The preparation and characterization of Schottky contacts on differently grown n-type ZnO crystals was studied. We demonstrate that depending on the crystal growth process different procedures of surface treatment should be used to achieve good Schottky contacts. A treatment in a dc-hydrogen plasma enables us to fabricate Schottky contacts on hydrothermally grown ZnO, whereas a similar treatment of the vapor phase and melt grown ZnO resulted in the reduction in the diode rectification ratio. In the later samples a treatment in H2O2 significantly improves the quality of Schottky contacts. The origin of the different behavior will be discussed.