Inversion Domain Research Articles

Continental-scale joint inversion of Alaska and Yukon gravity and magnetic data

The 3D inversion of potential field data constitutes an increasingly important method of interpretation of geophysical data. A generalized inversion method first discretizes the 3D earth models into cells of constant density, susceptibility, or magnetization vector. In the case of continental-scale geophysical data collected by a combination of land, airborne and satellite measurements, the survey area may cover thousands and even millions of square kilometres, which makes the size of the inversion domain and the number of inverse model parameters extremely large. It is well known that for potential field data the computational complexity increases linearly with the size of the problem. Even a small-sized 3D inversion of huge amounts of data to 3D earth models with hundreds of thousands of cells can exceed the memory available on a desktop computer. In the case of several millions of discretization cells, the memory requirements may exceed the capacity even of the PC clusters. The second obstacle is the amount of CPU time required to apply a huge, dense matrix of the forward modelling operator to the data and model vectors, even using parallel computing.

Origin and effective reduction of inversion domains in aluminum nitride grown by a sublimation method

Owing to the large differences in the chemical properties between Al and N polarities in aluminum nitride (AlN), the choice of the polar direction for crystal growth strongly affects not only the quality but also the shape (facet formation) of the grown crystal. In particular, N-polar (000−1) has been considered to be a more preferable direction than Al-polar (0001) for sublimation growth because compared to Al-polar (0001), N-polar (000−1) exhibits better stability at high growth rate (high supersaturation) conditions and enables easier lateral enlargement of the crystal. However, some critical growth conditions induce polarity inversion and hinder stable N-polar growth. Furthermore, the origin of the polarity inversion in AlN growth by the sublimation method is still unclear. To ensure stable N-polar growth without polarity inversion, the formation mechanism of the inversion domain during AlN sublimation growth must be elucidated. Therefore, herein, we demonstrate homoepitaxial growth on an N-polar seed and carefully investigate the obtained crystal that shows polarity inversion. Annular bright-field scanning transmission electron microscopy reveals that polarity is completely converted to the Al polarity via the formation of a 30nm thick mixed polar layer (MPL) just above the seed. Moreover, three-dimensional atom probe tomography shows the segregation of the oxygen impurities in the MPL with a high concentration of about 3atom%. Finally, by avoiding the incorporation of oxygen impurity into the crystal at the initial stage of the growth, we demonstrate an effective reduction (seven orders of magnitude) of the inversion domain boundary formation.

AlN grown on Si(1 1 1) by ammonia-molecular beam epitaxy in the 900–1200 °C temperature range

We present a comprehensive study of AlN growth on Si(111) substrate by gas source molecular beam epitaxy with ammonia as nitrogen precursor in the high temperature range. We first demonstrate that the observation of the silicon 7×7 surface reconstruction by reflection high energy electron diffraction can be misleading as this technique is not sensitive to low density surface defects like SiC crystallites. A careful in situ cleaning procedure with annealing cycles at 1100°C allows getting rid of any surface defects, as shown by atomic force microscopy imaging. Then, we explore the effect of the growth temperature on the surface morphology and structural properties of 100nm thick AlN epilayers. At 1200°C, the growth proceeds with the step flow mode regime, which induces spiral-growth around screw-type dislocations and therefore surface roughening. On the other hand, a smooth surface morphology can be achieved by setting the temperature at 1100°C, which corresponds to the growth mode transition from two-dimensional nucleation to step flow. A further decrease of the growth temperature to 900°C leads to surface defects ascribed to polarity inversion domains. Similar defects are observed for growths performed at 1100°C when the NH3 flow is reduced below 100sccm. This points out the sensitivity of AlN to the surface stoichiometry.

Energy-Efficient Versatile Memories With Ferroelectric Negative Capacitance by Gate-Strain Enhancement

In this brief, we reported a ferroelectric versatile memory with strained-gate engineering. The versatile memory with high-strain-gate showed a >40% improvement on ferroelectric hysteresis window, compared to low-strain case. The high compressive stress induced from high nitrogen-content TaN enhances monoclinic-to-orthorhombic phase transition to reach stronger ferrolectric polarization and lower depolarization field. The versatile memory featuring ferroelectric negative capacitance exhibited excellent transfer characteristics of the sub-60-mVdec subthreshold swing, ultralow off-state leakage of $\mu \text{m}$ and $> 10^{\mathsf {8}}$ on/off current ratio. Furthermore, the ferroelectric versatile memory can be switched by ±5 V under 20-ns speed for a long endurance cycling (~1012 cycles). The low-power operation can be ascribed to the amplification of the surface potential to reach the strong inversion and fast domain polarization at the correspondingly low program/erase voltages.

