We investigated the effects of annealing gas on the physical and electrical properties of ZrSixOy/ZrO2 high-k gate oxide. We found that the post oxidation annealing under oxidizing ambient (e.g., N2O or O-2) results in the additional interfacial oxide layer between silicate film and Si substrate, causing the degradation of equivalent oxide thickness (EOT) property. On the other hand, no clear evidence for the growth of interfacial oxide layer was observed if annealing was performed in N-2 ambient. Furthermore, we found that N-2 annealing effectively annihilates the defect center in ZrSixOy/ZrO2 layer. Therefore, we suggest that annealing in N-2 ambient is necessary to improve the characteristics of ZrSixOy/ZrO2 high-k gate oxide. (c) 2005 Elsevier B.V. All rights reserved.