Inversion Domain Research Articles

(Invited, Digital Presentation) Wafer-Scale Epitaxial Growth of Transition Metal Dichalcogenide Monolayers on Sapphire for Large Area Device Applications

Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on c-plane sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in twin boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is demonstrated on 2” diameter c-plane sapphire substrates enabling large area transfer of monolayers for characterization and device fabrication and testing. Applications for wafer-scale TMD monolayers in nanoelectronics, sensing and photonics will be discussed.

In-Grain Ferroelectric Switching in Sub-5 nm Thin Al0.74 Sc0.26 NFilms at 1 V.

Analog switching in ferroelectric devices promises neuromorphic computing with the highest energy efficiency if limited device scalability can be overcome. To contribute to a solution, one reports on the ferroelectric switching characteristics of sub-5 nm thin Al0.74 Sc0.26 Nfilms grown on Pt/Ti/SiO2 /Siand epitaxial Pt/GaN/sapphiretemplates by sputter-deposition. In this context, the study focuses on the following major achievements compared to previously available wurtzite-type ferroelectrics: 1) Record low switching voltages down to 1V are achieved, which is in a range that can be supplied by standard on-chip voltage sources. 2) Compared to the previously investigated deposition of ultrathin Al1-x Scx Nfilms on epitaxial templates, a significantly larger coercive field (Ec ) to breakdown field ratio is observed for Al0.74 Sc0.26 Nfilms grown on silicon substrates, the technologically most relevant substrate-type. 3) The formation of true ferroelectric domains in wurtzite-type materials is for the first time demonstrated on the atomic scale by scanning transmission electron microscopy (STEM) investigations of a sub-5 nm thin partially switched film. The direct observation of inversion domain boundaries (IDB) within single nm-sized grains supports the theory of a gradual domain-wall driven switching process in wurtzite-type ferroelectrics. Ultimately, this should enable the analog switching necessary for mimicking neuromorphic concepts also in highly scaleddevices.

Virtual data-based generative optimization using domain-adaptive designable data augmentation (DADDA): Application to electric vehicle design

In the field of virtual product development, product performance is evaluated at an early stage through metamodel-based optimization using simulation, reducing development time and cost, and improving product quality. However, it takes a lot of time to construct a simulation model. The model must go through calibration and validation process to reduce errors with actual systems. This study proposes a novel engineering design process that can perform virtual data-based generative optimization by adapting the design domain of existing systems to those of new systems without requiring many simulations. A domain-adaptive designable data augmentation (DADDA) algorithm is proposed using inverse generator-implemented data augmentation, domain adaptation, and design optimization techniques. The DADDA algorithm can quickly provide optimal design variables for new systems compared to the genetic algorithm-based approximate optimization. The proposed process was applied to electric vehicle design. Driving responses and design variables related to safety performance were selected, and a small amount of training data was obtained using Modelica-based electric vehicle. As a result, it was shown that DADDA could significantly reduce the time required to derive optimal design variables by approximately 53% compared with the existing method. In addition, it is possible to derive the optimal performance for a new electric vehicle, which is approximately 21% better than that of an existing vehicle.

A Transceiver-Configuration-Independent 2-D Electromagnetic Full-Wave Inversion Scheme Based on End-to-End Artificial Neural Networks

This communication presents a novel end-to-end artificial neural network (ANN) based two-dimensional (2-D) electromagnetic (EM) full-wave inversion (FWI) scheme in which the transceiver configurations are allowed to be different in training and testing. The equivalent current on the boundary of the numerical computational domain bridges the different scattered field data excited or recorded by different transceivers used in the training and testing. The transformation is accomplished through the following three steps: First, the fictitious equivalent current is solved from the scattered field recorded in the testing by multiplying it with the inverse of the radiation matrix whose kennel is the Green’s function of the background medium; then, let the obtained fictitious equivalent current radiate under the configuration of the transceivers adopted during training; finally, the scattered field data recorded by the receivers used in the training are input into the trained ANN to predict the dielectric parameters of the scatterers enclosed in the inversion domain. This rigorous transformation enables the end-to-end ANN training and testing for FWI to be implemented for different transceiver configurations and has many potential applications. Several numerical examples are used to validate the feasibility of the proposed inversion scheme.

Molecular dynamics of the ERRγ ligand-binding domain bound with agonist and inverse agonist.

