Here, z is the depth into the substrate, starting from the epitaxial film. Here z =0 refers to the point, at the SiP/substrate interface, where the doping starts falling for the first time; k is defined as a fitting constant; ND0 is the dopant concentration at z ≤ 0, which is the dopant concentration in SiP layer, assuming a flat profile in SiP layer. Here we also assume that n > 0, in order to give that hyperbolic decay profile observed in Fig 2 . We will later find that this approximation of dopant concentration gradient agrees well with the final electron holographic phase shift observed.