Cathodoluminescent Imaging of ZnO:N Films: Study of Annealing Processes Leading to Enhanced Acceptor Luminescence

A worldwide effort is under way to understand the activation of acceptor states in ZnO with the motivation to achieve persistent p‐type conductivity. In this study, cathodoluminescent (CL) imaging, electron microscopy (SEM), secondary‐ions mass spectrometry (SIMS), and X‐ray diffraction (XRD) are used to compare ZnO:N films subjected to a rapid thermal annealing process (RTP) in N2 and O2 atmosphere at 400–900 °C. The study, performed for ZnO:N films with nitrogen concentration of 2 × 1018 at cm−3 grown under O‐rich conditions is directed to establish the optimal atmosphere and temperature at which acceptor‐related CL is enhanced and correlated with structural properties. XRD shows that crystallite size increases from ≈100 to ≈250 nm with increasing annealing temperature up to 800 °C. The low‐temperature (LT) CL maps reveal that the acceptor‐ and donor‐related CL mostly derives from different crystallites. Both annealing medium and temperature influence acceptor‐related CL intensity, which is higher under oxygen annealing. It is observed that the intensity of acceptor‐related CL increases with annealing temperature up to 800 °C and then decreases. Noticeable donor‐related emission appears only after RTP at 700 °C and becomes prominent for RTP at 900 °C Cathodoluminescence (CL) imaging performed on ZnO:N films cross sections reveals that acceptor‐ and donor‐related luminescence comes from separated crystallites. Intensity of acceptor‐related CL increases gradually with annealing temperature up to 800 °C but at higher annealing temperature, donor‐related CL dominates. The mechanism of creation of shallow acceptor states in ZnO:N films grown under oxygen‐rich conditions is proposed.