Estrogen-related receptor gamma (ERRγ), the latest member of the ERR family, does not have any known reported natural ligands. Although the crystal structures of the apo, agonist-bound, and inverse agonist-bound ligand-binding domain (LBD) of ERRγ have been solved previously, their dynamic behavior has not been studied. Hence, to explore the intrinsic dynamics of the apo and ligand-bound forms of ERRγ, we applied long-range molecular dynamics (MD) simulations to the crystal structures of the apo and ligand-bound forms of the LBD of ERRγ. Using the MD trajectories, we performed hydrogen bond and binding free energy analysis, which suggested that the agonist displayed more hydrogen bonds with ERRγ than the inverse agonist 4-OHT. However, the binding energy of 4-OHT was higher than that of the agonist GSK4716, indicating that hydrophobic interactions are crucial for the binding of the inverse agonist. From principal component analysis, we observed that the AF-2 helix conformation at the C-terminal domain was similar to the initial structures during simulations, indicating that the AF-2 helix conformation is crucial with respect to the agonist or inverse agonist for further functional activity of ERRγ. In addition, we performed residue network analysis to understand intramolecular signal transduction within the protein. The betweenness centrality suggested that few of the amino acids are important for residue signal transduction in apo and ligand-bound forms. The results from this study may assist in designing better therapeutic compounds against ERRγ associated diseases.

Moiré patterns and inversion boundaries in graphene/hexagonal boron nitride bilayers

In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moir\'e patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moir\'e patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.

About switching of polarization in a thin ferroelectric film

By using the Ginzburg–Landau theory in the nonlinear case, the shape of the inverse domain nucleus in a thin ferroelectric film in a weak electric field is determined. The dependencies of the critical size of the inverse domain nucleus on the film thickness, the strength of external electric field, the temperature and the parameter [Formula: see text], which determines the polarization pinning at the film planes, are calculated. We have established that when the parameter [Formula: see text] increases above a certain critical value, which depends on film thickness and external field strength, the polarization switches uniformly without domain formation. In the framework of Zeldovich theory of nucleation, the average repolarization velocity has been determined as a function of the film thickness, the external field strength and the parameter [Formula: see text].

Origin of extra diffraction spots for high crystalline alpha-Ga2O3

This work has investigated the microstructure characteristics of high-quality alpha-Ga2O3 thin film grown on the Al2O3 single crystal substrate membrane. Hetero-epitaxial alpha Ga2O3 crystals reveal the formation of a three-fold symmetry at the initial stage of the growth by the oxygen template provided by the Al2O3. Inversion domains are found, and they have a 180° inverted configuration from the surroundings. These IDs lead to extra diffraction spots when observed along [110] and [010].

Polarity Control by Inversion Domain Suppression in N-Polar III-Nitride Heterostructures

Nitrogen-polar III-nitride heterostructures offer advantages over metal-polar structures in high frequency and high power applications. However, polarity control in III-nitrides is difficult to achieve as a result of unintentional polarity inversion domains (IDs). Herein, we present a comprehensive structural investigation with both atomic detail and thermodynamic analysis of the polarity evolution in low- and high-temperature AlN layers on on-axis and 4° off-axis carbon-face 4H-SiC (0001̅) grown by hot-wall metal organic chemical vapor deposition. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrate misorientation angle in order to achieve the desired growth mode and polarity. We demonstrate that IDs are completely suppressed for high-temperature AlN nucleation layers when a step-flow growth mode is achieved on the off-axis substrates. We employ this approach to demonstrate high quality N-polar epitaxial AlGaN/GaN/AlN heterostructures.

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.

A Robust Hierarchical Domain Constraint Strategy for Multiscale Contrast Objects Reconstruction

A robust hierarchical domain constraint (HDC) strategy is proposed for the multi-scale contrast objects reconstruction in this letter. The Monte Carlo method and point evaluation are adopted and modified for the classification of background and scatterers cells, the parameters of the background cells are set as the same as the actual background and removed from the inversion. The major work of the HDC strategy is to divide the inversion process into several stages, and the classification of cells is evaluated separately at each stage, making the classification reversible. And the HDC can effectively classify the inversion domain of low-contrast objects without negligence in the multi-scale contrast objects case, as well as the continuously varying inhomogeneous objects’ low-contrast parts. Another advantage of HDC is the threshold determining the classification of cells can be flexibly chosen. This robust constraint of the inversion domain compresses the solution space so that the ill-posedness of the inverse problem is mitigated, and better accuracy thus is achieved. In this letter, the HDC strategy is applied to the Variational Born Iterative Method (VBIM), numerical results validate the accuracy, robustness, and antinoise ability of the proposed VBIM-HDC.

Optical properties of ZnFe2O4 nanoparticles and Fe-decorated inversion domain boundaries in ZnO.

