Silicon oxynitride (SiOxNy) thin films, deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition, have been used to induce blueshifting of the emission wavelength of 1.55 μm partial three quantum well laser structures after an annealing treatment. The SiOxNy caps had thicknesses of about 1000 Å and refractive indices ranging from 1.45 to 1.85. Room temperature photoluminescence was used to measure the magnitude of the cap-induced blueshift. It was found that the maximum blueshift occurs for an index of refraction of 1.63. Rapid thermal annealing was applied after the deposition and the importance of accurate temperature control is discussed. Finally, the role of the microwave power is analyzed and its impact on the blueshift is discussed.