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.

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.

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 ͑DFT͒ is used to determine the dominant intrinsic defects in LiNbO 3 under various conditions. In particular, in an Nb 2 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 Li 2 O-rich conditions, the lithium Frenkel defect is predicted to be the most stable, with a positive defect formation energy ͑DFE͒. This is proposed as the underlying reason that the vapor-transport equilibration ͑VTE͒ method can grow stoichiometric LiNbO 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.

Defying the requirements of translational periodicity in 3D, rotation of the lattice orientation within an otherwise single crystal provides a new form of solid. Such rotating lattice single (RLS) crystals are found, but only as spherulitic grains too small for systematic characterization or practical application. Here we report a novel approach to fabricate RLS crystal lines and 2D layers of unlimited dimensions via a recently discovered solid-to-solid conversion process using a laser to heat a glass to its crystallization temperature but keeping it below the melting temperature. The proof-of-concept including key characteristics of RLS crystals is demonstrated using the example of Sb2S3 crystals within the Sb-S-I model glass system for which the rotation rate depends on the direction of laser scanning relative to the orientation of initially formed seed. Lattice rotation in this new mode of crystal growth occurs upon crystallization through a well-organized dislocation/disclination structure introduced at the glass/crystal interface. Implications of RLS growth on biomineralization and spherulitic crystal growth are noted.

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