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Three-dimensional measurement of Mg dopant distribution and electrical activity in GaN by correlative atom probe tomography and off-axis electron holography
摘要： The distribution and electrical activity of p-type doping (Mg) in gallium nitride (GaN) grown by metal organic chemical vapor deposition was investigated by correlating atom probe tomography (APT) and off-axis electron holography. APT results revealed that high Mg concentrations promote the formation of Mg-rich clusters. This is associated with the formation of pyramidal inversion domains (PIDs). The direct measurement of the doping concentration outside the clusters provided by APT suggests a saturation in the p-type electrical activity for Mg concentrations above 7 × 1019 cm?3. Maps of the electrostatic potential provided by off-axis electron holography confirm that the highest carrier concentration was achieved in the regions with the highest dopant concentration of 2 × 1020 cm?3, despite the presence of a high density of Mg-rich clusters revealed by APT. The correlation of these techniques suggests that PIDs are not the major cause of the reduction in electrostatic potential.
关键词： Mg doping,GaN,atom probe tomography,electrical activity,off-axis electron holography
更新于2025-09-23 15:19:57
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Electron holography for vortex beams
摘要： A combined technology of vortex beam technique and electron holography has been developed. A range of about 10 times the diameter of the grating opening was irradiated and electron holography, electron vortex beam, orbital angular momentum, helical wavefront, wave propagation two-wave interference between a Bragg diffraction wave as a vortex beam and transmitted wave from and around a fork-shaped grating was recorded as a hologram. Reconstruction by using the Fourier transform method has been simultaneously performed for amplitude and phase distributions. Furthermore, by using a wave aberration function, amplitude and phase distributions at various propagation positions were reconstructed numerically, resulting in confirmation of vortex beam twists.
关键词： helical wavefront,electron holography,electron vortex beam,orbital angular momentum,wave propagation
更新于2025-09-19 17:13:59
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Visualization of different carrier concentrations in n-type-GaN semiconductors by phase-shifting electron holography with multiple electron biprisms
摘要： Phase-shifting electron holography (PS-EH) using a transmission electron microscope (TEM) was applied to visualize layers with different concentrations of carriers activated by Si (at dopant levels of 1019, 1018, 1017 and 1016 atoms cm?3) in n-type GaN semiconductors. To precisely measure the reconstructed phase pro?les in the GaN sample, three electron biprisms were used to obtain a series of high-contrast holograms without Fresnel fringes generated by a biprism ?lament, and a cryo-focused-ion-beam (cryo-FIB) was used to prepare a uniform TEM sample with less distortion in the wide ?eld of view. All layers in a 350-nm-thick TEM sample were distinguished with 1.8-nm spatial resolution and 0.02-rad phase-resolution, and variations of step width in the phase pro?le (corresponding to depletion width) at the interfaces between the layers were also measured. Thicknesses of the active and inactive layers at each dopant level were estimated from the observed phase pro?le and the simulation of theoretical band structure. Ratio of active-layer thickness to total thickness of the TEM sample signi?cantly decreased as dopant concentration decreased; thus, a thicker TEM sample is necessary to visualize lower carrier concentrations; for example, to distinguish layers with dopant concentrations of 1016 and 1015 atoms cm?3. It was estimated that sample thickness must be more than 700 nm to make it be possible to detect sub-layers by the combination of PS-EH and cryo-FIB.
关键词： phase-shifting electron holography,dopant pro?ling,carrier concentration,GaN,inactive layer,active layer
更新于2025-09-16 10:30:52
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Quantitative Mapping of the Charge Density in a Monolayer of MoS <sub/>2</sub> at Atomic Resolution by Off-Axis Electron Holography
摘要： The electric potential, electric field and charge density of a monolayer of MoS2 has been quantitatively measured at atomic scale resolution. This has been performed by off-axis electron holography using a double aberration-corrected transmission electron microscope operated at 80 kV and a low electron beam current density. Using this low dose rate and acceleration voltage, the specimen damage is limited during imaging. In order to improve the sensitivity of the measurement, series of holograms have been acquired. Instabilities of the microscope such as the drifts of the specimen, biprism and optical aberrations during the acquisition have been corrected by data processing. Phase images of the MoS2 monolayer have been acquired with a sensitivity of 2π/698 rad associated to a spatial resolution of 2.4 ?. The improvement in the signal-to-noise ratio allows the charge density to be directly calculated from the phase images using Poisson’s equation. Density functional theory simulations of the potential and charge density of this MoS2 monolayer were performed for comparison to the experiment. The experimental measurements and simulations are consistent with each other and notably, the charge density in a sulphur mono-vacancy (VS) site is shown.
