Structural Stability and Defect Energetics of ZnO from Diffusion Quantum Monte Carlo

TL;DR

This study uses diffusion quantum Monte Carlo to accurately analyze the structural stability and defect energetics of ZnO, providing insights that surpass traditional density functional theory in precision.

Contribution

The paper demonstrates that DMC is a practical and accurate method for characterizing complex properties of ZnO, including defect formation energies and electronic properties, with better agreement to experiments than DFT.

Findings

DMC agrees with experimental measurements within 0.3 eV.

Oxygen vacancy acts as a deep donor with a formation energy of 5.0 eV.

Defect formation energies differ significantly between DMC and hybrid DFT methods.

Abstract

We have applied the many-body ab-initio diffusion quantum Monte Carlo (DMC) method to study Zn and ZnO crystals under pressure, and the energetics of the oxygen vacancy, zinc interstitial and hydrogen impurities in ZnO. We show that DMC is an accurate and practical method that can be used to characterize multiple properties of materials that are challenging for density functional theory approximations. DMC agrees with experimental measurements to within 0.3 eV, including the band-gap of ZnO, the ionization potential of O and Zn, and the atomization energy of O$_2$, ZnO dimer, and wurtzite ZnO. DMC predicts the oxygen vacancy as a deep donor with a formation energy of 5.0(2) eV under O-rich conditions and thermodynamic transition levels located between 1.8 and 2.5 eV from the valence band maximum. Our DMC results indicate that the concentration of zinc interstitial and hydrogen impurities in ZnO should be low under n-type, and Zn- and H-rich conditions because these defects have formation energies above 1.4 eV under these conditions. Comparison of DMC and hybrid functionals shows that these DFT approximations can be parameterized to yield a general correct qualitative description of ZnO. However, the formation energy of defects in ZnO evaluated with DMC and hybrid functionals can differ by more than 0.5 eV.