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.
@article{arxiv.1803.06656,
title = {Optical Property Study of Charge Compensated (Si, Na) Co-doped ZnO},
author = {Tulika Srivastava and Gaurav Bajpai and Gyanendra Rathore and Prashant Mishra and Shun Wei Liu and Sajal Biring and Somaditya Sen},
journal= {arXiv preprint arXiv:1803.06656},
year = {2018}
}