In Situ TEM Study of the Amorphous-to-Crystalline Transition during Dielectric Breakdown in TiO₂ Film

Dielectric breakdown of oxides is a main limiting factor for improvement of the performance of electronic devices. Present understanding suggests that defects produced by intense voltage accumulate in the oxide to form a percolation path connecting the two electrodes and trigger the dielectric breakdown. However, reports on directly visualizing the process at nanoscale are very limited. Here, we apply in situ transmission electron microscopy to characterize the structural and compositional changes of amorphous TiO₂ under extreme electric field (∼100 kV/mm) in a Si/TiO₂/W system. Upon applying voltage pulses, the amorphous TiO₂ gradually transformed into crystalline substoichiometric rutile TiO_2- and the Magnéli phase Ti₃O₅. The transitions started from the anode/oxide interface under both field polarities. Preferred growth orientation of rutile TiO_2- with respect to the Si substrate was observed when Si was the anode, while oxidation and melting of the W probe occurred when W was the anode. We associate the TiO₂ crystallization process with the electrochemical reduction of TiO₂, polarity-dependent oxygen migration, and Joule heating. The experimental results are supported by our phase-field modeling. These findings provide direct details of the defect formation process during dielectric breakdown in amorphous oxides and will help the design of electronic devices with higher efficiency and reliability.