The properties of the HfO2 ultra-thin films high-k gate stack deposited on surface passivated (GeON) germanium with three different orientations viz. Ge (100), Ge (110) and Ge (111) for CMOS applications have been studied. The effect of post deposition annealing (PDA) at temperatures 350 degrees C and 400 degrees C has been investigated. The high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy were used to study the interfacial, chemical and surface properties of atomic layer deposited-HfO2 ultrathin films, respectively. The high frequency conductance method was applied to evaluate the value of interface trap density (D-it) and the lowest Dit of 6.24 x 10(12) cm(-2)ev(-1) was evaluated for (111) orientation annealed at 400 degrees C than (100) and (110) orientation. The effective barrier height extracted from the Fowler-Nordheim conduction transport is in the range of 0.29-0.70 eV for all samples and shows the significant dependence of the substrate orientations and PDA temperature.