We have investigated the technique of implantation enhanced interdiffusion (IEI) for optical nanostructure fabrication in strained InxGal-xAs/GaAs quantum wells. Implantation masks with widths from 40 nm to 40 μm were fabricated on the surface of InxGal-xAs/GaAs (x+0.1, 0.2) 3.5 nm quantum well material which was implanted with 100 kV As+ with doses ranging from 5 × 1012 to 8.5× 1013 ions/cm2. After mask removal and a high temperature anneal, cathodoluminescence (CL) spectroscopy was used to investigate the optical properties of the resulting structures. We have measured electron-heavy hole recombination energy shifts due to quantum well interdiffusion of up to 60 meV for the highest doses used here with broad area implants. However, while quantum well emission under large (40 μm) masks is preserved, smaller masks show an emission blue shift not due to ions penetrating through the mask. A simple model of the width dependence of this shift yields an enhanced lateral diffusion length of approximately 1 μm which is many times larger than the lateral straggle of the implanted As+. We conclude that lateral diffusion effects may impose a limit on nanostructure fabrication in the InxGal-xAs/GaAs system with this technique.