  Several ten $\mu$m GaN have been deposited on a silicon substrate using a
two-step hydride vapor phase epitaxy (HVPE) process. The substrates have been
covered by AlN layers and GaN nanostructures grown by plasma-assisted
molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step,
stacking faults (SF) form, which show distinct luminescence lines and
stripe-like features in cathodoluminescence images of the cross-section of the
layers. These cathodoluminescence features allow for an insight into the growth
process. During a second high-temperature (high-T) step, the SFs disappear, and
the luminescence of this part of the GaN layer is dominated by the donor-bound
exciton. For templates consisting of both a thin AlN buffer and GaN
nanostructures, a silicon incorporation into the GaN grown by HVPE is not
observed. Moreover, the growth mode of the (high-T) HVPE step depends on the
specific structure of the AlN/GaN template, where in a first case, the epitaxy
is dominated by the formation of slowly growing facets, while in a second case,
the epitaxy proceeds directly along the c-axis.
