• Miakota, Denys Igorevich

Abstract

2D semiconductors are among the most promising materials for next-generation electronic and photonic devices owing to their strong light-matter coupling and low thickness. However, the growth of 2D monolayer semiconductors at the wafer scale possesses many unsolved challenges. This thesis’s most scientifically relevant results are 2D transition metal dichalcogenides (TMDs) synthesis using pulsed laser deposition (PLD). The aim is advanced studies of 2D materials growth conditions in PLD, both in direct and two-step processes. The key challenges faced in 2D TMD crystal synthesis via direct deposition of chalcogenides in PLD are discussed, and a detailed study of the precursor influence in a two-step 2D TMDs synthesis is presented.<br/>First, we synthesize 2D MoS₂ monolayers at 700^oC using direct PLD deposition and a MoS₂ target. Then, we systematically study intrinsic defects in 2D TMDs grown in a single-step PLD synthesis using atomic resolution imaging and first-principles calculations. We use the MoS₂ monolayer as a prototype material and show that sulfur vacancies, antisite defects, and mirror twin GBs with 4|8-membered ring motifs are predominant defects in PLD specimens. The intrinsic point defects are thermo-dynamically favourable under Mo-rich/S-poor conditions specific to the PLD-grown films.<br/>Second, the synthesis of 2D MoSe₂ monolayers using PLD is explored. The question is if it is possible to overcome the challenges faced by a single-step PLD using chalcogenide-rich targets. The formation of the intrinsic point defects in 2D MoSe₂ monolayers can be slightly suppressed under Mo-poor/Se-rich conditions. Additionally, we find that the photoluminescence (PL) response of the PLD-grown films is dominated by A⁰ neutral excitons, unlike the PL for chemical vapour deposition (CVD) grown 2D MoSe₂, which is dominated by negatively charged A⁻ trions.<br/>Third, the synthesis of 2D WS₂ monolayers using a PLD-assisted CVD process was studied. Here, we explore how intrinsic oxygen vacancies present in a non-stoichiometric WO_3−x solid oxide precursor lead to a more facile conversion from WO_3−x to WS₂ monolayers. A two-stage growth process was developed, employing tunability of the oxygen vacancies in uniform WO_3−x precursors to control the nucleation, lateral growth independently, and ultimately, the WS₂ domain size. This study suggests that native oxygen vacancies in the oxide films can serve as active sites through which sulfur atoms enter the lattice and facilitate the growth of WS₂ crystals with high PL emission and large domain size. <br/>Fourth, we apply our findings from WS₂ monolayer growth on the influence of the precursor composition and crystallinity to synthesize MoS₂ monolayers in a similar PLD-CVD process. Oxygen vacancies in a reduced MoO_3−x solid oxide precursor (0 &lt; x &lt; 1) lead to an easier conversion from MoO_3−x to 2D MoS₂. It was found that the resulting MoS₂ monolayers demonstrate good optical response and strong photo-luminescence, and the PL signal for 2L MoS₂ is dominated by negatively charged X⁻ trions.<br/>Fifth, based on the previous outcomes on individual 2D TMDs synthesis both in a single-step and a two-step processes, a comprehensive investigation on a few possible ways to grow 2D MoS₂-WS₂ heterostructures directly was performed. These results follow the advanced studies of the growth parameters in PLD and present a few possible routes to synthesize 2D MoS₂-WS₂ heterostructures. The heterostructures can be grown using bilayer MoO_x/WO_x oxide conversion and sequential synthesis. Here, different stages of the heterostructure synthesis are investigated, and an optical characterization of their various stacking types is presented. For all the cases, the PL emission from the heterostructures appears to be quenched as it emerges from the overlapping areas. The resulting films were classified as n-type/n-type heterostructures because the constituent semiconducting materials exhibit different bandgaps.<br/>Sixth, the influence of the efficiency of various growth promoters, which aims to minimize 2D TMD growth temperature and maximize area coverage by inducing an intermediate liquid phase, was studied. It was found that upon certain growth conditions, vapour-liquid-solid (VLS) phase reaction leads to the formation of long single crystalline MoS₂ quasi-1D nanoribbons. Notably, the alkali metal halide layer (NaF) acts as a promoter for the directional growth of quasi-1D MoS₂ nanoribbons. Here, a possible growth mechanism of such MoS₂ nanoribbons was suggested, and an intrinsic biaxial strain present in the nanoribbons was discu...

Topics

• impedance spectroscopy
• photoluminescence
• Oxygen
• semiconductor
• pulsed laser deposition
• crystallinity
• chemical vapor deposition
• liquid phase
• Alkali metal
• point defect