Advanced preparation of plan-view specimens on a MEMS chip for in situ TEM heating experiments.

In situ transmission electron microscopy (TEM) is a powerful tool for advanced material characterization. It allows real-time observation of structural evolution at the atomic level while applying different stimuli such as heat. However, the validity of analysis strongly depends on the quality of the specimen, which has to be prepared by thinning the bulk material to electron transparency while maintaining the pristine properties. To address this challenge, a novel method of TEM samples preparation in plan-view geometry was elaborated based on the combination of the wedge polishing technique and an enhanced focused ion beam (FIB) workflow. It involves primary mechanical thinning of a broad sample area from the backside followed by FIB-assisted installation on the MEMS-based sample carrier. The complete step-by-step guide is provided, and the method's concept is discussed in detail making it easy to follow and adapt for diverse equipment. The presented approach opens the world of in situ TEM heating experiments for a vast variety of fragile materials. The principle and significant advantage of the proposed method are demonstrated by new insights into the stability and thermal-induced strain relaxation of Ge Stranski-Krastanov islands on Si during in situ TEM heating. Supplementary Information: The online version contains supplementary material available at 10.1557/s43577-021-00255-5.

Monitoring the Electrochemical Failure of Indium Tin Oxide Electrodes via Operando Ellipsometry Complemented by Electron Microscopy and Spectroscopy.

Transparent conductive oxides such as indium tin oxide (ITO) are standards for thin film electrodes, providing a synergy of high optical transparency and electrical conductivity. In an electrolytic environment, the determination of an inert electrochemical potential window is crucial to maintain a stable material performance during device operation. We introduce operando ellipsometry, combining cyclic voltammetry (CV) with spectroscopic ellipsometry, as a versatile tool to monitor the evolution of both complete optical (i.e., complex refractive index) and electrical properties under wet electrochemical operational conditions. In particular, we trace the degradation of ITO electrodes caused by electrochemical reduction in a pH-neutral, water-based electrolyte environment during electrochemical cycling. With the onset of hydrogen evolution at negative bias voltages, indium and tin are irreversibly reduced to the metallic state, causing an advancing darkening, i.e., a gradual loss of transparency, with every CV cycle, while the conductivity is mostly conserved over multiple CV cycles. Post-operando analysis reveals the reductive (loss of oxygen) formation of metallic nanodroplets on the surface. The reductive disruption of the ITO electrode happens at the solid-liquid interface and proceeds gradually from the surface to the bottom of the layer, which is evidenced by cross-sectional transmission electron microscopy imaging and complemented by energy-dispersive X-ray spectroscopy mapping. As long as a continuous part of the ITO layer remains at the bottom, the conductivity is largely retained, allowing repeated CV cycling. We consider operando ellipsometry a sensitive and nondestructive tool to monitor early stage material and property changes, either by tracing failure points, controlling intentional processes, or for sensing purposes, making it suitable for various research fields involving solid-liquid interfaces and electrochemical activity.

Interfacial Resistive Switching of Niobium-Titanium Anodic Memristors with Self-Rectifying Capabilities.

A broad compositional range of Nb-Ti anodic memristors with volatile and self-rectifying behaviour was studied using a combinatorial screening approach. A Nb-Ti thin-film combinatorial library was co-deposited by sputtering, serving as the bottom electrode for the memristive devices. The library, with a compositional spread ranging between 22 and 64 at.% Ti was anodically oxidised, the mixed oxide being the active layer in MIM-type structures completed by Pt discreet top electrode patterning. By studying I-U sweeps, memristors with self-rectifying and volatile behaviour were identified. Moreover, all the analysed memristors demonstrated multilevel properties. The best-performing memristors showed HRS/LRS (high resistive state/low resistive state) ratios between 4 and 6 × 105 and very good retention up to 106 successive readings. The anodic memristors grown along the compositional spread showed very good endurance up to 106 switching cycles, excluding those grown from alloys containing between 31 and 39 at.% Ti, which withstood only 10 switching cycles. Taking into consideration all the parameters studied, the Nb-46 at.% Ti composition was screened as the parent metal alloy composition, leading to the best-performing anodic memristor in this alloy system. The results obtained suggest that memristive behaviour is based on an interfacial non-filamentary type of resistive switching, which is consistent with the performed cross-sectional TEM structural and chemical characterisation.

Elucidating the Origins of High Preferential Crystal Orientation in Quasi-2D Perovskite Solar Cells.

Incorporating large organic cations to form 2D and mixed 2D/3D structures significantly increases the stability of perovskite solar cells. However, due to their low electron mobility, aligning the organic sheets to ensure unimpeded charge transport is critical to rival the high performances of pure 3D systems. While additives such as methylammonium chloride (MACl) can enable this preferential orientation, so far, no complete description exists explaining how they influence the nucleation process to grow highly aligned crystals. Here, by investigating the initial stages of the crystallization, as well as partially and fully formed perovskites grown using MACl, the origins underlying this favorable alignment are inferred. This mechanism is studied by employing 3-fluorobenzylammonium in quasi-2D perovskite solar cells. Upon assisting the crystallization with MACl, films with a degree of preferential orientation of 94%, capable of withstanding moisture levels of 97% relative humidity for 10 h without significant changes in the crystal structure are achieved. Finally, by combining macroscopic, microscopic, and spectroscopic studies, the nucleation process leading to highly oriented perovskite films is elucidated. Understanding this mechanism will aid in the rational design of future additives to achieve more defect tolerant and stable perovskite optoelectronics.

