Surface functionalization and device applications of MoS₂ and WSe₂ realized by remote N₂ plasma treatment

Abstract

MoS₂ and WSe₂ are two-dimensional semiconductors from the transition metal dichalcogenide family. Owing to their good thermal stability, atomic layer thickness, and the extraordinary mechanical, electric and optical properties, they have been under extensive research in recent years. First, MoS₂ and WSe₂ can be used as ideal channel materials to suppress the short-channel effect. Moreover, single-layer MoS₂ and WSe₂ also provide a great platform to exploit the valley degree of freedom of electrons in solids for the possible electronic applications. By stacking them together with other layered two-dimensional materials, such as BN, versatile heterostructures of MoS₂ and WSe₂ with novel properties can be created. However, even though with all these promises, the practical applications of MoS₂ and WSe₂ still face many challenges. Due to the lack of dangling bonds and thus the lack of nucleation sites, it is challenging to integrate high-quality high-k dielectric on MoS₂ and WSe₂, which has become a big obstacle in building MoS₂/WSe₂ devices with versatile structures or complicated circuits. Besides the dielectric integration difficulty, metal contacts to MoS₂ and WSe₂ often show strong Fermi level pinning, as a result, the devices often show large Schottky barrier and contact resistance. Both the above challenges will unavoidably degrade the device performance and hinder the further practical applications of MoS₂ and WSe₂. To address these challenges, in this work, remote N₂ plasma treatment is explored as a surface functionalization technique to enhance the dielectric deposition on MoS₂ and WSe₂. First-principles calculations were also conducted to provide guidelines for using in-situ Raman spectroscopy as a characterization tool to realize the real-time and quantitative monitoring of the surface functionalization conditions and the possible defects. With remote N₂ plasma treatment, ultrathin high-k dielectric was integrated on single-layer MoS₂, which was further utilized as tunneling contact layer to effectively reduce the contact resistance of top-gate single-layer MoS₂ metal-oxide-semiconductor field-effect transistors (MOSFETs). To realize these objectives, the work in this thesis was divided into several parts: 1. Remote N₂ plasma was directly used as N source in a PEALD (plasma enhanced atomic layer deposition) system to deposition AlN on MoS₂. This AlN layer was further used as an interfacial layer to fabricate top-gate single-layer MoS₂ MOSFETs. The devices had improved gate dielectric deposition and showed enhanced electrical stability. 2. The remote N₂ plasma treatment was explored as a surface functionalization technique to promote the initial precursor adsorptions during ALD (atomic layer deposition) for MoS₂ and WSe₂. The N atom adsorptions on MoS₂ and WSe₂ surface are proved to function as anchors to fasten the precursors. Uniform dielectric deposition on MoS₂ and WSe₂ was achieved. The possible damages caused by remote N₂ plasma were characterized by Raman spectroscopy, with remote O₂ plasma treatment as a comparison. 3. First-principles calculations were 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. The theoretical results 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. 4. The acoustic-phonon Raman scattering, which is a reflection of crystal defects or adsorptions, was measured and analyzed for multilayer MoS₂ and WSe₂. The acoustic-phonon Raman intensities in the multilayer MoS₂ or WSe₂ show a weaker resonance enhancement than the first-order Raman intensities. In other words, if the Raman spectra are analyzed by the first-order Raman peak normalization, multilayer MoS₂ or WSe₂ will intrinsically have a weaker acoustic-phonon Raman intensity. This finding can contribute to the analysis of the crystal defects in multilayer MoS₂ or WSe₂ by acoustic-phonon Raman scattering. 5. Using remote N₂ plasma treatment as the surface functionalization method, ultrathin high-k dielectric was deposited on single-layer MoS₂, which was further used as a tunneling contact layer to fabricate top-gate single-layer MoS₂ MOSFET. With an optimized layer thickness, the high-k tunneling layer can effectively reduce the contact resistance.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology