A multi-layer superlattice quantum well thermoelectric material comprising at least 10 alternating layers has a layer thickness of each less than 50 nm, the alternating layers being electrically conducting and barrier layers, wherein the layer structure shows no discernible interdiffusion leading to a break-up or dissolution of the layer boundaries upon heat treatment at a temperature in the range from 50 to 150° C. for a time of at least 100 hours and the concentration of doping materials in the conducting layers is 1018 to 1023 cm−3 and in the barrier layers is 1013 to 1018 cm−3.