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

Abstract

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

Effect of Strain and Surface Proximity on the Acceptor Grouping in ZnO

“…Rapid thermal annealing (RTA, 800 °C, 3 min) in oxygen atmosphere was applied in order to further enhance layer quality and to facilitate the formation of zinc vacancies. As previously shown, ZnO samples grown by ALD have a high hydrogen content that after RTA was reported at the level of 10 19 /cm 3 . , For this reason, hydrogen atoms are included in the theoretical calculations.…”

“…About 120 nm thick films were deposited simultaneously on c -Al 2 O 3 and a -Al 2 O 3 substrates in the same ALD process. As previously shown, deposition on differently oriented sapphire substrates strongly influences orientation of ZnO films over a wide growth temperature range, especially for limited layer thicknesses . The studied layers were deposited at a temperature of 300 °C, which is high for the ZnO–ALD process, in order to get a high-intensity photoluminescence signal despite the limited layer thickness.…”

“…As previously shown, deposition on differently oriented sapphire substrates strongly influences orientation of ZnO films over a wide growth temperature range, especially for limited layer thicknesses. 15 The studied layers were deposited at a temperature of 300 °C, which is high for the ZnO−ALD process, in order to get a high-intensity photoluminescence signal despite the limited layer thickness. Rapid thermal annealing (RTA, 800 °C, 3 min) in oxygen atmosphere was applied in order to further enhance layer quality and to facilitate the formation of zinc vacancies.…”

“…This phenomenon translates into the experimentally observed differences in PES between crystallites containing different acceptor-related complexes . According to a number of experimental and theoretical works, native defects and their complexes, often containing hydrogen, such as V Zn , V O , V Zn – n H ( n = 2, 3), Zn i V O H, and others, determine ZnO conductivity as they introduce shallow and deep, donor and acceptor levels, − and the acceptor-related sample properties depend on a number of factors introduced by growth and/or annealing conditions. − For example, for ZnO/N it has been shown that both annealing medium and temperature influence acceptor-related cathodoluminescence intensity, which is higher under oxygen annealing . Generally, the reasons for donor and acceptor domains are not clear, but it might be due to the formation of defect clusters that require distortion of the crystal lattice.…”

“…12−15 For example, for ZnO/N it has been shown that both annealing medium and temperature influence acceptor-related cathodoluminescence intensity, which is higher under oxygen annealing. 15 Generally, the reasons for donor and acceptor domains are not clear, but it might be due to the formation of defect clusters that require distortion of the crystal lattice. Hence, it can be assumed that microstrain plays a role here.…”