtracting the contact resistance. Average and maximum values of intrinsic field-effect mobility are plotted in Fig. 6 with the highest mobility of 21.6 cm 2 /V·s. In conclusion, we synthesized monolayer MoS2 films by CVD method. We systematically studied the device performance based on these CVD films at various channel length. We achieved the highest drain current of 62.5 mA/mm at 100 nm channel length and maximum intrinsic mobility of 21.6 cm 2 /V·s.