Cross-sectional Transmission Electron Microscopy Research Articles

A van der Waals Integrated Damage-Free Memristor Based on Layered 2D Hexagonal Boron Nitride.

2D materials with intriguing properties have been widely used in optoelectronics. However, electronic devices suffered from structural damage due to the ultrathin materials and uncontrolled defects at interfaces upon metallization, which hindered the development of reliable devices. Here, a damage-free Au/h-BN/Au memristor is reported using a clean, water-assisted metal transfer approach by physically assembling Au electrodes onto the layered h-BN which minimized the structural damage and undesired interfacial defects. The memristors demonstrate significantly improved performance with the coexistence of nonpolar and threshold switching as well as tunable current levels by controlling the compliance current, compared with devices with evaporated contacts. The devices integrated into an array show suppressed sneak path current and can work as both logic gates and latches to implement logic operations allowing in-memory computing. Cross-sectional scanning transmission electron microscopy analysis validates the feasibility of this nondestructive metal integration approach, the crucial role of high-quality atomically sharp interface in resistive switching, and a direct observation of percolation path. The underlying mechanism of boron vacancies-assisted transport is further supported experimentally by conductive atomic force microscopy free from process-induced damage, and theoretically by ab initio simulations.

Identification of type of threading dislocation causing reverse leakage in GaN p\u2013n junctions after continuous forward current stress

Power devices are operated under harsh conditions, such as high currents and voltages, and so degradation of these devices is an important issue. Our group previously found significant increases in reverse leakage current after applying continuous forward current stress to GaN p–n junctions. In the present study, we identified the type of threading dislocations that provide pathways for this reverse leakage current. GaN p–n diodes were grown by metalorganic vapor phase epitaxy on freestanding GaN(0001) substrates with threading dislocation densities of approximately 3 × 105 cm−2. These diodes exhibited a breakdown voltage on the order of 200 V and avalanche capability. The leakage current in some diodes in response to a reverse bias was found to rapidly increase with continuous forward current injection, and leakage sites were identified by optical emission microscopy. Closed-core threading screw dislocations (TSDs) were found at five emission spots based on cross-sectional transmission electron microscopy analyses using two-beam diffraction conditions. The Burgers vectors of these dislocations were identified as [0001] using large-angle convergent-beam electron diffraction. Thus, TSDs for which b = 1c are believed to provide current leakage paths in response to forward current stress.

Improvement of electrical performance using PtSe2/PtTe2 edge contact synthesized by molecular beam epitaxy

One-dimensional (1D) edge contact is considered critical to reducing contact resistance between two-dimensional transition-metal dichalcogenides (TMDs) and contact metal and overcoming surface contact effects, such as surface damage and orbital hybridization. Although several studies have investigated edge contact, some problems such as inevitable etching damage and a limited area persist. Herein, a tellurization method is applied to a specific lateral region to fabricate PtTe2/PtSe2/PtTe2 edge-contact field-effect transistors (FETs) without etching damage. High-resolution X-ray photoelectron spectroscopy and Raman spectroscopy are used to verify tellurization completion, which is evidenced by the complete transformation of PtSe2 to PtTe2 by the substitution of Se atoms with evaporated Te atoms. Furthermore, tellurization is applied to a specific lateral region by utilizing the hexagonal boron nitride to block the interdiffusion of evaporated Te atoms, which is confirmed by Raman spectroscopy and cross-sectional annular dark-field scanning transmission electron microscopy. Finally, the 1D edge-contact FETs fabricated by tellurization demonstrate a higher on–off ratio and carrier mobility than the surface-contact FET. Therefore, this method can be applied to various FETs based on TMDs for improving electrical performance through a reduction in contact resistance.

