As noted above, Si can be doped with Er or Tm by ion-implantation but the suitability of this approach depends on what you are trying to achieve. i.e. what dopant concentrations and distributions do you need? Depending on the answer to these questions my group may be able to help. We are actively involved in ion-implantation research and also provide implant services (see: www.AFAiiR.org.au).
thanks for your answer. I already discussed this option with a friend of mine who is working on ion implantation here in Jena. The best case scenario for me would be a homogeneous distribution over a 1mm thick wafer. No chance to do that with ion implantation.
I need a bit more time to think about my own idea, whether there is a chance to do it with only a thin layer of Erbium or not. If I find that there is a chance and my friend (or their accelerator) aren't available, I will come back to you ;-)
What kind of doping concentration do you need? I have never heard of this type of wafer, but I suggest that you get in touch with a group that grows their own silicon ingots. I don't think ion implantation or diffusion doping will get the kind of depth you want.