Possible electrochemical origin of ferroelectricity in HfO2 thin films

Abstract

Recent observations of unusual ferroelectricity in thin films of HfO 2 and related materials have attracted broad interest to the materials and led to the emergence of a number of competing models for observed behaviors. Here we develop the electrochemical mechanism of observed ferroelectric-like behaviors, namely the collective phenomena of elastic and electric dipoles originated from oxygen vacancies formation in the vicinity of film surfaces, as well as from grain boundaries and other types of inhomogeneities inside the film. The ferroelectric phase is induced by the "electrochemical" coupling, that is the joint action of the omnipresent electrostriction and "chemical" pressure, which lead to the sign change of the positive coefficient α in the quadratic term αP 2 in the order-disorder type thermodynamic functional depending on polarization P. Negative coefficient α becomes the driving force of the transition to the long-range ordered ferroelectric phase with the spontaneous polarization P in the direction normal to the film surface. Using the above ideas, we estimated that the reversible ferroelectric polarization, as high as (0.05 -0.2) C/m 2 , can be induced by oxygen vacancies in HfO 2 films of thickness less than (20 -30) nm. Semi-quantitative agreement with available experimental data is demonstrated.

ZrOx Negative Capacitance Field-Effect Transistor with Sub-60 Subthreshold Swing Behavior

“…The ferroelectric-like behavior of the amorphous ZrO x film in the Fig. 2 a is proposed to be originated from the voltage-driven oxygen vacancy dipoles [ 35 ]. Figure 2 b shows the measured P–V curves for the Ge/ZrO x /TaN capacitors under different frequencies from 200 to 10 kHz.…”

Origin of Ferroelectricity and Multiferroicity in Binary Oxide Thin Films

“…Allowing for all the above facts, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remanent polarization in the binary oxide films [25]. Notably, that results of Ref.…”

Ferroelectricity in Si-Doped Hafnia: Probing Challenges in Absence of Screening Charges

“…In fact, even considering the probe as perfectly positioned on a well oriented orthorhombic crystal, the contribution of trap states might overwhelm the role of the ferroelectric dipole switching, thus limiting the insurgence of nonlinear cKPFM spectra with band formation. In addition, recent studies suggest that the ferroelectric contribution relies on the distribution and concentration of oxygen vacancies in the oxide [42]. Therefore, a relatively open environment such as the probe in direct contact with the oxide, would allow the oxygen vacancies to interact with other defects, absorb humidity near the sample surface, or to migrate and redistribute inside the bulk.…”

“…Consequently, while the use of a conductive probe in direct contact with the oxide represents a valid solution to sense the pristine distribution of ferroelectric domains, the strong electric field confinement must be considered when attempting point-contact tip-induced switching experiments. It should be mentioned that recent reports suggest that the ferroelectricity in FE-HfO 2 could be induced by the oxygen vacancies, whose segregation at interfaces and grain boundaries can play a major role for polarization hysteresis [42]. In this scenario, the probe being a non-blocking contact, a strong interaction of oxygen vacancies with the environment should be expected for the presence of water and oxygen radicals.…”