Silicon implantation and annealing in β-Ga₂O₃: Role of ambient, temperature, and time

Optimizing thermal anneals of Si-implanted β-Ga₂O₃ is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga₂O₃ at concentrations from 5 × 10¹⁸ to 1 × 10²⁰ cm⁻³, demonstrating full activation (>80% activation, mobilities >70 cm²/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga₂O₃ films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 10¹⁸, 5 × 10¹⁹, and 1 × 10²⁰ cm⁻³. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga₂O₃ stability, p_O2 must be greater than 10⁻⁹ bar. Anneals up to p_O2 = 1 bar achieve full activation at 5 × 10¹⁸ cm⁻³, while 5 × 10¹⁹ cm⁻³ must be annealed with p_O2 ≤ 10⁻⁴ bar, and 1 × 10²⁰ cm⁻³ requires p_O2 < 10⁻⁶ bar. Water vapor prevents activation and must be maintained below 10⁻⁸ bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperatures up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 min with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5 × 10¹⁹ cm⁻³ and occurs rapidly at 1 × 10²⁰ cm⁻³. Rutherford backscattering spectrometry (channeling) suggests that damage recovery is seeded from remnant aligned β-Ga₂O₃ that remains after implantation; this conclusion is also supported by scanning transmission electron microscopy showing retention of the β-phase with inclusions that resemble the γ-phase.