We present a comprehensive study of Si/SiO2/Hf((1-x))SixO2 structures using optical second-harmonic generation (SHG), supported by spectroscopic ellipsometry, photoelectron spectroscopy, and noncontact electrical measurements. Rotationally anisotropic and spectroscopic SHG depend strongly on dielectric composition and on postdeposition annealing in NH3. Spectroscopic SHG shows that these variations are linked to variations in charge trapped in the high-K material, which influences SHG via electrostatic-field-induced SH (EFISH) generation. Supporting measurements corroborate this interpretation. Noncontact electrical measurements and x-ray photoelectron spectroscopy show annealing-dependent trends in interface charge and nitrogen incorporation, respectively, that mirror annealing-dependent trends in EFISH generation. The results show that SHG can potentially be used as an in situ, real-time monitor of internal electric fields attributable to composition- and annealing-dependent fixed charge in the oxide layers. (c) 2006 American Vacuum Society.