Technobius Physics

Local phase transformations during current-induced forming and degradation in TiN/HfO₂/Pt metal/oxide/metal heterostructures

Aikerul Ece — Sat, 20 Dec 2025 00:00:00 +0500

This work investigates how current-induced forming and electrical stressing give rise to local structural and compositional transformations in TiN/HfO₂/Pt metal/oxide/metal devices used as model resistive memory cells. TiN/HfO₂/Pt structures with an approximately nine-nanometre-thick atomic-layer-deposited HfO₂ layer and device diameters of 5–20 micrometres were fabricated using CMOS-compatible processes. Electrical current–voltage and conductance–current characteristics were combined with in situ X-ray diffraction under bias and ex situ focused-ion-beam-prepared transmission electron microscopy and scanning electron microscopy, including energy-dispersive X-ray spectroscopy, on devices with well-documented electrical histories. Pristine devices showed uniform, area-scaled leakage currents of a few tens of nanoamperes at 0.1 volt, indicating a structurally homogeneous oxide. Electrical forming occurred reproducibly at about 2.5–3.0 volts with current compliance in the 100–500 microampere range and produced low-resistance states with conductance of approximately 2–3 millisiemens. With increasing cycle number and stronger current stressing, low-resistance conductance increased in a statistically significant way, while parts of the conductance–current curves became irreversible. In situ X-ray diffraction revealed the emergence of weak additional diffraction features attributed to transformed hafnium oxide only after strong current loading, while the metal electrodes remained largely unchanged. Cross-sectional microscopy showed that these electrical changes correlate with the appearance of localized filament-like regions, oxygen-deficient zones and mild interfacial reactions near the top electrode. Taken together, these observations establish a direct correlation between electrical forming and degradation regimes and spatially confined structural transformations within the HfO₂ layer and at metal/oxide interfaces. This understanding provides a basis for engineering more reliable resistive memory devices by defining electrical stress windows that avoid the onset of irreversible structural changes.

Nonlinear conductivity in SrTiO₃-based oxide heterostructures under strong electric fields

Ruslan Kalibek, Daria Sopyryaeva — Tue, 23 Dec 2025 00:00:00 +0500

This study explores the origin of nonlinear conductivity in epitaxial oxide heterostructures subjected to strong electric fields. We investigate vertical transport in Pt/SrTiO₃/Nb:SrTiO₃ and Pt/La₀.₇Sr₀.₃MnO₃/SrTiO₃/Nb:SrTiO₃ stacks with SrTiO₃ barrier thicknesses of 5, 10, and 20 nanometres. Heterostructures were grown by pulsed laser deposition and characterized structurally by X-ray diffraction, atomic force microscopy and cross-sectional transmission electron microscopy. Current–voltage measurements were performed over a wide voltage and temperature range, followed by model-based analysis to distinguish between Ohmic, space-charge-limited and trap-assisted conduction. Post-mortem electron microscopy was used to assess structural changes after strong-field stressing. All devices show a clear crossover from nearly linear conduction at low bias to a nonlinear regime at higher fields, with the threshold field increasing from about 110 kilovolts per centimetre for 5 nanometres to about 320 kilovolts per centimetre for 20 nanometres. Double-layer structures with La₀.₇Sr₀.₃MnO₃ exhibit systematically lower threshold fields (for example, about 140 kilovolts per centimetre for 10 nanometres) and stronger Poole–Frenkel-like response, indicating an enhanced role of interface-related trap states. Quantitative analysis of transformed current–voltage plots yields effective space-charge exponents between 1.6 and 2.1 and Poole–Frenkel slopes corresponding to activation energies of 50–120 millielectronvolts that decrease with increasing field. Electron microscopy confirms that the oxide remains structurally intact throughout the nonlinear regime and shows noticeable interface roughening only close to breakdown. These results demonstrate that nonlinear conduction in SrTiO₃-based heterostructures is governed by a field-induced crossover from bulk-limited, trap-assisted transport to increasingly interface-influenced conduction, and they define thickness and field windows where strong nonlinearity can be exploited without triggering irreversible structural damage.

Electron hydrodynamics in ultra-clean conductors: from Dirac fluids in graphene to viscous metals

David Paulsen — Sat, 20 Dec 2025 00:00:00 +0500

This work examines the emerging field of electron hydrodynamics in ultra-clean conductors, where charge carriers behave collectively as a viscous fluid rather than as independent quasiparticles. The objective is to provide a unified perspective that connects theoretical frameworks with key experimental realizations in graphene, delafossite metals and topological semimetals. To this end, we performed a structured literature search across major databases and preprint servers, applied explicit inclusion and exclusion criteria to identify genuinely hydrodynamic studies, and carried out a comparative, narrative analysis of transport, thermal and imaging experiments. The collected evidence shows that hydrodynamic transport arises when electron–electron collisions dominate over momentum-relaxing processes and when device dimensions are comparable to characteristic scattering lengths. In this regime, experiments reveal geometry-dependent resistivity, negative nonlocal signals, super-ballistic conductance, strong violations of the Wiedemann–Franz law and, in some cases, Hall viscosity. Graphene provides the clearest realization of a relativistic Dirac fluid, while PdCoO₂ and WP₂ demonstrate that viscous electron flow also occurs in anisotropic and multi-band metals. This work highlights that boundaries, disorder and Fermi-surface geometry critically shape hydrodynamic signatures and must be incorporated into any quantitative interpretation. It identifies open issues concerning the roles of phonons and Umklapp processes, the reliable extraction of viscosity, and the extension of hydrodynamics to more complex correlated and topological phases. Finally, it outlines priorities for future “benchmark” experiments that combine nonlocal transport, thermal measurements and real-space imaging within the same devices.