Grain Boundary Structures and Collective Dynamics of Inversion Domains in Binary Two-Dimensional Materials.

Understanding and controlling the properties and dynamics of topological defects is a lasting challenge in the study of two-dimensional materials, and is crucial to achieve high-quality films required for technological applications. Here grain boundary structures, energies, and dynamics of binary two-dimensional materials are investigated through the development of a phase field crystal model that is parametrized to match the ordering, symmetry, energy, and length scales of hexagonal boron nitride. Our studies reveal some new dislocation core structures for various symmetrically and asymmetrically tilted grain boundaries, in addition to those obtained in previous experiments and first-principles calculations. We also identify a defect-mediated growth dynamics for inversion domains governed by the collective atomic migration and defect core transformation at grain boundaries and junctions, a process that is related to inversion symmetry breaking in binary lattice.

Linearized 3-D Electromagnetic Contrast Source Inversion and Its Applications to Half-Space Configurations

One of the main computational drawbacks in the application of 3-D iterative inversion techniques is the requirement of solving the field quantities for the updated contrast in every iteration. In this paper, the 3-D electromagnetic inverse scattering problem is put into a discretized finite-difference frequency-domain scheme and linearized into a cascade of two linear functionals. To deal with the nonuniqueness effectively, the joint structure of the contrast sources is exploited using a sum-of-l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -norm optimization scheme. A cross-validation technique is used to check whether the optimization process is accurate enough. The total fields are, then, calculated and used to reconstruct the contrast by minimizing a cost functional defined as the sum of the data error and the state error. In this procedure, the total fields in the inversion domain are computed only once, while the quality and the accuracy of the obtained reconstructions are maintained. The novel method is applied to ground-penetrating radar imaging and through-the-wall imaging, in which the validity and the efficiency of the method are demonstrated.

Inversion domain boundaries in Mn and Al dual‐doped ZnO: Atomic structure and electronic properties

AbstractThe atomic and electronic structures of inversion domain boundaries in Mn‐Al dual‐doped ZnO (Zn0.89Mn0.1Al0.01O) have been investigated. Using atomic‐resolution scanning transmission electron microscopy, a head‐to‐head c‐axis configuration and cation stacking sequence of αβαβ|γ|αβαβ along the c‐axis were observed at the basal‐plane inversion domain boundary. Energy‐dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy revealed significant localization of Mn and minor localization of Al at the basal‐plane inversion domain boundary. Based on experimental findings, a Mn‐doped basal‐plane inversion domain boundary slab model was constructed and refined by first principles calculations. The model is in agreement with atomic‐resolution images. The local electronic density of states of the slab model basal‐plane inversion domain boundary shows a hybridization of the Mn d and O p states within the valence band and localized Mn d states in the conduction band. The thermoelectric properties of Zn0.99−xMnxAl0.01O ceramics have been reported in a previous work. In this work, the effects of inversion domain boundaries on the thermoelectric properties are discussed. In comparison to Zn0.99−xMnxAl0.01O ceramics with x≤0.05, inversion domain boundaries in Zn0.89Mn0.1Al0.01O caused thermal and electrical conductivity reduction due to interface scattering of phonons and electrons. The Seebeck coefficient increased, suggesting electron filtering at inversion domain boundaries.

Domain structure analysis of (AgSbTe2)15(GeTe)85 thermoelectric compounds fabricated by rapid solidification process (RSP) and spark plasma sintering (SPS)

The domain structure of the thermoelectric compound (AgSbTe2)15(GeTe)85 was analyzed using high resolution transmission electron microscopy. Numerous coherent domain boundaries of two different types were observed, formed by the polar phase transition from the cubic NaCl-type phase to a rhombohedral structure with an angular distortion of 0.5–1.5°. One type has twin boundaries parallel to [001] and [110] directions, the other has inversion domain boundaries parallel to [111] and [112] directions. This high density of internal interfaces could reduce lattice thermal conductivity due to phonon scattering on the interfaces, and less deleterious on electronic transport processes, because they are coherent.

