Switching polarity in nitride films
A German-French research collaboration has demonstrated the method to revert the polarity in III-group nitride films by annealing under oxygen flow. This approach will be particularly beneficial for suppressing the undesirable N-polarity in mixed-polar nanowires and thin films, which deteriorates the properties of devices based on such structures, for example, in deep-UV LEDs.
The considerations about polarity in III-nitride films have remained within the scope of pragmatic observations since the first successful experiments of Amano, Akasaki and Nakamura on the structures for blue LEDs. Only recently, the group from Leibniz Institut fuer Kristallzuechtung in collaboration with Centre de Recherche sur l'Hétéro-Epitaxie et ses Applications (CRHEA-CNRS) and North Carolina State University (NCSU) have demonstrated that metal-polarity in conventional LEDs structures is established through the aluminum-oxynitride inversion domain that forms the initial growth step "“ sapphire nitridation.
In the current work, these authors went further and showed that the direct annealing of AlN layer in oxygen-rich atmosphere also forms an aluminum-oxynitride structure, which can transform the mixed-polar film into metal-polar at any moment of growth in just one step.
The researchers applied advanced electron microscopy together with electron energy-loss spectroscopy analysis with high spatial resolution and were able to distinguish single nitrogen, oxygen, and aluminum atomic columns. They have proved that oxygen incorporates in the N-polar surfaces of mixed-polar AlN and drives the polarity inversion. Interesting to note, that the discovered mechanism is self-limited "“ only 1.5 monolayers of oxygen is sufficient to form AlxOyNz and establish metal polarity.
In the future, the authors plan to continue experimenting with oxygen annealing of AlN films to investigate how the annealing parameters influence the polarity inversion mechanism, and whether the method is suitable for the reverse switch "“ from Al to N-polarity. The possibility to alternate the polarity in-situ along the growth direction offers a promising approach to design novel electronic devices based on hetero-polar junctions, e.g., for a second harmonic generation.
'Intentional polarity conversion of AlN epitaxial layers by oxygen' by N. Stolyarchuk et al; Scientific Reports volume 8, Article number: 14111 (2018)