Magnetic properties and microstructure of FePt (Ag, C) films with MgTiOBN/CrRu underlayer

The (001) textured L10 FePt film is the promising candidate material for coming heat assisted magnetic recording technology. The material of epitaxial intermediate layer for FePt based granular film was considered by the surface energy and lattice mismatch between FePt and MgO-based materials. The (00L) textured FePt (Ag, C)/MgTiOBN/CrRu film was prepared and the MgTiOBN intermediate layer was prior explored in this study. This layer was sputtered by MgTiOBN target at 435 °C and formed the (002) texture on CrRu (002)/glass. The 6–12 nm thick FePt films were finally deposited on MgTiOBN at 470 °C. The FePt (Ag, C) films illustrate high perpendicular magneto- crystalline anisotropy (Ku) (3.85–3.97 × 107 erg/cm3) and out-of-plane coercivity (17.3–20.1 kOe). The lower in-plane magnetization is evidenced by the lower remanence ratio, [Mr(in-plane)/Mr(out-of-plane) ~0.08–0.14]. The saturation magnetization (Ms) of FePt films are around 752–775 emu/cm3 after co-sputtering with 40 vol% (Ag, C) segregants. From cross-sectional transmission electron microscopy image, the FePt shows island-like grains with contact angle between 35 and 67° and the average FePt grains size is around 20 nm estimated from plane view microstructural images. The reason for nonuniform trapezoidal islands is that diffusive C can penetrate and separated the FePt grains but agglomerated again during thin film formation and the lower grains contact angles was due to close surface energy between MgTiOBN and FePt layer. The release of lattice strain energy was formed the misfit dislocation in between FePt/MgTiOBN interface and epitaxial domain match was evidenced in the (111) plane system.

Tin monosulfide (SnS) epitaxial films grown by RF magnetron sputtering and sulfurization on MgO(100) substrates

The crystallographic and electrical properties of tin monosulfide (SnS) epitaxial thin films grown by RF magnetron sputtering and sulfurization were investigated. The SnS(040)-oriented films were grown on a MgO(100) substrate. Two types of four-fold rotational symmetrical in-plane orientations, offset by 45° from each other, were observed using X-ray diffraction. The rotational symmetry was also observed using cross-sectional transmission electron microscopy. The electrical properties of the SnS films were controlled by varying the sulfurization temperature, and the carrier transport of all the SnS epitaxial films was mainly limited by grain boundary scattering. The activation energies of the carrier concentration before and after sulfurization of the films were estimated to be approximately 0.26 ± 0.02 eV and 0.20 ± 0.01 eV, respectively, based on temperature-dependent Hall measurements. These values mainly correspond to the acceptor level energy of Sn vacancy with a high/low potential barrier height around the grain boundary.

Oxygen scavenging of HfZrO2-based capacitors for improving ferroelectric properties.

HfO2-based ferroelectric (FE) materials have emerged as a promising material for non-volatile memory applications because of remanent polarization, scalability of thickness below 10 nm, and compatibility with complementary metal–oxide–semiconductor technology. However, in the metal/FE/insulator/semiconductor, it is difficult to improve switching voltage (Vsw), endurance, and retention properties due to the interfacial layer (IL), which inevitably grows during the fabrication. Here, we proposed and demonstrated oxygen scavenging to reduce the IL thickness in an HfZrOx-based capacitor and the thinner IL was confirmed by cross-sectional transmission electron microscopy. Vsw of a capacitor with scavenging decreased by 18% and the same Pr could be obtained at a lower voltage than a capacitor without scavenging. In addition, excellent endurance properties up to 106 cycles were achieved. We believe oxygen scavenging has great potential for future HfZrOx-based memory device applications.

Improved resistive switching characteristics of a multi-stacked HfO2/Al2O3/HfO2 RRAM structure for neuromorphic and synaptic applications: experimental and computational study.

