To enhance the performance of 4H-SiC switching devices associated with power electronics, a decrease in the resistivity of the inversion channel at the SiO2/4H-SiC interface is required. It is necessary to increase the free carrier density by eliminating interface traps as well as to increase the free carrier mobility by eliminating interface scattering centers. However, the origin of the dominant scattering centers has not yet been clarified, and its scattering mechanism has not yet been experimentally investigated. Thus, in this study, the dominant scattering mechanism in the inversion channel formed on the m- and a-faces of 4H-SiC by nitridation was studied. To characterize the electron scattering in the inversion channel, temperature dependences of both the Hall effect mobility and the Hall scattering factor (gamma(s)) were investigated. Because the interface traps prevent the accurate estimation of the free carrier density by conventional capacitance measurements, evaluating gamma(s) for the SiO2/4H-SiC interface requires different approaches. Therefore, we developed and applied the "gamma(s)d alpha method" for regions in which the effect of the interface traps can be ignored. By experimentally observing the temperature dependent change of gamma(s) for the interface, it was revealed that the dominant scattering mechanism is Coulomb scattering. The possible origins of the mobility-limiting Coulomb scattering at the interface are discussed, including the ionized impurities in the substrate, trapped electrons at the interface, and potential fluctuations at the 4H-SiC surface.