Yazhong Wang

Experimental demonstration of hybrid improper ferroelectricity and the presence of abundant charged walls in (Ca,Sr)3Ti2O7 crystals

On the basis of successful first-principles predictions of new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Using trilinear coupling of two types of octahedron rotation, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca3Ti2O7, Ca3Mn2O7 and (Ca/Sr/Ba)3(Sn/Zr/Ge)2O7). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we provide the first experimental demonstration of room-temperature switchable polarization in bulk crystals of Ca3Ti2O7, as well as Sr-doped Ca3Ti2O7. Furthermore, (Ca, Sr)3Ti2O7 is found to exhibit an intriguing ferroelectric domain structure resulting from orthorhombic twins and (switchable) planar polarization. The planar domain structure accompanies abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, which exhibit a conduction difference of two orders of magnitude. These discoveries provide new research opportunities, not only for new stable ferroelectrics of Ruddlesden-Popper compounds, but also for meandering conducting domain walls formed by planar polarization.

Phase field microelasticity theory and modeling of multiple dislocation dynamics

The phase field theory and model of a multidislocation dynamics in an elastically anisotropic crystal under applied stress is developed. The proposed three-dimensional (3D) model is a particular case of our phase field microelasticity model of the stress-induced martensitic transformations. A spontaneous self-organization of dislocations in the evolving ensemble, which involves multiplication/annihilation and movement of dislocations controlled by their elastic interaction, is described by the Ginzburg–Landau kinetic equation. Examples of 3D computer simulation of dislocation dynamics are considered.

In situ infrared spectroscopy of hafnium oxide growth on hydrogen-terminated silicon surfaces by atomic layer deposition

The interface formation between HfO2 and H-terminated Si(111) and Si(100) is studied by in situ infrared absorption spectroscopy during atomic layer deposition using alternating tetrakis-ethylmethylamino hafnium (TEMAH) and deuterium oxide (D2O) pulses. The HfO2 growth is initiated by the reaction of TEMAH with Si–H rather than D2O, and there is no evidence for SiO2 formation at moderate growth temperatures (∼100°C). Although Rutherford backscattering shows a linear increase of Hf coverage, direct observations of Si–H, Si–O–Hf, and HfO2 phonons indicate that five cycles are needed to reach the steady state interface composition of ∼50% reacted sites. The formation of interfacial SiO2 (∼0.7nm) is observed after postdeposition annealing at 700°C in ultrapure nitrogen.

Three-dimensional phase field microelasticity theory and modeling of multiple cracks and voids

It is proved that the stress-free strain distribution minimizing the strain energy of the homogeneous modulus body fully determines the elasticity of the discontinuous body. This result is used as a basis for the proposed three-dimensional phase field microelasticity theory and model of a discontinuous body with cracks and voids in elastically anisotropic crystal under applied stress. The elastic equilibrium and spontaneous evolution of these defects are described by the Ginzburg–Landau kinetic equation. Examples of computations of elastic equilibrium and evolutions of systems with cracks and/or voids are considered.

Infrared characterization of hafnium oxide grown by atomic layer deposition using ozone as the oxygen precursor

HfO2 growth on H-terminated Si surfaces by atomic layer deposition (ALD) is studied with in situ Fourier transform infrared spectroscopy and ex situ Rutherford backscattering, using tetrakis-(ethyl-methyl-amino) hafnium and ozone as the hafnium and oxygen precursors, and compared to water-vapor-based ALD growth. The reaction pathways are different for the two oxygen precursors, leading to a lower growth rate for ozone (∼0.05nm∕cycle) than for water-based growth and to incorporation of different impurities in the HfO2 film. Furthermore, interfacial SiO2 is readily formed with ozone at the growth temperature (∼100°C), in contrast to water-based HfO2 growth.

Multiferroicity in doped hexagonal LuFeO3

Neutron scattering techniques are used to investigate the crystalline and magnetic structure of LuFe

Carrier capture at the SiO2–Si interface: A physical model

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Substrate effect on the thickness of spin-coated ultrathin polymer film

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Unveiling hidden ferrimagnetism and giant magnetoelectricity in polar magnet Fe2Mo3O8

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Interrelation between domain structures and polarization switching in hybrid improper ferroelectric Ca3(Mn,Ti)2O7

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