Growth of Transition Metal Dichalcogenides Thin Films via Pulsed Laser Deposition and their Characterization

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2, WS2, and WSe2, have emerged as a next-generation building block in modern devices due to their unique characteristics beyond conventional three-dimensional (3D) materials: atomic-scale thickness, high electrical mobility, tunable bandgap, excellent catalytic property, mechanical flexibility, and sharp heterointerfaces with other materials without atomic commensurability due to absence of dangling bond. Although TMDs have these excellent properties, several challenges must be addressed to realize TMDs-based devices. Particularly, since exfoliated or synthesized TMD thin films by conventional chemical vapor deposition (CVD), in general, have poor coverage and uneven thickness on the entire substrate area, large-scale synthesis of TMDs having high spatial uniformity is required. This thesis focuses on the growth of TMDs via pulsed laser deposition (PLD), which is a physical vapor deposition technique to fabricate highly crystalline thin film, to fabricate large-scale homostructure and heterostructure TMD thin films directly on silicon substrate because thermodynamically non-equilibrium growth is possible without the triangular island growth of TMDs. In the first part of this thesis, the fabrication of thickness-adjustable WSe2 thin film using PLD was introduced. Highly uniform monolayer and few-layer WSe2 thin films were grown on centimeter-scale SiO2/Si substrates, and confirmed using Raman mapping and transmission electron microscopy analysis. Patterned field-effect transistors based on the PLD-grown WSe2 were also fabricated using conventional lithography processes to analyze their electrical characteristics and suggest the potential of PLD-grown large-scale and highly uniform TMDs as electronic devices. In the second part of this thesis, the vertically stacked centimeter‐scale multi‐heterojunction TMD (MoS2/WS2/WSe2 and MoS2/WSe2) thin films were grown directly on p-type Si (p-Si) substrate by sequential PLD. The fabricated multi-TMD thin films have multi-staggered gaps (multi type‐II band structure) with p-Si and show excellent thickness and spatial uniformity on the entire substrate area, which was confirmed by Raman mapping analysis. These multi-TMD thin films were applied as a thin film catalyst to p‐Si photocathodes for photo‐electrochemical (PEC) hydrogen evolution. The multi-TMDs/p-Si photocathodes show highly efficient photoelectrochemical properties due to more effective charge transfer of photo-generated electrons and holes than homostructure TMDs/p-Si photocathodes owing to the built-in electric field by multi-staggered gaps.