Effects of HfO 2 encapsulation on electrical performances of few-layered MoS 2 transistor with ALD HfO 2 as back-gate dielectric

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Abstract
The carrier mobility of MoS 2 transistors can be greatly improved by the screening role of high-k gate dielectric. In this work, atomic-layer deposited (ALD) HfO 2 annealed in NH 3 is used to replace SiO 2 as the gate dielectric to fabricate back-gated few-layered MoS 2 transistors, and good electrical properties are achieved with field-effect mobility ( μ ) of 19.1 cm 2 V –1 s –1 , subthreshold swing (SS) of 123.6 mV dec –1 and on/off ratio of 3.76 × 10 5 . Furthermore, enhanced device performance is obtained when the surface of the MoS 2 channel is coated by an ALD HfO 2 layer with different thicknesses (10, 15 and 20 nm), where the transistor with a 15 nm HfO 2 encapsulation layer exhibits the best overall electrical properties: μ = 42.1 cm 2 V –1 s –1 , SS = 87.9 mV dec –1 and on/off ratio of 2.72 × 10 6 . 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 (HfO 2 ) is only 6.58 nm, which is conducive to scaling of the MoS 2 transistors.
Funding Information
  • National Natural Science Foundation of China (61774064)
  • the Fundamental Research Funds for the Central Universities (2015TS042)

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