The detrimental sub-oxide (SiOx) interfacial layer formed during the 4H-SiC metal-oxide-semiconductor (MOS) capacitor fabrication will drastically damage its device performance. In this work, an ultrathin lanthanum silicate (LaSiOx) passivation layer was introduced to enhance the interfacial and electrical characteristics of 4H-SiC MOS capacitor with Al2O3 gate dielectric. The interfacial LaSiOx formation was investigated by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The 4H-SiC MOS capacitor with ultrathin LaSiOx passivation interlayer shows excellent interfacial and electrical characteristics, including lower leakage current density, higher dielectric breakdown electric field, smaller C-V hysteresis, and lower interface states density and border traps density. The involved mechanism implies that the LaSiOx passivation interlayer can effectively restrain SiOx formation and improve the Al2O3/4H-SiC interface quality. This technique provides an efficient path to improve dielectrics/4H-SiC interfaces for future high-power device applications. (C) 2017 Elsevier B.V. All rights reserved.