Computational Fermi Level Engineering and Doping-Type Conversion of Ga₂O₃ via Three-Step Processing

Ga₂O₃ is being actively explored for optoelectronic (phosphorus, and electroluminescent devices, solar-blind photodetectors) applications due to its ultra-wide bandgap and low projected fabrication cost of large-size and high-quality crystals. Efficient <i>n</i>-type doping of Ga₂O₃ has been achieved, but <i>p</i>-type doping faces fundamental obstacles due to compensation, deep acceptor levels, and the polaron transport mechanism of free holes. However, aside from achieving <i>p</i>-type conductivity, plenty of opportunity exists to engineer the position of the Fermi level for improved design of Ga₂O₃ based devices. We use first-principles defect theory and defect equilibrium calculations to simulate a 3-step growth-annealing-quench synthesis protocol for hydrogen assisted Mg doping in β-Ga₂O₃, considering the gas phase equilibrium between H₂, O₂ and H₂O, which determines the H chemical potential. We predict Ga₂O₃ doping-type conversion to a net <i>p</i>-type regime after growth under reducing conditions in the presence of H₂followed by O-rich annealing, which is a similar process to the Mg acceptor activation by H removal in GaN. We show that there is an optimal temperature that maximizes the Ga₂O₃ net acceptor density for a given Mg doping level. After quenching to operating temperature, the Ga₂O₃ Fermi level drops below mid-gap down to about +1.5 eV above the valence band maximum, creating a significant number of uncompensated neutral Mg_Ga⁰ acceptors. The resulting free hole concentration in Ga₂O₃ is very low due to deep energy level of these Mg acceptors, and hole conductivity is further impeded by the polaron hopping mechanism. However, the Fermi level reduction down to +1.5 eV and suppression of free electron density in this doping type converted (<i>N</i>_A &gt; <i>N</i>_D) Ga₂O₃ material is of significance and impact for the design of Ga₂O₃ based optoelectronic devices.<br/>Reference: J. Appl. Phys. 129, 245704 (2021); https://doi.org/10.1063/5.0051788