Scanning Transmission Electron Microscopy Probe Research Articles

Ingredients for enhanced thermoelectric power at cryotemperatures in the correlated semiconductor CoSbS revealed by its optical response

Semiconducting CoSbS is well known for its thermoelectric performance at high temperatures but it is also of interest because of its colossal low-temperature thermopower. Here, we address the temperature dependence of its optical response over a broad spectral range, from which we reveal several ingredients determining the thermoelectric properties at cryotemperatures. We discover coherent phonon modes and with the additional support of scanning transmission electron microscopy investigations we provide evidence for in-gap impurity states driving the formation of correlated electrons in the valence band. Their implications, with respect to the high thermoelectric power at low temperatures, are discussed within the framework of the phonon-drag transport of low-mobility heavy quasiparticles.

Different Oxygen Desorption Durabilities of Lithium Titanium Oxides as Confirmed by Transmission Electron Energy-Loss Spectroscopy Analysis of Li4Ti5O12 and β-Li2TiO3 Biphase Specimens

Spinel-type Li4Ti5O12 and monoclinic β-Li2TiO3 phases are well known as typical functional materials in lithium titanium oxide. The former phase is practically applied as a negative electrode material for Li-ion battery systems, while the latter phase is used as a tritium breeder blanket material for thermonuclear reactors; thus, the material stability of these phases is essential for their functions. In this study, we investigate the oxygen desorption durability of oxide materials with different structures and chemical formulas by transmission electron microscopy-based electron energy-loss spectroscopy. To compare the different properties of these two phases, biphase specimens that consisted of Li4Ti5O12 and β-Li2TiO3 phases with a homogeneous thickness were successfully prepared by a two-step thermal process to perform suitable analysis by transmission electron microscopy. Irradiation with a scanning transmission electron microscopy probe, followed by electron energy-loss spectroscopy analysis, was performed in the Li4Ti5O12/β-Li2TiO3 interphase region of the biphase specimen, revealing that the oxygen desorption feature of β-Li2TiO3 was approximately one-third that of the Li4Ti5O12 phase. The stability of the cation–oxygen bond in each oxide crystal was also simulated by reaction enthalpy analysis based on density functional theory calculations, and the lattice stability of the Li4Ti5O12 phase was estimated to be three times higher than that of the β-Li2TiO3 phase. The oxygen vacancy formation energy of the β-Li2TiO3 lattice was approximately 0.8 eV higher than that of the Li4Ti5O12 phase. Thus, the reason for the different oxygen desorption durabilities of these oxide phases is not the stability of the cation–oxygen bond but the ease of reduction phase formation via oxygen desorption.

Materials characterization of TiO2 nanotubes decorated by Au nanoparticles for photoelectrochemical applications.

The structural and chemical modification of TiO2 nanotubes (NTs) by the deposition of a well-controlled Au deposit was investigated using a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), Raman measurements, UV-Vis spectroscopy and photoelectrochemical investigations. The fabrication of the materials focused on two important factors: the deposition of Au nanoparticles (NPs) in UHV (ultra high vacuum) conditions (1–2 × 10−8 mbar) on TiO2 nanotubes (NTs) having a diameter of ∼110 nm, and modifying the electronic interaction between the TiO2 NTs and Au nanoparticles (NPs) with an average diameter of about 5 nm through the synergistic effects of SMSI (Strong Metal Support Interaction) and LSPR (Local Surface Plasmon Resonance). Due to the formation of unique places in the form of “hot spots”, the proposed nanostructures proved to be photoactive in the UV-Vis range, where a characteristic gold plasmonic peak was observed at a wavelength of 580 nm. The photocurrent density of Au deposited TiO2 NTs annealed at 650 °C was found to be much greater (14.7 μA cm−2) than the corresponding value (∼0.2 μA cm−2) for nanotubes in the as-received state. The IPCE (incident photon current efficiency) spectral evidence also indicates an enhancement of the photoconversion of TiO2 NTs due to Au NP deposition without any significant change in the band gap energy of the titanium dioxide (Eg ∼3.0 eV). This suggests that a plasmon-induced resonant energy transfer (PRET) was the dominant effect responsible for the photoactivity of the obtained materials.