Area-preserving colour coding of inverse pole figure domain.

The inverse pole figure (IPF) map is a routinely displayed output in microtexture studies, interpreted using the attached colour legend/diagram. An area-preserving relation between the IPF domain and RGB colour domain has been developed here, and the resultant IPF colour diagrams of different crystal point group symmetries are presented.

Effect of screw threading dislocations and inverse domain boundaries in GaN on the shape of reciprocal-space maps.

The microstructure of polar GaN layers, grown by upgraded high-temperature vapour phase epitaxy on [001]-oriented sapphire substrates, was studied by means of high-resolution X-ray diffraction and transmission electron microscopy. Systematic differences between reciprocal-space maps measured by X-ray diffraction and those which were simulated for different densities of threading dislocations revealed that threading dislocations are not the only microstructure defect in these GaN layers. Conventional dark-field transmission electron microscopy and convergent-beam electron diffraction detected vertical inversion domains as an additional microstructure feature. On a series of polar GaN layers with different proportions of threading dislocations and inversion domain boundaries, this contribution illustrates the capability and limitations of coplanar reciprocal-space mapping by X-ray diffraction to distinguish between these microstructure features.

Defect structure of high temperature hydride vapor phase epitaxy–grown epitaxial (0 0 0 1) AlN/sapphire using growth mode modification process

Defect structures were investigated by transmission electron microscopy for AlN/sapphire (0001) epilayers grown by high temperature hydride vapor phase epitaxy using a growth mode modification process. The defect structures, including threading dislocations, inversion domains, and voids, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. AlN film growth was initiated at 1450°C with high V/III ratio for 8min. This was followed by low V/III ratio growth for 12min. The near-interfacial region shows a high density of threading dislocations and inversion domains. Most of these dislocations have Burgers vector b=1/3〈1120〉 and were reduced with the formation of dislocation loops. In the middle range 400nm<h<2μm, dislocations gradually aggregated and reduced to ∼109cm−2. The inversion domains have a shuttle-like shape with staggered boundaries that deviate by ∼±5° from the c axis.Above 2μm thickness, the film consists of isolated threading dislocations with a total density of 8×108cm−2. Most of threading dislocations are either pure edge or mixed dislocations. The threading dislocation reduction in these films is associated with dislocation loops formation and dislocation aggregation-interaction during island growth with high V/III ratio.

Heteroepitaxy mechanisms of AlN on nitridated c- and a-plane sapphire substrates

We investigate the metalorganic vapor phase epitaxy of c-oriented AlN on c- and a-plane sapphire substrates, focusing on the effect of sapphire nitridation on the AlN structure. Prior to AlN growth, the sapphire surface is subjected to nitridation via an in-situ NH3 treatment. We demonstrate that nitridation without H2 thermal etching treatment realizes high quality AlN on both c- and a-plane sapphires, indicating that a reaction between NH3 and oxygen on the sapphire surface is a critical factor in the material growth. It is proposed that nitridation initially creates nanometer-scale inversion domains in the AlN epilayer, but as growth proceeds, the N-polar domains are annihilated, leaving voids. Such growth behaviors can be regarded as spontaneous selective area growth with strain-adsorbing void formation, and lead to crack-free, ∼5 μm thick AlN layers, which produce x-ray line widths as narrow as 180 and 483 arc sec for the (0002) and (101¯2) reflections, respectively, on c-plane sapphire, and 237 and 433 arc sec for these reflections on a-plane sapphire.

High quality AlN epilayers grown on nitrided sapphire by metal organic chemical vapor deposition

Influence of sapphire pretreatment conditions on crystalline quality of AlN epilayers has been investigated by metal organic chemical vapor deposition (MOCVD). Compared to alumination treatment, it is found that appropriate sapphire nitridation significantly straightens the surface atomic terraces and decreases the X-ray diffraction (0002) full width at half maximum (FWHM) to a minimum of 55 arcsec, indicating a great improvement of the tilting feature of the grain structures in the AlN epilayer. More importantly, there is no inversion domains (IDs) found in the AlN epilayers, which clarifies that optimal sapphire nitridation is promising in the growth of high quality AlN. It is deduced that the different interfacial atomic structures caused by various pretreatment conditions influence the orientation of the AlN nucleation layer grains, which eventually determines the tilting features of the AlN epilayers.

