The reactively sputtered HfO2 and HfSixOy dielectrics have been investigated comparatively for metal-insulator-metal (MIM) capacitor applications. X-ray photoelectron spectroscopy analyses reveal the presence of Hf-O, Hf-O-Si and Si-O chemical bonds in the HfSixOy films as well as lots of oxygen vacancies. The relative concentrations of Hf-O-Si and Si-O bonds increase with an increment of the power applied to the Si target. Further, it is found that the quadratic voltage coefficient of MIM capacitor decreases with increasing the Si content in the HfSixOy dielectric in despite of a decrease in the resulting capacitance density. The HfSixOy dielectric MIM capacitors with a capacitance density of ~8.4fF/μm2 exhibit a quadratic voltage coefficient of 1840 ppm/V2 at 100kHz, which is much smaller than 2750 ppm/V2 for the HfO2 dielectric MIM capacitors with a density of ~11.8fF/μm2.