Suppression of Na interstitials in Na-F codoped ZnO.

Controlling the formation of interstitial Na (<italic>Na</italic>i) self-compensating defects has been a long-term physics problem for effective Na doping in ZnO. Herein, we present an experimental approach to the suppression of <italic>Na</italic>i defects in ZnO via Na and F codoping under an oxygen-rich condition during the molecular beam epitaxy growth process. It is found that the incorporation of such large numbers of Na and F dopants (∼1020 cm−3) does not cause an obvious influence on the lattice parameters. Hall-effect measurements demonstrate that F doping efficiently raises the Fermi level (<italic>E</italic>F) of ZnO films, which is expected to make the formation energy of <italic>Na</italic>i and <italic>Na</italic>Zn increase and decrease, respectively. Most of the Na atoms occupy the substitutional Zn sites, and the formation of <italic>Na</italic>i is suppressed consequently. Secondary ion mass spectrometry measurements reveal that F and Na atoms are tightly bonded together due to their strong Coulomb interaction. The enhanced deep level emission (DLE) in ZnO:Na-F is ascribed to the considerable amount of isolated Zn vacancy (<italic>V</italic>Zn) defects induced by the elevated <italic>E</italic>F and the formation of neutral F O + − N a Zn − 0 complexes. On the other hand, formation of F O + − V Zn 2 − − complexes in ZnO:F exhausts most of the isolated Zn vacancies, leading to the disappearance of the DLE band. [ABSTRACT FROM AUTHOR]