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

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, LiZn and GaZn 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 LiZn acceptor faces a severe issue in that LiZn is the metastable structure and will transform into the most stable Jahn-Teller-distorted structure (LiZnJT) 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.