High mobility ambipolar MoS$_{2}$ field-effect transistors

Unlike graphene, single and multilayer MoS2 have a 1-1.8eV band gap, which makes MoS2 an promising candidate for future semiconducting industry. However many groups have observed poor charge carrier mobility for thin MoS$_{2}$ crystals deposited on silicon dioxide substrates. Here we report on MoS$_{2}$ field effect transistors on SiO$_{2}$ and on polymethyl methacrylate (PMMA) dielectric. We measure the conductivity in a four-probe configuration as a funcation of carrier density controlled by the back gate electrode. For multilayer MoS$_{2}$ on SiO$_{2}$, the mobility is on order 10-60 cm$^{2}$/Vs, and independent of thickness (5-80 nm), and most devices exhibit unipolar n-type behavior. In contrast, multilayer MoS$_{2}$ on PMMA shows mobility increasing with thickness, up to 500cm$^{2}$/Vs (electrons) and 400 cm$^{2}$/Vs (holes) at thickness $\sim$50 nm. We observe activated temperature dependence of the resistance consistent with optical phonon scattering-limited resistance in the highest mobility devices. The dependence of the mobility on thickness for thicknesses up to 70 nm is unexpected, and points to a long-range dielectric effect of the bulk MoS$_{2}$ in increasing mobility.