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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23586号 / エネ博第432号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 松田 一成, 教授 宮内 雄平 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Syftet med denna avhandling är att bidra till forskningen av befintliga TMD:er (på Engelska transition metal dichalcogenide-TMD) som visar laddningstäthetsvågor och supraledning vid låga temperaturer (som 2H-TaS2). 2H-TaS2 är också känt för att visa supraledning vid 2K. Dessutom kommer en betydande del av denna avhandling att ägnas åt analysen av den elektronstrukturen nära Ferminivån av Cu-interkalerad TiSe2 och speciellt dess laddningstäthets-beteende vid temperaturer under 200K. Medan de teoretiska modellerna överlåts till teoretikerna, är följande sidor tillägnad att ge ett kvalitativt perspektiv på materialen. Avhandlingen är uppdelad i fyra huvudavsnitt: grundläggande begrepp, experiment-ella tekniker, tidigare rön och dataanalys. Det första avsnittet syftar till att introducera de viktigaste relevanta begreppen för att spåra de många möjliga fenomen som händer i bulk-TMD, speciellt Fe-dopade TaS2 i 2H-fasen och Cu-interkalerade TiSe2 i 1T fas. Elektronisk dispersion i fasta ämnen kommer att diskuteras på ett inledande och fenomenologiskt sätt utan rigorösa härledningar och ska hjälpa läsaren att förstå kapitlen därefter. / Motivation: The family of transition metal dichalcogenides (TMDs) has captured the fascination of researchers worldwide due to their remarkable properties and vast potential for various applications. These 2D materials exhibit a wide range of electronic, optical, and mechanical characteristics, making them incredibly versatile. From semiconductors to superconductors, TMDs offer a rich playground for exploration in condensed matter physics and materials science. Their unique properties are paving the way for breakthroughs in electronics, optoelectronics, energy storage, and beyond. As we delve deeper into the world of TMDs, we uncover new opportunities to revolutionize technology and enhance our understanding of the fundamental principles governing the behavior of matter. Joining the journey of discovery within the TMD family promises exciting challenges and the potential to contribute to the forefront of scientific and technological advancement.  The aim of this thesis is to add to the canon of existing TMDs that display charge density waves and superconductivity at low temperatures (like 2H-TaS2). 2H-TaS2 is also known to display superconductivity at 2K. Additionally, a substantial part of this thesis will be dedicated to the analysis of the electronic structure near the Fermi level of Cu-intercalated TiSe2 and especially its charge-density behaviour at temperatures below 200K. While the theoretical models are left to the theoreticians, the following pages are dedicated to giving a qualitative perspective on the materials. Thesis Outline: The primary goal of this thesis is to provide an introduction to both widely utilized and cutting-edge experimental setups employed by physicists worldwide. This will enable the acquisition of practical experience, facilitating the mastery of best practices and analysis techniques within the realm of experimental condensed matter physics. The thesis is split into four main sections: fundamental concepts, experimental techniques, previous findings and data analysis. The first section is occupied to introduce the main relevant concepts to trace the many possible phenomena happening in bulk TMDs, specifically Fe-doped TaS2 in the 2H phase and Cu-intercalated TiSe2 in the 1T phase. Subjects such as electronic dispersion in solids will be discussed in a rather introductory and phenomenological manner without rigorous derivations and shall aid the reader in understanding the chapters thereafter.

The push for electronic devices on smaller and smaller scales has driven research in the direction of transition metal dichalcogenides (TMD) as new ultra-thin semiconducting materials. These ‘two-dimensional' (2D) materials are typically on the order of a few nanometers in thickness with a minimum all the way down to monolayer. These materials have several layer-dependent properties such as a transition to direct band gap at single-layer. In addition, their lack of dangling bonding and remarkable response to electric fields makes them promising candidates for future electronic devices. For the purposes of this work, two 2D TMDs were studied, MoS2 and MoTe2. This dissertation comprises of three sections, which report on exploration of charge lifetimes, investigation environmental stability at elevated temperatures in air, and establishing feasibility of UV laser annealing for large area processing of 2D TMDs, providing a necessary knowledge needed for practical use of these 2D TMDs in optoelectronic and electronic devices. (1) A study investigating the layer-dependence on the lifetime of photo-generated electrons in exfoliated 2D MoTe2 was performed. The photo-generated lifetimes of excited electrons were found to be strongly surface dependent, implying recombination events are dominated by Shockley-Read-Hall effects (SRH). Given this, the measured lifetime was shown to increase with the thickness of exfoliated MoTe¬2; in agreement with SRH recombination. Lifetimes were also measured with an applied potential bias and demonstrated to exhibit a unique voltage dependence. Shockley-Read-Hall recombination effects, driven by surface states were attributed to this result. The applied electric field was also shown to control the surface recombination velocity, which lead to an unexpected rise and fall of measured lifetimes as the potential bias was increased from 0 to 0.5 volts. (2) An investigation into the environmental stability of exfoliated 2D MoTe2 was conducted using a passivation layer of amorphous boron nitride as a capping layer for back-gated MoTe2 field effect transistor (FET) devices. A systematic approach was taken to understand the effects of heat treatment in air on the performance of FET devices. Atmospheric oxygen was shown to negatively affect uncoated MoTe2 devices while BN-covered FETs showed remarkable chemical and electronic characteristic stability. Uncapped MoTe2 FET devices, which were heated in air for one minute, showed a polarity switch from n- to p-type at 150 °C, while BN-MoTe2 devices switched only after 200 °C of heat treatment. Time-dependent experiments at 100 °C showed that uncapped MoTe2 samples exhibited the polarity switch after 15 min of heat treatment while the BN-capped device maintained its n-type conductivity. X-ray photoelectron spectroscopy (XPS) analysis suggests that oxygen incorporation into MoTe2 was the primary doping mechanism for the polarity switch. (3) The feasibility of UV laser annealing as a post-process technique to sinter 2D crystal structures from sputtered amorphous MoS2 was explored. Highly crystalline materials are sought after for their use in electron and opto-electronic devices. Sputtered MoS2 has the advantage of potential for large area deposition and high scalability, however, it requires high temperatures (>350 °C) for their crystalline growth. Which creates difficulty for devices grown on polymer substrates. Low-temperature and room temperature deposition results in amorphous films which is detrimental for electric devices. A one-step lase annealing procedure was developed to provide amorphous to crystalline conversion of nanometer thin MoS2 films. Samples were annealed using an unfocused laser beam from a KrF (248 nm) excimer source. The power density was found to be 1.04 mJ/mm2. Raman analysis of laser annealed MoS2 was shown to exhibit a significant improvement of the 2D MoS2 crystallinity compared to as-deposited films on both SiO2/Si, as well as polydimethylsiloxane (PDMS) substrates. Annealed samples showed improvement of their conductivity on an order of magnitude. A top-gated FET device was fabricated on flexible PDMS substrates using Al2O3 as a gate oxide. Measured field effect mobility of annealed samples showed significant improvement over as-deposited devices.

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