Structural Incorporation and Electrical Efficiency of Tl in PbTe

PbTe with a bandgap E = 0.21 eV (77 JS) is an important semiconductor for optoelectronic devices in the medium infrared region. Thallium ions T1' a r e known as acceptors in PbTe, if they a r e substituting lead ions, Pb. The electrical efficiency of T1 in PbTe should be poor /lh nevertheless there has been found a different electrical behaviour after 'additive' and 'substitutional' incorporation, respectively /2/, and even a rather high electrical activity of T1' /3/, a t least a t concentrations cT1 5 0.3 at% /4/. Of course, one has to take into consideration the electrical activity of point defects due to non-stoichiometry, dependent on the kind of producing the crystal, a t least at small dopant concentrations. g 2+ We were interested in the incorporation behaviour and the electrical activity of T1 in PbTe. Our crystals have been synthesized Te-rich and have been grown, after adding T1, by the travelling heater method from Te-rich solution /5/. W e investigated the lattice constant by the so-called Soller slit technique /6/, the carrier concentration by thermoprobe as well a s Hall measurements , and the T1 concentration by SIMS measurements, calibrated by T1 implantation investigations. Fig. 1 shows that the lattice constant decreases with increasing carrier con-1 1 centration. Fig. 2 shows that p77K% (to 5) ~ T l .