  GaAs nanopillars with 150 nm - 200 nm long axial InGaAs inserts are grown by
MOCVD via catalyst-free selective-area-epitaxy (SAE). The alloy composition of
the InGaAs region, as determined by room-temperature photoluminescence (PL),
depends critically on the pitch and diameter of the selective-area pattern
geometry. The PL emission varies based on pattern geometry from 1.0 \{mu}m to
1.25 \{mu}m corresponding to a In to Ga ratio from 0.15 to > 0.3. This In
enrichment is explained by a pattern dependent change in the incorporation rate
for In and Ga. Capture coefficients for Ga and In adatoms are calculated for
each pattern pitch. As the pitch decreases, these data reveal a contest between
a synergetic effect (related to nanopillar density) that increases the growth
rate and a competition for available material that limits the growth rate.
Gallium is more susceptible to both of these effects, causing the observed
changes in alloy composition.
