Titanium forms a resonant level in the conduction band of PbTe

Titanium is a donor in PbTe, leading to electron concentrations up to n {~} 1 ×{} 10¹⁹ cm^-3, above which it pins the Fermi level at about 52 meV above the conduction-band edge. While Ti is thus a resonant level, it does not enhance the thermopower above that of similarly doped PbTe, suggesting that the electrons on Ti are localized. Delocalized electrons appear again when the Ti concentration is increased so that 1 ×{} 10¹⁹ cm^-3 n 1 ×{} 10²⁰ cm^-3, suggesting the appearance of impurity-band conduction. The PbTe:Ti system is analogous to the well-studied HgSe:Fe system. A model for Fermi-level pinning is proposed that is based on the chemical equilibrium between different ionization states of the donor impurity.