Kelvin probe study on formation of electric dipole at direct-contact HfO2/Si interfaces

Abstract

Although the direct-contact HfO2/Si structure has been demonstrated to have advantages on an equivalent oxide thickness scaling of <1 nm, characteristic electric charges have been reported to be formed at the HfO2/Si interface. A Kelvin probe method was used to examine the electric charges in HfO2/Si structures. We first describe the basic principles behind the Kelvin probe measurements of electric charges in an oxide/Si structure, and then point out what effect the surface adsorbates have on the measured contact-potential-difference voltage (VCPD). A large VCPD difference (>0.5 V) was observed between the direct-contact HfO2/Si and HfO2/SiO2/Si stack surfaces, which suggests the existence of a strong interface dipole. This result is consistent with previous reports using electrical measurements of metal–oxide–semiconductor capacitances. We also found that the dipole completely disappeared after ultrahigh vacuum annealing at 700 °C, and appeared again after exposure to O2 at room temperature. The formation of a dipole by exposure to O2 had a correlation with the formation of Si–O bonds at the HfO2/Si interface. We propose that interface Si–O–Hf bonding naturally produces a strong interface dipole from simple electrostatic potential analysis mainly due to the effect of the large dielectric constant of HfO2.