Investigation of Electrical Properties of Transition Metal Dichalcogenides Transistors with High-K Dielectrics




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Recently, transition metal dichalcogenides (TMDs) have attracted intense attention due to their atomic layer-by-layer structure and unique electronic, optical and mechanical properties. Some of them, such as MoS₂ and WSe₂, have demonstrated satisfactory energy bandgap values and promising properties for future applications in electronics and optoelectronics. However, the relatively inert surface of these materials prevents the direct deposition of high-k dielectrics on these 2-D materials. Furthermore, capacitance-voltage (C-V) measurements of high-k dielectric on TMDs and interface defects analysis have not been researched sufficiently. In this dissertation, fabrication, electrical characterization, and simulation of top-gated few-layer TMD transistors are demonstrated with a major focus on interface property study of high-k/TMD. Top-gated capacitors on bulk MoS₂ with 30 nm HfO₂ and Al₂O₃ dielectrics are characterized with C-V and I-V measurements as the early work, showing the necessity of having a more robust test structure and an in-situ surface treatment to enable better interface assessment with quantitatively study. Top-gated few-layer MoS₂ field effect transistors are fabricated using photolithographic patterning, with less than 10 nm thin ALD HfO₂ on MoS₂ after in-situ UV-O₃ surface functionalization. C-V and I-V measurements are performed on these transistors. Interface defect density is extracted and analyzed from C-V measurement results. Annealing effects, such as cleaning effect of ultra-high vacuum annealing before high-k deposition, and N₂ or a forming gas anneal after device fabrication are demonstrated as well. As a comparison, Al₂O₃/MoS₂ interface is also investigated with/without anneals, and the simulation work demonstrates the energetic and spatial distributions of the interface traps. Furthermore, border traps, which are the dielectric traps close to the high-k/MoS₂ interface, are studied based on electrical characterization and simulation, along with the interface traps. The methodologies of fabrication and characterization are also extended to MoSe₂, to understand the high-k/MoSe₂ interface and annealing effects. The electrical characterization and analysis in this dissertation reveal the high-k/TMD interfacial properties, which potentially helps find the origins of those defects and ultimately improves the electrical performance of the TMD devices by passivating the defects.



Transition metals, Chalcogenides, Molybdenum disulfide, Dielectric devices, Annealing of metals