The maximum efficiency of solar cells utilizing a single layer for photovoltaic conversion is given by the single junction Shockley-Queisser limit. In tandem solar cells, a stack of materials with different band gaps contribute to the conversion, enabling tandem cells to exceed the single junction Shockley-Queisser limit. An intriguing variant of this approach is to embed semiconducting nanoparticles in a transparent conducting oxide (TCO) solar cell front contact. This alternative route would enhance the functionality of the TCO layer, allowing it to participate directly in photovoltaic conversion via photon absorption and charge carrier generation in the nanoparticles. Here, we demonstrate the functionalization of ZnO through incorporation of either ZnFe2O4 spinel nanoparticles (NPs) or inversion domain boundaries (IDBs) decorated by Fe. Diffuse reflectance spectroscopy and electron energy loss spectroscopy show that samples containing spinel particles and samples containing IDBs decorated by Fe both display enhanced absorption in the visible range at around 2.0 and 2.6 eV. This striking functional similarity was attributed to the local structural similarity around Fe-ions in spinel ZnFe2O4 and at Fe-decorated basal IDBs. Hence, functional properties of the ZnFe2O4 arise already for the two-dimensional basal IDBs, from which these planar defects behave like two-dimensional spinel-like inclusions in ZnO. Cathodoluminescence spectra reveal an increased luminescence around the band edge of spinel ZnFe2O4 when measuring on the spinel ZnFe2O4 NPs embedded in ZnO, whereas spectra from Fe-decorated IDBs could be deconvoluted into luminescence contributions from bulk ZnO and bulk ZnFe2O4.

Comparative studies of interatomic potentials for modeling point defects in wurtzite GaN

In this paper, a new version of the Stillinger–Weber (SW) potential for wurtzite GaN is presented, by which we systematically explore the structural and thermodynamical properties of native point defects and their complexes. In parallel, the semi-empirical Modified Embedded-Atom Method (MEAM) potential is selected for comparison. The SW and MEAM potentials are assessed by the reproduction of the fundamental properties of wurtzite GaN and by the ability to describe the inversion domain boundaries and the wurtzite–rocksalt phase transition. Then the structural search of native point defects and their complexes in GaN is implemented using both SW and MEAM potentials with the benchmark of Density Functional Theory (DFT) calculations. Besides vacancies and antisites, four N and five Ga interstitials are confirmed by refining the DFT calculations, among which two N split interstitials N+−N⟨21̄1̄0⟩ and N+−Ga⟨011̄0⟩, and two Ga split interstitials, Ga+−Ga⟨011̄0⟩−g and Ga+−N⟨011̄0⟩, are observed for the first time. The SW potential correctly predicts the octahedral occupation GaOct to be the most stable Ga interstitial, while the MEAM potential predicts the ground state of the N+−N⟨011̄0⟩ split interstitial (N+−N⟨011̄0⟩−g) as the most stable N interstitial. However, neither of the two potentials could simultaneously generate the most stable configurations of N and Ga interstitials. The investigations of point defect complexes reveal that N octahedral Frenkel [FrenkelOct(N)] and paired antisite (NGaGaN) defects are unstable and get converted into VN⊕N+−N⟨011̄0⟩−g configurations with different separations between VN and N+−N⟨011̄0⟩−g point defects based on the DFT calculations. The formation energies calculated by the DFT and SW potential demonstrate that Schottky, Ga octahedral Frenkel [FrenkelOct(Ga)], and VN⊕N+−N⟨011̄0⟩−g point defect complexes are energetically feasible and that they should not dissociate into two isolated point defects. In contrast, the MEAM potential predicts the dissociation to be exothermic for Schottky and VN⊕N+−N⟨011̄0⟩−g. Overall, the structural features concerned with N–N or Ga–Ga bonds relaxed by the SW potential are more consistent with DFT calculations than the MEAM counterpart.

Pressure and saturation changes estimated from extended elastic impedance properties using time-lapse seismic data: Enfield Field, NW Australia

The extended elastic impedance (EEI) concept has been used in the oil industry primarily for lithology and fluid prediction in exploration and development projects. We present a method of reservoir monitoring that identifies and maps changes in pressure and saturation in a producing reservoir by applying EEI to time-lapse seismic data. The method uses time-lapse seismic difference data rotated to specific EEI χ angles that are optimized for the changes expected in a given reservoir. One angle is found to be appropriate to identify predicted changes in saturation, using fluid-substitution models, while the other angle is found from rock-physics assumptions or laboratory measurements of fluid-pressure changes. Our technique is tested using time-lapse seismic data from the Enfield oil field in Australia's North West Shelf, with estimates of optimal EEI rotation angles χ based on log data and Biot-Gassmann modeling for the fluid changes (χ = +42°) and on rock-physics models fit to measurements made on core samples for the pressure changes (χ = –79°). Seismic reflectivity and inversion domains were used for comparison and analysis of the final rotated volumes. We used publicly available seismic data that had been recorded and processed in 2004 and 2007 when time-lapse processing was still being developed. The low reproducibility of these seismic data and the complexity of the reservoir architecture limited the extent of our interpretation and results. Nonetheless, our qualitative and quantitative results are encouraging and supported by field production data. This technically simple approach should be useful in the analysis of time-lapse seismic data processed by modern techniques and would help in the management of reservoirs using a straightforward and readily reproduced procedure.

