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High-temperature gallium oxide devices: improving stability for long-term operation
Callahan, William A.
Callahan, William A.
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2024
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Abstract
Due to its high utility and low cost, power devices and components that utilize silicon are widespread. For common applications, this is sufficient. However, harsh environments (high temperatures, strongly oxidizing or reducing conditions) require a higher degree of thermal and chemical stability for reliable device operation. We are entering into a new generation of power electronic devices, dominated by wide-bandgap (WBG) and ultrawide-bandgap (UWBG) materials, where the strongest candidate to emerge as a serious competitor is gallium oxide (Ga$_2$O$_3$). The most stable polymorph, $\beta$-Ga$_2$O$_3$, offers a high degree of chemical and thermal stability, as well as the potential to withstand high voltages before failure.
A significant amount of research has been conducted to improve the electrical performance of $\beta$-Ga$_2$O$_3$ devices, with an intense focus on optimizing the resilience to high electrical fields and the on-state performance, simultaneously. Very little is reported in the way of either high-temperature performance or long-term reliability in non-ambient conditions. The few demonstrations that defy this often use electrical contacts and interlayers that are not optimized for the conditions, invariably leading to performance degradation. In this thesis, I address this lack of reliability data by systematically examining current contact structures and redesigning them based on the fundamental principles of thermodynamics and kinetics of materials.
I find that Ohmic contacts to Ga$_2$O$_3$ can be formed by using an ultrathin layer of Ti (5 nm) with a Au capping layer (100 nm). These contacts show remarkable stability and excellent Ohmic performance after both extensive thermal cycling between 25 and 550$^{\circ}$C in and long-term thermal soaking at 600 $^{\circ}$C for $>$500 hours under vacuum conditions. Building on this, I then demonstrate reliable operation of Cr$_2$O$_3$:Mg/ $\beta$-Ga$_2$O$_3$ p-n heterojunction diodes for more than 140 hours at 600$^{\circ}$C. This experimental work is motivated by theoretical calculations that predict thermodynamically stable phase coexistence of n-Ga$_2$O$_3$ and p-Cr$_2$O$_3$ for an extremely wide range of temperatures and pO$_2$. Finally, my work culminates with the demonstration of a hydrogen sensitive Pt / Cr$_2$O$_3$:Mg/ $\beta$-Ga$_2$O$_3$ p-n heterojunction diode operating continuously at 600$^{\circ}$C for over 1000 hours. I find that, while the sensor signal and the sensitivity degrade over time, the response to low concentration of hydrogen (500 / 1000 / 1500 ppm) remains for the entire duration of the experiment.
This body of work addresses the lack of high-temperature reliability data for Ga$_2$O$_3$ devices. My work examines the current most common electrical contacts and structures that are used to fabricate devices for low-temperature operation, and redesigns them with a focus on high-temperature operation.
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