Shallow levels in virgin hydrothermally grown n-type ZnO studied by thermal admittance spectroscopy

Three n-type single crystal hydrothermally grown ZnO samples with resistivities of 5.1 +/- 0.6, 15 +/- 2 and 220 +/- 20 Omega cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D-1 and D-2 and a deeper one D-3 with activation energies of similar to 30, similar to 50 and similar to 290 meV, respectively, are detected. in addition, the TAS spectra reveal the presence of a fourth level, D-X, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with D-X being a negative-U defect of donor-type. An activation energy of similar to 80 meV for the ++/+ transition, a capture cross section equal to similar to 3 x 10(-17) cm(2) and an energy barrier for atomic reconfiguration of similar to 0.25 eV, respectively, deduced according to the assignment of D-X to a negative-U defect. A tentative assignment of the D-X defect with oxygen vacancies is discussed. (C) 2009 Elsevier B.V. All rights reserved.