| Author: | Zhang, Songge |
| Title: | Two-dimensional transition metal dichalcogenides and high-κ dielectrics for high-performance field-effect transistors |
| Advisors: | Tao, Xiaoming (SFT) Chai, Yang (AP) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | School of Fashion and Textiles |
| Pages: | xiv, 128 pages : color illustrations |
| Language: | English |
| Abstract: | Flexible electronics is an emerging electronic technology that fabricates electronic devices on flexible substrates. Broad applications prospects have been applied in fields such as information, energy, healthcare and national defense. It requires operating under a certain range of deformation, including bending, folding, twisting, compression and stretching, which puts forward requirements for new materials. Atomically thin two-dimensional (2D) materials, such as metallic graphene, semiconducting molybdenum disulfide (MoS2) and insulating hexagonal boron nitride (hBN), have attract interests because of their high mobility (for semiconductors and conductors) and good mechanical properties. In this case, developing 2D devices is important for flexible electronics and systems. This thesis developed a high-performance MoSe₂ field-effect transistors, an ultrathin high-κ dielectric materials for 8-in. level MoS₂ field-effect transistors, and 2D ferroelectric field-effect transistors with ferroelectric topological film, which is significant for 2D devices towards prolonging Moore's Law. The study focuses on the 2D materials synthesis and devices fabrication for high-performance with high on/off ratio, small SS and large on-state current. For MoSe₂ growth and high-performance transistors fabrication, high-quality continuous MoSe₂ films with salt as metal sources are successfully synthesized. The domain size is around 120 μm, which is larger than that in most litheratures and oxide sources. Raman and PL also demonstrate its high-crystalline quality. Electrical measurement shows that the MoSe₂ film is uniform and transistors coule reach a high on/off ratio of 109 and a high on-state current of 12 μA/μm at 1 V bias, which is superior than that in reported literatures. For ultrathin high-κ HfO₂ dielectrics, we utilize an optimized ALD process with two-oxidation step to deposit defect-free high-quality 1.3-nm-thick HfO₂ high-κ dielectric layers on 8-inch wafers at only 200 °C. Our ultrathin HfO₂ film shows an extraordinarily small EOT of 0.27 nm, a ultrasmall leakage current of 10-14 A/µm² at 1 V bias voltage, and a robust breakdown electric field of approximately 32 MV/cm. We also fabricate MoS₂ transistors and circuits with 1.3-nm-thick HfO₂ as dielectric layers to demonstrate its practicality and gate controllability. Typical transistors exhibit large on-state current density of 250 µA/µm and an impressive on/off ratio of 108, accompanied by an ultra-low subthreshold slope of 60 mV/dec. Logic gates and memory devices are also fabricated on 8-inch wafers to demonstrate its substantial implications for the forthcoming advanced semiconductor processes in the angstrom era. For topological ferroelectric field-effect transistors, ferroelectric topological structures are promising new candidates for high-performance memories with excellent stability and low power consumption. Abundant ferroelectric materials and topological structures have been found and investigated over the past decade; however, integration of them into memory devices has greatly delayed, mostly due to the constraint from their substrates. In this work, we demonstrate, for the first time, scalable topological ferroelectric field-effect transistors (Fe-FETs) based on large-scale uniform flux-closure topological ferroelectric films epitaxially grown on substrates with sacrificial layers. Unlike the previous coexistence of trivial and nontrivial domains, the rigid single flux-closure arrays can stably exist in freestanding films and can be erased by increasing temperature over a critical point, rather than undergoing a phase transition to ferroelastic domains as previously reported. Moreover, the flux-closure capacitors demonstrate exceptional performances such as small coercive voltages, negligible leakage currents, a remanent polarization reaching 195 μC/cm², and an endurance of up to 109 cycles. Furthermore, CMOS-compatible Fe-FET arrays are fabricated, featuring a large memory window of 7 V, high ON/OFF ratio of 108, good endurance and stability, and superior flexibility. Our work paves a way towards the ferroelectric memory devices in future semiconductor industries and flexible electronics. |
| Rights: | All rights reserved |
| Access: | open access |
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