TDDB Mechanism in a-Si/TiO2 Nonfilamentary RRAM Device

Mechanisms of time-dependent dielectric breakdown (TDDB) in nonfilamentary a-Si/TiO2 RRAM cell [a-vacancy modulated conductive oxide (VMCO)] have been examined in this paper, including defects generation in the grain boundary, defects clustering, and different defects generation rates in a-Si and TiO2 layers. The unique feature of a bimodal Weibull distribution at low-resistance state (LRS) and a single shallow slope distribution at high-resistance state (HRS) cannot be explained by the above mechanisms. By using a combination of constant voltage stress, time-to-breakdown Weibull distribution, and random telegraph noise (RTN)-based defect profiling in devices of various sizes, layer thicknesses, and processes, it is revealed that the defect profile is modulated when switching between HRS and LRS and the correlation of defect profile modulation with local defect generation rate can explain the difference in Weibull distributions at HRS and LRS. The transition from bimodal distribution at LRS to a single-steep slope with thinner a-Si layer, and the good area scaling of Weibull distribution at HRS but not at LRS, can also be explained. The critical layers affecting the TDDB in a-VMCO are identified, providing useful guidance for device performance improvement.