Lead telluride based semiconductors doped with group-III impurities are promising materials for possible applications in VLIR and submillimeter regions. Still up to now the nature of its photosensitivity is not clear. Doped with Ga lead telluride, in which high photoresponse in MWIR region was revealed, was taken as a model object to explain the nature of group-III deep levels in IV-VI semiconductors and to elucidate the doping mechanism.

Vapor phase doping technique when prefabricated PbTe thin films interact with the various gallium-containing molecules was considered. Formation energies for defects created in PbTe as a result of interaction the Ga₂Te molecules, Ga₂ dimers and Ga atoms with a host crystal were calculated using density functional theory. Particularly Ga_Pb and Ga_i together with formation of accompanied Pb_i and Te_i self interstitials were examined. We propose the new type of defects – the impurity complex (Ga₂)_Pb which looks like <111>-oriented gallium dumbbell. Calculations suggest that (Ga₂)_Pb centers are preferably formed under Ga₂Te doping while (Ga₂)_Pb+Pb_i ones are formed under Ga₂ or Ga doping.

The (2Ga)_Pb center has negative value of formation energy and resulting defect concentration is determined by the reaction kinetics only. It is a double donor with DX-like properties, ’negative-U’ behavior and metastable paramagnetic state corresponding to the positive defect charge. The neutral state has C_3V symmetry and the deep electronic level while positive state is characterized by the shallow level and D_3d symmetry. After ionization of the neutral defect under infrared illumination the large atomic relaxation (~0.6 Å) occurs. This sets conditions for the observed phenomenon of persistent photoconductivity.

The (2Ga)_Pb complex may be regarded as a new type of DX-like centers in the ionic semiconductors. Performed EPR experiments confirm the model proposed as well.

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