A single crystal silicon wafer has a single crystal silicon substrate (1) with either (a) a K concn. of max. 2 x 10<11> atoms/cm<2> at its outer surface or (b) a K concn. of max. 1 x 10<16> atoms/cm<3>. Also claimed are (i) similar wafers with a polysilicon layer on the bottom surface, the above K concn. (a) provided at the interface or the outer surface of the polysilicon layer or the above K concn. (b) provided in the polysilicon layer; (ii) a single crystal silicon wafer with a single crystal silicon substrate, a phosphor-contg. oxide layer on the substrate bottom surface and a polysilicon layer covering the oxide layer; (iii) a single crystal silicon wafer with first and second single crystal silicon substrates joined together at a surface with a K concn. of max. 2 x 10<11> atoms/cm<2>; and (iv) processes for thermally oxidising a surface of a single crystal silicon wafer.
