Low-resistive Ohmic contacts are needed in most microelectronics and photonics devices to connect a device to the electric circuit. Manufacturing of Ohmic contacts typically requires the doping of a semiconductor surface region as n-type or p-type (i.e., electron- or hole-doped, respectively). This task has, however, become challenging when the doping needs to be controlled with nanometer or even atomic level precision at lowered processing temperatures. In this work, we demonstrate a low-temperature method to tackle this contact manufacturing challenge using ultrathin antimony (Sb) doped germanium (Ge) nanolayers. We have integrated the method with the common lift-off processing to make Ohmic nickel (Ni) contacts on low-doped n-type Ge and Si substrates and on semi-insulating GaAs, which initially show the Schottky contacts. A proper combination of wet chemical cleaning plus depositing Sb and Ge atomic layers on the substrates, kept at room temperature, in a very clean environment of ultrahigh vacuum before the Ni-film deposition and postmetallization heating changes the Schottky contacts to Ohmic ones. Complementary methods are used to probe the physicochemical properties of interfaces during the manufacturing process to clarify the mechanisms behind the Ohmic-contact formation.