An Inverse Direct Time Domain Boundary Element Method for the Reconstruction of Transient Acoustic Field

An inverse direct time domain boundary element method (IDTBEM) is proposed for the reconstruction of transient acoustic field radiated by arbitrarily shaped sources. The method is based on the theory of direct time domain boundary element method (DTBEM), which is free from the calculation of hypersingular integrals, and thus, its reconstruction process is relatively simple and easy to implement. However, the formulations of DTBEM cannot be used directly for the reconstruction of transient acoustic field, and therefore, new formulations with a modified time axis are derived. With these new formulations, a linear system of equations is formed and the reconstruction is performed in a marching-on-time (MOT) way. Meanwhile, to deal with the ill-posedness involved in the inverse process, the truncated singular value decomposition (TSVD) is employed. Numerical simulations with three examples of a sphere, a cylinder, and a simplified car model are carried out to verify the validity of IDTBEM, and the results demonstrate that the IDTBEM is effective in reconstructing the transient acoustic fields radiated by arbitrarily shaped sources in both time and space domains.

Multi-objective inverse scheduling optimization of single-machine shop system with uncertain due-dates and processing times

Generally, ideal manufacturing system environments are assumed before determining effective scheduling. However, the original schedule is no longer optimal or even to be infeasible due to many uncertain events. This paper investigates a multi-objective inverse scheduling problem in single-machine shop system with due-dates and uncertain processing parameters. Moreover, in order to more close the addressed problem into the situations encountered in real world, the processing parameters are considered to be uncertain stochastic parameters. First, a comprehensive mathematical model for multi-objective single-machine inverse scheduling problem (MSMISP) is addressed. Second, an effective hybrid multi-objective evolutionary algorithm (HMNL) is proposed to handle uncertain processing parameters (uncertainties) and multiple objectives at the same time. In HMNL, using an effective decimal system encoding scheme and genetic operators, the non-dominated sorting based on NSGA-II is adapted for the MSMISP. In addition, hybrid HMNL are proposed by incorporating an adaptive local search scheme into the well-known NSGA-II, where applies a separate local search process, total six strategies, to improve quality of solutions. Furthermore, an on-demand layered strategy is embedded into the elitism strategy to keep the population diversity. Afterwards, an external archive set is dynamically updated, where a non-dominated solution is selected to participate in the creation of the new population. Finally, 36 public problem instances with different scales and statistical performance comparisons are provided for the HMNL algorithm. This paper is the first to propose a mathematical model and develop a hybrid MOEA algorithm to solve MSMISP in inverse scheduling domain.

Nanoscale adhesion and sliding on biased semiconductors.

We present experimental and theoretical results on controlling nanoscale sliding friction and adhesion by electric fields on model contacts realized by bringing a conductive atomic force microscope tip into contact with the surface of a silicon-oxide/silicon wafer. We find that applying a bias voltage on silicon (or on the conductive tip) enables a noticeable control of the sliding forces. Two electrostatic interactions are identified as being relevant for the friction variation as a function of applied voltage. The first is a short-range electrostatic interaction between opposite charges localized at oxide-silicon/silicon and tip/silicon-oxide interfaces. This attractive interaction results from the high capacity of the oxide-semiconductor interface to change its charge density in response to a bias voltage. Various regimes of charging resulting from silicon electronic bands' alignment and deformation are evidenced. We mainly focused here on the strong charge accumulation and inversion domains. The second longer-range electrostatic interaction is between the voltage-induced bulk and surface charges of both tip and sample. This interaction decreases very slowly with the distance between tip and silicon surface, i.e. oxide thickness, and can be attractive or repulsive depending on voltage polarity. Our results demonstrate the possibility of controlling nanoscale friction/adhesion in nanoscale contacts involving semiconductors. These results are relevant for the operation of nanoscale devices or for on-surface nanomanipulation of metallic nanoparticles. We model the experimental results by adding an electrostatic energy contribution to the tip-surface binding energy, which translates into an increase or decrease of the normal force and ultimately of the sliding friction.

