UC San Diego

The design and development of future electronics must incorporate sustainable design principles throughout the process, including the use of emerging materials, low-power operations, and environmentally friendly manufacturing practices. The objectives of the thesis are to develop oxide semiconductors and devices that pave the way to revolutionize future sustainable electronic applications, including large-area flexible integrated circuits, back-end-of-line (BEOL) electronics in monolithic 3D integration for CMOS, and emerging computing devices for non-traditional computing platforms.

Amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors have been successfully commercialized in the flat panel display industry. The next significant challenge in oxide semiconductor technology lies in the development of high-performance p-channel oxide thin-film transistors, as their absence currently hinders the progress of low-power oxide complementary metal-oxide semiconductor (CMOS) circuits. Overcoming this challenge is crucial for realizing future cost-effective, energy-efficient, and flexible oxide circuit applications. Part I of this dissertation emphasizes the development of high-performance p-channel oxide TFTs, including material design, defect termination techniques, and atomically thin oxide semiconductor development. In Chapter 2, we introduce a scalable liquid element treatment technique to address structural defects in p-type SnO, particularly oxygen and metal cation vacancies. This technique contributes to enhancing the performance of p-channel SnO TFTs. Additionally, leveraging the liquid Bi-Sn eutectic alloy system allows us to control Sn cation treatment at a low temperature of approximately 140°C. This capability facilitates the modulation of hole concentration and a wide range of threshold voltage adjustments for p-channel SnO TFTs. In Chapter 3, we develop high-performance p-channel oxide TFTs using atomically thin p-type tin monoxide (SnO) channels with a thickness of approximately 1 nm, exhibiting extremely low off-current suitable for low-power oxide CMOS circuits.

Exploring new ultra-widegap oxide materials with a bandgap larger than the current commercialized a-IGZO holds promise for reducing TFT off-current and decreasing power consumption. Part II of this dissertation explores new oxide channel materials for low-temperature processed, high-mobility, and wide-bandgap oxide TFTs. In Chapter 4, we introduced a pressure-assisted liquid-metal printing technique that enables low-temperature crystallization of β-Ga2O3 for wide bandgap and high-performance n-channel oxide TFTs with a high saturation mobility of ~11 cm2V-1s-1.

Research into new computing devices and systems based on novel computing concepts, rather than traditional digital computers, is an important direction for developing future energy-efficient and sustainable computing. Oxide TFTs also show the potential to be essential building blocks for next-generation analog neuromorphic computing systems. Part III of this dissertation focuses on investigating oxide TFTs as emerging computing devices for non-traditional computing platforms. In Chapter 5, we designed the low-temperature processed ambipolar SnO-based TFT to demonstrate dynamic reconfigurable excitatory and inhibitory synaptic responses in a single synaptic device, emulating complex biological functions in emerging neuromorphic hardware. In Chapter 6, we introduced a novel gate-tunable memristor, referred to as a memtransistor, which utilizes non-layer two-dimensional (2D) oxide semiconductor material, specifically SnO2. The memtransistor combines memristor and transistor functionalities to advance next-generation normochromic computing technology. We successfully demonstrated that the gate-tunable synaptic device dynamically modulated analog switching behavior with good linearity and improved conductance change ratio for high recognition accuracy learning.

Finally, the remaining challenges and opportunities in oxide semiconductors and devices for future advanced electronics are discussed.