Cross-sectional High-resolution Transmission Electron Microscopy Research Articles

Synthesis of multilayered h-BN film on the molten Ni–B and the influence of underneath W substrate

High-quality two-dimensional hexagonal boron nitride (h-BN) film with tens of layers has been used as a universal substrate and capping layer for the van der Waals devices. Various approaches have been carried out for the synthesis of multilayered h-BN. Among them, the metal flux method is reliable in yielding h-BN crystals with high crystalline quality. However, this time- and energy-demanding method hinders its scale application. Herein, inspired by the metal flux method, we reported the time-effective growth of high-quality multilayered h-BN film (ca. 20 nm) on a molten Ni–B layer wetting on W substrate with chemical vapor deposition method. The film exhibits an excellent stacking sequence and a full-width at half maximum of the Raman E2g peak narrow to 9.5 cm−1. Cross-sectional high-resolution transmission electron microscopy and in-situ x-ray diffraction spectroscopy were carried out to investigate the crystal structure evolution of Ni–B layer wetting on W substrate. It is found that the low surface tension caused by the spreading of Ni–B alloy on the W substrate and the presence of the Ni subsurface may be responsible for the formation of multilayered h-BN with excellent crystalline quality. Meanwhile, the W diffusion in Ni–B-based melt can hinder the formation of h-BN under certain growth conditions. The approach demonstrates the feasibility of large-scale growth of multilayered h-BN, paving the way to future applications in van der Waals electronic and optoelectronic devices.

Angle-resolved X-ray photoelectron spectroscopy intensity modeling of SiNx ultrathin layer grown on Si (100) and Si (111) substrates by N2 plasma treatment

In this work, we investigate the silicon nitridation process using a N2 plasma Glow Discharge Source (GDS) in an ultra-high vacuum chamber. In situ Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS) measurements were performed to determine various chemical environments of the surface species. The main goals of this study are determination of the film chemical composition, to estimate its thickness and identify various phenomena taking place during the nitridation process under different experimental conditions. To achieve these objectives, we developed an ARXPS intensity model for an ultrathin SiNx overlayer. Based on the ARXPS results, which indicate that the silicon nitride film is not homogenous, we adopted a bi-layer model composed of two SixNyOz layers with varying thickness and chemical composition. The model is built from analysis of photoelectron intensities, considering a comparison between experimental and theoretical data. This bi-layer model revealed that the silicon nitride layer consists of a surface layer composed of approximately equal amounts of silicon and nitrogen (50% each), and an interfacial layer containing varying amounts of oxygen depending on experimental parameters. The presence of oxygen is due to residual air contamination in the vacuum chamber and/or to Si native oxide. The estimated thickness of silicon nitride film is less than 9 nm, depending on the nitridation time and temperature.The model developed in this work enabled us to study the effects of initial surface state (as received or chemically cleaned), Si substrate crystallographic orientation (such as (100) and (111)), and the nitridation temperature (room temperature and 500°C). The diffusing species (N+, N2+, O+), which play a crucial role during nitridation, were found to be influenced by temperature, treatment time, and substrate orientation. The diffusion phenomenon competed with other processes such as NO desorption, leading to a significant impact on formation of the SiNx film. Furthermore, we compared the thickness obtained from the ARXPS model using cross-sectional High-Resolution Transmission Electron Microscopy (HR-TEM) images. Additional Low-Energy Electron Diffraction (LEED) measurements indicated that the SiNx film was amorphous.

Plasma-activated high-strength non-isothermal anodic bonding for efficient fabrication of the micro atomic vapor cells

Microfabricated alkali metal vapor cells are the core components for miniaturizing quantum devices, such as atomic clocks, atomic gyroscopes, and atomic magnetometers. Alkali metals are prone to thermal migration at the bonding interface during the sealing of the vapor cell, which leads to bonding failure. In this study, a non-isothermal anodic bonding process based on plasma activation was developed to achieve high-strength bonding at low temperatures using the temperature gradient to keep the alkali metal away from the bonding interface. This procedure eliminates interference from the alkali metal and significantly increases the success rate of anodic bonding of vapor cells. Simulation results for the temperature field and the residual stresses during anodic bonding demonstrate that the procedure is theoretically feasible. The effect of plasma treatment on the contact angle and roughness of the bonding surface revealed a mechanism for improving the bond strength. In addition, the construction of an integrated processing platform made it possible to perform non-isothermal anode bonding, injection of alkali metal, filling of inert gases, and recovery and recycling of noble gases. The high strength and good hermeticity of the bonding interface were demonstrated by the bond strength, cross-sectional high-resolution transmission electron microscopy observations, and leakage rate tests. Finally, the absorption spectrum and free-induction decay signal of the fabricated vapor cell were measured.

