  Numerous efforts have been devoted to improve the electronic and optical
properties of III-V compound materials via reduction of their nonradiative
states, aiming at highly-efficient III-V sub-micrometer devices. Despite many
advances, there is still a controversial debate on which combination of
chemical treatment and capping dielectric layer can best reproducibly protect
the crystal surface of III-Vs, while being compatible with readily available
plasma deposition methods. This work reports on a systematic experimental study
on the role of sulfide ammonium chemical treatment followed by dielectric
coating in the passivation effect of GaAs/AlGaAs nanopillars. Our results
conclusively show that the best surface passivation is achieved using ammonium
sulfide followed by encapsulation with a thin layer of silicon nitride by low
frequency plasma enhanced chemical deposition. Here, the sulfurized GaAs
surfaces, the high level of hydrogen ions and the low frequency (380 kHz)
excitation plasma that enable intense bombardment of hydrogen, all seem to
provide a combined active role in the passivation mechanism of the pillars. We
observe up to a 29-fold increase of the photoluminescence (PL) integrated
intensity for the best samples as compared to untreated nanopillars. X-ray
photoelectron spectroscopy analysis confirms the best treatments show
remarkable removal of gallium and arsenic native oxides. Time-resolved micro-PL
measurements display nanosecond lifetimes resulting in a record-low surface
recombination velocity for dry etched GaAs nanopillars. We achieve robust,
stable and long-term passivated nanopillar surfaces which creates expectations
for remarkable high internal quantum efficiency (IQE>0.5) in nanoscale
light-emitting diodes. The enhanced performance paves the way to many other
nanostructures and devices such as miniature resonators, lasers, photodetectors
and solar cells.
