Computational Fermi Level Engineering and Doping-Type Conversion of Mg:Ga2O3 via Three-Step Synthesis Process

Anuj Goyal, Andriy Zakutayev, Vladan Stevanovic, Stephan Lany

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14 Scopus Citations

Abstract

Gallium oxide (Ga2O3) is being actively explored for electronics that can operate at high power, temperature, and frequency as well as for deep-ultraviolet optoelectronics and other applications due to its ultra-wide bandgap (UWBG) and low projected fabrication cost of large-size and high-quality crystals. Efficient n-type doping of monoclinic beta-phase of Ga2O3 has been achieved, but p-type doping faces fundamental obstacles due to compensation, deep acceptor levels, and the polaron transport mechanism of free holes. However, aside from the challenges of achieving p-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 three-step growth-annealing-quench synthesis protocol for hydrogen-assisted Mg doping in β-Ga2O3. The simulations take into account the gas phase equilibrium between H2, O2, and H2O, which determines the H chemical potential. We predict Ga2O3 doping-type conversion to a net p-type regime after growth under reducing conditions in the presence of H2 followed by O-rich annealing, which is a similar process to Mg acceptor activation by H removal in GaN. For equilibrium annealing with re-equilibration of compensating O vacancies, there is an optimal temperature that maximizes the Ga2O3 net acceptor density for a given Mg doping level; the acceptor density is further increased in the non-equilibrium annealing scenario without re-equilibration. 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 MgGa0 acceptors. The resulting free hole concentration in Ga2O3 is very low even at elevated operating temperature (∼108 cm−3 at 400 °C) due to the deep energy level of these Mg acceptors, and hole conductivity is further impeded by the polaron hopping mechanism. However, the Fermi-level reduction and suppression of free electron density in this doping-type converted (NA > ND) Ga2O3 material are important for improved designs of Ga2O3 electronic devices. These results illustrate the power of computational predictions not only for new materials but also for their synthesis science.

Original languageAmerican English
Article numberArticle No. 245704
Pages (from-to)1ENG
Number of pages11
JournalJournal of Applied Physics
Volume129
Issue number24
DOIs
StatePublished - 28 Jun 2021

Bibliographical note

Publisher Copyright:
© 2021 Author(s).

NREL Publication Number

  • NREL/JA-5K00-79593

Keywords

  • density functional theory calculations
  • Ga2O3
  • wide bandgap semiconductor

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