The absolute epitaxial relationships between III-nitride films and R-plane sapphire are investigated using convergent beam electron diffraction. The resulting relationships are found to be[1100]III-N[1120]sapphire and [0001]III-N[1101]sapphire, respectively. The O–Al bonds in the sapphire surface are antiparallel to the N-III bonds in the III-N films. High-resolution transmission electron microscopy shows that the AlN nucleation layer is asymmetrically strained. Along its c direction, the AlN layer is in extension whereas it is relaxed perpendicular to c.

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