  Single GaN nanowires formed spontaneously on a given substrate represent
nanoscopic single crystals free of any extended defects. However, due to the
high area density of thus formed GaN nanowire ensembles, individual nanowires
coalesce with others in their immediate vicinity. This coalescence process may
introduce strain and structural defects, foiling the idea of defect-free
material due to the nanowire geometry. To investigate the consequences of this
process, a quantitative measure of the coalescence of nanowire ensembles is
required. We derive objective criteria to determine the coalescence degree of
GaN nanowire ensembles. These criteria are based on the area-perimeter
relationship of the cross-sectional shapes observed, and in particular on their
circularity. Employing these criteria, we distinguish single nanowires from
coalesced aggregates in an ensemble, determine the diameter distribution of
both, and finally analyze the coalescence degree of nanowire ensembles with
increasing fill factor.
