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Semiconductor nanowaveguides are the key structure for light-guiding nanophotonics applications. Efficient guiding and confinement of single-mode light in these waveguides require high aspect ratio geometries. In these conditions, sidewall verticality becomes crucial. We fabricated such structures using a top-down process combining electron beam lithography and inductively coupled plasma (ICP) etching of hard masks and GaAs/AlGaAs semiconductors with Al concentrations varying from 0 to 100%. The GaAs/AlGaAs plasma etching was a single-step process using a Cl 2/BCl 3/Ar gas mixture with various fractions of N 2. Scanning electron microscope (SEM) observations showed that the presence of nitrogen generated the deposition of a passivation layer, which had a significant effect on sidewall slope. Near-ideal vertical sidewalls were obtained over a very narrow range of N 2, allowing the production of …