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

Abstract

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 to crystalline sub-stoichiometric rutile TiO2-x and the Magnéli phase Ti3O5. The transitions started from the anode/oxide interface under both field polarities. Preferred growth orientation of rutile TiO2-x 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.

Direct Observations of Field-Intensity-Dependent Dielectric Breakdown Mechanisms in TiO2 Single Nanocrystals

“…During the process, the upper part of the nanocrystal (next to the Si anode) remains crystalline. A similar structurecrystalline at anode and amorphous at cathodewas observed in the crystallization of the amorphous TiO 2 film in our previous work …”

“…A similar structurecrystalline at anode and amorphous at cathodewas observed in the crystallization of the amorphous TiO 2 film in our previous work. 19 The recorded electric current shows relatively steady increases in the first 30 s and some spikes in the third application of bias (Figure 3b), presumably resulting from the buildup of charged point defects under intense field and the subsequent formation of temporary percolation pathways across the nanocrystal. 4,11 The large current density causes local resistive heating of the nanocrystal when the bias is on and fast quenching when the bias is off, which is responsible for the changes in the nanocrystal contour.…”

“…11,13,[16][17][18] Studying dielectric breakdown micromechanisms with in-situ TEM must avoid surface tracking in the vacuum, which poses special requirements on specimen geometry and electrode configuration. 19,20 The common approach of specimen preparation from a bulk piece via focused ion beam inevitably leads to contaminated specimens and makes them unsuitable for such studies. 21 Nanocrystals synthesized with wet chemistry would be good specimens for in-situ TEM breakdown studies due to their small dimensions, high purity, 22 and absence of grain boundaries, if they can be individually placed between two electrodes without mechanical damage and chemical contamination.…”

Insertion Trade-off Effects on the Spin-Transfer Torque Memory Explored by In Situ X-ray

“…The efforts to engineer novel interface structures are often hindered by difficulties reconciling the properties of ultrathin films at macroscopic and microscopic scales. − The CoFeB/MgO interface forms the basis of magnetic tunnel junction (MTJ) technology due to its tunneling magnetoresistance (TMR) and perpendicular magnetic anisotropy (PMA). − Subjecting the thin MgO tunnel barrier in MTJs to repeated electrical stress induces breakdown, which limits the endurance of magnetoresistive random-access memory (MRAM) devices. This situation is similar to the time-dependent dielectric breakdown (TDDB) in complementary metal-oxide–semiconductor (CMOS) gate oxides. − Researchers have reported that the breakdown can be delayed by inserting an ultrathin film of metallic Mg into the CoFeB/MgO interface during MgO barrier formation to prevent CoFeB oxidization. − Mg termination has also been shown to suppress the magnetic dead layer in ferromagnetic electrodes. − …”

In Situ Transmission Electron Microscopy Study of Conductive Filament Formation in Copper Oxides

“…In situ TEM techniques using a specimen holder equipped with various capabilities enable the direct characterization of structure and physical properties of materials simultaneously. To study the dielectric breakdown in copper oxides, we coated copper oxides on a wedge shape Si substrate and loaded the substrate on a Hysitron PI95 STEM specimen holder which we have successfully used to study the effects of applied voltages on different oxides [10], [11]. Different parts of the copper oxide coating were selectively studied by placing a W probe in contact with the region of interest.…”