Chemistry, Microstructure, and Interphases of Magnesium Metal Anodes Captured Via Cryogenic Electron Microscopy

Lithium-ion batteries are an important component in mobile electronics; however, their modest energy density makes them less than ideal for use in all-electric vehicles or localized grid residential/commercial energy storage. Multivalent-ion batteries (MIBs), including Mg2+, are promising high-energy density technologies that are cheaper and more abundant than lithium. Realizing the high theoretical energy density of MIBs requires the native multivalent-ion metal to be used as the anodic electrode, where densely controlled metal plating and stripping has yet to be achieved. The mechanisms governing plating/stripping of MIBs and the resulting microstructural impact on the battery performance must be understood before MIBs can be employed commercially. Understanding the role that the solid electrolyte interface has on multivalent-ion metal plating/stripping may lead to solutions for controlling reversible anodic cycling.Recent advances in cryogenic electron microscopy(cryo-EM) preparation have allowed for scanning transmission electron microscopy (STEM) investigation of intact solid/liquid interfaces and electron-beam-sensitive materials. Here we present results of our chemical and microstructural investigation of plated and stripped Mg2+ anodes, collected with cryo-FIB preparation, cryo-transfer, and cryo-STEM. Details will be presented on the anode porosity, grain orientation maps, chemical maps from the electrode/electrolyte interfaces, and high-resolution structural imaging of the electrode interfaces.This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

Phase transformations in a heterogeneous Ti-xNb-7Zr-0.8O alloy prepared by a field-assisted sintering technique

Biomedical metastable alloy Ti-xNb-7Zr-0.8O with a compositional gradient of Nb was prepared from elemental powders by a field-assisted sintering technique (FAST). The aim was to investigate phase transformations over a wide range of compositions, facilitating the designing of biomedical Ti alloys. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) investigations revealed that Nb-rich regions retained the β phase, surrounded by transition region consisting of the β and ω phases, while Nb-lean regions consisted of the α and β phases. The Nb concentration, above which formation of the ω phase occurs during cooling instead of the α phase, was determined to be 22 wt%, an important parameter for the low-modulus alloy design. The paper validates the viability of using FAST to prepare heterogeneous Ti alloys permitting to study microstructure over a wide range of compositions. This technique could also be readily used as a high-throughput method for designing other alloy systems.The experimental results were supplemented by calculation of Gibbs energy curves and schematic phase diagrams, which allowed to explain a competition between α and ω formation depending on alloy composition. Such semi-empirical approach can serve as a useful tool for general alloy design, in particular for biomedical Ti alloys.

Oxidation behavior of arc evaporated TiSiN coatings investigated by in-situ synchrotron X-ray diffraction and HR-STEM

Owing to its excellent mechanical and thermal properties and outstanding oxidation resistance, TiSiN is used for protective hard coatings for cutting applications. While several reports confirm the oxidation stability of TiSiN up to temperatures above 800 °C, literature is currently lacking a thorough investigation of the oxidation sequence of this coating system. Thus, in this study the oxidation mechanism of TiSiN was monitored via in-situ synchrotron X-ray diffraction (XRD) and complemented by a detailed analysis of the microstructure and elemental composition of oxidized coatings. A TiSiN coating was deposited by cathodic-arc evaporation in an industrial scale deposition plant. In-situ synchrotron XRD experiments of the powdered coating showed an oxidation stability up to ~830 °C, followed by the formation of both, rutile and anatase TiO2 with increasing temperature. The formation of anatase during oxidation was confirmed by Raman and XRD investigations on a solid coating. High-resolution scanning transmission electron microscopy investigations revealed the oxidation of only several hundred nm of the coating surface after oxidation at 930 °C for 5 min, while increasing the temperature to 1130 °C resulted in full oxidation of the Ti(Si)N nanocrystals, accompanied by high porosity and significant grain coarsening. Furthermore, elemental analysis showed the presence of TiO2 grains surrounded by an amorphous Si-O-N phase as well as the formation of a TiO2 top layer due to diffusion of Ti to the surface. The obtained results provide detailed and novel insight into the oxidation mechanism of TiSiN as well as on the microstructure of oxidized TiSiN coatings.

