Doping in the two-dimensional limit: p/n -type defects in monolayer ZnO

Abstract

The thickness limit is utilized to investigate the doping physics in ZnO, i.e., monolayer (ML) ZnO. First-principles study demonstrates that the $p/n$-type defects in ML ZnO still have doping asymmetry. Among the doping defect models widely studied in bulk ZnO, ${\mathrm{Li}}_{\text{Zn}}$ and ${\mathrm{Ga}}_{\text{Zn}}$ with ionization energies of 0.86 and 0.82 eV are the optimal $p$- and $n$-type doping defects in ML ZnO, respectively. Their ionization energies are comparable with those of relatively shallow defects in other ML semiconductors. However, the ${\mathrm{Li}}_{\text{Zn}}$ acceptor faces a severe issue in that ${\mathrm{Li}}_{\text{Zn}}$ is the metastable structure and will transform into the most stable Jahn-Teller-distorted structure (${\mathrm{Li}}_{\text{Zn}}^{\text{JT}}$) with increasing its ionization energy to 1.53 eV. Furthermore, our scanning tunneling microscopy simulations show even a little structural distortion of the doping defects can be easily detected with the appropriate positive bias voltage on a sample of ML ZnO. The present study reveals the $p/n$-type defects' properties in ML ZnO and offers a way to understand and directly identify defect behaviors in wide-band-gap semiconductors in their two-dimensional limit form.