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Several scholars have pointed to the increasing salience of cities in the global endeavour to reduce dangerous climate change to 1.5°C. However, we still lack systematic comparative analyses on how urban climate initiatives evolve in different political-administrative systems of countries in the global South and what role transnational city networks play in that context. This question is specifically important with regard to the role of cities in contributing to reaching the targets of the Paris Agreement. Against this backdrop, this article explores the vertical and horizontal integration of cities’ climate actions in the multi-level climate governance landscapes in Brazil, India, Indonesia and South Africa. We contend that while transnational city networks provide significant support to cities in some cases, their tools and practices can only reach their full potential where they encounter committed local administrations and when they are not constrained by domestic political-administrative and economic factors.

Ferroelectric hafnium oxide (HfO2) is considered a very prospective material for applications in integrated devices due to its considerably large spontaneous polarization and superior thickness scaling. In fact, the evolution of the ferroelectric hysteresis upon field cycling plays an important role in most applications; especially the so-called wake-up effect that describes the increase of remanent polarization for initial field cycling, needs a profound understanding in HfO2. Herein, the discovery of electric field-induced crystallization in hafnium oxide is reported. In addition, differences in the wake-up behavior are addressed that finally can be categorized into five different cases, all being relevant when describing the evolution of ferroelectricity. Moreover, analysis of the temperature dependence and transmission Kikuchi diffraction provides insight into the underlying physical mechanisms of different wake-up behavior scenarios, and proves ferroelastic switching as the origin for the observed antiferroelectric-like behavior. This knowledge provides clear procedures of 1) how to experimentally quantify and 2) how to prepare and manufacture hafnium oxide phases for the five different wake-up types.

Thin-film transistors (TFTs) based on amorphous indium-gallium-zinc-oxide (a-IGZO) have attracted vast attention for use in organic light-emitting diode (AMOLED) displays due to their high electron mobility and large current on off ratio. Although amorphous oxide semiconductors show considerably less threshold voltage (Vth) variation than poly-silicon, large-area processing and degradation effects can impede the characteristic parameters of a-IGZO TFTs, which manifests in an uneven brightness distribution across the display panel. Such Vth variations are usually reduced by additional compensation circuits consisting of TFTs and capacitors. Herein, a new approach to compensate such variabilities is demonstrated: the integration of a programmable ferroelectric (FE) film in the gate stack o f the TFT. This simplifies the complexity of the pixel cell and potentially minimizes the need for compensation circuits, which is crucial for transparent displays. To test this new approach, fully integrated FE-TFTs (i.e., with vias contacting a structured bottom gate electrode from the top) based on a-IGZO and FE hafnium-zirconium oxide (HZO) are developed. A single low-temperature post-fabrication treatment at 350 °C for 1 h in air is used to simultaneously crystallize the HZO film in the FE phase and to reduce the number of defects in the a-IGZO channel. The structural and electrical characterizations provide comprehensive guidance for the design of effective FE-TFT gate stacks and device geometries. An accurate control of the polarization state and linear switching between multiple intermediate states is shown by using programming pulses of various amplitudes and widths. Furthermore, a direct correlation between the channel length and the applied pulse width for programming is observed.

Piezoelectric thin films are of current interest in science and industry for highly integrated nano-electro-mechanical-systems and sensor devices. In this study, the dependence of the piezoelectric properties on the doping concentration in Si:HfO2 thin films and their crystallographic origin are investigated. The Si:HfO2 films with a thickness of 20 nm and various Si doping concentrations in the range of 2.7-5.6 cat.% were examined. The relationship between the piezoelectric displacement and remanent polarization is studied during wake-up from the antiferroelectric-like pristine state until the cycled ferroelectric state, which reveals an application-dependent optimal doping concentration. Furthermore, the piezoelectric and ferroelectric properties, as well as the relative permittivity, were measured over wake-up, thus giving a glimpse at the underlying mechanism of the transition from a pristine antiferroelectric-like behavior to a ferroelectric/piezoelectric one, revealing a pre-existing polar phase that is reorienting during wake-up. The studied samples show a strong displacement and polarization dependence on the doping concentration. Hence, the stoichiometry is an excellent parameter for the application-specific adjustment of complementary metal-oxide-semiconductor compatible piezoelectric thin films.

