In this paper, we present an enhanced Differential Hall Effect method (DHE) for Si and SiGe ultrathin layers for the investigation of dopant activation in the surface region with sub-nanometre resolution. In the case of SiGe case, which constitutes the most challenging process, we show the reliability of the SC1 chemical solution (NH4OH:H2O2:H2O) thanks to its slow etch rate, stoichiometry conservation and low roughness generation. The reliability of a complete DHE procedure, with an etching step as small as 0.5 nm, is demonstrated on a dedicated 20 nm-thick SiGe test structure fabricated by CVD and uniformly doped in situ during growth. The developed method is finally applied to the investigation of dopant activation achieved by advanced annealing methods (including millisecond and nanosecond laser anneal) in two material systems: 6 nm-thick SiGeOI and 11 nm-thick SOI. In both cases, DHE is shown to be a unique sensitive characterisation technique for a detailed investigation of dopant activation in ultra-shallow layers, providing sub-nm resolution for both dopant concentration and carrier mobility depth profiles.