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STRUCTURAL AND MATERIAL INNOVATIONS FOR HIGH PERFORMANCE BETA-GALLIUM OXIDE NANO-MEMBRANE FETS
Beta-gallium oxide (β-Ga2O3) is an emerging wide bandgap semiconductor for next generation power devices which offers the potential to replace GaN and SiC. It has an ultra-wide bandgap (UWBG) of 4.8 eV and a corresponding Ebr of 8 MV/cm. β-Ga2O3 also possesses a decent intrinsic electron mobility limit of 250 cm2/V·s, yielding high Baliga’s figure of merit of 3444. In addition, the large bandgap of β-Ga2O3 gives stability in harsh environment operation at high temperatures.
Although low-cost large-size β-Ga2O3 native bulk substrates can be realized by melt growth methods, the unique property that (100) surface of β-Ga2O3 has a large lattice constant of 12.23 Å allows it to be cleaved easily into thin and long nano-membranes. Therefore, β-Ga2O3 FETs on foreign substrates by transferring can be fabricated and investigated before β-Ga2O3 epitaxy technology becomes mature and economical viable. Moreover, integrating β-Ga2O3 on high thermal conductivity materials has an advantage in terms of suppressing self-heating effects.
In this dissertation, structural and material
innovations to overcome and improve critical challenges are summarized as
follows: 1) Top-gate nano-membrane β-Ga2O3 FETs on a high thermal conductivity diamond
substrate with record high maximum drain current densities are demonstrated.
The reduced self-heating effect due to high thermal conductivity of the
substrate was verified by thermoreflectance measurement. 2) Local
electro-thermal effect by electrical bias was applied to enhance the electrical
performance of devices and improvements of electrical properties were shown
after the annealing. 3) Thin thermal bridge materials such as HfO2 and ZrO2 were inserted between β-Ga2O3 and
a sapphire substrate to reduce self heating effects without using a diamond
substrate. The improved thermal performance of the device was analyzed by
phonon density of states plots of β-Ga2O3 and the thin film materials. 4) Nano-membrane
tri-gate β-Ga2O3 FETs on SiO2/Si substrate fabricated via exfoliation have been demonstrated for the
first time. 5) Using the robustness of β-Ga2O3 in harsh environments, β-Ga2O3 ferroelectric
FETs operating as synaptic devices up to 400 °C were demonstrated. The result
offers the potential to use the novel device for ultra-wide bandgap logic
applications, specifically neuromorphic computing exposed to harsh