Optimizing Si Implantation and Annealing in β-Ga₂O₃ and β-(Al_xGa_1-x)₂O₃

Optimizing thermal activation anneals for silicon ion implantation in β-Ga₂O₃ is critical for achieving low resistance contacts and selective area doping for advanced device structures. While activation has been widely reported, annealing conditions vary significantly and systematic annealing studies are lacking. We report the impact of time, temperature, and especially the annealing ambient, on the activation of Si in β-Ga₂O₃ at concentrations from 5×10¹⁸ cm^-3, to 1×10²⁰ cm^-3, and in β-(Al_xGa_1-x)₂O₃ (x≤25%) at concentrations up to 5×10¹⁹ cm^-3. Under optimized conditions, nearly full activation (&gt;80%) and high mobility (&gt;70 cm²/V-s) are achieved in β-Ga₂O₃ at implant concentrations to 1×10²⁰ cm^-3, with contact resistances below 0.16 Ω-mm for 5×10¹⁹ cm^-3. Similar optimized anneal conditions for β-(Al_xGa_1-x)₂O₃ show promising results with high activation (50%) and mobility recovery (&gt;60 cm²/V-s) at 9% Al.<br/><br/>Unintentionally doped (UID) β-Ga₂O₃ films (&gt;400 nm) were grown by plasma assisted molecular beam epitaxy, and β-(Al_xGa_1-x)₂O₃ layers by metal-organic chemical vapor deposition, on Fe-doped (010) β-Ga₂O₃ substrates. Samples were ion implanted with Si at multiple energies to yield 65 or 100 nm box profiles with concentrations of 5×10¹⁸ cm^-3, 5×10¹⁹ cm^-3, or 1×10²⁰ cm^-3. Dopant activation was achieved by annealing as-implanted films in a load-locked ultrahigh vacuum compatible quartz tube furnace at 1 bar pressure with precisely controlled gas ambient.<br/><br/>The impact of O₂ (partial pressure &lt;10^-6 to 1 bar) and H₂O (to 25 ppm) in the annealing ambient condition was determined by mixing trace gases in research plus grade nitrogen (&lt;1 ppm total impurities). To maintain the stability of the β-Ga₂O₃, P_O2 must be greater than 10^-9 bar, with annealing in vacuum or forming gas resulting in poor activation or decomposition, respectively. The upper limit for P_O2 is strongly dependent on Si concentration: P_O2 below 10^-4 bar ensures Si activation (&gt;85%) at 5×10¹⁹ cm^-3 while lower concentration implants (5×10¹⁸ cm^-3) maintain high carrier activation to oxygen partial pressures above 10^-2 bar. Water vapor is even more critical at ppm concentrations; 25 ppm H₂O reduces active carriers by an order of magnitude 5×10¹⁹ cm^-3 implants. The effect is reversible, and activation can be recovered by annealing in a “dry” ambient. Optimal results were consistently obtained with the gas ambient intentionally dried below 10 ppb of H₂O.<br/><br/>Recovery of the mobility requires annealing temperatures above 900 °C, with increasing carrier activation and mobility to temperatures as high as 1050 °C, though SIMS shows substantial Si diffusion at temperatures above 1000 °C. 950 °C is identified as an optimized temperature with minimal Si diffusion, with optimal times between 5 and 30 minutes; shorter times show lower mobilities with longer durations showing carrier deactivation. “Over-annealing” behavior is observed at all temperatures and becomes increasingly significant with higher Si concentrations.<br/><br/>Lattice damage and recovery were determined using Rutherford Backscattering Channeling (RBS/C), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD). XRD showed no evidence of second phase formation and RBS/C and STEM show only partial damage from even 1×10²⁰ cm^-3 implants. In STEM, damage regions remain crystal-like with no evidence of full amorphization. The remnant aligned β-Ga₂O₃ is hypothesized to volumetrically seed lattice recovery during thermal annealing. Varying implant doses and implant temperatures (from liquid nitrogen cooled to 600 °C) were used to assess the impact of implant damage on lattice recovery.<br/> <br/>In conclusion, we demonstrate the importance of optimizing anneal conditions to activate implanted Si carriers in β-Ga₂O₃ and extend this understanding to higher concentration Si implants and to activating implants in β-(Al_xGa_1-x)₂O₃.