Chen Li

DPC in Scanning Transmission Electron Microscopy (STEM) is a valuable method for mapping the electric fields in semiconductor materials. However, optimising the experimental conditions can be challenging. In this paper, we test and compare critical experimental parameters, including the convergence angle, camera length, acceleration voltage, sample configuration, and orientation using a four-quadrant segmented detector and a Si specimen containing layers of different As concentrations. The DPC measurements show a roughly linear correlation with the estimated electric fields, until the field gets close to the detection limitation, which is similar to 0.5 mV/nm with a sample thickness of similar to 145 nm. These results can help inform which technique to use for different user cases: When the electric field at a planar junction is above similar to 0.5 mV/nm, DPC with a segmented detector is practical for electric field mapping. With a planar junction, the DPC signal-to-noise ratio can be increased by increasing the specimen thickness. However, for semiconductor devices with electric fields smaller than similar to 0.5 mV/nm, or for devices containing curved junctions, DPC is unreliable and techniques with higher sensitivity will need to be explored, such as 4D STEM using a pixelated detector.

Article published online: 14 October 2023 © 2023. Thieme. All rights reserved Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Indium phosphide quantum dots (QDs) have drawn attention as alternatives to cadmium and lead based QDs that are currently used as phosphors for various opto-electronic applicationsin lamps and displays. The main drawbacks of InP QDs are, in generally, a lower photoluminescence quantum yield (PLQY), a decreased color purity and poor chemical stability. In this research, we attempted to increase the PLQY and stability of indium phosphide QDs by developing lattice matched InP/MgSe core-shell nano-heterostructures. The choice of MgSe comes from the fact that, in theory, it has a near-perfect lattice match with InP, provided MgSe is grown in the zinc blende crystal structure, which can be achieved by alloying with Znzinc. To retain lattice matching we used zinc Zn in both the core and shell and we fabricated InZnP/ZnxMg1-xSe core/shell QDs. To identify the most suitable conditions for shell growth, we first developed a synthesis route to ZnxMg1-xSe nanocrystals (NCs) wherein Mg is effectively incorporated. Our optimized procedure was employed for the successful growth of ZnxMg1-xSe shells around In(Zn)P QDs. The corresponding core/shell systems exhibit PLQYs higher than those of the starting In(Zn)P QDs and, more importantly, a higher color purity for decreasing values of xupon increasing the Mg content. The results are discussed in the context of a reduced density of interface states upon using better lattice matched ZnxMg1-xSe shells.

The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4 layers have been grown between Al2O3 and MgO, and the atomic structure of Al2O3/MgAl2O4 interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (hcp) stacking in Al2O3 to cubic close-packed (ccp) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4 interface into Al2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4 grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent {\Sigma}3 grain boundaries form. The newly grown MgAl2O4 grains compete with each other, leading to a growth-selection and successive coarsening of the MgAl2O4 grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar hcp/ccp transition.

The structure of crystalline interfaces plays an important role in solid-state reactions. The Al₂O₃/MgAl₂O₄/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl₂O₄ layers have been grown between Al₂O₃ and MgO, and the atomic structure of Al₂O₃/MgAl₂O₄ interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in Al₂O₃ to cubic close-packed (c.c.p.) stacking in MgAl₂O₄. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al₂O₃/MgAl₂O₄ interface into Al₂O₃ is accomplished by the glide of partial dislocations accompanied by the exchange of Al³⁺ and Mg²⁺ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl₂O₄ grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAl₂O₄ grains compete with each other, leading to a growth selection and successive coarsening of the MgAl₂O₄ grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.