Comparison of density functionals for nitrogen impurities in ZnO

TL;DR

This paper compares various density functional theory methods to evaluate their accuracy in predicting the formation energy and electronic levels of nitrogen impurities in ZnO, highlighting the strengths and limitations of each approach.

Contribution

It provides a systematic comparison of hybrid functionals, empirical correction schemes, and semi-local DFT in modeling nitrogen impurities in ZnO, revealing their relative predictive capabilities.

Findings

Generalized gradient approximation fails to accurately describe N impurity electronic structure.

Methods that increase ZnO's band gap improve agreement with hybrid functional results.

N impurities at oxygen sites are likely slightly endothermic and create deep levels above the valence band.

Abstract

Hybrid functionals and empirical correction schemes are compared to conventional semi-local density functional theory (DFT) calculations in order to assess the predictive power of these methods concerning the formation energy and the charge transfer level of impurities in the wide-gap semiconductor ZnO. While the generalized gradient approximation fails to describe the electronic structure of the N impurity in ZnO correctly, methods that widen the band gap of ZnO by introducing additional non-local potentials yield the formation energy and charge transfer level of the impurity in reasonable agreement with hybrid functional calculations. Summarizing the results obtained with different methods, we corroborate earlier findings that the formation of substitutional N impurities at the oxygen site in ZnO from N atoms is most likely slightly endothermic under oxygen-rich preparation conditions, and introduces a deep level more than 1eV above the valence band edge of ZnO. Moreover, the comparison of methods elucidates subtle differences in the predicted electronic structure, e.g. concerning the orientation of unoccupied orbitals in the crystal field and the stability of the charged triplet state of the N impurity. Further experimental or theoretical analysis of these features could provide useful tests for validating the performance of DFT methods in their application to defects in wide-gap materials.