THE INFLUENCE OF GROWTH CONDITIONS AND DONOR DOPING ON CONDUCTIVITY MODE AND DEEP TRAPS SPECTRA IN TLBR SINGLE CRYSTALS

Studies of electrical characteristics, deep traps spectra, microcathodoluminescence (MCL) spectra of undoped and donor (Pb, Ca) doped TlBr crystals as influenced by growth conditions (Br pressure, Ar pressure, growth in air) are presented. It is shown that, for the 85−320 K temperature range, the crystal conductivity was determined not by ionic conductance but by the density of electrons and holes supplied by the ionization of deep centers. Centers with activation energies of 1−1.2 eV that pin the Fermi level in donor doped crystals are shown to play a prominent role in the recombination of nonequilibrium charge carriers. In undoped crystals the Fermi level is pinned near Ev+0.8 eV and these centers are also active in the recombination of charge carriers and are responsible for the MCL band peak near 1.85 eV. The temperature dependence of photocurrent in undoped crystals is strongly influenced by electron trapping on relatively shallow centers located 0.1−0.2 eV below the conduction band edge. Deep traps spectra revealed the presence of centers with activation energies 0.36, 0.45, 0.6 eV whose concentration increases with donor doping. Doping with Pb or Ca increases the dark resistivity of the crystals by about an order of magnitude, but Pb doping enhances the density of deep traps, which is not favorable for use of this material in radiation detectors.

TlBr, deep levels, photoinduced current transient spectros-copy of deep traps, microcathodoluminescence, radiation detectors, ionic conductance, electronic conductance

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