(Invited) Consideration on Gate Stack Formation Processes of Ultrawide Bandgap Ga2O3 with SiO2 Dielectric Layer
Authors: Kita Koji 
Published: August 2024

MOSFETs on β-Ga2O3 are expected to be a high-efficient and cost-effective alternative option of high-voltage power devices. Taking account of the reliability and the large conduction-band offset against β-Ga2O3, it would be reasonable to employ SiO2 as the gate dieletric. In this study we investigated the effects of post-deposition annealing (PDA) on SiO2/β-Ga2O3 MOS gate stacks, in terms of the MOS electrical characteristics and the band alignment.
β-Ga2O3 (001) wafers with ~2×1016 cm-3 n-type doped epitaxial layers were cleaned in diluted HF solution, and SiO2 films were deposited by electron-beam (EB) evaporation of Si in O2 ambient of ~1×10-2 Pa, followed by PDA at various temperatures from 600 to 1000℃ in either O2 or N2 ambient. Au was evaporated as gate electrode to form MOS capacitors. X-ray photoelectron spectroscopy (XPS) was conducted on the stacks with ~3nm-thick SiO2 for the evaluation of band alignment. Ultraviolet photoelectron spectroscopy (UPS) was also employed to determine the band diagram of β-Ga2O3 (001).
Nearly-ideal capacitance-voltage characteristics were obtained for the MOS capacitor fabricated with O2-PDA at 1000℃. The interface state density (Dit) determined by conductance method decreased down to ~1010 cm-3 at the energy level of EC−0.2 eV by PDA at higher temperature. The hysteresis width (ΔVhys) of C-V curves was smaller for the samples annealed at higher temperatures. The flatband voltage (VFB) shift after O2-PDA resulted in VFB close to the ideal value estimated from the physical properties of Au and Ga2O3, suggesting that fixed charge density at the interface would be minimized. However, PDA in N2 at 1000℃ resulted in a negative shift of VFB, which indicates the formation of positive fixed charges in the stack. The beneficial influences of O2-PDA on those characteristics suggest that an oxygen deficiency introduced near the interface is one of the origins of interface defect states. It is noteworthy that Ga3d XPS on the surface of Ga2O3 substrate always shows a tailing in lower binding energy side, which is indicating oxygen deficiency on the surface. The intensity of this lower-binding-energy component reduced significantly when a bare-Ga2O3 substate was annealed in O2, however, it did not decrease efficiently even after O2-PDA when the surface was covered with SiO2. This is probably due to the oxygen rearrangement at SiO2/Ga2O3 interface determined by the difference of material properties of those oxides. This fact suggests that the Ga2O3 surface easily forms oxygen vacancies when it is covered with SiO2. This would be the reason why the formation of interface defect states is not suppressed without O2-PDA.
Next the band diagram of β-Ga2O3 (001) substrate was investigated by UPS analysis where the energy level of valence band edge referring to the vacuum level was evaluted from the energy difference between minimum (cut-off) and maximum edges of the obtained kinetic energy spectra of photoelectrons. As a result we found the valence band edge of of β-Ga2O3 (001) was ~8.2 eV below the vacuum level, where the influences of O2 annealing at 1000℃ on the valence band edge energy level was limited to the shift within ~0.1eV. This would tell us the valence band offset between SiO2 and Ga2O3 should be around ~1.1eV, and the conduction band offset between them is to be around 2.8-2.9eV, by taking account of the well-reported SiO2 band diagram and assuming the bandgap of β-Ga2O3 as 4.7eV. On the other hand, the valence band offset estimated from the deconvolution of the XPS valence band spectrum for a ~3nm-thick SiO2/β-Ga2O3 stack from the energy edge difference between the deconvoluted spectra corresponding to SiO2 and Ga2O3, was a few hundreds of mV smaller than the above expected value (~1.1eV). This discrepancy is indicating the formation of an interface dipole layer between SiO2 and Ga2O3 with a pair of nagative and positive charges aligned at the interface to cause a shift in the band alignment. The magnitude of such dipole was indicated to be more significant (~0.5eV) for the stack after PDA at 1000℃, whereas it was less (0.2~0.3eV) after PDA at 600℃.
In conclusion, we demonstrated that SiO2/β-Ga2O3 (001) MOS capacitors showed nearly-ideal electrical characteristics with significantly reduced interface defect density and small fixed charge density by applying O2-PDA. The band alignment of SiO2/β-Ga2O3 (001) MOS stack was clarified by photoelectron spectroscopy to confirm a very large conduction band offset of the stack, but an influence of the formation of interface dipole layer between SiO2 and Ga2O3 on the band alighment was also indicated.