Abstract

Ultra-wide bandgap (UWBG) Gallium Oxide (Ga₂O₃), particularly in its monoclinic beta phase (β-Ga₂O₃) has recently gained significant interest due to its high breakdown electric field, high electron mobility, and the potential for large-area, low-cost substrate production. Various studies have demonstrated the β-Ga₂O₃-based devices, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), metal-semiconductor field-effect transistors (MESFETs), and Schottky diodes for applications in power electronics and high-frequency devices. However, the development of β-Ga₂O₃ electronics for extreme temperature applications remains limited. This thesis investigates the development and optimization of UWBG β-Ga₂O₃ semiconductors, for extreme-temperature electronics, including both cryogenic and high-temperature environments.

Due to the growth capability of β-Ga₂O₃ on foreign substrates, heteroepitaxial β- Ga₂O₃ devices were initially investigated on sapphire (α-Al₂O₃) substrates. This includes the epitaxial growth of β-Ga₂O₃ on sapphire using pulsed laser deposition (PLD), as well as the demonstration of β-Ga₂O₃ inverter integrated circuits (ICs) and non-volatile flash memory at high temperatures. The investigation further reveals the operation of β-Ga2O3 MOSFETs from 10 K to 573 K and β-Ga₂O₃ inverter ICs down to 30 K.

The research identifies critical factors influencing the quality of these β-Ga₂O₃ films grown using PLD, such as growth temperature and ambient conditions. The introduction of a novel argon/oxygen (Ar/O₂) growth environment for PLD led to significant improvements in carrier mobility and conductivity, crucial for device applications. These improvements enabled the realization of state-of-the-art crystalline and conductive heteroepitaxial β-Ga₂O₃ on sapphire. Moreover, this technique, along with the introduction of buffer layers, was employed to optimize homoepitaxial β-Ga₂O₃ (010) films using PLD, enhancing conductivity and carrier concentration control while addressing challenges related to doping and defect density.

This thesis delves into the study of homoepitaxial Si-doped β-Ga₂O₃ (010) films, which exhibit the notable phenomenon of "no-carrier freeze-out" down to 2 K enabled by impurity band conduction. This is crucial for achieving the cryogenic operation of β-Ga₂O₃ devices. With this advancement, high-performance β-Ga₂O₃ FinFETs and integrated circuits were demonstrated, showing excellent performance at temperatures as low as 2 K. Additionally, a floating gate flash memory device was developed, achieving a remarkable memory window of over 10 V at 77 K.