We present a comprehensive investigation of reconstructions on β -Ga 2 O 3 ( 001 ) combining first-principles calculations with experimental observations. Using atomistic thermodynamics and replica-exchange grand-canonical molecular dynamics simulations, we explore the configurational space of possible reconstructions under varying chemical potentials of oxygen and gallium. Our calculations reveal several stable surface reconstructions, most notably a previously unreported 1 × 2 reconstruction consisting of paired GaO 4 tetrahedra that exhibits remarkable stability across a wide range of experimental growth conditions. In this reconstruction, two Ga atoms share one oxygen bond and are separated by a distance of 2.64 Å along the [010] direction. High-angle annular dark-field scanning transmission electron microscopy imaging of homoepitaxially grown (001) layers is consistent with the predicted structure. Additional investigations of possible indium substitution at the surface sites, which can occur during indium-mediated metal-exchange catalysis via molecular beam epitaxial growth, reveal a cooperative effect in In incorporation, with distinct stability regions for In-substituted structures under O-rich conditions. Our findings provide an understanding for controlling surface properties during epitaxial growth of β -Ga 2 O 3 ( 001 ) .

Recovery of epitaxial AlN films on sapphire at high temperatures is now an established process to produce pseudo-substrates with high crystalline perfection, which can be used to grow epitaxial structures for UV-light-emitting devices. To elucidate the elementary mechanisms taking place during the thermal treatment of MOVPE-grown films, we studied as-grown and annealed samples combining transmission electron microscopy techniques and secondary ion mass spectrometry (SIMS). By using SIMS, we find a temperature-dependent increase in the overall oxygen content of the films, which cannot be explained quantitatively with either simple bulk or pure pipe-diffusion from the sapphire substrate. Instead, we propose a lateral outdiffusion from the dislocation cores to explain qualitatively and quantitatively the presence of observed oxygen concentration plateaus. Based on the formation enthalpy of various atomic defects and complexes found in literature, we conclude that the di-oxygen/aluminum vacancy complex (VAl–2ON) is the dominant point defect controlling the annealing process. The formation of this defect at high temperatures promotes a dislocation core climb process, which causes the annihilation/fusion of the threading dislocation segments.

Homoepitaxial growth of SrTiO 3 thin films on 0.5 wt% niobium doped SrTiO 3 (100) substrates with high structural perfection was developed using liquid-delivery spin metal–organic vapor phase epitaxy (MOVPE). Exploiting the advantage of adjusting the partial pressures of the individual constituents independently, we tuned the Sr/Ti ratio of the gas phase for realizing, stoichiometric, as well as Sr deficient layers. Quantitative energy dispersive X-ray spectroscopy in a scanning transmission electron microscope confirm Sr deficiency of up to 20% in nominally off-stoichiometrically grown films. Our MOVPE process allows to grow such layers in phase pure state and without extended defect formation. Indications for oxygen deficiency could not be identified. Sr deficient layers exhibit an increased permittivity of ɛ r = 202 and a larger vertical lattice parameter. Current–voltage characteristics (IVCs) of metal–oxide–semiconductor (Pt/SrTiO 3 /SrTiO 3 :Nb) structures reveal that Sr deficient SrTiO 3 films show an intrinsic resistive switching with on–off ratios of three orders of magnitude at RT and seven orders of magnitude at 10 K. There is strong evidence that a large deviation from stoichiometry pronounces the resistive switching behavior. IVCs conducted at 10 K indicate a defect-based mechanism instead of mass transport by ion diffusion. This is supported by in-situ STEM investigations that show filaments to form at significant higher voltages than those were resistive switching is observed in our samples.

In this work, we study the thermal degradation of In-rich InxGa1-xN quantum wells (QWs) and propose explanation of its origin based on the diffusion of metal vacancies. The structural transformation of the InxGa1-xN QWs is initiated by the formation of small initial voids created due to agglomeration of metal vacancies diffusing from the layers beneath the QW. The presence of voids in the QW relaxes the mismatch stress in the vicinity of the void and drives In atoms to diffuse to the relaxed void surroundings. The void walls enriched in In atoms are prone for thermal decomposition, what leads to a subsequent disintegration of the surrounding lattice. The phases observed in the degraded areas of QWs contain voids partly filled with crystalline In and amorphous material, surrounded by the rim of high In-content InxGa1-xN or pure InN; the remaining QW between the voids contains residual amount of In. In the case of the InxGa1-xN QWs deposited on the GaN layer doped to n-type or on unintentionally doped GaN, we observe a preferential degradation of the first grown QW, while doping of the underlying GaN layer with Mg prevents the degradation of the closest InxGa1-xN QW. The reduction in the metal vacancy concentration in the InxGa1-xN QWs and their surroundings is crucial for making them more resistant to thermal degradation.