Comparison of convergent beam electron diffraction and annular bright field atomic imaging for GaN polarity determination.

A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM mode. Both measurements were made at nominally the same locations on a variety of GaN nanowires. In all cases the two techniques gave the same polarity result. An important aspect of the study was the calibration of the CBED pattern rotation relative to the TEM image. Three different microscopes were used for CBED measurements. For all three instruments there was a substantial rotation of the diffraction pattern (120 or 180°) relative to the image, which, if unaccounted for, would have resulted in incorrect polarity determination. The study also shows that structural defects such as inversion domains can be readily identified by ABF imaging, but may escape identification by CBED. The relative advantages of the two techniques are discussed.

Direct Hydraulic Parameter and Function Estimation for Diverse Soil Types Under Infiltration and Evaporation

A new computationally efficient direct method is applied to estimating unsaturated hydraulic properties during steady-state infiltration and evaporation at soil surface. For different soil types with homogeneous and layered heterogeneity, soil hydraulic parameters and unsaturated conductivities are estimated. Unlike the traditional indirect inversion method, the direct method does not require forward simulations to assess the measurement-to-model fit; thus, the knowledge of model boundary conditions (BC) is not required. Instead, the method employs a set of local approximate solutions to impose continuity of pressure head and soil water fluxes throughout the inversion domain, while measurements act to condition these solutions. Given sufficient measurements, it yields a well-posed system of nonlinear equations that can be solved with optimization in a single step and is thus computationally efficient. For both Gardner’s and van Genuchten’s soil water models, unsaturated hydraulic conductivities and pressure heads (including the unknown BC) can be accurately recovered. When increasing measurement errors are imposed, inversion becomes less accurate, but the solution is stable, i.e., estimation errors remain bounded. Moreover, when the unsaturated conductivity model is known, inversion can recover its parameters; if it is unknown, inversion can recover a nonparametric, piecewise continuous function to which soil parameters can be obtained via fitting. Overall, inversion accuracy of the direct method is influenced by (1) measurement density and errors; (2) rate of infiltration or evaporation; (3) variation of the unsaturated conductivity; (4) flow direction; (5) the number of soil layers.

A Color-Doppler Shear-Wave-Imaging Phase-reconstruction Method Using Four Color Flow Images.

This study investigates shear wave phase map reconstruction using a limited number of color flow images (CFIs) acquired with a color Doppler ultrasound imaging instrument. We propose an efficient reconstruction method to considerably reduce the number of CFIs required for reconstruction and compare this method with Fourier analysis-based color Doppler shear wave imaging. The proposed method uses a two-step phase reconstruction process, including an initial phase map derived from four CFIs using an advanced iterative algorithm of optical interferometry. The second step reduces phase artifacts in the initial phase map using an iterative correction procedure that cycles between the Fourier and inverse Fourier domains while imposing directional filtering and total variation regularization. We demonstrate the efficacy of this method using synthetic and experimental data of a breast phantom and human breast tissue. Our results show that the proposed method maintains image quality and reduces the number of CFIs required to four; previous methods have required at least 32 CFIs to achieve equivalent image quality. The proposed method is applicable to real-time shear wave elastography using a continuous shear wave produced by a mechanical vibrator.

V-shaped pits in HVPE-grown GaN associated with columnar inversion domains originating from foreign particles of α-Si3N4 and graphitic carbon

The v-shaped pits (so-called V-pits) observed in hydride-vapor-phase-epitaxy-grown GaN and associated with the columnar inversion domains originating from foreign particles were investigated. The inversion domains on the front and back surfaces of the test sample were recognized after chemical mechanical polishing. It was found that the V-pits originate from the columnar inversion domains. The inversion domains, in turn, arise from the particles that exist on a low-temperature GaN buffer layer on sapphire substrate. Using transmission electron microscopy, these particles were found to be of α-Si3N4 and graphitic carbon. Such particles are attributable to the components of the reactor and adhere to the low-temperature GaN buffer layer, which has a surface roughness of the order of several nanometers. Thus, an effective way of obtaining HVPE-grown thick GaN layers without the V-pits associated with columnar IDs is to maintain the parts of the HVPE chamber properly to prevent foreign particles from being generated.