Abstract
Nitrogen doping is a commonly employed and effective strategy to achieve p-type ZnO films. A detailed understanding of the ZnO film growth behavior resulting in epitaxy is essential for elucidating the doping mechanism. In this study, ZnO films are grown on nonpolar (101̅0) ZnO substrates using plasma-assisted molecular-beam epitaxy (PA-MBE). Introducing nitrogen during PA-MBE growth is expected to lead to the incorporation of N, thus affecting the film properties. Growth kinetics, surface morphology, and chemical structure of the films are thus investigated through in situ reflection high-energy electron diffraction, in-system scanning tunneling microscopy, and ex situ synchrotron-based X-ray absorption spectroscopy. Well-ordered "stripe-like" structures are observed on the surface of the N-free ZnO films that evolve into "corn-like" nanostructures upon nitrogen addition and further transform into a "particle-like" morphology as the N(2)/O(2) partial pressure ratio increases. Several nitrogen species, including N(2), NO, NO(2), and N(2)O, are expected to exist during the growth process, with NO and NO(2) molecules suggested to predominantly adsorb onto (101̅0) ZnO surfaces, where N-Zn bonds are likely to form. A pathway for this chemical structural change is proposed. This work aims to explore whether the supply of nitrogen during PA-MBE growth results in the incorporation of nitrogen into the ZnO lattice and whether this can be considered a doping mechanism. Hence, this work might contribute to a better understanding of nitrogen incorporation, potentially realizing (and deliberately optimizing the) p-type doping of ZnO films.