Oxide ion conductivity, Resistive-switching and Ferroelectricity of Doped-HfO2

Yttria-stabilised hafnia, Hf1-xYxO2-x/2 (x=0.15, 0.30 and 0.45), ceramics were prepared by solid
state synthesis and characterised by ac impedance spectroscopy. Electrical conductivity of
x=0.15 is one order of magnitude higher than x=0.30 and 0.45. All compositions are foremost
oxide ion conductors but they contain a small p-type electronic conduction via increasing Y3+
concentration and pO2. YSH materials therefore are mixed conductors. P-type conductivity
is attributed to created holes that is believed to be located on under-bonded oxide ions.
A study on the effect of a small dc bias on YSH ceramics shows enhancement of their
conductivity, transforming into a resistive switching behaviour. This behaviour is novel for
any bulk ceramics other than Ca-doped BiFeO3. Switching of YSH depends on both oxygen
partial pressure, pO2, and temperature. At low applied voltage, the increase in the
conductivity is associated with p-type behaviour commenced at the positive
electrode/ceramic interface, but with further applied voltage, YSH underwent a transition
from p-type to n-type behaviour which is commenced at the negative electrode/ceramics
interface. With increasing applied voltage, the total conductivity enhanced sharply by 1.5-
2.5 orders of magnitude that is reversible with hysteresis under removal of the applied
voltage.
The ac impedance results of Hf1-xZrxO2 thin films demonstrate a single homogeneous
component attributed to the bulk layer. Resistance of all compositions decreased with
increasing pO2 (N2-Air-O2). This decrease in resistance is associated with p-type electronic
conduction. In p-type materials, a decrease in resistance is related to the increase in number
of holes that can probably be located on under-bonded oxygen or created during the
preparation of the thin films. The equivalent circuit of a resistor in parallel with a seriesconnected CPE-C element gave best fitting to HZO thin film samples. Results of fixed
frequency variable temperature sweeps of permittivity, er, of HZO show a sharp peak at 464-
474 °C with a maximum permittivity of ~ 11100. This result is the first report of a peak in the
permittivity of HZO thin films that is attributed to a transition from ferroelectric into
paraelectric behaviour at Tc.