Investigation of Dielectric Reliability with ?-Gallium Oxide for Future Power Devices

Abstract

The growing dependence on electrical energy has made the development of high-performing power electronics crucial; this has led to significant interest in the use of ultra-wide bandgap semiconductors (UWBG) in power devices as they demonstrate improved efficiencies compared to silicon. Research in β-Gallium Oxide or β-Ga2O3 has grown in popularity due to its large bandgap (4.6 – 4.9 eV) and high expected breakdown field (>6 MV/cm). The development of high-performing, stable β-Ga2O3 metal-oxide-semiconductor (MOS) devices requires the optimization of the gate dielectric-semiconductor interface. The unique aspects of β-Ga2O3, including its ultra-large bandgap and its self-trapping hole behavior, presents many questions about the impact of the UWBG semiconductor on MOS reliability. In this thesis I will investigate the impact of dielectric material, deposition technique, and nonplanar MOS structure on interface defects, reliability and photo-response with β-Ga2O3. First, I demonstrated a solution-processed high-κ dielectric, (Y0.6Sc0.4)2O3, with β-Ga2O3 in MOS capacitors (MOSCAPs) and analyzed the interface quality between the ternary dielectric and the UWBG semiconductor. A low total interface trap charge (Dit) of < 1012 cm 2 was extracted using the photo-assisted capacitance - voltage (C-V) technique, which indicates a clean interface between (Y0.6Sc0.4)2O3 and β-Ga2O3. Due to the need of a conformal dielectric for nonplanar structures, I next investigated the reliability of atomic layer deposited (ALD) HfO2/β-Ga2O3 MOSCAPs. Positive bias stress measurements were used to investigate charge trapping in these MOSCAPs, and a comparison to HfO2/Si was made to determine how the unique characteristics of β-Ga2O3 impacted the charge trapping and recovery observed here as opposed to a smaller bandgap semiconductor (i.e. Si). β-Ga2O3 MOSCAPs are shown to be more susceptible to charge trapping at lower electric stress fields, and border traps play a significant role in the initial flatband voltage shifts observed when C-V sweeps are repeated due to the lack of mobile holes in β-Ga2O3. Next, the thermal reliability of the ALD HfO2/β-Ga2O3 interface was investigated due to the known diffusion issues β-Ga2O3 has with many materials including Al2O3. Secondary ion mass spectrometry (SIMS) depth profile showed negligible Ga diffusion in the HfO2 layer up to annealing temperatures of 850°C in N2 environment, but increases in forward bias leakage and Dit were observed in MOSCAPs annealed at 700°C compared to MOSCAPs annealed below 500°C. Annealing at 900°C resulted in significant Ga diffusion into the HfO2 layer providing poor electrical insulation for the UWBG semiconductor. Lastly, I studied vertical MOS structures on etched β-Ga2O3, inspired by the vertical fin-MOSFET structure, which shows promising enhancement-mode behavior as a β-Ga2O3 MOS device. A fabrication process for HfO2/β-Ga2O3 fin MOSCAPs arrays has been developed, and initial analysis shows that the MOSCAPs on plasma-etched fin sidewalls of β-Ga2O3 may have significantly higher Dit compared to planar MOSCAPs using the same ALD dielectric. The techniques used throughout this thesis can be utilized in future UWBG semiconductor MOS device research.