Influence of Biaxial Strain and Interfacial Layer Growth on Ferroelectric Wake-Up and Phase Transition Fields in ZrO2.

Investigations on fluorite-structured ferroelectric HfO2/ZrO2 thin films are aiming to achieve high-performance films required for memory and computing technologies. These films exhibit excellent scalability and compatibility with the complementary metal-oxide semiconductor process used by semiconductor foundries, but stabilizing ferroelectric properties with a low operation voltage in the as-fabricated state of these films is a critical component for technology advancement. After removing the influence of interfacial layers, a linear correlation is observed between the biaxial strain and the electric field for transforming the nonferroelectric tetragonal to the ferroelectric orthorhombic phase in ZrO2 thin films. This observation is supported by applying the principle of energy conservation in combination with ab initio and molecular dynamics simulations. According to the simulations, a rarely reported Pnm21 orthorhombic phase may be stabilized by tuning biaxial strain in the ZrO2 films. This study demonstrates the significant influence of interfacial layers and biaxial strain on the phase transition fields and shows how strain engineering can be used to improve ferroelectric wake-up in ZrO2.

Ferroelectric Domain Wall p-n Junctions.

We have used high-voltage Kelvin probe force microscopy to map the spatial distribution of electrical potential, dropped along curved current-carrying conducting domain walls, in x-cut single-crystal ferroelectric lithium niobate thin films. We find that in-operando potential profiles and extracted electric fields, associated with p-n junctions contained within the walls, can be fully rationalized through expected variations in wall resistivity alone. There is no need to invoke additional physics (carrier depletion zones and space-charge fields) normally associated with extrinsically doped semiconductor p-n junctions. Indeed, we argue that this should not even be expected, as inherent Fermi level differences between p and n regions, at the core of conventional p-n junction behavior, cannot occur in domain walls that are surrounded by a common matrix. This is important for domain-wall nanoelectronics, as such in-wall junctions will neither act as diodes nor facilitate transistors in the same way as extrinsic semiconducting systems do.

The Effect of Chemical Environment and Temperature on the Domain Structure of Free-Standing BaTiO3 via In Situ STEM.

Ferroelectrics, due to their polar nature and reversible switching, can be used to dynamically control surface chemistry for catalysis, chemical switching, and other applications such as water splitting. However, this is a complex phenomenon where ferroelectric domain orientation and switching are intimately linked to surface charges. In this work, the temperature-induced domain behavior of ferroelectric-ferroelastic domains in free-standing BaTiO3 films under different gas environments, including vacuum and oxygen-rich, is studied by in situ scanning transmission electron microscopy (STEM). An automated pathway to statistically disentangle and detect domain structure transformations using deep autoencoders, providing a pathway towards real-time analysis is also established. These results show a clear difference in the temperature at which phase transition occurs and the domain behavior between various environments, with a peculiar domain reconfiguration at low temperatures, from a-c to a-a at ≈60 °C. The vacuum environment exhibits a rich domain structure, while under the oxidizing environment, the domain structure is largely suppressed. The direct visualization provided by in situ gas and heating STEM allows to investigate the influence of external variables such as gas, pressure, and temperature, on oxide surfaces in a dynamic manner, providing invaluable insights into the intricate surface-screening mechanisms in ferroelectrics.

Effects of Multiple Local Environments on Electron Energy Loss Spectra of Epitaxial Perovskite Interfaces.

The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments.

Towards 3D characterisation of site-controlled InGaAs pyramidal QDs at the nanoscale.

In this work, we report an extensive investigation via transmission electron microscopy (TEM) techniques of InGaAs/GaAs pyramidal quantum dots (PQDs), a unique site-controlled family of quantum emitters that have proven to be excellent sources of single and entangled photons. The most striking features of this system, originating from their peculiar fabrication process, include their inherently 3-dimensional nature and their interconnection to a series of nanostructures that are formed alongside them, such as quantum wells and quantum wires. We present structural and chemical data from cross-sectional and plan view samples of both single and stacked PQDs structures. Our findings identify (i) the shape of the dot, being hexagonal and not triangular as previously assumed, (ii) the chemical distribution at the facets and QD area, displaying clear Indium diffusion, and (iii) a near absence of Aluminium (from the AlAs marker) at the bottom of the growth profile. Our results shed light on previously unreported structural and chemical features of PQDs, which is of extreme relevance for further development of this family of quantum emitters. Supplementary Information: The online version contains supplementary material available at 10.1007/s10853-022-07654-2.

Elastic distortion determining conduction in BiFeO3 phase boundaries.

It is now well-established that boundaries separating tetragonal-like (T) and rhombohedral-like (R) phases in BiFeO3 thin films can show enhanced electrical conductivity. However, the origin of this conductivity remains elusive. Here, we study mixed-phase BiFeO3 thin films, where local populations of T and R can be readily altered using stress and electric fields. We observe that phase boundary electrical conductivity in regions which have undergone stress-writing is significantly greater than in the virgin microstructure. We use high-end electron microscopy techniques to identify key differences between the R-T boundaries present in stress-written and as-grown microstructures, to gain a better understanding of the mechanism responsible for electrical conduction. We find that point defects (and associated mixed valence states) are present in both electrically conducting and non-conducting regions; crucially, in both cases, the spatial distribution of defects is relatively homogeneous: there is no evidence of phase boundary defect aggregation. Atomic resolution imaging reveals that the only significant difference between non-conducting and conducting boundaries is the elastic distortion evident - detailed analysis of localised crystallography shows that the strain accommodation across the R-T boundaries is much more extensive in stress-written than in as-grown microstructures; this has a substantial effect on the straightening of local bonds within regions seen to electrically conduct. This work therefore offers distinct evidence that the elastic distortion is more important than point defect accumulation in determining the phase boundary conduction properties in mixed-phase BiFeO3.

Correction: Giant resistive switching in mixed phase BiFeO3via phase population control.

Correction for 'Giant resistive switching in mixed phase BiFeO3via phase population control' by David Edwards et al., Nanoscale, 2018, 10, 17629-17637.

Giant resistive switching in mixed phase BiFeO3via phase population control.

Highly-strained coherent interfaces, between rhombohedral-like (R) and tetragonal-like (T) phases in BiFeO3 thin films, often show enhanced electrical conductivity in comparison to non-interfacial regions. In principle, changing the population and distribution of these interfaces should therefore allow different resistance states to be created. However, doing this controllably has been challenging to date. Here, we show that local thin film phase microstructures (and hence R-T interface densities) can be changed in a thermodynamically predictable way (predictions made using atomistic simulations) by applying different combinations of mechanical stress and electric field. We use both pressure and electric field to reversibly generate metastable changes in microstructure that result in very large changes of resistance of up to 108%, comparable to those seen in Tunnelling Electro-Resistance (TER) devices.