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 Ω cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D1 and D2 and a deeper one D3 with activation energies of ∼30, ∼50 and ∼290 meV, respectively, are detected. In addition, the TAS spectra reveal the presence of a fourth level, DX, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with DX being a negative-U defect of donor-type. An activation energy of ∼80 meV for the ++/+ transition, a capture cross section equal to ∼3x10-17 cm2 and an energy barrier for atomic reconfiguration of ∼0.25 eV, respectively, deduced according to the assignment of DX to a negative-U defect. A tentative assignment of the DX defect with oxygen vacancies is discussed.