A process for forming a doped nc-Si thin film thermoelectric material. A nc-Si thin film is slowly deposited on a substrate, either by hot-wire CVD (HWCVD) with a controlled H2:SiH4 ratio R=6-10 or by plasma-enhanced (PECVD) with a controlled R=80-100, followed by ion implantation of an n- or p-type dopant and a final annealing step to activate the implanted dopants and to remove amorphous regions. A doped nc-Si thin film thermoelectric material so formed has both a controllable grain size of from a few tens of nm to 3 nm and a controllable dopant distribution and thus can be configured to provide a thermoelectric material having predetermined desired thermal and/or electrical properties. A final annealing step is used to activate the dopants and remove any residual amorphous regions.