Technobius Physics

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

Aikerul Ece — 2025-12-20

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 — 2025-12-23

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.

Low-temperature magneto-transport properties of nanostructured alloys with disordered magnetic subsystems

Tomiris Beisenbayeva — 2025-12-30

This work investigates how nanostructuring and controlled disorder govern low-temperature magnetotransport in Fe–Cr–Ni–B alloys with a deliberately disordered magnetic subsystem. Bulk alloys of nominal composition Fe₇₃Cr₁₀Ni₇B₁₀ were prepared by arc melting, high-energy ball milling, and short-time annealing between 450 and 550 degrees Celsius. The resulting microstructures were characterized by X-ray diffraction with profile analysis, scanning and transmission electron microscopy, and magnetization measurements under zero-field-cooled and field-cooled protocols. Electrical resistivity, magnetoresistance, and Hall effect were measured from 2 to 300 kelvin in magnetic fields up to 14 tesla using four-probe and five-contact configurations. The alloys form nanocrystalline Fe-based solid solutions with characteristic crystallite sizes increasing from about 18 to 38 nanometres as the annealing temperature rises, while microstrain and porosity decrease. Magnetization data reveal glassy freezing and unsaturated hysteresis, confirming a frustrated, disordered magnetic subsystem. All samples show high resistivity at room temperature, around 185 to 230 microohm centimetres, with low residual resistivity ratios and a crossover of the low-temperature exponent from approximately 1.3 to 1.9 as structural disorder is reduced. At 5 kelvin and 14 tesla, a sizable negative magnetoresistance of roughly 7, 5, and 3 percent is observed for the three annealing states, accompanied by metallic, electron-like Hall response. Together, these results demonstrate a clear correlation between nanostructure, magnetic disorder, and magnetotransport, and show that modest changes in crystallite size and microstrain provide an efficient handle to tune spin-disorder scattering in disordered magnetic alloys.

Statistical properties and scaling of 1/f noise in disordered nicr thin-film resistors

Azamat Sharipov — 2025-12-30

This work investigates the statistical properties and scaling behaviour of 1/f noise in resistive elements based on disordered nickel–chromium films. Thin Ni₈₀Cr₂₀ layers with thicknesses of 20, 40 and 80 nanometres were sputtered onto oxidized silicon substrates, patterned into micron-scale devices, and structurally characterized by atomic force microscopy and transmission electron microscopy. Low-frequency voltage noise was measured in a closed-cycle cryostat between 90 and 300 kelvin under strictly ohmic bias conditions, and converted into resistance noise spectra and time-domain fluctuation series for statistical analysis. All devices exhibit near-ideal 1/f noise, with spectral exponents between about 0.9 and 1.1 over roughly two decades in frequency. The normalized noise level at 1 hertz decreases strongly with thickness, from approximately 3.2 × 10⁻¹⁰ hertz⁻¹ for 20 nanometres to 0.7 × 10⁻¹⁰ hertz⁻¹ for 80 nanometres, while cooling modestly increases the noise and slightly steepens the spectra. Normalized resistance fluctuations are nearly Gaussian for thicker films and higher temperatures, but develop heavier tails in thinner films at low temperature, consistent with a reduced effective number of active fluctuators. Rescaling the spectra by a characteristic amplitude and correlation time produces an approximate collapse onto a common curve, indicating a nearly universal scaling function within the studied NiCr films rather than a strictly material-independent universality. Estimates of the Hooge parameter place the NiCr resistors in the low-noise range typical for precision thin-film technologies. These results show that 1/f noise in disordered nickel–chromium resistors is governed by a robust ensemble of relaxation processes whose collective behaviour is largely independent of microscopic details within this material class, providing guidance for designing low-noise resistive elements and for testing models of noise in disordered conductors.

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

David Paulsen — 2025-12-20

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.