Synthesis of Nanoparticles and Theoretical Model of Their Retention in Plasma of RF Capacitive Discharge with Vertically Arranged Electrodes in Acetylene.

In the present research, experiments on the formation and retention of nanoparticles (NPs) in the plasma of radio frequency (RF) capacitive discharge in acetylene were carried out with vertically positioned internal electrodes. It has been shown via SEM and TEM techniques that NPs found on the horizontal tube wall after the discharge operation have a spherical shape with a predominant diameter of approximately 400-600 nm. HRTEM analysis reveals their amorphous structure. At the same time, such NPs were not found on vertical electrodes, only a polymer film was deposited. To elucidate the possibility of NPs leaving the plasma in the direction of vertical electrodes, a model of NP retention in the near-electrode sheath of an RF capacitive discharge was elaborated. The model has shown that nanometer- and even micrometer-sized particles formed in the plasma cannot cross the near-electrode sheath and reach the electrode surface. For the plasma consisting of three charged components (positive ions, electrons, and NPs), an analytical model of ambipolar diffusion was developed. Applying this model, it has been shown that the ambipolar electric field can keep the micrometer-sized NPs in the plasma if their concentration is low. However, in the case of a high concentration of NPs, they can be retained with a diameter of no more than a few hundred nanometers due to a significant decrease in the ambipolar electric field. The calculation results are in agreement with our experimental data.

Composite Memristors by Nanoscale Modification of Hf/Ta Anodic Oxides.

Composite memristors based on anodic oxidation of Hf superimposed on Ta thin films are studied. A layered structure is obtained by successive sputtering of Ta and Hf thin films. The deposition geometry ensured components' thickness gradient profiles (wedges) aligned in opposite directions. Anodization in citrate buffer electrolyte leads to a nanoscale columnar structuring of Ta2O5 in HfO2 due to the higher electrical resistance of the latter. Following the less resistive path, the ionic current forces Ta oxide to locally grow toward the electrolyte interface according to the Rayleigh-Taylor principle. The obtained composite oxide memristive properties are studied as a function of the Hf/Ta thickness ratio. One pronounced zone prominent for memristive applications is found for ratios between 4 and 5. Here, unipolar and bipolar memristors are found, with remarkable endurance and retention capabilities. This is discussed in the frame of conductive filament formation preferentially along the interfaces between oxides.

Gallium-Enhanced Aluminum and Copper Electromigration Performance for Flexible Electronics.

Wide range binary and ternary thin film combinatorial libraries mixing Al, Cu, and Ga were screened for identifying alloys with enhanced ability to withstand electromigration. Bidimensional test wires were obtained by lithographically patterning the substrates before simultaneous vacuum co-deposition from independent sources. Current-voltage measurement automation allowed for high throughput experimentation, revealing the maximum current density and voltage at the electrical failure threshold for each alloy. The grain boundary dynamic during electromigration is attributed to the resultant between the force corresponding to the electron flux density and the one corresponding to the atomic concentration gradient perpendicular to the current flow direction. The screening identifies Al-8 at. % Ga and Cu-5 at. % Ga for replacing pure Al or Cu connecting lines in high current/power electronics. Both alloys were deposited on polyethylene naphthalate (PEN) flexible substrates. The film adhesion to PEN is enhanced by alloying Al or Cu with Ga. Electrical testing demonstrated that Al-8 at. % Ga is more suitable for conducting lines in flexible electronics, showing an almost 50% increase in electromigration suppression when compared to pure Al. Moreover, Cu-5 at. % Ga showed superior properties as compared to pure Cu on both SiO2 and PEN substrates, where more than 100% increase in maximum current density was identified.

Electrolyte-Dependent Modification of Resistive Switching in Anodic Hafnia.

Anodic HfO2 memristors grown in phosphate, borate, or citrate electrolytes and formed on sputtered Hf with Pt top electrodes are characterized at fundamental and device levels. The incorporation of electrolyte species deep into anodic memristors concomitant with HfO2 crystalline structure conservation is demonstrated by elemental analysis and atomic scale imaging. Upon electroforming, retention and endurance tests are performed on memristors. The use of borate results in the weakest memristive performance while the citrate demonstrates clear superior memristive properties with multilevel switching capabilities and high read/write cycling in the range of 106. Low temperature heating applied to memristors shows a direct influence on their behavior mainly due to surface release of water. Citrate-based memristors show remarkable properties independent on device operation temperatures up to 100 °C. The switching dynamic of anodic HfO2 memristors is discussed by analyzing high resolution transmission electron microscope images. Full and partial conductive filaments are visualized, and apart from their modeling, a concurrency of filaments is additionally observed. This is responsible for the multilevel switching mechanism in HfO2 and is related to device failure mechanisms.