Non-metal to metal transition in n-type ZnO single crystal materials

The electrical properties of ZnO mono-crystalline materials, either in the form of bulk crystals or epitaxial films, were investigated for a large range of un-intentional or intentional doping concentrations extending from $4.0×1015 cm−3$ up to $1.3×1020 cm−3$⁠. Hall and resistivity measurements were carried out from 10 K to 300 K, yielding the temperature dependent carrier densities and carrier mobilities. This allowed for an unambiguous determination of the dopant ionization energies, taking into account the concentration of compensation centers. The ionization energy variation as a function of dopant concentration was found to follow Mott's law, being consistent with the hydrogenic behavior of all involved donors; an effective critical Mott's concentration for the insulator to metal transition was found to be around $4.2×1018 cm−3$⁠, while the apparent value of the isolated donor ionization energy was determined as being 60 meV.