Structure and electrical conductivity of Ti-Zr-Ni films of quasicrystalline and related crystalline phases

The temperature dependence of electrical resistivity in the temperature range from 4.4 to 300 K for Ti41Zr41Ni18 (at.%) thin coatings of various crystal perfection and phase compositions has been investigated. The films were deposited by the magnetron sputtering method. The following vacuum annealing in various modes provided several combinations of the film phases such as 1/1 and 2/1 crystal-approximant phases, the quasicrystalline icosahedral phase, Laves (Ti,Zr)2Ni phase and α-Ti(Zr) solid solution phase. The structure and composition of the films before and after annealing were studied in detail by X-ray diffractometry and cross-sectional high-resolution transmission electron microscopy. Samples with predominant amorphous, quasi-crystalline, and approximant 2/1 phases have a temperature coefficient of resistance less than zero, while samples dominated by crystalline phases exhibit the characteristic metallic conductivity. For the quasicrystalline Ti41Zr41Ni18 film, the temperature dependence of the electrical resistance turned out to be similar to that of bulk samples with the composition Ti40Zr40Ni20. A significant effect of the presence of phases with metallic conduction on the overall conductivity of the coating is described. The electrical resistance of the 2/1 approximant phase with a low content of the additional Laves phase in the film has an almost constant value over the entire temperature range under study.

Thick p-type Mg2Si film on Si: Growth, structure and transport properties

A ∼ 0.8–0.9 µm thick Mg2Si film was grown by pulsed deposition of Mg on Si(111) at 340 °C in UHV. The X-Ray Diffraction (XRD) and cross-sectional High-resolution Transmission Electron Microscopy (x-HRTEM) data demonstrate the high crystal quality and nearly 100% texture of the film. Energy Dispersive X-ray spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS) depth profiling show that the synthesized Mg2Si contains some oxygen. Hall measurements reveal the p-type conductivity of 130 S/cm and carrier concentration of 6 × 1018 cm−3 at room temperature (RT), the acceptor concentration of ∼ 1.4 × 1018 cm−3 at 190 K, hole activation energy Ea ≈ 26 meV, mobility μ∼5050 cm2/V∙s at 75 K and μ∼134 cm2/V∙s at RT. The p-type conductivity is related to either unintentional doping with oxygen or/and Mg vacancies occurred during the non-equilibrium process of silicide ultra-fast formation. The outstanding transport parameters of the film are explained by its high crystal quality and texture. The films with such thickness and transport properties are suitable for photovoltaic and thermoelectric applications.

Growth and characterization of GePb/Ge multiple quantum wells

GePb/Ge multiple quantum well (MQW) structures with a Pb content of up to 7.2% were successfully grown on Ge(100) substrates via sputtering epitaxy. Scanning electron microscopy revealed that both the Pb content in the GePb layer and Ge layer thickness affect the Pb surface segregation. High-resolution X-ray diffraction and cross-sectional transmission electron microscopy indicated that the GePb/Ge MQW structures had a high crystal quality. Moreover, the thermal stability of the GePb/Ge MQW structures was investigated, and the MQW structures were found to be stable at an annealing temperature of 500 °C. The band gaps of the GePb/Ge MQWs were measured, and the band gap regulation caused by the quantum confinement effect was verified. These results indicate that the MQW structure is a promising approach for realizing efficient GePb light sources.

Strong perpendicular magnetic anisotropy of Pt/Co/AlN structure

In this study, the perpendicular magnetic anisotropy (PMA) and microstructure of Pt/Co/AlN stacks were investigated. At a Co thickness of 0.5 nm, the magnetic moment was not observed in the as-deposited state, but after annealing, it increased significantly. Furthermore, a strong PMA was obtained in the annealed samples; after annealing at 300 °C and 400 °C, the effective PMA energy densities were 2.28 and 1.96 × 107 erg/cm3, respectively. Similar behavior was observed for samples with thicker Co layers. The magnetic dead layer and interfacial PMA energy density were also investigated to determine the interfacial effects on PMA. The occurrence of strong PMA was supported by high-resolution cross-sectional transmission electron microscopy results, demonstrating a well-defined layered structure for the annealed Pt/Co/AlN samples.