Atomic Layer Deposition (ALD) was used for a tri-layer structure (HfO2/Al2O3/HfO2) at low temperature over an Indium Tin Oxide (ITO) transparent electrode. First, the microstructure of the fabricated TaN/HfO2/Al2O3/HfO2/ITO RRAM device was examined by the cross-sectional High-Resolution Transmission Electron Microscopy (HRTEM). Then, Energy Dispersive X-ray Spectroscopy (EDS) was performed to probe compositional mapping. The bipolar resistive switching mode of the device was confirmed through SET/RESET characteristic plots for 100 cycles as a function of applied biasing voltage. An endurance test was performed for 100 DC switching cycles @0.2 V wherein; data retention was found up to 104 s. Moreover, for better insight into the charge conduction mechanism in tri-layer HfO2/Al2O3/HfO2, based on oxygen vacancies (VOX), total density of states (TDOS), partial density of states (PDOS) and isosurface three-dimensional charge density analysis was performed using WEIN2k and VASP simulation packages under Perdew–Burke–Ernzerhof _Generalized Gradient approximation (PBE-GGA). The experimental and theoretical outcomes can help in finding proper stacking of the active resistive switching (RS) layer for resistive random-access memory (RRAM) applications.

Facile growth of epitaxial vanadium monoxide on SrTiO3 via substrate oxygen scavenging

The deposition of vanadium metal on SrTiO3 results in the spontaneous scavenging of oxygen ions from SrTiO3 to oxidize vanadium to VOx, where x = 0.3–1.1, depending on the deposition temperature. At temperatures above 700 °C, an epitaxial film of the rock salt compound vanadium monoxide is formed on both SrTiO3 (111) and SrTiO3 (100) substrates. Surprisingly, oxygen scavenging and epitaxy persist for thicknesses over 800 Å with no sign of degradation. We describe the growth process and layer-by-layer characterization of the films using in situ reflection high-energy electron diffraction and x-ray photoelectron spectroscopy, as well as ex situ in-plane and out-of-plane x-ray diffraction and cross-sectional scanning transmission electron microscopy. This easy method of growing vanadium monoxide with controlled stoichiometry can provide new opportunities to study this normally hard to synthesize material known for the highly correlated nature of its electronic structure.

Electrical properties of NbN/NbN x /NbN Josephson junctions

In this work, we report the electrical properties of NbN internally shunted Josephson junctions with NbN x barriers. Cross-sectional scanning transmission electron microscopy analysis shows that all layers have the same cubic structure; NbN/NbN x /NbN trilayers were epitaxially grown on MgO substrates. The resistivity of the NbN x films could be varied in the range of 1–104 mΩ cm by controlling both the N2 partial pressure and the deposition time during reactive sputtering. The temperature dependence of the critical current density (J c) and characteristic voltage (I c R n) of the junctions with different barrier resistivities were measured for various barrier thicknesses. For the 10 nm-thick NbN x layer with resistivities of 73.44, 385.72, and 711 mΩ cm, the coherence length of the barrier was determined to be 5.55 ± 0.07, 4.88 ± 0.06, and 1.40 ± 0.13 nm, respectively, corresponding to carrier diffusion rates of 2.741 ± 0.004, 1.211 ± 0.002, and 0.008 ± 0.001 cm2 s−1, respectively. Thus, the reduction in barrier resistivity leads to a larger coherence length and a faster diffusion rate, which will further increase the J c and I c R n of the junction. By adjusting the barrier resistivity and thickness, the J c of the junction can be easily tuned over more than four orders of magnitude, and an I c R n value of 0.97 ± 0.07 mV was obtained at 10 K. The results indicate that the all-NbN self-shunt junction is a promising candidate in high-speed and high-temperature applications.

Infrared stimulated emission with an ultralow threshold from low-dislocation-density InN films grown on a vicinal GaN substrate

Near-infrared stimulated emission from a high-quality InN layer under optical pumping was observed with a threshold excitation power density of 0.3 and 4 kW cm−2 at T = 8 and 77 K, respectively. To achieve such a low threshold power density, vicinal GaN substrates were used to reduce the edge-component threading dislocation (ETD) density of the InN film. Cross-sectional transmission electron microscopy images reveal that the annihilation of ETDs can be divided into two steps, and the ETD density can be reduced to approximately 5 × 108 cm−2 near the surface of the 5-µm-thick film. The well-resolved phonon replica of the band-to-band emission in the photoluminescence spectra at 9 K confirm the high quality of the InN film. As a result, the feasibility of InN-based photonic structures and the underlying physics of their growth and emission properties are demonstrated.

