The invention relates to a method for fabricating a semiconductor substrate by providing a silicon on insulator type substrate that includes a base, an insulating layer and a first semiconductor layer, doping the first semiconductor layer to thereby obtain a modified first semiconductor layer, and providing a second semiconductor layer with a different dopant concentration than the modified first semiconductor layer over or on the modified first semiconductor layer. With this method, an improved dopant concentration profile can be achieved through the various layers which makes the substrates in particular more suitable for various optoelectronic applications.