Mg segregation at inclined facets of pyramidal inversion domains in GaN:Mg

Structural defects in Mg-doped GaN were analyzed using high-resolution scanning transmission electron microscopy combined with electron energy loss spectroscopy. The defects, in the shape of inverted pyramids, appear at high concentrations of incorporated Mg, which also lead to a reduction in free-hole concentration in Mg doped GaN. Detailed analysis pinpoints the arrangement of atoms in and around the defects and verify the presence of a well-defined layer of Mg at all facets, including the inclined facets. Our observations have resulted in a model of the pyramid-shaped defect, including structural displacements and compositional replacements, which is verified by image simulations. Finally, the total concentration of Mg atoms bound to these defects were evaluated, enabling a correlation between inactive and defect-bound dopants.

(Invited) Epitaxial Growth of Transition Metal Dichalcogenide Monolayers for Large Area Device Applications

Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on c-plane sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in twin boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is demonstrated on 2” diameter c-plane sapphire substrates enabling large area transfer of monolayers for characterization and device fabrication and testing. Applications for wafer-scale TMD monolayers in nanoelectronics, sensing and photonics will be discussed.

The Study of the Possibility of Applying Parallel Programming to the Algorithms of Space-Time Adaptive Processing

The article presents the description, assumptions and subsequent steps of the space-time adaptive processing (STAP) algorithms used as a signal processing tool in radars. The possibilities of object detection using the Sample Matrix Inversion (SMI) and Data Domain Least Squares (DDLS) algorithms were compared and showned. The article shows the impact of the use of parallel programming on the computation time of both algorithms. The main aim of this study was to propose an efficient method for the real-time implementation of the STAP algorithm in airborne radar systems. The idea of using parallel programming in STAP, supported only by the preliminary research results presented above, gives a real chance for the casual implementation of the STAP algorithm in a radar operating in close to real time mode.

N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD Part-II: Microstructural investigation

The present work is part-II of a study that addresses in detail the microstructural features of nitrogen-polar gallium nitride (N-polar GaN) deposited on c-plane sapphire wafers under the metalorganic chemical vapor deposition conditions described in the part-I work. The growth process was interrupted at various stages and characterized the depositions using transmission electron microscopy. The plan view diffraction study in combination with the Moiré fringe analysis revealed the presence of rotational domains of 4° in the low-temperature GaN nucleation layer. A crystallographic basis for the occurrence of these rotational domains is proposed based on the arrangement of surface atoms in the underlying sapphire substrate. Pure a dislocations are found to accommodate the twist between the rotational domains. In addition, Ga-polar inversion domains are observed in the matrix of N-polar GaN, and they originate at atomic level steps induced by the initial thermal cleaning and nitridation process of sapphire wafers at 1100°C. The rotational domains, the accommodating a dislocations and the inversion domains are replicated into the N-polar GaN epilayers.

Molecular beam homoepitaxy of N-polar AlN on bulk AlN substrates

N-polar AlN epilayers were grown on the N-face of single-crystal bulk AlN substrates by plasma-assisted molecular beam epitaxy. A combination of in situ thermal deoxidation and Al-assisted thermal desorption at high temperature aided in removing native surface oxides and impurities from the N-polar surface of the substrate enabling successful homoepitaxy. Subsequent epitaxial growth of the AlN layer on the in situ cleaned substrates, grown in a sufficiently high Al droplet regime, exhibited smooth surface morphologies with clean and wide atomic steps. KOH etch studies confirmed the N-polarity of the homoepitaxial films. Secondary ion mass spectrometry profiles show Si and H impurity concentrations below the noise levels, whereas O and C impurity concentrations of ∼8×1017 and ∼2×1017 atoms/cm3 are observed, respectively. Although the structural defect densities are low, they interestingly appear as inversion domains of different dimensionalities.

Enhanced high-temperature volume resistivity of AlN by doping

The addition of various oxide dopants for AlN were investigated aiming to increase the AlN volume resistivity at high temperature. It was confirmed that the volume resistivity of 1 mol% MgO doped AlN shows four orders of magnitude higher than that of non-doped AlN. To further clarify the effect of MgO doping, we sintered various kinds of AlN with different MgO concentration and grain sizes, and the microstructures were investigated by transmission electron microscopy. The results suggest the inversion domain boundary appeared in the MgO doped AlN might be the possible reason for the increase of volume resistivity.