III-V compound semiconductor quantum dots photonic devices are very attractive because of their low power consumption, temperature stability, and high-speed modulation. We studied and developed a defect-free top-down fabrication process for sub-20-nm GaAs nanodisks (NDs) that uses bio-template and neutral beam etching. We successfully fabricated 100-nm-high nanopillars embedding 4- and 8-nm-thick GaAs quantum well and 30-nm-thick Al 0.3Ga0.7As barrier-stacked structures. The nanopillars were mounted by metalorganic vapor phase epitaxy. We measured visible light photoluminescence at a low temperature originating from the GaAs NDs. Nanodisks fabricated by the top-down process have a great potential for use in high-performance III-V photonic devices.