Schottky barrier contacts have been formed by electron beam evaporation of Pd on the (0001)-face of hydrothermally grown n-type single-crystalline ZnO. The contacts can be operated under reverse bias voltages up to -5 V and in the temperature range between 130 and 350 K. A barrier height of 0.83 eV is deducted at room temperature, which is in reasonable agreement with the value predicted by fundamental theory. The ideality factor for the current-voltage characteristics varies between 1.01 and 1.03 in the voltage range of -5.0 to +0.5 V, indicating that thermionic emission is a dominant mechanism for charge-carrier transport. Thermal admittance measurements were performed between 15 and 350 K, and two prominent levels were resolved at 0.05(1) and 0.33(2) eV below the conduction band edge, respectively. The origin of these levels is not known, but possible candidates are complexes involving hydrogen, Zn interstitials, and vacancy-oxygen centers.