【Member Papers】High-performance HfO₂ dielectric gated Ge-doped κ-Ga₂O₃ thin-film transistors

      Researchers from Hubei University, Wuhan Textile University and Austrian Academy of Sciences have published an article titled “High-performance HfO₂ dielectric gated Ge-doped κ-Ga₂O₃ thin-film transistors” in Applied Physics Letters.

Background

      κ-Ga₂O₃ is a highly promising ultra-wide bandgap semiconductor for next-generation power electronics and thin-film transistors due to its spontaneous polarization and high breakdown field. Conventional Si and Sn doping easily induce lattice distortion, and heteroepitaxy on regular c-plane sapphire tends to generate in-plane rotational domains and grain boundaries, which significantly degrade mobility. Meanwhile, traditional gate dielectrics exhibit weak electrostatic control, making it difficult to achieve high on/off ratio and enhancement-mode operation, thus limiting the development of high-performance κ-Ga₂O₃ transistors. At present, studies on integrating Ge doping for phase stabilization, off-cut substrates for epitaxy quality improvement, and HfO₂ high-k dielectric for enhanced gate control are still insufficient, and no reproducible strategy for high-performance enhancement-mode transistors has been established.

Abstract

      Orthorhombic κ-Ga₂O₃ has emerged as a promising candidate for next-generation power electronics due to its ultra-wide bandgap and high breakdown field. Herein, Ge-doped Ga₂O₃ thin films were grown via pulsed laser deposition as active channels on c-sapphire, 6° off-axis c-sapphire (6°/c-sapphire), and MgO (100) substrates to explore thin-film transistors (TFTs). Ge was strategically introduced to stabilize the κ-phase and control carrier concentrations in the range of 10¹⁶–10¹⁷ cm⁻³. Top-gate TFTs were fabricated utilizing HfO₂ as the high-k gate dielectric and Ti/Au as source/drain Ohmic contacts. Structural analyses indicate that substrate choice is critical: MgO(100) favors β-Ga₂O₃, while sapphire promotes κ-Ga₂O₃. Notably, the 6°/c-sapphire substrates yielded smooth films with minimal defects, which achieved an optimal carrier concentration (3.98 ×10¹⁷ cm⁻³) and a peak Hall mobility of 3.95 cm² V⁻¹ s⁻¹. Corresponding TFTs exhibited superior enhancement-mode characteristics with a threshold voltage of 2.6 V, subthreshold swing of 1.48 V/dec, and a high current on/off ratio of 7.16 × 10⁵. Technology-computer-aided-design simulations suggest a theoretical breakdown voltage of 298 V, limited primarily by gate-edge field crowding. This work demonstrates that synergistic strategy of Ge doping and 6°/c-sapphire substrate use can lead to high-quality κ-phase Ga₂O₃:Ge for high-performance power electronics.

Highlights

      Realize high-performance enhancement-mode κ-Ga₂O₃:Ge thin-film transistors by Ge doping and 6° off-axis sapphire substrate.

      Achieve phase stabilization of κ-Ga₂O₃ and carrier concentration modulation via Ge doping.

      Obtain low-defect, smooth κ-Ga₂O₃:Ge films on 6°/c-sapphire with improved mobility.

      Demonstrate top-gate TFTs with high current on/off ratio of 7.16 ×10⁵ and threshold voltage of 2.6 V.

      Reveal breakdown mechanism dominated by gate-edge electric field crowding via TCAD simulation.

Conclusion

      In conclusion, this work systematically investigates the structural and electrical properties of Ga₂O₃:Ge thin films across three distinct substrates, profoundly elucidating the decisive role of heteroepitaxial substrate engineering in modulating the performance of ultra-wide bandgap semiconductor devices. The results demonstrate that κ-Ga₂O₃:Ge films grown on 6°/c-sapphire exhibit enhanced surface planarity and superior carrier mobility. Evaluation of the 6°/c-sapphire-based devices reveals significant performance leaps, which achieve a high μ_sat of 13.7 cm² V⁻¹ s⁻¹ and an impressive I_on/I_off of 7.16 ×10⁵. While the observed SS remains relatively high, it presents a clear pathway for further optimization through refined interface engineering. This study presents a simple and effective method to enhance the mobility and overall performance of κ-Ga₂O₃ TFTs by the introduction of Ge as dopant and 6°/c-sapphire as template for Ga₂O₃.

Project Support

      This work was supported by the National Natural Science Foundation of China (Grant Nos. 62274057, 11975093, and 52202132), the Sino-German Mobility Program (Grant No. M-0764), the Hubei International Science and Technology Cooperation Project (Grant No. 2025EHA006), and the Wuhan Municipal Bureau of Science and Technology Innovation (Grant Nos. 2024040801020306 and 2025011202030396).