Epitaxial perovskite oxide interfaces with different symmetry of the epitaxial layers have attracted considerable attention due to the emergence of novel behaviors and phenomena. In this paper, we show by aberration-corrected transmission electron microscopy (TEM) that orthorhombic LaInO3 films grow in form of three different types of domains on the cubic BaSnO3 pseudosubstrate. Quantitative evaluation of our TEM data shows that c(pc)-oriented and a(pc)/b(pc)-oriented domains are present with similar probability. While continuum elasticity theory suggests that c(pc)-oriented domains should exhibit a significantly higher strain energy density than a(pc)/b(pc)-oriented domains, density-functional calculations confirm that c(pc)- and a(pc)-oriented domains on BaSnO3 have similar energies.

We present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate (SrTiO3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the essentially omnipresent 180–190 ps lifetime component is most likely the TiSr antisite defect, possibly coupled with one or more oxygen vacancies, supporting the importance of the TiSr antisite related defects in SrTiO3.

In this work, we emphasize the important contribution of the 2s Bloch wave state to the properties of a STEM electron probe propagating on an atomic column. For a strong enough column potential, the confinement of the 2s state leads to a long-period oscillation of the electron wave function, which is reflected in the resulting STEM-HAADF intensity. We show how this influences STEM composition quantification even at large thicknesses. We found additionally that the excitation of the 2s state affects the intensity of alloys where long-range order phenomena are present, which in turn provides a way to probe the degree of order in alloys.

Crystalline silicon is grown onto an amorphous silicon (a-Si) seed layer from a liquid tin solution (steady state liquid phase epitaxy, SSLPE). To investigate the crystallization of embedded a-Si during our process, we adapted Raman measurements for fast mapping, with a dwell time of just one second per single measurement. A purposely developed imaging algorithm which performs point-by-point gauss fitting provides adequate visualization of the data. We produced scans of a-Si layers showing crystalline structures formed in the a-Si matrix during processing. Compared to scanning electron microscopy images which reveal merely the topography of the grown layer, new insights are gained into the role of the seed layer by Raman mapping. As part of a series of SSLPE experiments, which were interrupted at various stages of growth, we show that plate-like crystallites grow laterally over the a-Si layer while smaller, randomly orientated crystals arise from the a-Si layer. Results are confirmed by an in situ TEM experiment of the metal-induced crystallization. Contrary to presumptions, initially formed surface crystallites do not originate from the seed layer and are irrelevant to the final growth morphology, since they dissolve within minutes due to Ostwald ripening. The a-Si layer crystallizes within minutes as well, and crystallites of the final morphology originate from seeds of this layer.

The metal-insulator transition of NbO2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbOx that may be inherently different to crystalline NbO2. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at similar to 810 degrees C. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.

Journal Article In situ Transmission Electron Microscopy Annealing for Crystallization and Phase Stability Studies in the Ga2O3-In2O3 System Get access Charlotte Wouters, Charlotte Wouters Leibniz-Institut für Kristallzüchtung, Berlin, Germany Corresponding author: charlotte.wouters@ikz-berlin.de Search for other works by this author on: Oxford Academic Google Scholar Toni Markurt, Toni Markurt Leibniz-Institut für Kristallzüchtung, Berlin, Germany Search for other works by this author on: Oxford Academic Google Scholar Oliver Bierwagen, Oliver Bierwagen Paul-Drude-Institut für Festkörperelektronik, Berlin, Germany Search for other works by this author on: Oxford Academic Google Scholar Christopher Sutton, Christopher Sutton Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany Search for other works by this author on: Oxford Academic Google Scholar Martin Albrecht Martin Albrecht Leibniz-Institut für Kristallzüchtung, Berlin, Germany Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1890–1891, https://doi.org/10.1017/S1431927619010183 Published: 01 August 2019

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.

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. We present results on the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.

There is a growing interest in exploiting the functional properties of niobium oxides in general and of the T-Nb2O5 polymorph in particular. Fundamental investigations of the properties of niobium oxides are, however, hindered by the availability of materials with sufficient structural perfection. It is expected that high-quality T-Nb2O5 can be made using heteroepitaxial growth. Here, we investigated the epitaxial growth of T-Nb2O5 on a prototype perovskite oxide, SrTiO3. Even though there exists a reasonable lattice mismatch in one crystallographic direction, these materials have a significant difference in crystal structure: SrTiO3 is cubic, whereas T-Nb2O5 is orthorhombic. It is found that this difference in symmetry results in the formation of domains that have the T-Nb2O5 c-axis aligned with the SrTiO3 <001>(s) in-plane directions. Hence, the number of domain orientations is four and two for the growth on (100)(s)- and (110)(s)-oriented substrates, respectively. Interestingly, the out-of-plane growth direction remains the same for both substrate orientations, suggesting a weak interfacial coupling between the two materials. Despite challenges associated with the heteroepitaxial growth of T-Nb2O5, the T-Nb2O5 films presented in this paper are a significant improvement in terms of structural quality compared to their polycrystalline counterparts.

