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.