Electrical Characterization of Hydrothermally Grown ZnO Annealed in Different Atmospheres

Hydrothermally grown ZnO crystals were investigated by capacitance techniques, such as deep level transient spectroscopy (DLTS) and thermal admittance spectroscopy (TAS). N-type ZnO samples were treated by high-temperature annealing in different gas ambients, followed by Pd Schottky contact formation on the zinc-polar surface. The DLTS measurements show three main levels in the band gap with the position of E-c -0.19, -0.30, and -0.54 eV, labelled E2, E3, and E4, respectively (E-c denotes the conduction band edge). The TAS measurements reveal the E2 and E3 levels in the 70-120 K range, and demonstrate complete carrier freeze-out at 50 K corresponding to a shallow donor at 30 meV below E-c. The data are consistent with an assignment of the E2 and E3 levels to oxygen deficient defects while the E4 level appears to originate from a zinc-deficient defect. Further, the role of residual impurities on the observed defects cannot be excluded and this holds especially for E2 and E3.