Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.

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Domain wall nanoelectronics

Optical Waveguide Theory

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

Biogenic calcium carbonate forms the inorganic component of seashells, otoliths, and many marine skeletons, and its formation is directed by an ordered template of macromolecules. Classical nucleation theory considers crystal formation to occur from a critical nucleus formed by the assembly of ions from solution. Using cryotransmission electron microscopy, we found that template-directed calcium carbonate formation starts with the formation of prenucleation clusters. Their aggregation leads to the nucleation of amorphous nanoparticles in solution. These nanoparticles assemble at the template and, after reaching a critical size, develop dynamic crystalline domains, one of which is selectively stabilized by the template. Our findings have implications for template-directed mineral formation in biological as well as in synthetic systems.

The initial stages of template-controlled CaCO3 formation revealed by Cryo-TEM

Grain Boundary Complexions

Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Domains and domain walls are a fundamental property of interest in ferroelectrics, magnetism, ferroelastics, superconductors, and multiferroic materials. Unlike magnetic Bloch walls, ideal ferroelectric domain walls are well accepted to be only one to two lattice units wide, over which polarization and strain change across the wall. However, walls in real ferroelectrics appear to show unexpected property variations in the vicinity of domain walls that can extend over micrometer length scales. This chapter specifically reviews the local electrical, elastic, optical, and structural properties of antiparallel domain walls in the trigonal ferroelectrics lithium niobate and lithium tantalate. It is shown that extrinsic point defects and their clustering play a key role in the observed local wall structure and influence macroscale properties by orders of magnitude. The review also raises broader and yet unexplored fundamental questions regarding intrinsic widths, defect–domain wall interactions, and static versus dynamic wall structure.

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Defect–Domain Wall Interactions in Trigonal Ferroelectrics

Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.

Growths of staggered InGaN quantum wells light-emitting diodes emitting at 520–525 nm employing graded growth-temperature profile

Staggered InGaN quantum wells (QWs) are investigated both numerically and experimentally as improved active region for light-emitting diodes (LEDs) emitting at 520–525 nm. Based on a self-consistent six-band k·p method, band structures of both two-layer staggered InxGa1−xN/InyGa1−yN QW and three-layer staggered InyGa1−yN/InxGa1−xN/InyGa1−yN QW structures are investigated as active region to enhance the spontaneous emission radiative recombination rate (Rsp) for LEDs emitting at 520–525 nm. Numerical analysis shows significant enhancement of Rsp for both two-layer and three-layer staggered InGaN QWs as compared to that of the conventional InzGa1−zN QW. Significant reduction of the radiative carrier lifetime contributes to the enhancement of the radiative efficiency for both two-layer and three-layer staggered InGaN QW LEDs emitting at 520–525 nm. Three-layer staggered InGaN QW LEDs emitting at 520–525 nm was grown by metal-organic chemical vapour deposition (MOCVD) by employing graded-temperature profile. Power density-dependent cathodoluminescence (CL) measurements show the enhancement of peak luminescence by up to 3 times and integrated luminescence by 1.8–2.8 times for the three-layer staggered InGaN QW LED. Electroluminescence (EL) output power of the staggered InGaN QW LED exhibits 2.0–3.5 times enhancement as compared to that of the conventional InGaN QW LED. The experimental results show the good agreement with theory.

Design and characteristics of staggered InGaN quantum-well light-emitting diodes in the green spectral regime

A large experimental body of literature on lithium niobate, a technologically important ferroelectric, suggests that nonstoichiometric defects dominate its physical behavior, from macroscale switching to nanoscale wall structure. The exact structure and energetics of such proposed intrinsic defects and defect clusters remains unverified by either first-principles calculations or experiments. Here, density functional theory (DFT) is used to determine the dominant intrinsic defects in $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ under various conditions. In particular, in an ${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$-rich environment, a cluster consisting of a niobium antisite compensated by four lithium vacancies is predicted to be the most stable defect structure, thereby verifying what was thus far a conjecture in the literature. Under ${\mathrm{Li}}_{2}\mathrm{O}$-rich conditions, the lithium Frenkel defect is predicted to be the most stable, with a positive defect formation energy (DFE). This is proposed as the underlying reason that the vapor-transport equilibration (VTE) method can grow stoichiometric $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$. The effects of temperature and oxygen partial pressure are also explored by combining the DFT results with thermodynamic calculations. These predictions provide a picture of a very rich defect structure in lithium niobate, which has important effects on its physical behavior at the macroscale.

Volkmar Dierolf

Papers

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Defect–Domain Wall Interactions in Trigonal Ferroelectrics

Growths of staggered InGaN quantum wells light-emitting diodes emitting at 520–525 nm employing graded growth-temperature profile

Design and characteristics of staggered InGaN quantum-well light-emitting diodes in the green spectral regime

Stability of intrinsic defects and defect clusters in LiNbO3 from density functional theory calculations