Charge Transport Mechanisms in a Pb₂P₂Se₆ Semiconductor

Charge transport in semi-insulating Pb₂P₂Se₆ single crystals was investigated. The dark current was dominated by the ionization of deep-level defects within the gap of the material, with activation energies between 0.6 and 0.8 eV. A model for charge transport was developed where a continuum of these midgap defect levels determined the conductivity of Pb₂P₂Se₆. Current-voltage characteristics in Pb₂P₂Se₆ single crystals showed nonlinear behavior at high voltages. The nonlinear characteristics are attributed to competing Poole-Frenkel emission and phonon-assisted tunneling processes, such that at lower fields the former effect dominates, while at higher electric fields the latter mechanism emerges. Calculated tunneling times in the 250-500 fs range indicate that the deep traps promote weak electron-phonon coupling and that the tunneling involves deep defect levels. Transient multi-terahertz spectroscopy and temperature-dependent photoconductivity measurements reveal signatures of dispersive transport and low mobility on the order of 10 cm²/(V s), consistent with a disordered potential energy landscape in Pb₂P₂Se₆. Photoresponse in these crystals is therefore limited by a distribution of trapping and recombination sites within the band gap.