Doped with Ga lead telluride was taken as a model object to explain the nature of group-III deep levels in
IV-VI semiconductors and to elucidate the vapour phase doping mechanism. For this goal interaction of various
gallium-containing molecules with defect-free crystal as well as with native defects in PbTe were considered.
The first-principle density functional theory calculations using
216-atoms supercell were performed. Formation
energies for different point defects created in PbTe as a result of interaction the Ga<sub>2</sub>Te molecules, Ga<sub>2</sub> dimers
and single Ga atoms with a host crystal were calculated. Particularly Ga<sub>Pb</sub> and Gai together with formation
of accompanied Pb<sub><i>i</i></sub> and Te<sub><i>i</i></sub> self interstitials in various charge states were examined. In addition we propose
the new type of defects - the impurity complex (Ga<sub>2</sub>)<sub>Pb</sub> which looks like < 111 >-oriented gallium dumbbell.
Calculations suggest the double donor behavior and DX-like properties of this defect together with extremely
low formation energy values. Namely, Ga<sub>Pb</sub> centers are preferably formed under Ga<sub>2</sub>Te doping while (Ga<sub>2</sub>)<sub>Pb</sub>
+ Pb<sub><i>i</i></sub> ones are formed under Ga<sub>2</sub> or Ga doping. In all cases formation energies are negative and resulting defect
concentration is determined by the reaction kinetic only. Mechanisms of the lead vacansy compensation with
the vapour phase doping is considered as well.