We present the results of an investigation of impurity free vacancy diffusion (IFVD) post-growth treatments of p-i-n modulator structures. The investigation is in two parts. We first establish that gallium vacancies (VGa) are produced during IFVD (by measuring the intensity of the low temperature 1.2 eV signal from Si-VGa complexes) in a thick Si-doped GaAs sample. The second part of this work investigates the degree of intermixing of three 80A GaAs quantum wells embedded in the intrinsic region of a p-i-n modulator at depths between 1–2 μm from the surface. Photoluminescence studies on etched samples and cathodoluminescence showed that no significant depth dependence occurs as a result of IFVD.