Microstructure-informed prediction and measurement of nanoindentation hardness of an Fe-9Cr alloy irradiated with Fe-ions of 1 and 5 MeV energy

Hardening of Fe-9%Cr alloys exposed to irradiation with Fe2+ ions of two different energies, 1 and 5 MeV, is investigated using nanoindentation. The limited penetration depth of the ions causes steep damage gradients in the near-surface volumes of the irradiated samples. This damage gives rise to graded microstructures resulting in depth-dependent irradiation hardening. Nanoindentation integrates the depth-dependent hardness over the indentation plastic zone. Our study combines quantitative analysis of the ion-irradiated microstructures with the determination of the full depth dependence of irradiation hardening. The microstructure-informed model incorporates direct experimental evidence revealed by a former investigation using cross-sectional scanning transmission electron microscopy. For both ion energies, the observed microstructures consist of dislocation loops with band-like distributions and are concluded to be the dominant source of the measured irradiation-induced hardening. The model predictions are found to be in reasonable agreement with the as-measured nanoindentation response. The comparison of hardening predictions with different kinds of superposition rules of hardening contributions suggests linear superposition as the most appropriate selection under the present conditions. The possible transferability of these results to neutron irradiation is also discussed.

Morphology and Degradation of Multicompartment Microparticles Based on Semi-Crystalline Polystyrene-block-Polybutadiene-block-Poly(L-lactide) Triblock Terpolymers.

The confinement assembly of block copolymers shows great potential regarding the formation of functional microparticles with compartmentalized structure. Although a large variety of block chemistries have already been used, less is known about microdomain degradation, which could lead to mesoporous microparticles with particularly complex morphologies for ABC triblock terpolymers. Here, we report on the formation of triblock terpolymer-based, multicompartment microparticles (MMs) and the selective degradation of domains into mesoporous microparticles. A series of polystyrene-block-polybutadiene-block-poly(L-lactide) (PS-b-PB-b-PLLA, SBL) triblock terpolymers was synthesized by a combination of anionic vinyl and ring-opening polymerization, which were transformed into microparticles through evaporation-induced confinement assembly. Despite different block compositions and the presence of a crystallizable PLLA block, we mainly identified hexagonally packed cylinders with a PLLA core and PB shell embedded in a PS matrix. Emulsions were prepared with Shirasu Porous Glass (SPG) membranes leading to a narrow size distribution of the microparticles and control of the average particle diameter, d ≈ 0.4 µm–1.8 µm. The core–shell cylinders lie parallel to the surface for particle diameters d < 0.5 µm and progressively more perpendicular for larger particles d > 0.8 µm as verified with scanning and transmission electron microscopy and particle cross-sections. Finally, the selective degradation of the PLLA cylinders under basic conditions resulted in mesoporous microparticles with a pronounced surface roughness.

Strain-Induced Domain Structure and Its Impact on Magnetic and Transport Properties of Gd0.6Ca0.4MnO3 Thin Films.

The evolution of lattice strain on crystallographic domain structures and magnetic properties of epitaxial low-bandwidth manganite Gd0.6Ca0.4MnO3 (GCMO) films have been studied with films on different substrates: SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrLaAlO3, and MgO. The X-ray diffraction data reveals that all of the films, except the films on MgO, are epitaxial and have an orthorhombic structure. Cross-sectional transmission electron microscopy (TEM) shows lattice mismatch-dependent microstructural defects. Large-enough tensile strain can increase oxygen vacancies concentration near the interface and can induce vacancies in the substrate. In addition, a second phase was observed in the films with tensile strain. However, compressive strain causes dislocations in the interface and a mosaic domain structure. On the other hand, the magnetic properties of the films, including saturation magnetization, coercive field, and transport property depend systematically on the substrate-induced strain. Based on these results, the choice of appropriate substrate is an important key to obtaining high-quality GCMO film, which can affect the functionality of potential device applications.

