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We present an overview of the atomic structure of extended defects in wurtzite III-nitrides, focusing on GaN heterostructures applicable to light-emitting optoelectronic devices (LEDs and lasers) and radio frequency (RF) power electronics. We provide experimental routines to visualize and recognize these defects with aberration-corrected scanning transmission electron microscopy (STEM). Line, planar, and volume defects are imaged, with examples taken from InGaN multi quantum well heterostructures, epitaxially grown on Si. We demonstrate how the different line and planar defects can be identified by visualizing either the extra or sheared planes that are characteristic to each defect or by differentiating their unique atomic column projections through depth-sectioning. Volume defects in the form of locally phase-separated InN and metallic indium precipitates were also identified via spatially resolved electron energy-loss spectroscopy. The classification of defects established in this work and the methods to visualize them will be a reference to researchers in this field and will provide a foundation for the targeted engineering of future III-nitrides. [ABSTRACT FROM AUTHOR]

Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single‐photon sources and valleytronic devices. For these applications, defects with well‐defined optical characteristics need to be generated in a precisely controlled manner. However, defect‐induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton‐beam irradiation. Defect‐induced emissions are found to occur ≈100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high‐excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe‐Salpeter‐equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state. [ABSTRACT FROM AUTHOR]

The exploration of n‐type PbTe as thermoelectric materials always falls behind its p‐type counterpart, mainly due to their quite different electronic band structure. In this work, elemental Sb and Cu2Te are introduced into an n‐type base material (PbTe)81‐Sb2Te3. The introduction of extra Sb can effectively tune the concentration of electrons; meanwhile, Sb precipitates can also scatter low‐energy electrons (negatively contribute to the Seebeck coefficient) thus enhance the overall Seebeck coefficient. The added Cu2Te is found to always co‐precipitate with Sb, forming an interesting Sb/CuTe core/shell structure; moreover, the interface between core/shell precipitates and PbTe matrix simultaneously shows coherent lattice and strong strain contrast, beneficial for electron transport but adverse to phonon transport. Eventually, a peak figure of merit ZTmax ≈ 1.6 @ 823K and simultaneously an average ZT ≈ 1.0 (323–823 K) are realized in the (PbTe)81Sb2Te3‐0.6Sb‐2Cu2Te sample, representing the state of the art for n‐type PbTe‐based thermoelectric materials. Moreover, for the first time the three existing forms of Cu atoms in Cu2Te alloyed PbTe are unambiguously clarified with aberration‐corrected scanning transmission electron microscopy (Cs‐STEM). [ABSTRACT FROM AUTHOR]

Abstract: We study atomic-resolution annular electron energy-loss spectroscopy (AEELS) in scanning transmission electron microscopy (STEM) imaging with experiments and numerical simulations. In this technique the central part of the bright field disk is blocked by a beam stop, forming an annular entry aperture to the spectrometer. The EELS signal thus arises only from electrons scattered inelastically to angles defined by the aperture. It will be shown that this method is more robust than conventional EELS imaging to variations in specimen thickness and can also provide higher spatial resolution. This raises the possibility of lattice resolution imaging of lighter elements or ionization edges previously considered unsuitable for EELS imaging. [Copyright &y& Elsevier]