Silicon Implantation and Annealing in β-Ga2O3: Role of Ambient, Temperature, and Time
Abstract
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 activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5x1018 to 1x1020 cm-3, demonstrating full activation (>80% activation, mobilities >70 cm2/Vs) with contact resistances below 0.29 -mm. Homoepitaxial β-Ga2O3 films, grown by plasma assisted MBE on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5x1018, 5x1019, and 1x1020 cm-3. Anneals were performed in a UHV-compatible quartz furnace at 1 bar with well-controlled gas composition. To maintain β-Ga2O3 stability, pO2 must be greater than 10-9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5x1018 cm-3, while 5x1019 cm-3 must be annealed with pO2 <10-4 bar and 1x1020 cm-3 requires pO2 <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 temperature up to 1050 C, though Si diffusion has been reported above 950 C. At 950 C, activation is maximized between 5 and 20 minutes with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5x1019 cm-3 and occurs rapidly at 1x1020 cm-3. RBS (channeling) suggests damage recovery is seeded from remnant aligned β-Ga2O3 that remains after implantation; this conclusion is also supported by STEM showing retention of the β-phase with inclusions that resemble the γ-phase.
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