Chemistry, Microstructure, and Interphases of Magnesium Metal Anodes Captured Via Cryogenic Electron Microscopy

Lithium-ion batteries are an important component in mobile electronics; however, their modest energy density makes them less than ideal for use in all-electric vehicles or localized grid residential/commercial energy storage. Multivalent-ion batteries (MIBs), including Mg2+, are promising high-energy density technologies that are cheaper and more abundant than lithium. Realizing the high theoretical energy density of MIBs requires the native multivalent-ion metal to be used as the anodic electrode, where densely controlled metal plating and stripping has yet to be achieved. The mechanisms governing plating/stripping of MIBs and the resulting microstructural impact on the battery performance must be understood before MIBs can be employed commercially. Understanding the role that the solid electrolyte interface has on multivalent-ion metal plating/stripping may lead to solutions for controlling reversible anodic cycling.Recent advances in cryogenic electron microscopy(cryo-EM) preparation have allowed for scanning transmission electron microscopy (STEM) investigation of intact solid/liquid interfaces and electron-beam-sensitive materials. Here we present results of our chemical and microstructural investigation of plated and stripped Mg2+ anodes, collected with cryo-FIB preparation, cryo-transfer, and cryo-STEM. Details will be presented on the anode porosity, grain orientation maps, chemical maps from the electrode/electrolyte interfaces, and high-resolution structural imaging of the electrode interfaces.This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

Interfacial atomic structure and electrical activity of nano-facetted CSL grain boundaries in high-performance multi-crystalline silicon

The interfacial structure of coincidence site lattice grain boundaries in multi-crystalline silicon plays a decisive role in their electrical behavior as revealed by high-resolution (scanning) transmission electron microscopy investigations. Considering only the global misorientation of the adjacent grains can lead to a false correlation between the structural and electrical properties of a grain boundary. The grain boundary habit plane as well as local deviations in the orientation and misorientation that induce additional structural defects need to be analyzed. Indeed, a Σ9 {221} grain boundary, presenting a perfect coincidence and periodicity at the atomic scale, appears electrically non-active. However, a grain boundary also identified as Σ9 {221} at the mesoscale is found to be composed of nano-sized triangular structures involving Σ3 {111} and Σ3 {112} facets at the nano-scale. This leads to the formation of grain boundary kinks and triple junctions that induce additional structural defects and turn the overall grain boundary interface electrically active. The possible origin of such a grain boundary dissociation, as well as its impact on the electrical activity, is discussed.

Coarsening- and creep resistance of precipitation-strengthened Al–Mg–Zr alloys processed by selective laser melting

The coarsening behavior of Al3Zr precipitates during aging was investigated for two Al–Mg–Zr alloys (Al–3.6Mg–1.2Zr and Al–2.9Mg–2.1Zr, wt%) processed by selective laser melting (SLM). Scanning transmission electron microscopy (STEM) investigations of peak-aged (400 °C, 8 h) samples reveal both continuous (~2 nm in diameter) and discontinuous (~5 nm wide and hundreds of nanometers in length) coherent, secondary L12-Al3Zr precipitates. In-situ STEM experiments showed that aging at 400 °C results in the appearance and growth of both grain-boundary Al3Zr precipitates, and intragranular nanometer-sized spherical Al3Zr precipitates in Zr-rich dendritic arms. Heating to 500 °C resulted in the disappearance of most Al3Zr precipitates and oxide particles. This microstructural evolution sheds light on the evolution of the alloy strength at elevated temperature. For short-term yield tests, as-fabricated samples displayed higher yield strengths than peak-aged samples at temperatures above 150 °C (e.g., 87 vs 24 MPa at 260 °C). This is attributed to coarsening of grain-boundary precipitates during aging, decreasing their ability to inhibit grain-boundary sliding (GBS) of the fine equiaxed grains (~1 µm). For longer term creep tests at 260 °C, both as-fabricated and peak-aged samples displayed near-identical creep behavior during a long-duration (168 h) creep test; by contrast, during a shorter duration creep test (8 h), as-fabricated samples are more creep-resistant than samples previously aged at 260 °C (threshold stresses of ~40 vs. ~14 MPa, respectively). Again, the creep behavior is consistent with coarsening of grain-boundary precipitates, occurring now during long-duration creep tests at 260 °C. An exact creep mechanism could not be isolated due to microstructural changes during testing but is believed to be a combination of GBS and dislocation motion.

