Electrical, optical and partly structural properties are investigated on very thin ALD HfO2/ultrathin NAOS SiO2/n-type Si structures. An ALD layer was deposited at 250 degrees C and it contains amorphous and crystalline-probably monoclinic HfO2 phases. HfO2 films with both types of structural phases were not stable if thermal treatment above 200 degrees C was applied. On as- prepared samples, deep interface traps with activation energy of Delta W = 0.23 eV have been determined. After annealing of the structure at 200 degrees C, the traps were partly transformed and a mid-gap level LxW= 0.49 eV was detected. FIR and AFM measurements confirmed presence of HfO2 monoclinic phase in the HfO2 films. On the other side, the density of interface defect states of the structure decreased from approx. 10(12) eV(-1) cm(-2) to 10(11) eV(-1) cm(-2) after low temperature annealing of the reference structure. The results are compared with very similar (almost identical) development of interface defect states on the very thin thermal SiO2/Si structure before and after passivation in a 0.1 M KCN methanol solution.PACS: 78.55.Qr; 78.66.Jg; 81.16.Pr; 85.40Ls (C) 2014 The Authors. Published by Elsevier B.V. All rights reserved.