【International Papers】Quasi-vertical β-Ga₂O₃ Schottky diodes on sapphire using all-LPCVD growth and plasma-free Ga-assisted etching

      Researchers from the  University of Massachusetts Lowell have published a dissertation titled "Quasi-vertical β-Ga₂O₃ Schottky diodes on sapphire using all-LPCVD growth and plasma-free Ga-assisted etching" in APL Electronic Devices.

Background

      β-Ga₂O₃ stands out as a promising ultra-wide bandgap semiconductor for next-generation high-voltage, high-efficiency power electronics, due to its ultra-wide bandgap (∼4.8 eV), high breakdown electric field (6–8 MV/cm), and the availability of low-cost, large-area native substrates grown by melt-based growth methods. These characteristics result in Baliga’s figure of merit that surpasses those of conventional wide bandgap power semiconductors such as SiC and GaN, making β-Ga₂O₃ an ideal candidate for next-generation power conversion systems, radio frequency amplification, and extreme-environment electronics. Substantial progress in high-quality β-Ga₂O₃ epitaxial film growth has enabled the realization of both lateral and vertical device architectures with excellent breakdown strength and thermal stability. To fully leverage the potential of β-Ga₂O₃ in high-power vertical device architectures, precise control over vertical isolation and trench formation is essential. This requires fabrication techniques capable of defining deep, high-aspect-ratio structures with minimal damage and high fidelity. However, the absence of p-type doping in β-Ga₂O₃ necessitates electric field control through mesa, trench, or fin geometries, structures that are highly sensitive to processing-induced surface degradation.

Abstract

      This work demonstrates quasi-vertical β-Ga₂O₃ Schottky barrier diodes (SBDs) fabricated on c-plane sapphire substrates using an all-low-pressure chemical vapor deposition (LPCVD)-based, plasma-free process flow that integrates both epitaxial growth of a high-quality β-Ga₂O₃ heteroepitaxial film with in situ Ga-assisted β-Ga₂O₃ etching. A 6.3 μm thick (⁠-201) oriented β-Ga₂O₃ epitaxial layer structure was grown on c-plane sapphire with 6° miscut, comprising a moderately Si-doped (2.1 × 10¹⁷ cm⁻³) 3.15 μm thick drift layer and a heavily doped (1 × 10¹⁹ cm⁻³) contact layer on an unintentionally doped buffer layer. Mesa isolation was achieved via Ga-assisted plasma-free LPCVD etching, producing ∼60° inclined mesa sidewalls with an etch depth of 3.6 μm. The fabricated SBDs exhibited excellent forward current–voltage characteristics, including a turn-on voltage of 1.22 V, an ideality factor of 1.29, and a Schottky barrier height of 0.83 eV. The minimum differential specific on-resistance was measured to be 8.6 mΩ cm², and the devices demonstrated high current density capability (252 A/cm² at 5 V). Capacitance–voltage analysis revealed a net carrier concentration of 2.1 × 10¹⁷ cm⁻³, uniformly distributed across the β-Ga₂O₃ drift layer. Temperature-dependent J–V–T measurements, conducted from 25 to 250 °C, revealed thermionic emission-dominated transport with strong thermal stability. The Schottky barrier height increased from 0.80 to 1.16 eV, and the ideality factor rose modestly from 1.31 to 1.42 over this temperature range. Reverse leakage current remained low, increasing from ∼5 × 10⁻⁶ A/cm² at 25 °C to ∼1 × 10⁻⁴ A/cm² at 250 °C, with the I_on/I_off ratio decreasing from ∼1 × 10⁷ to 5 × 10⁵. The devices achieved breakdown voltages ranging from 73 to 100 V, corresponding to parallel-plate electric field strengths of 1.66–1.94 MV/cm. These results highlight the potential of LPCVD-grown and etched β-Ga₂O₃ devices for high-performance, thermally resilient power electronics applications.

Conclusion

      In summary, this work demonstrates the first realization of quasi-vertical β-Ga₂O₃ Schottky barrier diodes fabricated entirely using a plasma-free LPCVD-based process, from epitaxial growth to in situ etching for mesa isolation, on insulating sapphire substrates. Leveraging the in situ Ga-assisted LPCVD etching technique, we achieved plasma damage-free mesa isolation with an etch depth of 3.6 μm and ∼60° inclined sidewalls. The resulting SBDs exhibited excellent forward characteristics with an ideality factor of 1.29, a Schottky barrier height of 0.83 eV, and a low specific on-resistance of 8.6 mΩ cm². Temperature-dependent I–V–T analysis revealed thermionic emission-dominated transport with strong thermal stability and a modest increase in Schottky barrier height and ideality factor, likely attributed to interface inhomogeneities and barrier non-uniformity. The devices exhibited low and stable reverse leakage characteristics, achieving breakdown fields of up to 1.94 MV/cm. Although the breakdown voltage was limited by the relatively high drift layer doping concentration, the demonstrated reverse blocking capability, along with robust forward conduction and thermal performance, highlight the promise of LPCVD-grown and etched β-Ga₂O₃ devices. The integration of epitaxy and etching within a single LPCVD platform offers a scalable and low-damage fabrication route for next-generation UWBG power electronics. These results also provide a critical step toward enabling cost-effective, high-voltage, and thermally resilient β-Ga₂O₃-based device architectures on foreign substrates.