Thesis Examination Committee
Prof Ho Bun CHAN, PHYS/HKUST (Chairperson)
Prof Kevin Jing CHEN, ECE/HKUST (Thesis Supervisor)
Prof Yang CHAI, Department of Applied Physics, The Hong Kong Polytechnic University (External Examiner)
Prof Zhiyong FAN, ECE/HKUST
Prof Jianan QU, ECE/HKUST
Prof Jiannong WANG, PHYS/HKUST
MoS2 and WSe2 are two-dimensional semiconductors from the transition metal dichalcogenide family. Owing to their high thermal stability, atomic-scale thickness, tunable bandgap, high mobility and other extraordinary properties, they have been under intensive research recently. However, the practical applications of MoS2 and WSe2 still face many challenges. Due to the lack of dangling bonds, it is challenging to integrate high-quality high-k dielectric on MoS2 and WSe2, which has become a substantial obstacle in building MoS2/WSe2 devices with versatile structures or complicated circuits. Moreover, metal contacts to MoS2 and WSe2 often show strong Fermi level pinning and large contact resistance.
In this work, remote N2 plasma treatment has been developed as a critical means to realize the surface functionalization and device applications for MoS2 and WSe2. Remote N2 plasma is first directly used as N source to deposit AlN on MoS2. This AlN layer is further used as an interfacial layer to fabricate single-layer MoS2 MOSFETs. The devices show enhanced gate dielectric deposition and greatly improved electrical stability. Furthermore, remote N2 plasma treatment is explored as a surface functionalization technique for MoS2 and WSe2 to promote the initial precursor adsorptions during ALD, which is proven to result in uniform dielectric deposition.
First-principles calculations are conducted to study the Raman spectra of MoS2 and WSe2 with N and O atom adsorptions, aiming to explore the possibility of using in-situ resonant Raman spectroscopy to monitor the real-time surface adsorptions. Acoustic-phonon Raman scattering is also measured and analyzed for multilayer MoS2 and WSe2. The theoretical calculations suggest that in-situ Raman spectroscopy, specifically the acoustic-phonon Raman scattering, is capable of providing important information to quantify the surface adsorptions and to realize robust surface functionalization of MoS2 and WSe2.
Using remote N2 plasma treatment as the surface functionalization method, ultrathin high-k dielectric is deposited on single-layer MoS2, which is further used as a tunneling contact layer to fabricate top-gate single-layer MoS2 MOSFET. With an optimized layer thickness, this high-k tunneling layer can effectively reduce the contact resistance and boost the ON current of single-layer MoS2 MOSFET.