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Polarity of group-III nitrides

Feng Wu

August 25, 2016 - Posted in Discussion
For the group-III nitrides grown by MOCVD, it is well known that nitridation before the deposition is beneficial to the improvement of structural and optical properties of nitride layers. While the chemical processes of the nitridation step are studied in detail, very little and contradictory work is presented on structural aspects that mediate polarity. Recently, an international group from Germany, France, Italy, and USA just published a PRA paper showing that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. Through high resolution transmission electron microscopy, they observed an inversion domain boundary (IDB), which mediated the polarity from N-polar of the first few AlN layers to Al-polar of the AlN layer after the IDB, as shown in figure 3 in this paper. They proposed that this inversion domain boundary was consisted of various AlxOyNz phases of the pseudobinary Al2O3-AlN system, and demonstrated the model based on the well agreement of the experimental and simulated results.
2016-08-25 Feng_polarity.jpg‚Äč
 By comparing the HRTEM images of samples with and without low temperature buffer layer, they also demonstrated that metal-polarity was mediated during nitridation, regardless of whether a low GaN or AlN buffer layer was deposited. 
This new finding can help people get a deeper understanding of the polarity of III nitrides and how it changes during the growth process in MOCVD system. 
However, group-III nitrides nanowires grown by MBE are exclusively N-polar, is there still an inversion domain boundary existing on the substrate just like the one mentioned above?

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