FIG. 4B is a timing diagram showing the ALD deposition cycles for forming the stack type dual dielectric structure described in FIG. 4A. As shown, a metal organic compound such as Al(CH3)3 or Al(C2H5)3 is used as a precursor of the aluminum source gas for depositing the Al2O3 layer 22A. The oxidizing agent is selected from a group consisting of ozone (O3), oxygen (O2) and water (H2O) vapor.