DFB grating with dopant induced refractive index change

To make a grating substructure in semiconductor material for use in a DFB laser, a first layer of semiconductor material is doped at a first doping concentration. A second layer of the semiconductor material is formed over the first layer. The second layer is doped higher concentration than the first layer and sufficiently different to change the refractive index of the semiconductor material. A third layer doped at a concentration comparable with the first layer is formed over the second layer. An etch is performed through a mask to form spaced etched regions extending at least through the second and third layers. Then a further layer of the semiconductor material doped at a doping concentration comparable the first and third layers is overgrown on the wafer. This results in a composite layer of the semiconductor material doped at a low doping concentration containing spaced islands of the semiconductor material doped with a dopant at a high doping concentration and having a different refractive index from the composite layer. The semiconductor material is preferably silicon-doped InP.