关键词： MoS2,high-resolution,charge density,electric potential,electric field,electron holography,sulphur vacancy
更新于2025-09-12 10:27:22
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Nanosecond electron holography by interference gating
摘要： The interference gating is a novel method for robust time-resolved electron holographic measurements by directly switching the interference. Here, a new arrangement is presented in which a biprism in the condenser aperture as a fast electric phase shifter is used to control the interference pattern. High-frequency stimulation of the electric phase shifter in the gigahertz range are performed and observed via electron holography, proving the feasibility of interference gating in the upper picosecond range. Despite the bandwidth limitation of 180 MHz of the current signal generator, a time resolution of 100 ns is achieved through forward correction of the control signal. With this time resolution, it is already possible to measure the transient response of the biasing holder system. Our method paves the way towards a closer look on fast dynamic processes with high temporal and spatial resolution.
关键词： Interference gating,Time-resolved electron holography,Nanosecond time resolution,Condenser biprism,Electrical biasing
更新于2025-09-12 10:27:22
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Determination of electronic band structure by electron holography of etched-and-regrown interfaces in GaN <i>p-i-n</i> diodes
摘要： The electrostatic potential variation across etched-and-regrown GaN p-i-n diodes for power electronics has been studied using electron holography in a transmission electron microscope. The potential profiles have been correlated with the composition profiles of Mg, Si, and O obtained by secondary ion mass spectroscopy. Electronic charges obtained from the potential profiles correlate well with the presence of Si and O impurities at regrown interfaces. The overlap of Mg and Si when Mg doped GaN is grown directly over an etched undoped GaN surface results in the formation of a highly doped p-n junction. The introduction of a thin undoped layer over the etched GaN surface prevents the formation of such a junction as the regrowth interface is moved away from the Mg-doped GaN, and results in diodes with improved reverse leakage currents, close to the best values of continuously grown p-i-n diodes. Potential profiles of continuously grown (not etched) p-i-n diodes are compared to those of etched-and-regrown diodes.
关键词： Mg,O impurities,regrown interfaces,Si,secondary ion mass spectroscopy,electrostatic potential,GaN p-i-n diodes,electron holography
更新于2025-09-12 10:27:22
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European Microscopy Congress 2016: Proceedings || Analysis of GaAs compound semiconductors and the semiconductor laser diode using electron holography, Lorentz microscopy, electron diffraction microscopy and differential phase contrast STEM
摘要： The content within the paper discusses the development and application of semiconductor devices and their semiconductor layers using electron holography, electron microscopy, electron diffraction microscopy, and different phase contrast methods. It highlights the importance of these techniques in understanding the structural and electronic properties of semiconductor materials.
