First-principles study of defects and doping limits in CaO
Applied Physics Letters(2024)
Abstract
Calcium oxide (CaO) is a promising host for quantum defects because of its
ultrawide band gap and potential for long spin coherence times. Using hybrid
functional calculations, we investigate the intrinsic point defects and how
they limit Fermi-level positions and doping in CaO. Our results reveal calcium
and oxygen vacancies to be the most common intrinsic defects, acting as
compensating acceptors and donors, respectively. Oxygen interstitials are also
prevailing under O-rich conditions and act as compensating donors. Due to
compensation by these defects, O-poor conditions are required to dope CaO
n-type, while O-rich conditions are required for p-type doping. We find that,
at room temperature, intrinsic CaO can only achieve Fermi-level positions
between 1.76 eV above the valence-band maximum (VBM) and 1.73 eV below the
conduction-band minimum (CBM). If suitable shallow dopants can be found, the
allowed range of Fermi levels would increase to between VBM+0.53 eV and
CBM-0.27 eV and is set by the compensating intrinsic defects. Additionally, we
study hydrogen impurities, and show that hydrogen will limit p-type doping but
can also act as shallow donor when substituting oxygen (H_O
defects).
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