Interface defects as the origin of breakdown in ferroelectric TiN/HZO/TiN memory capacitors subjected to electrical stress

Abstract

Alloyed Hf0.5Zr0.5O2 (HZO) ultrathin layers crystallized in a metastable non-centrosymmetric phase are the leading candidate for implementation of scaled ferroelectric devices due to their excellent CMOS compatibility and demonstrated integrability with TiN in industrial process flows. One of the key problems on the route of using HZO-based capacitors as functional parts of ferroelectric memory cells is that the cyclic electrical stress required to write and read the stored information sets off a gradual increase in the leakage current, leading to eventual breakdown in the HZO layer. The breakdown in dielectric oxide layers sandwiched between two metal electrodes is generally viewed as a defect-assisted process; however, the relative contributions from the bulk and interfaces depend on the material system and are still under intense investigation. Here, we report on the dramatic effect of the polarity of constant-voltage electrical stress on the lifetime prior to breakdown in the nominally symmetric TiN/HZO/TiN ferroelectric capacitors. At the same time, we observed that pulsed stress with the same amplitude and equivalent total time under stress does not lead to the breakdown of the HZO layer. The step-recovery with multi-pulse test (SRMPT) technique was employed to quantify the energy distribution of chargeable defects in the HZO bandgap. Our analysis suggests that electrical breakdown in TiN/HZO/TiN devices is governed by the evolution of preexisting charged oxygen-vacancy-related defects associated with chemically nonequivalent TiN/HZO interfaces formed during HZO crystallization annealing.