关键词： semiconductor devices,phase contrast,electron diffraction,electron microscopy,electron holography
更新于2025-09-11 14:15:04
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European Microscopy Congress 2016: Proceedings || Measuring Charge Distribution in Nanoscale Magnesium Aluminate Spinel by Electron Energy-Loss Spectroscopy and Electron Holography
摘要： Charge distribution resulting in the formation of a space charge zone (SCZ) in ionic materials has a critical role on functional properties [1]. Even though significant advances in theoretical models have been accomplished, experimental evidence in nanoscale granular materials is indirect. Here, we investigated the distribution of cations and defects on the formation of a SCZ in a nanoscale granular model system of non-stoichiometric MgO?nAl2O3 (MAS, n= 0.95 and 1.07). The SCZ was investigated experimentally by electron energy-loss spectroscopy (EELS) and off-axis electron holography (OAEH). EEL spectra were collected along directions perpendicular to grain boundaries (GB’s), from which the magnesium-to-aluminum relative cation concentrations were calculated, as presented in Fig.1. We found that regardless of annealing processes, the vicinity of GB’s of the Mg rich spinel has excess Mg+2 cations while the vicinity of GB’s of the Al rich spinel has excess of Al+3 cations. Additionally, the cation distribution shows strong dependency on the grain size. For non-stoichiometric MAS, cation concentration is proportional to the defect concentration, because deviation from stoichiometry results in adjacent defects that compensate for the electric charge [2, 3, 4]. In both materials, the cation distribution is inhomogeneous for grains smaller than 40 nm. For larger grains, the defect concentration approaches the bulk value at the center of the grain. Furthermore, excess of Mg (Al) cations at the vicinity of the GB decreased with increase of grain size. Maier et al. [1] calculated that for grain size at the scale of the Debye length (estimated at 9nm for non-stoichiometric MAS studied here [7]), the GC is no longer electrically neutral, instead influenced by accumulation or depletion of charge at the boundaries. Due to the lack of accurate values for defect formation energy [5, 6], we applied OAEH to measure directly the electrostatic charge distribution in nano-sized MAS. We show that charge distribution and the buildup of electrostatic potential between GB and core are linked to the spatial distribution of defects rather than the overall composition of MAS (Fig. 2). At the vicinity of GB’s, excess Mg+2 or Al+3 cations accumulate depending on the composition, the magnitude of which increases with decreasing grain size. Indeed, the potential distributions show the relation between the excess cation species, grain size and the Debye length, in agreement with theoretical models [1].
关键词： Lattice ordering,Electron holography,Ionic nano-materials,Electron energy loss spectroscopy,Space charge potential
更新于2025-09-11 14:15:04
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European Microscopy Congress 2016: Proceedings || Visualization of 2-dimensional potential map in multilayer organic electroluminescence materials by phase-shifting electron holography
摘要： Electron holography (EH) is a TEM method which can quantitatively measure electromagnetic fields of various samples [1-3]. In this study, we tried observing a local two-dimensional electric distribution, formed in multilayer organic electroluminescence (OEL) quantitatively with phase-shifting EH [4-5] by HF-3300EH Cold-FE TEM operated at 300 kV equipped with multiple biprism system. An OEL multilayer sample (CuPc/α-NPD/Rubrene/Alq3/LiF/Ag) was fabricated on a Si substrate using a vacuum evaporation method. Each layer’s thickness and the surface morphology of the OEL multilayer sample were evaluated by X-Ray Reflectometry (XRR) and Atomic Force Microscopy (AFM), respectively. The sample for phase-shifting EH observation was fabricated by focused ion beam (FIB) technique. A part of the multilayer sample, which formed a multilayer structure on a Si substrate, was picked up by the microsampling technique and was fixed onto a W deposition on the mesh for TEM observation. Then, a thin film sample of thickness 450 nm was fabricated by FIB processing. Generally, the OEL sample is vulnerable to water, and the structure may change in quality in reaction to atmospheric water vapor, depending on the formed materials. Therefore, in this study, after thin film processing, contact with the atmosphere was prevented by using an air protection mesh holder. Figure 1 shows a TEM image, and a hologram by double-biprism EH technique [6]. The model structure of an OEL multilayer is inserted in the TEM image. Sample thickness was about 450 nm. From the TEM image, some contrast is observed in the position of the CuPc layer, but we cannot confirm the image contrast corresponding to the other layers. Figure 2 shows the result of the visualization of the 2-dimensional potential map of an OEL multilayer. We can clearly observe the contrast of the OEL multilayer, as shown in the reconstruction image (Figure 2b). Because the inner potential of the materials in each layer is constant, we can consider phase shifts to be an electric potential change in the sample. In the future, we will investigate the accuracy of this experimental data by comparing our data with a simulation.
关键词： 2-dimensional potential distribution,organic electroluminescence material,phase-shifting electron holography
更新于2025-09-11 14:15:04
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European Microscopy Congress 2016: Proceedings || Quantitative measurement of doping and surface charge in a ZnO nanowire using in-situ biasing and off-axis electron holography
摘要： The measurement of doping and surface charge in a Zn2N3 nanowire using in-situ biasing and off-axis electron holography
关键词： surface charge,in-situ biasing,off-axis electron holography,Zn2N3 nanowire,doping
更新于2025-09-11 14:15:04