Comparative study of hydrogen-induced intergranular fracture behavior in Ni and Cu–Ni alloy at ambient and cryogenic temperatures

In order to clarify the contribution of dislocation‒hydrogen interaction on the hydrogen embrittlement (HE) of pure Ni and of Cu‒55 wt% Ni binary alloy, slow strain rate tensile (SSRT) tests were conducted at room temperature (RT) and at 77 K on hydrogen-precharged specimens. Regarding the SSRT test at RT, hydrogen increased the flow stress and induced intergranular fracture in both pure Ni and Cu–Ni alloy. Furthermore, based on scanning transmission electron microscopy investigations, it was suggested that the evolution of dislocation structures had been enhanced by hydrogen, but only in the case of pure Ni. At 77 K, the ductility of pure Ni was degraded by hydrogen, whereas that of Cu–Ni alloy was not. The difference in temperature dependence of the dislocation‒hydrogen interaction between pure Ni and Cu–Ni alloy was discussed, based on the previously proposed HE mechanisms.

Isolating hydrogen in hexagonal boron nitride bubbles by a plasma treatment

Atomically thin hexagonal boron nitride (h-BN) is often regarded as an elastic film that is impermeable to gases. The high stabilities in thermal and chemical properties allow h-BN to serve as a gas barrier under extreme conditions. Here, we demonstrate the isolation of hydrogen in bubbles of h-BN via plasma treatment. Detailed characterizations reveal that the substrates do not show chemical change after treatment. The bubbles are found to withstand thermal treatment in air, even at 800 °C. Scanning transmission electron microscopy investigation shows that the h-BN multilayer has a unique aligned porous stacking nature, which is essential for the character of being transparent to atomic hydrogen but impermeable to hydrogen molecules. In addition, we successfully demonstrated the extraction of hydrogen gases from gaseous compounds or mixtures containing hydrogen element. The successful production of hydrogen bubbles on h-BN flakes has potential for further application in nano/micro-electromechanical systems and hydrogen storage.

Magnetoelectric and HR-STEM investigations on eutectic CoFe2O4–Ba1–xSrxTiO3 composites

Multiferroic Ba1–xSrxTiO3–CoFe2O4 (x = 0.03, 0.05) composites with rarely investigated 3-3 connectivity were prepared by eutectic crystallization in an optical floating zone furnace. High-resolution scanning transmission electron microscopy investigations of the CoFe2O4–BaTiO3 interface revealed an almost perfect connection between both components. These micrographs also showed that the impact of post-annealing in air was much larger than expected and resulted in formation of small BaTiO3 inclusions in the CoFe2O4 phase. The magnetoelectric coefficient αME was studied in detail with respect to its dependence on the static magnetic field, the frequency of the driving AC-field and temperature. Furthermore, the influence of different growth rates (5, 10 and 20 mm h−1), chemical composition, sample thickness and the alignment of electrical polarization and magnetic field (collinear or vertical) on the magnetoelectric properties were studied. The largest value of αME = 1.3 mV Oe−1 cm−1 was found for a sample grown at 5 mm h−1. For even slower growth rates, a higher Sr content was required to avoid the formation of impurity phases leading to a decrease of αME.

Rational Kinetics Control toward Universal Growth of 2D Vertically Stacked Heterostructures.

