EXPLORATION OF GALLIUM OXIDE DEVICES FOR POWER APPLICATIONS

Beta gallium oxide (β-Ga₂O₃) is an emerging candidate for the next generation of power devices due to its wide bandgap (~4.9 eV) property. This characteristic holds great promise for applications in high-power and high-radiation environments. The Baliga Figure of Merit (BFOM), calculated based on its high critical field (~8 MV/cm), is more than three times greater than the conventional wide bandgap materials like GaN and SiC Recent research has shown the potential of Ga₂O₃ devices, particularly Schottky barrier diode (SBD) and metal-oxide-semiconductor field-effect transistors (MOSFET). An intriguing finding shows that Ga₂O₃ device fabricated on wafer without packaging and stored in exposed environment (air) exhibits no degradation in electrical performance even after two years. This finding highlights the non-reactive nature of the oxide semiconductor and suggests a high level of reliability for Ga₂O₃-based devices. In addition, the use of the melt-growth technique for Ga₂O₃ offers a promising and cost-effective approach for bulk device platform. To date, single crystal Ga₂O₃ 6-inch wafer for power application has been reported by Novel Crystal Technology, Inc. 

The first part of the thesis focuses on enhancing the Ga₂O₃ Schottky interface. Nickel (Ni) with a high work function (5.01 eV) has been widely accepted as the anode metal for Ga₂O₃ SBD. However, Ni is a fast diffuser and will cause contamination at the Schottky interface during metal deposition and subsequent fabrication processes.¹ To counteract Ni diffusion, a thin interfacial layer is inserted between the metal and semiconductor. This interlayer is formed using the aluminum reaction method proposed in this work. The resulting metal-interlayer-semiconductor (MIS) SBD demonstrates improved subthreshold slope (61 mV/dec) and uniformity across the wafer.  Additionally, the forward voltage drop is significantly reduced from 1.28 V to 0.79 V. Furthermore, a modified thermionic emission (TE) model based on Gaussian distributed barrier heights is used to evaluate the role of the interlayer.

Despite the exceptional characteristics of the Ga₂O₃ SBD, the lack of p-type Ga₂O₃ is preventing it from achieving even higher voltage blocking capability. The second part of the thesis introduces various methods to address this issue. In order to achieve bipolar operation on Ga₂O₃, p-type alternatives are explored to form p-n heterojunction. One option involves using nickel oxide (NiO_x), which is patterned using lithography and lift-off process to form a trench merged p-i-n Schottky diode (TMPS). The resulting TMPS exhibits an improved breakdown voltage (~79%) and negligible degradation in on-resistance. A model is proposed to extract the optimal pattern design and is validated through experimental results. Another approach is to use magnesium-doped p-type gallium nitride (p-GaN) as an alternative for p-type Ga₂O₃. A high-quality and uniform 2-inch wafer-scale Ga₂O₃/p-GaN heterojunction is successfully demonstrated. The fabricated p-n diode manifests an advancement based on Ga₂O₃ for power applications. Finally, instead of forming a p-type heterojunction on Ga₂O₃, a Mg-doped Ga₂O₃ current blocking layer (CBL) is introduced, exhibiting exceptional peak electric field reduction as a homojunction. The Mg-doping is achieved via a low-cost spin-on-glass (SOG) technique, which does not cause lattice damage. 

The unipolar operation characteristic of Ga₂O₃ also raises concerns for Ga₂O₃-based metal-oxide-semiconductor field-effect-transistor (MOSFET), as conventional Ga₂O₃ MOSFET typically operate in depletion-mode (D-mode). In the final segment of the thesis, an innovative approach is presented, which introduces an enhancement-mode (E-mode) Ga₂O₃ MOSFET based on charge trapping layer (CTL) technique. This CTL method showcases a wide threshold voltage tuning window and exceptional reverse blocking capabilities, making it well-suited for power applications. To gain insights into the trap profile within the CTL and assess the reliability of the proposed technique, photon-stimulated characterization and time-dependent dielectric breakdown (TDDB) tests are conducted. 

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