Understanding W doping in wurtzite ZnO

Abstract

In the context of bandgap engineering of the ZnO photoactive material for solar harvesting applications via W doping, a number of a priori discrepant experimental observations in the literature concerning ZnO c axis expansion/contraction, bandgap red- or blue-shifting, the Zn-substitutional or interstitial nature of W atoms, or the W6+ charge compensation view with ZnO native defects are addressed by thorough density functional theory calculations on a series of bulk supercell models encompassing a large range of W contents. The present results reconcile the at first sight dissimilar observations by showing that interstitial W (Wi) is only energetically preferred over substitutional (WZn) at large large W doping concentrations; the WZn c lattice expansion can be compensated by a W triggered Zn-vacancy (VZn) c lattice contraction. The presence of WZn energetically fosters nearby VZn defects, and vice versa, up to a double VZn situation. The quantity of mono or divacancies explains the lattice contraction/expansion, and both limiting situations imply gap states which reduce the band gaps, but increase the energy gaps. Based on present results, the ZnO band gap red-shifting necessary for solar light triggered processes is achievable by W doping in Zn rich conditions.