Effects of HfO2 encapsulation on electrical performances of few-layered MoS2 transistor with ALD HfO2 as back-gate dielectric

The carrier mobility of MoS2 transistors can be greatly improved by the screening role of high-k gate dielectric. In this work, atomic-layer deposited (ALD) HfO2 annealed in NH3 is used to replace SiO2 as the gate dielectric to fabricate back-gated few-layered MoS2 transistors, and good electrical properties are achieved with field-effect mobility (μ) of 19.1 cm2 V-1 s-1, subthreshold swing (SS) of 123.6 mV dec-1 and on/off ratio of 3.76 × 105. Furthermore, enhanced device performance is obtained when the surface of the MoS2 channel is coated by an ALD HfO2 layer with different thicknesses (10, 15 and 20 nm), where the transistor with a 15 nm HfO2 encapsulation layer exhibits the best overall electrical properties: μ = 42.1 cm2 V-1 s-1, SS = 87.9 mV dec-1 and on/off ratio of 2.72 × 106. These improvements should be associated with the enhanced screening effect on charged-impurity scattering and protection from absorption of environmental gas molecules by the high-k encapsulation. The capacitance equivalent thickness of the back-gate dielectric (HfO2) is only 6.58 nm, which is conducive to scaling of the MoS2 transistors.