Study on the relationship between the damage of machined surface layer and the dispersion curve of laser-induced surface acoustic wave

Laser-induced surface acoustic wave is a new surface damage detection technology, which uses the transient thermal effect of pulse laser to excite the surface acoustic wave on the sample surface. The damage of machined surface layer will cause the velocity change of laser-induced surface acoustic wave. This phenomenon is introduced into the evaluation of the surface layer damage of silicon wafers in this paper. The three-dimensional propagation model of the single-layer structure is extended to the double-layer structure, and it is proved that the gradient change of the surface layer damage will change the trend of velocity curve. In order to characterize the change of velocity curve, the surface layer damage coefficients are introduced. The cross-sectional transmission electron microscope (TEM) image of the lapped silicon wafer shows that the surface of the silicon wafer after lapping will produce a surface damage layer. The test data of the nanoindenter shows that the modulus of the surface damage layer is lower than that of the bulk material. The modulus estimated by dispersion curve is very close to the data measured by nanoindenter, indicating that the dispersion curve can truly reflect the damage degree of silicon wafer surface layer.

Ultralow thermal conductivity and thermoelectric properties of Bi4GeTe7 with an intrinsic van der Waals heterostructure

Ternary chalcogenides, having large crystalline unit cells and van der Waals stacking of layers, are expected to be poor thermal conductors and good thermoelectric (TE) materials. We are reporting that layered Bi4GeTe7 with alternating quintuplet-septuplet layers of Bi2Te3 and Bi2GeTe4 has an ultralow thermal conductivity of κtotal ∼ 0.42 W m−1 K−1 because of a high degree of anharmonicity as estimated from the large Gru¨neisen parameter (γ ∼ 4.07) and low Debye temperature (θd ∼ 135 K). The electron dominated charge transport has been realized from the Seebeck coefficient, S ∼ −82 μV/K, at 380 K and a Hall carrier concentration of ne ∼ 9.8 × 1019 cm−3 at 300 K. Observation of weak antilocalization due to the spin–orbit coupling of heavy Bi and Te also advocates Bi4GeTe7 to be a topological quantum material. The cross-sectional transmission electron microscopy images show the inherent stacking of hetero-layers, which are leading to a large anharmonicity for poor phonon propagation. Thus, being a poor thermal conductor with a TE figure of merit, ZT ∼ 0.24, at 380 K, Bi4GeTe7 is a good material for TE applications.

Optical properties of differing nanolayered structures of divalent europium doped barium fluoride thin films synthesized by pulsed laser deposition

Optically-active thin films are employed in a variety of applications, such as LEDs and photovoltaics, due to their ability to act as up- or down-photon energy converters. Their performance depends critically on their composition and structure; thus, the use of novel synthesis techniques that allow for their control at the nanoscale level can result in improved efficiency and practicality. Layered thin films consisting of Eu2+- doped BaF2 nanocrystalline layers separated by amorphous Al2O3 were synthesized via sequential pulsed laser deposition using three separate targets for the different components; this synthesis technique provides precise control of layer thickness at the nanoscale along with dopant distribution within the film. Cross-sectional transmission electron microscopy analysis verified the desired nano-layering. Post-deposition heat treatments in a nitrogen atmosphere resulted in samples exhibiting steady emission with a broad peak ranging from 400 to 600 nm and a shoulder at 410 nm. The CIE 1931 chromaticity coordinates are x = 0.26–0.29 and y = 0.32–0.35 and vary as a function of the sample configuration. Because the chromaticity coordinates are close to those of a pure white light (x = 0.33,y = 0.33), these films demonstrate advantageous properties for applications with UV-pumped white light LEDs.

Nanocrystallized Ge-Rich SiGe-HfO2 Highly Photosensitive in Short-Wave Infrared.

