CBED study of grain misorientations in AlGaN epilayers

Large angle convergent beam electron diffraction (LACBED) has been used to examine AlGaN epilayers grown by facet-controlled epitaxial lateral overgrowth on GaN/(0 0 0 1) sapphire substrates in prototype UV laser structures. The substrates, defined by masks with seed openings along a 〈 1 0 - 1 0 〉 stripe direction, had GaN seed columns with { 1 1 - 2 2 } surfaces. Studies were carried out on cross-sectional samples cut perpendicular to the stripe axis. An LACBED analysis of the orientation of (0 0 0 2) planes, and of the ( 1 1 - 2 0 ) planes parallel to the stripe axis, revealed that the AlGaN wings were both rotated by angles of 1–2×10−2 radians about the 〈 1 0 - 1 0 〉 stripe axis with respect to the underlying GaN, and distorted due to misfit strains. It is shown that the results are consistent with the observed structure of the AlGaN/GaN and the wing/wing boundaries, and with a new model for the generation of a-type misfit dislocations at the AlGaN/GaN interface.