Effect of a Discontinuous Ag Layer on Optical and Electrical Properties of ZnO/Ag/ZnOStructures

ZnO/Ag/ZnO nanolaminate structures were deposited by consecutive RF sputtering at room temperature.The optical transparency, sheet resistance, and figure of merit are determined in relation to the deposition time of Ag and to the film thickness of the ZnO top layer. An improved transmittance has been found in the visible spectral range of the ZnO/Ag/ZnO structure compared to ZnO multilayers without Ag. High transmittance of 98% at 550 nm, sheet resistance of 8 Ω/sq, and figure of merit (FOM) of 111.01 × 10−3 Ω−1are achieved for an optimized ZnO/Ag/ZnO nanolaminate structure. It is suggested that the good optical and electrical properties are due to the deposition of the discontinuous Ag layer. The electrical metallic type conductivity is caused by planar located silver metal granules. The deposition of a discrete layer of Ag nano-granules is confirmed by atomic force microscopy (AFM) and cross-section high-resolution transmission electron microscopy (HRTEM) observations.

Interface effect on superlattice quality and optical properties of InAs/GaSb type-II superlattices grown by molecular beam epitaxy

We systematically investigate the influence of InSb interface (IF) engineering on the crystal quality and optical properties of strain-balanced InAs/GaSb type-II superlattices (T2SLs). The type-II superlattice structure is 120 periods InAs (8 ML)/GaSb (6 ML) with different thicknesses of InSb interface grown by molecular beam epitaxy (MBE). The high-resolution x-ray diffraction (XRD) curves display sharp satellite peaks, and the narrow full width at half maximum (FWHM) of the 0th is only 30–39 arcsec. From high-resolution cross-sectional transmission electron microscopy (HRTEM) characterization, the InSb heterointerfaces and the clear spatial separation between the InAs and GaSb layers can be more intuitively distinguished. As the InSb interface thickness increases, the compressive strain increases, and the surface “bright spots” appear to be more apparent from the atomic force microscopy (AFM) results. Also, photoluminescence (PL) measurements verify that, with the increase in the strain, the bandgap of the superlattice narrows. By optimizing the InSb interface, a high-quality crystal with a well-defined surface and interface is obtained with a PL wavelength of 4.78 μm, which can be used for mid-wave infrared (MWIR) detection.

Cross-sectional high-resolution transmission electron microscopy at Mo/Si multilayer stacks

Abstract Using a Mo/Si multilayer stack, capabilities and limits of high-resolution transmission electron microscopy with respect to thin film analysis are pointed out. Image processing is shown to provide non-subjective criteria for the determination of layer thicknesses. In comparison to measurements by X-ray reflectometry, an integrating method capable of determining multilayer periodicities, preparation and observation artefacts in transmission electron microscopy is discussed. Special emphasis is placed on cleavage as a powerful method for TEM sample preparation, its advantages and disadvantages.

Biotic Process Dominated the Uptake and Transformation of Ag+ by Shewanella oneidensis MR-1.

Silver ions (Ag+) directly emitted from industrial sources or released from manufactured Ag nanoparticles (AgNPs) in biosolid-amended soils have raised concern about the risk to ecosystems. However, our knowledge of Ag+ toxicity, internalization, and transformation mechanisms to bacteria is still insufficient. Here, we combine the advanced technologies of hyperspectral imaging (HSI) and single-particle inductively coupled plasma mass spectrometry to visualize the potential formed AgNPs inside the bacteria and evaluate the contributions of biological and non-biological processes in the uptake and transformation of Ag+ by Shewanella oneidensis MR-1. The results showed a dose-dependent toxicity of Ag+ to S. oneidensis MR-1 in the ferrihydrite bioreduction process, which was primarily induced by the actively internalized Ag. Moreover, both HSI and cross-section high-resolution transmission electron microscopy results confirmed that Ag inside the bacteria existed in the form of particulate. The Ag mass distribution in and around live and inactivated cells demonstrated that the uptake and transformation of Ag+ by S. oneidensis MR-1 were mainly via biological process. The bioaccumulation of Ag+ may be lethal to bacteria. A better understanding of the uptake and transformation of Ag+ in bacteria is central to predict and monitor the key factors that control Ag partitioning dynamics at the biointerface, which is critical to develop practical risk assessment and mitigation strategies.

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.

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.

