Role of Si Doping in Reducing Coercive Fields for Ferroelectric Switching in HfO2

The ferroelectricity of HfO2 thin films is technologically useful with various advantages compared to conventional ferroelectrics. However, the application of orthorhombic HfO2 has been limited by its large coercive field compared to perovskite-based ferroelectrics. Using first-principles calculations, we extensively search for 34 dopant elements to reduce the problematic coercive fields and discover that the coercive fields exhibit a simple volcano shape against the dopant's size. We also discover that the Si dopant is a critical element in stabilizing tetragonal phase HfO2 (transition state) because of its intrinsic sp(3) bond favoring characteristics with oxygen, thereby notably lowering the coercive fields. We provide an atomic scale picture to understand the excellent role of Si in effective ferroelectric switching and a simple rule to tune coercive fields with various doping agents

Facile Ferroelectric Phase Transition Driven by Si Doping in HfO2

The recently discovered ferroelectricity in thin-film orthorhombic HfO2, which can be directly integrated into complementary metal-oxide semiconductor technology, has become an important research target. However, the use of orthorhombic HfO2 in practical devices has been limited by undesirable mixing with the monoclinic phase, which is nonpolar and thus degrades the ferroelectric properties. Here, we demonstrate that a Si dopant significantly stabilizes the ferroelectric phase because of its unique bonding characteristics, particularly its intrinsic tendency to form strong covalent bonds with O, thereby weakening the phase boundary to stabilize the ferroelectric orthorhombic phase over 20 the nonpolar monoclinic phase, relatively. On the basis of our theoretical predictions, we conducted transmission electron microscopy measurements and confirmed that Si substitution doping indeed induced monoclinic structural components into the orthorhombic phase, which is a strong indication of the weakened phase boundary and subsequent facilitation of the ferroelectric transition. This work thus provides an atomic-scale picture for understanding the unique role of Si in promoting the ferroelectric phase and the dopant dependence on the wake-up effect in HfO2, offering a substantial advancement toward integrating ferroelectrics into practical devices

Scale-free ferroelectricity induced by flat phonon bands in HfO2

Discovery of robust yet reversibly switchable electric dipoles at reduced dimensions is critical to the advancement of nanoelectronics devices. Energy bands flat in momentum space generate robust localized states that are activated independently of each other. We determined that flat bands exist and induce robust yet independently switchable dipoles that exhibit a distinct ferroelectricity in hafnium dioxide (HfO2). Flat polar phonon bands in HfO2 cause extreme localization of electric dipoles within its irreducible half-unit cell widths (~3 angstroms). Contrary to conventional ferroelectrics with spread dipoles, those intrinsically localized dipoles are stable against extrinsic effects such as domain walls, surface exposure, and even miniaturization down to the angstrom scale. Moreover, the subnanometer-scale dipoles are individually switchable without creating any domain-wall energy cost. This offers unexpected opportunities for ultimately dense unit cell???by???unit cell ferroelectric switching devices that are directly integrable into silicon technology

High-Output Triboelectric Nanogenerator Based on Dual Inductive and Resonance Effects-Controlled Highly Transparent Polyimide for Self-Powered Sensor Network Systems

High-output triboelectric nanogenerators (TENGs) are demonstrated based on polyimide (PI)-based polymers by introducing functionalities (e.g., electron-withdrawing and electron-donating groups) into the backbone. The TENG based on 6FDA-APS PI, possessing the most negative electrostatic potential and the low-lying lowest unoccupied molecular orbital level, produces the highest effective charge density of about 860 mu C m(-2) in practical working conditions with the ion injection process. This may be ascribed to the excellent charge-retention characteristics as well as the enhanced charge transfer capability, which increases the output power by 7 times compared to the commercially available Kapton film-based TENG. Finally, a 6FDA-APS-driven sensor network system is demonstrated, providing the identity of three gases (H-2, CO, and NO2) by illuminating the light-emitting diodes within several seconds

Sustainable highly charged C-60-functionalized polyimide in a non-contact mode triboelectric nanogenerator

In this paper, we report on a new dielectric, a C-60-containing block polyimide (PI-b-C-60). This was realized by introducing C-60 as pendent groups into a polymer backbone. When this dielectric was used in a non-contact mode triboelectric nanogenerator (TENG), it achieved high output power and reliable operation. Compared with perfluoroalkoxy alkane film-based TENGs, the TENG based on PI-b-C-60 generated 4.3 times higher output power and a superior charge density of over 300 mu C m(-2) with a 3 times slower charge decay rate. These results are most likely due to the excellent charge-retention characteristics induced by the most negative electrostatic potential of C-60 within the backbone, and these characteristics were confirmed by surface potential measurements. Furthermore, in the course of our work, two non-contact mode applications, a keyless electronic door lock system and a speed sensor with a tone wheel for a car, were developed. Without an ion injection process being needed, very sensitive and reliable operations of the speed sensor were successfully demonstrated, even under very humid conditions (similar to 99% RH)