Abstract Owing to the large differences in the chemical properties between Al and N polarities in aluminum nitride (AlN), the choice of the polar direction for crystal growth strongly affects not only the quality but also the shape (facet formation) of the grown crystal. In particular, N-polar (0 0 0 −1) has been considered to be a more preferable direction than Al-polar (0 0 0 1) for sublimation growth because compared to Al-polar (0 0 0 1), N-polar (0 0 0 −1) exhibits better stability at high growth rate (high supersaturation) conditions and enables easier lateral enlargement of the crystal. However, some critical growth conditions induce polarity inversion and hinder stable N-polar growth. Furthermore, the origin of the polarity inversion in AlN growth by the sublimation method is still unclear. To ensure stable N-polar growth without polarity inversion, the formation mechanism of the inversion domain during AlN sublimation growth must be elucidated. Therefore, herein, we demonstrate homoepitaxial growth on an N-polar seed and carefully investigate the obtained crystal that shows polarity inversion. Annular bright-field scanning transmission electron microscopy reveals that polarity is completely converted to the Al polarity via the formation of a 30 nm thick mixed polar layer (MPL) just above the seed. Moreover, three-dimensional atom probe tomography shows the segregation of the oxygen impurities in the MPL with a high concentration of about 3 atom%. Finally, by avoiding the incorporation of oxygen impurity into the crystal at the initial stage of the growth, we demonstrate an effective reduction (seven orders of magnitude) of the inversion domain boundary formation.

中文翻译：

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升华法生长的氮化铝中反转域的起源和有效减少

摘要 由于氮化铝 (AlN) 中 Al 和 N 极性之间化学性质的巨大差异，晶体生长极性方向的选择不仅对晶体的质量而且对晶体的形状（小面形成）有很大影响。特别是，对于升华生长，N-极性（0 0 0 -1）被认为是比Al-极性（0 0 0 1）更优选的方向，因为与Al-极性（0 0 0 1）相比，N- polar (0 0 0 -1) 在高生长速率（高过饱和度）条件下表现出更好的稳定性，并使晶体更容易横向扩大。然而，一些关键的生长条件会导致极性反转并阻碍稳定的 N 极性生长。此外，升华法在 AlN 生长中极性反转的起源仍不清楚。为了确保稳定的 N 极性生长而不发生极性反转，必须阐明 AlN 升华生长过程中反转域的形成机制。因此，在这里，我们展示了在 N 极性种子上的同质外延生长，并仔细研究了所获得的显示极性反转的晶体。环形明场扫描透射电子显微镜显示，通过在种子正上方形成 30 nm 厚的混合极性层 (MPL)，极性完全转换为 Al 极性。此外，三维原子探针断层扫描显示 MPL 中氧杂质的偏析具有约 3 原子％的高浓度。最后，通过避免在生长初期将氧杂质掺入晶体中，