Structural and electrical characterisation of PtS from H2S-converted Pt

A manufacturing-compatible 300 mm chamber reactor for atomic layer deposition or chemical vapour deposition, and pre-fitted with H2/H2S gases that can be uniformly delivered to the wafer surface, is employed to thermally convert Pt to PtS in a H2/H2S gaseous atmosphere for 7 hours at a chamber temperature of 550 °C. Prior to conversion, platinum layers 5 nm thick are uniformly deposited by electron beam evaporation onto ∼30 nm of amorphous aluminium sesquioxide deposited by atomic layer deposition on, (a) p-type silicon, and (b) c-plane sapphire. Structural characterisation is performed by high-resolution cross-sectional transmission-electron microscopy, scanning-electron microscopy and Raman spectroscopy, confirming the formation of continuous films of polycrystalline platinum monosulfide (PtS) with a ∼15 nm thickness. Electrical characterisation is performed by 4-point resistivity and Hall-effect transport measurements on van der Pauw structures of PtS on aluminium sesquioxide on c-plane sapphire, and by back-gate junctionless MOSFET device measurements for PtS on aluminium sesquioxide on p-type silicon, showing that PtS behaves as a semiconductor with a mobility of ∼16 cm2/V.s and with an n-type carrier concentration of ∼1.2 × 1015 cm−3. Advanced commercial-grade Sentaurus simulations, alongside density-functional theory calculations, agree well with the experimental observations and suggest a large bandgap of ∼1.58 eV may be possible that could lead to a low off-current and a high Ion/Ioff ratio, suggesting that PtS may be an advanced material candidate for future device integration with CMOS and for 3D integration applications in Beyond-CMOS and More-than-Moore technologies.

Nb Texture Evolution and Interdiffusion in Nb/Si-Layered Systems.

In this paper, we present a detailed study on the microstructure evolution and interdiffusion in Nb/Si-layered systems. Interlayer formation during the early stages of growth in sputter-deposited Nb-on-Si and Si-on-Nb bilayer systems is studied in vacuo using a high-sensitivity low-energy ion-scattering technique. An asymmetric intermixing behavior is observed, where the Si-on-Nb interface is ∼2× thinner than the Nb-on-Si interface, and it is explained by the surface-energy difference between Nb and Si. During Nb-on-Si growth, the crystallization of the Nb layer occurs around 2.1 nm as-deposited Nb thickness with a strong Nb(110)-preferred orientation, which is maintained up to 3.3 nm as-deposited Nb thickness. A further increase in the Nb layer thickness above 3.3 nm results in a polycrystalline microstructure with a reduced degree of texture. High-resolution cross-sectional transmission electron microscopy imaging is performed on Nb/Si multilayers to study the effect of the Nb layer texture on interdiffusion during low-temperature annealing. Nb/Si multilayers with amorphous 2 nm Nb layers and strongly textured 3 nm thick Nb layers, with limited grain-boundary pathways for diffusion, show no observable interdiffusion during annealing at 200 °C for 8 h, whereas in a Nb/Si multilayer with polycrystalline 4 nm thick Nb layers, a ∼1 nm amorphous Nb/Si interlayer is formed at the Si-on-Nb interface during annealing.

Growth of centimeter-scale single crystal MoO3 ribbons for high performance ultraviolet photodetectors

Molybdenum oxides have attracted much interest due to their unique electronic properties. Here, we report a convenient and efficient method to synthesize centimeter-scale single crystal of MoO3 ribbons through an atmospheric pressure physical vapor deposition approach. Optical microscopy, atomic force microscopy, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction, and x-ray diffraction measurements reveal that the MoO3 ribbons grow along the MoO3⟨100⟩ direction and the top surface is MoO3(010) plane. Raman spectroscopy, x-ray photoelectron spectroscopy, and cross-sectional HRTEM results disclose that the surface of MoO3 ribbons can be transformed from MoO3 to MoO2 through H2 annealing treatment forming MoO2/MoO3 heterostructure. We found that the conductance and ultraviolet photoresponsivity of the MoO3 ribbons can be improved by ∼11 and ∼5 orders of magnitude, respectively, after H2 annealing treatment, which will be helpful for the applications of MoO3 in the optoelectronic field.

GaAs layer on c-plane sapphire for light emitting sources

High-quality cubic GaAs (111)A buffer layers have been grown on an atomically flat c-plane trigonal sapphire substrate having well-defined steps and terraces. A two-step growth method has been used where, at an early stage, a GaAs layer has been grown at low temperature (LT), followed by second high-temperature GaAs growth layer. In addition to the two-step growth process, an AlAs nucleation layer and multiple annealing steps have been employed. The effectiveness of the LT GaAs layer in this highly dissimilar epitaxy was then investigated. An LT GaAs layer resulted in a relaxed GaAs buffer with smooth surface morphology and high crystalline quality. An InGaAs quantum well (QW) was epitaxially grown on the 70 nm GaAs buffer and compared with a reference InGaAs QW grown on a GaAs (111)A substrate. Along with x-ray and high-resolution cross-section transmission electron microscopy, comparable QW photoluminescence intensity and linewidth with respect to reference InGaAs QW confirmed the effectiveness of our growth strategies to produce high-quality GaAs on sapphire. This demonstrates the opportunity for GaAs photonics on sapphire and the potential to realise an integrated microwave photonic chip on a sapphire platform.