The rational control of the nucleation and growth kinetics to enable the growth of 2D vertical heterostructure remains a great challenge. Here, an in-depth study is provided toward understanding the growth mechanism of transition metal dichalcogenides (TMDCs) vertical heterostructures in terms of the nucleation and kinetics, where active clusters with a high diffusion barrier will induce the nucleation on top of the TMDC templates to realize vertical heterostructures. Based on this mechanism, in the experiment, through rational control of the metal/chalcogenide ratio in the vapor precursors, effective manipulation of the diffusion barrier of the active clusters and precise control of the heteroepitaxy direction are realized. In this way, a family of vertical TMDCs heterostructures is successfully designed. Optical studies and scanning transmission electron microscopy investigations exhibit that the resulting heterostructures possess atomic sharp interfaces without apparent alloying and defects. This study provides a deep understanding regarding the growth mechanism in terms of the nucleation and kinetics and the robust growth of 2D vertical heterostructures, defining a versatile material platform for fundamental studies and potential device applications.

Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM): From Scanning Nanodiffraction to Ptychography and Beyond.

Scanning transmission electron microscopy (STEM) is widely used for imaging, diffraction, and spectroscopy of materials down to atomic resolution. Recent advances in detector technology and computational methods have enabled many experiments that record a full image of the STEM probe for many probe positions, either in diffraction space or real space. In this paper, we review the use of these four-dimensional STEM experiments for virtual diffraction imaging, phase, orientation and strain mapping, measurements of medium-range order, thickness and tilt of samples, and phase contrast imaging methods, including differential phase contrast, ptychography, and others.

Oxygen filling voids and direct element imaging of metastable ZnSb structures by aberration-corrected scanning transmission electron microscopy

Void formation in metastable ZnSb structures has yet to be investigated. We developed a technique to fill voids caused by Sb diffusion, through the introduction of oxygen dopants into the metastable ZnSb phase. This filling of voids enables their role in phase-change processes to be accurately investigated. Our scanning transmission electron microscopy investigations showed that oxygen dopants repaired voids through recovery of bond connectivity. This approach stabilizes a metastable phase and avoids the metastable-to-stable phase transformation. Our characterization of void defects sheds light on the long-standing puzzle of the metastable-to-stable phase transition mechanism.

Gallstone-Formation-Inspired Bimetallic Supra-nanostructures for Computed-Tomography-Image-Guided Radiation Therapy.

Inspired by the gallstone formation mechanism, we report a fast one-pot synthesis of high-surface-area bimetallic hierarchical supra-nanostructures. As gallstones are generated from metal cholate complexes, cholate bile acid molecules with Au/Ag metal precursors formed stable nanocomplexes aggregated with metal Au ions and preformed ~2 nm silver halide nanoparticles before reduction. When a reducing agent was added, the metal cholate nanocomplexes quickly formed noble bimetallic hierarchical supra-nanostructures. The morphology of bimetallic supra-nanostructures could be tailored by changing the feeding ratio of each metal precursor. In situ synchrotron small-angle X-ray scattering measurement with a custom-designed reaction cell showed two-step growth and attachment behavior toward hierarchical supra-nanostructures from the gallstone-formation-inspired metal cholate nanocomplexes in a 60 s reaction. Additional wide-angle X-ray scattering, X-ray absorption near-edge structure, in situ Fourier transform infrared, and high-resolution scanning transmission electron microscopy investigations subsequently revealed the mechanism for the evolution of bimetallic hierarchical supra-nanostructures. The gallstone-formation-inspired synthesis mechanism can be universally applied to other metals, for example, Pt-Ag and Pd-Ag bimetallic nanostructures. Finally, the synthesized high-surface-area bimetallic supra-nanostructures demonstrated significantly enhanced X-ray computed tomography imaging contrast and radiosensitizing effect for a potential image-guided nanomedicine application. We believe that our synthetic method inspired by gallstone formation and understanding represents an important step toward the development of hierarchical nanoparticles for various applications.

