【Member Papers】Improved Interface Quality in β-Ga₂O₃ MOSFETs With a Crystalline (AlGa)₂O₃ Cap

      A research team from Xidian University has reported a crystalline (AlGa)₂O₃-cap engineering approach for improving the interface quality and electrical performance of β-Ga₂O₃ MOS devices. The work, entitled “Improved Interface Quality in β-Ga₂O₃ MOSFETs With a Crystalline (AlGa)₂O₃ Cap,” was published in IEEE Transactions on Electron Devices. The corresponding authors are Yibo Wang and Genquan Han.

Background

      β-Ga₂O₃ is considered a promising ultrawide-bandgap semiconductor for next-generation power electronics because of its high critical electric field and the availability of native substrates.

      However, β-Ga₂O₃ lacks a high-quality native oxide. Therefore, foreign high-k dielectrics such as Al₂O₃ are typically employed to construct MOS gate stacks. Direct contact between amorphous Al₂O₃ and crystalline β-Ga₂O₃ can introduce incomplete bonding and structural disorder during the initial dielectric-growth stage, resulting in interface traps, near-interface oxide traps, and dielectric-related defects.

      These defects can degrade subthreshold characteristics and channel transport, induce threshold-voltage instability, and increase gate leakage. Improving the dielectric/β-Ga₂O₃ interface therefore remains essential for achieving high-performance and reliable β-Ga₂O₃ MOS power devices.

      In this work, a crystalline (AlGa)₂O₃ cap layer was introduced between the amorphous Al₂O₃ dielectric and β-Ga₂O₃, providing a structurally improved transition layer at the MOS interface.

Abstract

      A ~5-nm Al₂O₃ layer was first deposited on β-Ga₂O₃ by ALD, followed by annealing at 1000 °C for 3 min in N₂. A 25-nm Al₂O₃ layer was subsequently deposited to complete the gate stack. Interfacial Ga-Al interdiffusion during annealing transformed the initially deposited layer into an approximately 5-nm-thick crystalline (AlGa)₂O₃ cap.

      Cross-sectional HRTEM revealed an epitaxial-like alignment between the crystalline cap and the underlying β-Ga₂O₃. Elemental mapping further confirmed Ga-Al interdiffusion at the interface.

      MOS capacitor measurements showed a pronounced reduction in interface traps after introducing the crystalline cap. The capped MOSCAPs also exhibited lower gate leakage and higher breakdown fields over the investigated temperature range, demonstrating improved overall gate-stack quality.

      For β-Ga₂O₃ MOSFETs, the representative hysteresis was reduced from 0.57 to 0.43 V, while the subthreshold swing was improved from 909 to 734 mV/dec. The specific ON-resistance decreased from 46.2 to 38.8 mΩ·cm². Statistical measurements on 12 devices in each group confirmed reductions of approximately 32% in hysteresis and 16% in SS.

      The capped MOSFETs also exhibited lower OFF-state leakage and improved breakdown characteristics. Combined with the reduced ON-resistance, the power figure of merit increased from 95.5 to 136.3 MW/cm², corresponding to a 42.7% enhancement.

      Positive-bias instability measurements further demonstrated suppressed electron trapping. The capped devices exhibited smaller threshold-voltage shifts over the investigated stress-time and gate-bias ranges. A ΔV_TH of only 0.82 V after 1000 s at V_G,stress = 4 V was obtained, indicating improved threshold-voltage stability.

Highlights

      The key innovation of this work lies in the use of a crystalline oxide transition layer instead of relying solely on direct optimization of the amorphous dielectric/semiconductor interface.

      Through thin-Al₂O₃ predeposition and high-temperature PDA, a crystalline (AlGa)₂O₃ cap with epitaxial-like alignment to β-Ga₂O₃ was formed in situ. This approach reduces direct contact between amorphous Al₂O₃ and β-Ga₂O₃ and suppresses interface-related defects.

      More importantly, the improved interface quality was consistently verified from structural characterization and MOSCAP measurements to transistor performance and bias-stress reliability. The resulting devices achieved simultaneous improvements in hysteresis, subthreshold swing, channel transport, breakdown performance, PFOM, and threshold-voltage stability.

Conclusion

      A crystalline (AlGa)₂O₃ cap was successfully introduced into β-Ga₂O₃ MOS gate stacks through thin Al₂O₃predeposition followed by high-temperature PDA.

      The crystalline cap suppresses interface defects and electron trapping, improves the overall gate-stack quality, and leads to enhanced subthreshold characteristics, channel transport, breakdown performance, PFOM, and positive-bias stability.

      These results demonstrate that crystalline oxide interfacial engineering provides an effective route toward high-performance and reliable β-Ga₂O₃ MOS power devices.

Project Support

      This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Key Program of Shaanxi Provincial Department of Science and Technology, the Key Program of Hangzhou City Department of Science and Technology, the Zhejiang Provincial Natural Science Foundation of China, the Leading Innovation and Entrepreneurship Project of Zhejiang Province, and the Hangzhou Leading Talent Program.