Group IV nanocrystals (NCs), in particular from the Si–Ge system, are of high interest for Si photonics applications. Ge-rich SiGe NCs embedded in nanocrystallized HfO2 were obtained by magnetron sputtering deposition followed by rapid thermal annealing at 600 °C for nanostructuring. The complex characterization of morphology and crystalline structure by X-ray diffraction, μ-Raman spectroscopy, and cross-section transmission electron microscopy evidenced the formation of Ge-rich SiGe NCs (3–7 nm diameter) in a matrix of nanocrystallized HfO2. For avoiding the fast diffusion of Ge, the layer containing SiGe NCs was cladded by very thin top and bottom pure HfO2 layers. Nanocrystallized HfO2 with tetragonal/orthorhombic structure was revealed beside the monoclinic phase in both buffer HfO2 and SiGe NCs–HfO2 layers. In the top part, the film is mainly crystallized in the monoclinic phase. High efficiency of the photocurrent was obtained in a broad spectral range of curves of 600–2000 nm at low temperatures. The high-quality SiGe NC/HfO2 matrix interface together with the strain induced in SiGe NCs by nanocrystallization of both HfO2 matrix and SiGe nanoparticles explain the unexpectedly extended photoelectric sensitivity in short-wave infrared up to about 2000 nm that is more than the sensitivity limit for Ge, in spite of the increase of bandgap by well-known quantum confinement effect in SiGe NCs.

Effect of low-dose Xe20+ ion irradiation on the deformation behavior of the magnetron sputtered Cr coatings under nanoindentation

The magnetron sputtered Cr coatings on zirconium (Zr) alloy substrates were irradiated by the 5 MeV Xe20+ ions at room temperature, followed by the studies on microstructure, mechanical properties, and deformation behavior. The coatings exhibit structural integrity without blistering or flaking effects after the 1 × 1014 ions/cm2 irradiation, but the irradiation caused the surface particle refining and the hardening effect. The hardness of the irradiated coating increased ~60% and the indentation modulus decreased ~10%. Moreover, the microstructural characterization was conducted on the tetrahedron indentations by using scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM). A piling-up of matter was observed in the indentation of the as-deposited coating, whereas some micro-cracks appeared near the indentation zones on the irradiated coatings. The cross-sectional TEM micrograph of an indentation further revealed the disordering of lattices and the bending of columns in the irradiated area, maybe for the irradiation made the column interface strong so that the column sliding hardly occurred.

Nucleation retardation of cubic boron nitride films caused by the addition of oxygen in argon‑nitrogen sputtering gas

Cross-sectional transmission electron microscopy (TEM) was employed to study the effect of the additive oxygen gas on the microstructure of boron nitride (BN) films especially focusing on the intervening turbostratic BN (tBN) layer's behavior. Films in this study were deposited on diamond coated Si substrates, which were biased with negative voltages, using an unbalanced magnetron sputtering method with a boron carbide target. Various amounts of oxygen were added into argon‑nitrogen mixed sputtering gas. TEM showed that the tBN intervening layer was also observed to form prior to cubic BN (cBN) layer formation for all films, irrespective of oxygen addition. The tBN layer thickness increased with increasing oxygen content. This thickness increase implied that the addition of oxygen delayed cBN phase nucleation. On the other hand, the alignment of the hexagonal BN planes in the tBN layer, estimated from FTIR spectra, was observed to be enhanced with increasing oxygen content. Such alignment enhancement is considered as a result of oxygen adsorption on the tBN layer surface, which suppresses breaking sp2 bonds caused by Ar ion bombardments. The suppression of the sp2 bond breakage, consequently, causes the difficulty of forming sp3 bonds and delays the cBN phase nucleation.

Evolution of a notched surface under glancing-angle broad ion beam erosion

Recently, a novel approach to sample preparation for microstructure analyses has been introduced where a surface possessing initial notches is eroded by a broad ion beam at gracing angles. In this contribution, details of the evolution of the sample surface and the geometry obtained are investigated in depth, using both simulation and experiments on single-crystalline Si substrates. A framework is introduced which describes the surface by a linear model and allows easy application for given sample preparation tasks. Furthermore, the surface amorphization introduced during sample preparation is evaluated using cross-sectional high-resolution transmission electron microscopy.