Coulomb repulsion and Wigner glass formation of localized electrons in HfO2

Hafnium oxide (HfO2) is a key gate insulator in silicon devices. The electron transport in HfO2 is described by a model of phonon-assisted tunneling between neighboring traps. The trap ionization energy obtained from charge transport experiments is 1.25 ± 0.1 eV, which independently agrees, within the experimental accuracy, with the activation energy of 1.25 ± 0.02 eV derived from thermally stimulated depolarization experiments. This paper demonstrates—both experimentally and through Monte Carlo simulations—that the concentration of occupied traps is two orders of magnitude lower than the total concentration of neutral traps in HfO2. This finding indicates Coulomb repulsion between free electrons and localized electrons. Additional numerical simulations show that electrons localized in HfO2 form a short-range ordered structure known as Wigner glass.