One‐Pot Synthesis of Au Embedded ZnO Nanorods Composite Heterostructures with Excellent Photocatalytic Properties

AbstractHere, we have designed a noble composite nanostructure by embedding gold (Au) nanoparticles into zinc oxide (ZnO) nanorods surface in one pot synthesis as a photocatalyst. The formation the composite nanostructure was confirmed by X‐ray diffraction, X‐ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy and transmission electron microscopy investigations. Microscopic studies suggest that spherical Au nanoparticles are nucleated on the ZnO nanorods surface. XPS shows shifting of peak positions towards higher binding energy which indicates charge transfer from ZnO to Au in the composite nanostructures. This is unambiguously confirmed by the steady state spectroscopic studies. It was found that 95.7% (1.8 × 10−5 mM) of Methylene blue dye is degraded by the composite nanostructure after 140 min under UV light illumination and the apparent rate constant was found to be 0.015 min−1. This new class of Au nanoparticles embedded ZnO nanorods composite nanostructure opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications.

High-Temperature Thermoelectricity in LaNiO3-La2CuO4 Heterostructures.

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal K2NiF4-type insulating La2CuO4 (LCO) and perovskite-type conductive metallic LaNiO3 (LNO) layers with different thicknesses to assess the heterostructure-thermoelectric property-relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contribution of the individual layers where possible interfacial contributions cannot be ruled out. High-resolution scanning transmission electron microscopy investigations showed that a substitutional solid solution of La2(CuNi)O4 was formed when the thickness of the constituent layers was decreased.

Interface enhanced superconductivity in monolayer FeSe films on MgO(001): charge transfer with atomic substitution

Interface enhanced superconductivity over 50 K has been discovered in monolayer FeSe films grown on several TiO2-terminated oxide substrates. Whether such phenomenon exists in other oxide substrates remains an extremely interesting topic. Here we report enhanced superconductivity with an onset transition temperature of 18 K in monolayer FeSe on MgO(001) substrate by transport measurement. Scanning transmission electron microscopy investigation on the interface structure indicates that FeSe films grow epitaxially on MgO(001) and that overlayer Fe atoms diffuse into the top two layers of MgO and substitute Mg atoms. Our density functional theory calculations reveal that this substitution promotes the charge transfer from the MgO substrate to the FeSe films, an essential process that also occurs in monolayer FeSe on TiO2-terminated oxides and contributes to the enhanced superconductivity therein. Our finding suggests that superconductivity enhancement in monolayer FeSe films on oxides substrates is rather general as long as charge transfer is allowed at the interface, thus pointing out an explicit direction for searching for new high temperature superconductivity by interface engineering.

MOVPE Growth of Smooth and Homogeneous Al0.8Ga0.2N:Si Superlattices as UVC Laser Cladding Layers

Smooth, uniform, and conductive AlxGa1−xN layers with high aluminum mole fractions of x > 0.7 are required as cladding layers for laser diodes emitting in the deep ultraviolet spectral region. In this paper, the growth of silicon‐doped AlGaN/AlGaN superlattices by metal‐organic vapor phase epitaxy is investigated and compared to bulk AlGaN layers. It is found that the superlattice approach enables the growth of AlGaN layers with improved lateral uniformity of composition and strain state. In order to reduce the surface roughness, growth interruptions between the superlattice layers are investigated. With increasing growth interruption time, the average aluminum content is increasing and the superlattice period thickness is decreasing. Scanning transmission electron microscopy investigations show that the growth interruptions lead to more abrupt interfaces and to the formation of one to two monolayer thin aluminum‐rich AlGaN layers at each interface. This also leads to a significantly smoother surface morphology. Low resistivities of 0.025 Ω cm are obtained for AlGaN:Si superlattices with average aluminum content of x = 0.8. The influence of the surface morphology of the AlGaN cladding layers on optically pumped laser heterostructures is investigated. By using a smooth AlGaN super­lattice, the lasing threshold decreases by a factor of 2 compared to lasers with bulk AlGaN cladding layers.