  Films of Hf0.5Z0.5O2 (HZO) contain a network of grain boundaries. In (111)
HZO epitaxial films on (001) SrTiO3, for instance, twinned orthorhombic (o-HZO)
ferroelectric crystallites coexist with grain boundaries between o-HZO and a
residual paraelectric monoclinic (m-HZO) phase. These grain boundaries
contribute to the resistive switching response in addition to the genuine
ferroelectric polarization switching and have detrimental effects on device
performance. Here, it is shown that, by using suitable nanometric capping
layers deposited on HZO film, a radical improvement of the operation window of
the tunnel device can be achieved. Crystalline SrTiO3 and amorphous AlOx are
explored as capping layers. It is observed that these layers conformally coat
the HZO surface and allow to increase the yield and homogeneity of functioning
ferroelectric junctions while strengthening endurance. Data show that the
capping layers block ionic-like transport channels across grain boundaries. It
is suggested that they act as oxygen suppliers to the oxygen-getters grain
boundaries in HZO. In this scenario it could be envisaged that these and other
oxides could also be explored and tested for fully compatible CMOS
technologies.
