【International Papers】Low temperature (600 °C) formation of Al₂O₃/β Ga₂O₃ interface with low interface state density employing substrate surface O₃ treatment processes

      Researchers from the University of Tokyo have published a dissertation titled "Low‑temperature (600 °C) formation of Al₂O₃/β‑Ga₂O₃ interface with low interface state density employing substrate‑surface O₃ treatment processes" in Applied Physics Letters.

Background

      β‑Ga₂O₃ is expected to be a next‑generation power device material due to its large bandgap (4.4–4.7 eV) and high breakdown electric field. Additionally, the availability of bulk crystal growth from molten Ga₂O₃ using the EFG method is beneficial for the production of large‑diameter wafers, thereby enabling low‑cost wafer production. However, it remains challenging to realize high‑performance β‑Ga₂O₃‑based MOSFETs, and the key challenge is the formation of MOS gate stacks with low interface state density. Conventional processes require high‑temperature annealing over 1000 °C to reduce interface state density, but such high‑temperature oxidation will reduce carrier concentration and increase resistance of β‑Ga₂O₃ significantly, deteriorating its electrical properties. How to realize selective surface oxidation at low temperature while avoiding bulk oxidation is still an unsolved technical gap.

Abstract

      The benefit of employing O₃ surface treatment of the β‑Ga₂O₃ substrate to fabricate metal–oxide–semiconductor (MOS) capacitors with atomic‑layer‑deposited (ALD) Al₂O₃ as dielectrics was investigated systematically to design a fabrication process at a sufficiently low temperature (600 °C) which is expected to avoid high‑temperature‑induced deterioration of the electrical characteristics of the β‑Ga₂O₃ substrate. With a post‑deposition annealing (PDA) at 600 °C, the MOS interface characteristics with low interface state density (Dᵢₜ) were achieved by performing O₃ surface treatment at 300 °C prior to deposition of Al₂O₃ and/or by using O₃ as the oxidant in ALD. The impact of the gas composition of the 600 °C PDA ambient for the stack fabricated with the above O₃ treatments was further investigated to reduce Dᵢₜ down to ~ 2 × 10¹⁰ eV⁻¹ cm⁻² at an energy level of 0.2 eV below the conduction band edge.

Highlights

      Adopting 300 °C O₃ substrate surface pretreatment and O₃ oxidant during ALD to realize Al₂O₃/β‑Ga₂O₃ interface with low interface state density at 600 °C low temperature.

      Optimizing gas composition of 600 °C post‑deposition annealing ambient, the interface state density can reach ~ 2 × 10¹⁰ eV⁻¹ cm⁻², comparable to 1000 °C high‑temperature annealing.

      Low‑temperature process avoids bulk oxidation of β‑Ga₂O₃ and prevents deterioration of material electrical properties.

      Low oxygen partial pressure post‑deposition annealing can reduce both interface state density and oxide trapped charge density simultaneously.

Conclusion

      In conclusion, We investigated a low‑thermal‑budget process to suppress Al₂O₃/β‑Ga₂O₃ (001) MOS interface defects using a low‑temperature PDA at 600 °C. By conducting O₃ surface pretreatment and employing O₃ instead of H₂O as the oxidant for ALD growth of Al₂O₃, Dᵢₜ as low as ~ 2 × 10¹⁰ eV⁻¹ cm⁻² at the energy level of 0.2 eV below the conduction band edge of β‑Ga₂O₃ was achieved. Low p_O₂ PDA was found to be beneficial in reducing both Dᵢₜ and Qₜᵣₐₚ than PDA in O₂ when O₃‑Al₂O₃ was employed. Consequently, the employment of O₃ surface treatments combined with low‑p_O₂ PDA enables the formation of β‑Ga₂O₃ MOS interfaces with low defect densities even at a low PDA temperature of 600 °C.