Using the photoluminescence surface state spectroscopy (PLS 3) technique, attempts were made to determine the surface state density ( N ss) distribution on Al x Ga 1− x As ( x≈0.3) surfaces passivated by the Si interface control layer (ICL) technique. Air-exposed AlGaAs epitaxial wafers which are technologically important for fabrication of various devices were passivated ex situ by forming a SiO 2/Si 3N 4/Si ICL/AlGaAs structure after the HCl treatment and their photoluminescence behavior was investigated in detail. The result of the PLS 3 analysis indicated that Si ICL-based passivation reduces the minimum interface state density value down to 10 10 cm −2 eV −1 range. Some indication was also obtained that further improvements are possible by using electron cyclotron resonance (ECR)-enhanced N 2 plasma for Si 3N 4/Si ICL interface formation.