The discovery of ferroelectricity in thin doped hafnium oxide films revived the interest in ferroelectric (FE) memory concepts. Zirconium‐doped hafnium oxide (HZO) crystallizes at low temperatures (e.g., 400 °C), which makes this material interesting for the implementation of FE functionalities into the back end of line (BEoL). So far, the FE phase of prior amorphous HZO films is achieved by using a dedicated rapit thermal annealing (RTA) treatment. However, herein, it is shown that this dedicated anneal is not needed. A sole furnace treatment given by the thermal budget present during the interconnect formation is sufficient to functionalize even ultrathin 5 nm HZO films. This result helps to optimize the integration sequence of HZO films (e.g., involving a minimum number of BEoL process steps), which saves process time and fabrication costs. Herein, metal-FE-metal capacitors with Hf0.5Zr0.5O2 films of different thicknesses (5-20 nm) are fabricated annealed at 400 °C for various durations within different types of ovens (RTA and furnace). Structural and electrical characterization confirms that all furnace‐annealed samples have similar X‐ray diffraction patterns, remanent polarization, endurances, and thickness dependencies as RTA‐annealed ones. With respect to remanent polarization, leakage current, and endurance, the HZO film of 10 nm thickness shows the most promising results for the integration into the BEoL.

We report on the high-temperature operation and reliability of the Si-doped hafnium oxide (HSO) ferroelectric FET (FeFET) emerging memory. In this study, we explore the role of high-temperature operation of the ferroelectric (FE) material on the FeFET in the temperature range of 40-120 °C. The inverse memory window (MW) dependence on temperature leads to a very small MW size (~300 mV) at 120 °C. The recovery of MW size at room temperature (RT) indicates a potential pyroelectric effect as a cause for MW closure upon high-temperature operation. The FeFET state readout shows pronounced effects to erase (ER) threshold voltage (Vth) shift, due to the decrease in remnant polarization and improved substrate Vth shift as the temperature increases. The endurance reliability measured for temperature range (-40 °C to 40 °C) with 105 cycles shows a maximized initial MW at low temperatures, whereas higher postcycling interface trap generation occurs as the temperature increases. The FeFET 10 h retention tests at 0 °C and 40 °C were extrapolated to ten years and indicated stable properties independent of temperature. The depolarization field (Ed) dependence on the FeFET stack parameters is studied based on an analytical formula. The Ed decreases with increased permittivity of the FE, interface layer (IL), and increased FE film thickness. Substrate doping and temperature seem to have a small impact on E d , whereas the FE-IL area ratio tuning below unity lowers the E d . The role of pyroelectric effect on the FeFET memory and operating temperature-induced endurance and retention reliability concerns are discussed.

We report on the temperature-dependent operation of fluorite-structure-based ferroelectric FET (FeFET) emerging memory. A temperature range (-40 °C to 40 °C) is used to explore the FeFET characteristic relation to operating temperature. The memory window (MW) shows a modulated response that features a reciprocal MW dependence on temperature, such that a maximum of the MW is realized at -40 °C. The gradual MW closure upon temperature increase is attributed to the ferroelectric (FE) polarization change with temperature. On the contrary, the FE coercive field shows a minor variation with operating temperature. The FeFET state readout shows a trend of Vth shift with temperature such that the decrease in remnant polarization, as well as the substrate effects, causes a maximized shift for erase (ER) state compared to the program (PG) one. The benchmark of Si-doped hafnium oxide (HSO) and hafnium zirconium oxide (HZO) shows comparable trends for dependence on temperature. The temperature cycling by repetitive sweep from -40 °C to 40 °C shows reproducible MW and PG/ER readout trends with a predictable FeFET response over temperature. This suggests system design techniques for mitigating the variation effects. The FeFET characteristics are explored with insight on physical mechanisms and FE response to temperature variation.

Long data retention is a critical requirement for many of the potential applications of HfO 2 -based ferroelectric field-effect transistors (FeFETs). However, methods for its rapid assessment are still missing. In this article, we report a detailed investigation of the retention and switching properties of FeFETs fabricated in the 28-nm high-k metal gate technology. We identify a clear correlation between the two properties and propose a method for fast prediction of the device retention behavior, which can be easily adopted to judge different fabrication processes. Finally, we extend the validity of the method to a direct assessment of disturb-free operating conditions, which may be particularly valuable for FeFET array operation.