Reversible changes in the conductivity of HfO2 dielectric film between
high and low resistive states of a metal-insulator-metal memory cell were
attributed to the formation of oxygen vacancies and their clustering across the
insulator layer. In this study we present an innovative model which includes
generation of two-charged states of oxygen vacancies at the anode, their
diffusion to the cathode, transformation to one-charged state, and then to
neutral vacancies. Vacancy clusters in the insulator layer are built from only
neutral vacancies, while the kinetics of the clustering process is controlled by
diffusion of mobile one-charged state vacancies. Resistive switching is treated
as the formation of critical size vacancy cluster which provides continuous
conductive path through the dielectric layer. Good agreement between the
experimental data and the theoretical bias and temperature dependences for the
delay time was obtained.