Do A or B Steps Have Faster Kinetics during Hexagonal Crystal Growth?

The stacking sequence of hexagonal close-packed and related crystals typically results in steps on vicinal $\{0001\}$ surfaces that have alternating $A$ and $B$ structures and growth behaviors. However, because it is difficult to experimentally identify which step is $A$ or $B$, it has not been possible to determine which has faster adatom attachment kinetics. We show that surface X-ray scattering can unambiguously differentiate the growth behavior of $A$ and $B$ steps. Measurements performed in situ during growth of (0001) GaN 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.