  A new doping mechanism is described, whereby a doping impurity does not
simply transfer charge to the bands of a host semiconductor or semimetal, but
rearranges the core energy levels deep in the valence band of the host. This,
in turn, leads to a redistribution of all electrons in the host, and, if
designed properly, changes the location of the Fermi level EF and the density
of conducting charge carriers near EF. The principle is proven experimentally
in elemental Bi, whereby group III elements In and Ga dope Bi p-type, in spite
of the fact that all three atoms are trivalent in the solid state. Electronic
structure calculations show the formation of a hyperdeep defect state (HDS) and
its effect on the EF in Bi doped with In (Bi:In) and Ga (Bi:Ga). The HDS at -5
to -6 eV below the EF of elemental Bi hybridizes with the Bi 6p electrons, and
deprives the Bi valence band of two electrons per impurity atom. This then
perturbs the electron count in the solid and lowers the EF. The charge on the
impurity atoms is unchanged. In principle, this doping action does not result
in the appearance of ionized impurities that scatter conduction electrons and
holes in conventional doping. Experimentally, Shubnikov - de Haas and Hall
Effect measurements show that adding In to Bi results in an increase in the
density of holes. Thermoelectric, galvanomagnetic and thermomagnetic data are
given for single-crystal and polycrystalline samples. In-doping leads to an
enhancement of the thermoelectric figure of merit, which suggests that the new
doping mechanism also gives a route to develop better thermoelectric materials.
The same mechanism is at work for Sn-doped Bi (Bi:Sn), although here ionized
impurity scattering is not avoided.
