Valence-Band Electronic Structure of ZnO and ZnO:N: Experimental and Theoretical Evidence of Defect Complexes

Undoped and N-doped $\mathrm{Zn}\mathrm{O}$ films are grown by atomic layer deposition under O-rich conditions. Scanning photoelectron microscopy studies carried out on the films cross sections with a state-of-the-art resolution of 130 nm, allowed the study of the electronic structure of individual crystallites. In has been found that crystallites can be divided into two types, that differ in the electronic structure of the valence band. This finding, together with the cathodoluminescence images showing clustering the acceptor- and donor-related emission, unveiled that acceptors and donors are grouped in separate regions of the $\mathrm{Zn}\mathrm{O}$ and $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$ films. Density-functional-theory (DFT) calculations reveal that the complexes involving zinc vacancy, hydrogen, and nitrogen (in the case of $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$) modify the density of states in the valence-band region, so the experimentally observed differences in photoelectron spectra between crystallites evidence the grouping of acceptor complexes in some crystallites. DFT calculations also suggest that shallow acceptor states might be of $n\ensuremath{\cdot}{V}_{\mathrm{Zn}}$ origin. Nitrogen willingly joins such complexes and facilitates their formation. The separation of donors and acceptors and its tendency to group along the growth columns is of great relevance for future applications, as it indicates that eventual electronic devices should rather be constructed in a vertical architecture.