Chemical vapor deposited two-dimensional material based high frequency flexible field-effect transistors
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Flexible nanoelectronics have attracted great attention due to interesting concepts such as wearable electronics and internet of things, which requires high speed and low power consumption flexible smart system with functions ranging from sensing, computing to wireless communicating. In this dissertation, transparent and solution processable nanoscale polyimide film for highly flexible gate dielectrics is demonstrated by in-situ opto-electro-mechanical measurement and utilized for two-dimensional nanomaterials based field-effect transistors (FETs). Graphene thin film transistor with the nanoscale polyimide dielectric on flexible glass is operated in extremely high frequency regime and shows the highest experimental saturation velocity (~8.4 × 10⁶ cm/s) in any materials in any flexible transistors. Molybdenum disulfide (MoS₂) based transistors with embedded gate structure on rigid substrate are demonstrated with enhancement mode operation, ON/OFF ratio over 10⁸, the highest transconductance (~ 70 µS/µm) and saturation velocity (~1.8 × 10⁶ cm/s). CVD MoS₂ FETs on flexible plastic substrates are also demonstrated, showing enhancement mode operation, ON/OFF radio over 10¹⁰ and transconductance (~6 µS/µm). The flexible CVD MoS₂ transistors with embedded gate structure were employed to study effects of substoichiometric doping by HfO [subscript 2-x]. After the doping layer, the flexible MoS₂ transistors show ×8 higher source-drain current density as well as more than ×2 mobility improvements. For the another first demonstration, GHz operation and flexibility of graphene and MoS₂ based FETs are realized on commercial available paper substrates, which indicates flexible two-dimensional material based nanoelectronics can be implemented on paper substrates for systems, sensors, and Internet of Things.