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Recent observations of unusual ferroelectricity in thin films of HfO_2 andrelated materials have attracted broad interest to the materials and led to theemergence of a number of competing models for observed behaviors. Here wedevelop the electrochemical mechanism of observed ferroelectric-like behaviors,namely the collective phenomena of elastic and electric dipoles originated fromoxygen vacancies formation in the vicinity of film surfaces, as well as fromgrain boundaries and other types of inhomogeneities inside the film. Theferroelectric phase is induced by the "electrochemical" coupling, that is thejoint action of the omnipresent electrostriction and "chemical" pressure, whichlead to the sign change of the positive coefficient alfa in the quadratic termalfa*P^2 in the order-disorder type thermodynamic functional depending onpolarization P. Negative coefficient alfa becomes the driving force of thetransition to the long-range ordered ferroelectric phase with the spontaneouspolarization P in the direction normal to the film surface. Using the aboveideas, 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 ofthickness less than (20 - 30) nm. Semi-quantitative agreement with availableexperimental data is demonstrated.

Possible Electrochemical Origin of Ferroelectricity in HfO2 Thin Films