We have developed an in situ method for removing a native silicon oxide layer from an amorphous silicon (a-Si) surface using a UV laser. The a-Si film containing crystalline silicon seeds is used for the subsequent growth of crystalline Si layers by steady-state liquid phase epitaxy (SSLPE). The main goal of this technique is to grow crystalline silicon layers on low-cost glass substrates which can be used as absorber layers for thin film solar cells. We have investigated the interaction between a-Si and laser pulses as well as the growth results by scanning force microscopy (SFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The heating of the a-Si surface by a laser pulse is modelled by numerical simulations using a finite element approach in COMSOL-Multiphysics. The simulations verify that the laser pulse heats a-Si to temperatures sufficient for the thermal desorption of the native oxide layer but lower than both the crystallization temperature of a-Si and the glass transition temperature.

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 article number 1800005 by Christian Kuhn and co-workers, the growth of silicon doped AlGaN/ AlGaN superlattices by metal-organic vapor phase epitaxy is investigated and compared to bulk AlGaN layers. The authors find 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 1 to 2 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.

A novel concept to obtain a ferroelectric material with enhanced piezoelectric properties is proposed. This approach is based on the combination of two pathways: (i) the evolution of a ferroelectric monoclinic phase and, (ii) the coexistence of different types of ferroelectric domains leading to polarization discontinuities at the domain walls. Each of these pathways enables polarization rotation in the material which is responsible for giant piezoelectricity. Targeted incorporation of anisotropic epitaxial lattice strain is used to implement this approach. The feasibility of our concept is demonstrated for K0.9Na0.1NbO3 epitaxial layers grown on NdScO3 substrates where the coexistence of (100)pc and (001)pc pseudocubic oriented monoclinic domains is experimentally verified. This coexistence results in a complex periodic domain pattern with alternating emergence of ferroelectric in-plane a1a2 and inclined MC monoclinic phases, which differ in the direction of the electrical polarization vector. Our approach opens the possibility to exploit ferroelectric properties in both vertical and lateral directions and to achieve enhanced piezoelectric properties in lead-free material caused by singularities at the domains walls.

2‐dimensional nanostructures consisting only of a single layer of material have attracted great research interest in the past years. Among them are e.g. transition metal dichalcogenides like MoSe 2 , WS 2 and their ternary alloys. But also In x Ga 1‐x N/GaN short period superlattices (SPSL) built up of In x Ga 1‐x N monolayer quantum wells belong to this material class [1]. For such kind of nanostructures relevant structural parameters, which determine material properties and thus device performance, are interface quality, alloy composition and possible ordering phenomena. Characterization of these quantities, at atomic scale, is commonly performed by high angle annular dark field imaging using a scanning transmission electron microscope (STEM HAADF). In the past years even quantitative composition analysis at atomic scale by STEM HAADF imaging has been demonstrated for various material systems, including In x Ga 1‐x N quantum wells [2]. What makes STEM HAADF imaging so attractive for that purpose is the generally valid monotonic relationship between image intensity and the mean atomic number Z of the probed material (commonly expressed by the Z 1.7 rule of thumb for the image intensity). However, in our combined experimental and theoretical STEM HAADF analysis of In 0.33 Ga 0.67 N/GaN SPSL consisting of ordered In 0.33 Ga 0.67 N monolayers we have observed an anomalous contrast behavior. Within the ordered In 0.33 Ga 0.67 N monolayers In atoms are arranged in a periodic √3x√3R30° structure, resulting in pure In atomic columns in a GaN matrix along the cross‐sectional viewing directions of the wurtzite lattice (see Fig. 1). This has been experimentally confirmed ex‐situ by high resolution (S)TEM and in‐situ by reflection high‐energy electron diffraction (RHEED). Expecting intuitively a high contrast in high‐resolution STEM HAADF images of the ordered In 0.33 Ga 0.67 N monolayers, the experimental contrast between pure In and Ga atomic columns, however, was far below the Z 1.7 rule of thumb. To verify this result, we have performed frozen phonon simulations of a relaxed structure model consisting of a √3x√3R30° ordered In 0.33 Ga 0.67 N monolayer coherently embedded in a GaN matrix. Although the simulations agree with our experimental finding, even on a quantitative level, the explanation for the low contrast is far from intuitive. Even more surprisingly, for specimen thicknesses above 45 nm a contrast inversion occurs, i.e. the peak intensity of pure In atomic columns becomes lower than that of adjacent Ga atomic columns (see Fig. 2). Our frozen phonon simulations reveal that the origin for this anomalous contrast behavior lies in a strongly enhanced de‐channeling of the electron probe if it is positioned on the In atomic column of the ordered In 0.33 Ga 0.67 N monolayer. This in turn is caused by a complex interplay of increased disorder in the direct vicinity of the In atomic column in terms of chemistry (In atomic column is surrounded by material with a different atomic potential) and lattice periodicity (stronger local distortions around the In column because of differences in the In‐N vs. Ga‐N bond length).

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. GaN or ZnO on sapphire are prominent examples for that. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. In this work, we study the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy as an example. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.