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In Situ Microbeam Surface X-Ray Scattering Reveals Alternati


Ju, Guangxu; Xu, Dongwei; Thompson, Carol; Highland, Matthew; Eastman, Jeffrey; Walkosz, Weronika; Zapol, Peter; Stephenson, G.


The stacking sequence of hexagonal close-packed (HCP) and related crystals typically results in steps on vicinal {0001} surfaces that have alternating A and B structures with different growth kinetics. This produces fascinating surface morphologies with alternating terrace arrangements. However, because it is difficult to experimentally identify which step has the A or B structure, it has not been possible to determine which has faster adatom attachment kinetics. Here we show that a new method, in situ microbeam surface X-ray scattering, can settle the long-standing question of whether A or B steps have faster kinetics under specific growth conditions. We demonstrate this for organo-metallic vapor phase epitaxy (OMVPE) of (0001) GaN. X-ray measurements performed during growth find that the average width of terraces above A steps increases with growth rate, indicating that attachment rate constants are higher for A steps, in contrast to most predictions. Our results have direct implications for understanding the atomic-scale mechanisms of GaN growth by OMVPE. The X-ray method demonstrated can be applied to a wide variety of HCP and related crystals.



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