Thesis Examination Committee
Prof Philip Kwok Tai MOK, ECE/HKUST (Chairperson)
Prof Man Sun CHAN, ECE/HKUST (Thesis Supervisor)
Prof George Jie YUAN, ECE/HKUST
Molybdenum disulfide (MoS2), among many other two-dimensional (2D) materials, holds great promise for future applications in next-generation logic devices due to its ultra-thin nature, desired bandgaps, superior mobility, and excellent thermal stability. However, there are two main challenges that block the development of MoS2-based metal oxide semiconductor field effect transistor (MOSFETs). Firstly, the performances of MoS2 MOSFETs are severely limited by the large contact resistance and low carrier mobility. Secondly, fabricating a unipolar MoS2 p-MOSFETs remains a challenging task, due to severe Fermi Level pinning at metal/ MoS2 contacts blocking the hole injection.
First, high performance multilayer MoS2 n-MOSFETs of 1-µm channel length have been achieved on ultrathin high-k dielectrics (∼ZrO2/Si). Highest drain current exceeding ~ 220 µA/µm and On/Off ratio about ~ 106 have been achieved. High extrinsic mobility ~ 69 cm2/Vs and intrinsic mobility ~ 132 cm2/Vs have been extracted, enhanced by the strong screening effect of high-k dielectric on the Coulomb scattering. A low contact resistance (below 1 kΩ-μm) has also been achieved, which is attributed to the significant electrostatic doping on contact-to-gate overlapping by using ultrathin high-k dielectric. Second, enhancement-mode unipolar MoS2 p-MOSFETs with high drive current and suppressed ambipolar behavior have been achieved. A reactive metal, Scandium, is used to form the source/drain contacts, which together with the interface dipoles contributed by ZrO2 pins the Fermi level near the valence band of the MoS2, leading to a clear p-type characteristic. Niobium (Nb) dopes the MoS2 p-type reducing the depletion region width at the contact, enhancing holes tunneling. Furthermore, the larger barrier height at the conduction band suppresses the ambipolar electron current at the off-state. The fabricated device shows unipolar characteristics with high on-current of ~10μA/μm while the ambipolar current of ~10nA/μm. This work contributes towards monolithic integration of high performance MoS2-based complementary logic circuits.