【International Papers】Evidence for carrier compensation by gallium vacancies during annealing of highly Si-doped β-Ga₂O₃

      Researchers from the Cornell University have published a dissertation titled " Evidence for carrier compensation by gallium vacancies during annealing of highly Si-doped β-Ga₂O₃" in Journal of Applied Physics.

Background

      Beta-phase gallium oxide (β-Ga₂O₃) has received attention in recent years due to its ultrawide bandgap (∼4.8 eV), high breakdown strength (∼8 MV/cm), and UV optical transparency. While other metastable polymorphs are also of interest, large single-crystals of the thermodynamically stable monoclinic β-Ga₂O₃ phase can be grown from the melt, facilitating rapid transition to commercial applications. For these reasons, β-Ga₂O₃ has tremendous potential relative to other wide and ultrawide bandgap semiconductors.

      While there are significant challenges with p-type doping, n-type β-Ga₂O₃ can be readily achieved with various donors, including Si, Ge, and Sn substituting on Ga sites, F and Cl substituting on O sites, and H on interstitial sites. Unintentionally doped (UID) bulk β-Ga₂O₃ has been measured with >10¹⁸ cm⁻³ carriers, which was initially attributed to intrinsic oxygen vacancies (V_O²⁺) due to the material's conductivity dependence on P_O2 (σ ∼ P_O2^1/4). Annealing β-Ga₂O₃ under oxygen-rich ambients resulted in the formation of insulating layers, which could be largely reversed under subsequent low P_O2 annealing, or minimized with a SiO₂ cap present during annealing ambient. 

Abstract

      Net carrier densities in highly Si-doped β-Ga₂O₃ decrease during thermal annealing as a function of temperature, time, and P_O2 for both in situ and implant-doped films. The mechanism for this loss has been attributed to the formation of gallium vacancies (V_Ga³⁻), which act as compensating acceptors and form passivating defect complexes. Samples doped above 3 × 10¹⁹ cm⁻³, annealed at the same temperature and P_O2, ultimately approach a similar net carrier density of 1–3 × 10¹⁹ cm⁻³ (for 950 °C and P_O2 < 2 × 10⁻⁷ bar) independent of the starting dopant concentration or doping method. These highly doped samples, metastable under these conditions, generate V_Ga³⁻ to reach the steady state |N_D − N_A|, determined by the Fermi level. An activation energy of 1.6 ± 0.2 eV was observed for the rate of net carrier loss in high concentration in situ doped films. Scanning transmission electron microscopy and electrical results for implanted samples show that dopant activation, lattice recovery, and compensation are all active within the first minutes of annealing. Understanding the metastability of higher doping levels and V_Ga³⁻ formation allows for the development of mitigation strategies to achieve highly Si-doped β-Ga₂O₃ by implant (8 × 10¹⁹ cm⁻³ with >70 cm²/V s mobility), including using the minimum required anneal time to achieve lattice recovery and dopant activation, reducing vacancy formation. Compensation initially limited full activation of deeper implants to 1 × 10²⁰ cm⁻³; by protecting the free surface with a SiO₂ cap during annealing, the activation efficiency doubled, and the mobility increased by 30%, highlighting the importance of controlling V_Ga³⁻ formation and compensation.

Conclusions

      Findings presented in this work support the hypothesis that the decrease in net carriers as a function of extended annealing time, observed in both in situ and implant Si-doped films, arises from the formation of compensating gallium vacancies, V_Ga³⁻, and associated passivating defect complexes. Under given P_O2 and temperature anneal conditions, net carriers decrease to a common range independent of the starting doping density; these data show vacancies will continue forming until the energy barrier to generate more V_Ga³⁻ is no longer surmountable. The rate and extent of this V_Ga³⁻ generation depends on P_O2, impacting the defect formation energies and the reaction equilibrium, and temperature, impacting formation kinetics and diffusion. This process exhibits an activation energy of 1.6 ± 0.2 eV, linked to surface V_Ga³⁻ formation and vacancy diffusion. Doping levels below 3 × 10¹⁹ cm⁻³ show only limited deactivation (at 950 °C and P_O2 < 2 × 10⁻⁷ bar) with no significant mobility change, likely associated with only vacancy-dopant complex formation at longer times. Net active carriers rapidly decrease in highly doped samples with a decrease in mobility, due to increased ionized acceptors compensating Si donors and increasing ionized impurity scattering. For implanted films at higher doping concentrations, the mobility shows only limited changes with anneal time due to masking of the vacancy compensation at short times by lattice recovery processes. The use of a capping layer during annealing limits vacancy formation, enabling doubled activation to higher metastable net carrier densities near 1 × 10²⁰ cm⁻³ with a 30% increase in mobility. These results show that metastable heavily n-type doping within β-Ga₂O₃ is feasible by both in situ doping and ion implantation if the formation of gallium vacancies is minimized to prevent deactivation.