  GaAs-based nanowires are among the most promising candidates for realizing a
monolithical integration of III-V optoelectronics on the Si platform. To
realize their full potential for applications as light absorbers and emitters,
it is crucial to understand their interaction with light governing the
absorption and extraction efficiency, as well as the carrier recombination
dynamics determining the radiative efficiency. Here, we study the spontaneous
emission of zincblende GaAs/(Al,Ga)As core/shell nanowire arrays by
$\mu$-photoluminescence spectroscopy. These ordered arrays are synthesized on
patterned Si(111) substrates using molecular beam epitaxy, and exhibit an
exceptionally low degree of polytypism for interwire separations exceeding a
critical value. We record emission spectra over more than five orders of
excitation density for both steady-state and pulsed excitation to identify the
nature of the recombination channels. An abrupt Mott transition from excitonic
to electron-hole-plasma recombination is observed, and the corresponding Mott
density is derived. Combining these experiments with simulations and additional
direct measurements of the external quantum efficiency using a perfect diffuse
reflector as reference, we are able to extract the internal quantum efficiency
as a function of carrier density and temperature as well as the extraction
efficiency of the nanowire array. The results vividly document the high
potential of GaAs/(Al,Ga)As core/shell nanowires for efficient light emitters
integrated on the Si platform. Furthermore, the methodology established in this
work can be applied to nanowires of any other materials system of interest for
optoelectronic applications.
