  ZnO is co-doped with Na+ and Si4+ in the ratio 2:1. The ratio was
intentionally chosen so that net valence state of dopant theoretically matches
that of host. This is to avoid dependence in the amount of oxygen
vacancies/interstitials arising out of cationic valence state of the dopant.
With such a combination, modifications in structural and optical properties do
not depend on excess or deficit of the dopant charge state. For lower doping,
Na+ ions behave as interstitial sites which enhance strain, lattice disorder
and thereby creating defects. Formation of interstitial defects leads to
reduction in bandgap energy and produce orange-red luminescence. For higher
doping, Na+ starts substituting at Zn2+ site which helps in reducing strain and
lattice disorder and thereby increases bandgap. Inspite of presence of Si4+
with higher charge, there is a gradual increase in oxygen vacancies due to
lattice disorder.