Scanning probe-based nanolithography: nondestructive structures fabricated on silicon surface via distinctive anisotropic etching in HF/HNO3 mixtures

HF/HNO3 mixtures were typical isotropic etching systems for processing silicon (Si) surface, and still suffer from difficulties in controllably fabricating nanostructures. Here, fast and anisotropic etching approach with HF/HNO3 mixtures was developed to overcome this challenge. An atomic force microscope diamond tip was used for the preparation of friction-induced hillocks, mainly consisting of amorphous Si, through reciprocating sliding of Si surfaces below critical contact pressure. The hillock acted as a mask against HF/HNO3 mixtures etching, forming nanostructures through subsequent distinctive anisotropic etching. The effects of volume ratio of HF/HNO3 mixtures, sliding cycle, normal load, and etching time on anisotropic etching were systematically investigated to optimize fabrication parameters. Further analysis suggested that lower average dangling bond density in scanned regions may retard the breakup of Si–H bonds and polarized Si–Si backbonds, leading to lower dissolution rates compared to original Si surface. Cross-sectional high-resolution transmission electron microscope observations revealed defect-free nanostructures fabricated by the anisotropic etching. Also, several nondestructive nanostructures with different patterns were realized on Si surfaces by programming tip scanning traces. In sum, the proposed method is promising for the fabrication of controllable nano-sized functional devices by HF/HNO3 mixtures. Nanofabrication of nondestructive structures on silicon surface via distinctive anisotropic etching in HF/HNO3 mixtures

Scalable Design of Two-Dimensional Oxide Nanosheets for Construction of Ultrathin Multilayer Nanocapacitor.

Large size of capacitors is the main hurdle in miniaturization of current electronic devices. Herein, a scalable solution-based layer-by-layer engineering of metallic and high-κ dielectric nanosheets into multilayer nanosheet capacitors (MNCs) with overall thickness of ≈20nm is presented. The MNCs are built through neat tiling of 2D metallic Ru0.95 O2 0.2- and high-κ dielectric Ca2 NaNb4 O13 - nanosheets via the Langmuir-Blodgett (LB) approach at room temperature which is verified by cross-sectional high-resolution transmission electron microscopy (HRTEM). The resultant MNCs demonstrate a high capacitance of 40-52 µF cm-2 and low leakage currents down to 10-5 -10-6 A cm-2 . Such MNCs also possess complimentary in situ robust dielectric properties under high-temperature measurements up to 250°C. Based on capacitance normalized by the thickness, the developed MNC outperforms state-of-the-art multilayer ceramic capacitors (MLCC, ≈22 µF cm-2 /5 × 104 nm) present in the market. The strategy is effective due to the advantages of facile, economical, and ambient temperature solution assembly.

Real-time tracking of the self-assembled growth of a 3D Ge quantum dot lattice in an alumina matrix

A laboratory in situ grazing-incidence small-angle X-ray scattering (GISAXS) tracking of the self-assembled growth of a regular 3D Ge quantum dot (QD) structure in an amorphous Al2O3 matrix during the ion beam sputter deposition of a periodic Ge/Al2O3 multilayer on silicon is reported. A 573 K substrate temperature proved to be necessary to achieve the self-assembly effect. Relying on a fast repeated acquisition of GISAXS patterns, the temporal evolution of the growing 3D Ge QD structure was analyzed bilayer by bilayer to determine its type, lateral and vertical correlation lengths, and inter-QD distance. The QD structure was found to have body-centered tetragonal lattice type with ABA stacking, with the lattice parameters refined by fitting the final GISAXS pattern relying on a paracrystal model. A single set of paracrystal parameters enables one to simulate the temporal evolution of the in situ GISAXS patterns throughout the deposition process, suggesting that the Ge QD self-assembly is driven from the very beginning solely by the growing surface morphology. Ex situ GISAXS and X-ray reflectivity measurements along with a cross-section high-resolution transmission electron microscopy analysis complete the study.