  Images of the morphology of GaN (0001) surfaces often show
half-unit-cell-height steps separating a sequence of terraces having
alternating large and small widths. This can be explained by the $\alpha \beta
\alpha \beta$ stacking sequence of the wurtzite crystal structure, which
results in steps with alternating $A$ and $B$ edge structures for the lowest
energy step azimuths, i.e. steps normal to $[0 1 \bar{1} 0]$ type directions.
Predicted differences in the adatom attachment kinetics at $A$ and $B$ steps
would lead to alternating $\alpha$ and $\beta$ terrace widths. However, because
of the difficulty of experimentally identifying which step is $A$ or $B$, it
has not been possible to determine the absolute difference in their behavior,
e.g. which step has higher adatom attachment rate constants. Here we show that
surface X-ray scattering can measure the fraction of $\alpha$ and $\beta$
terraces, and thus unambiguously differentiate the growth dynamics of $A$ and
$B$ steps. We first present calculations of the intensity profiles of GaN
crystal truncation rods (CTRs) that demonstrate a marked dependence on the
$\alpha$ terrace fraction $f_\alpha$. We then present surface X-ray scattering
measurements performed \textit{in situ} during homoepitaxial growth on (0001)
GaN by vapor phase epitaxy. By analyzing the shapes of the $(1 0 \bar{1} L)$
and $(0 1 \bar{1} L)$ CTRs, we determine that the steady-state $f_\alpha$
increases at higher growth rate, indicating that attachment rate constants are
higher at $A$ steps than at $B$ steps. We also observe the dynamics of
$f_\alpha$ after growth conditions are changed. The results are analyzed using
a Burton-Cabrera-Frank model for a surface with alternating step types, to
extract values for the kinetic parameters of $A$ and $B$ steps. These are
compared with predictions for GaN (0001).
