Transmission Electron Microscopy Lattice Images Research Articles

Highly 28Si enriched silicon by localised focused ion beam implantation

Solid-state spin qubits within silicon crystals at mK temperatures show great promise in the realisation of a fully scalable quantum computation platform. Qubit coherence times are limited in natural silicon owing to coupling to the 29Si isotope which has a non-zero nuclear spin. This work presents a method for the depletion of 29Si in localised volumes of natural silicon wafers by irradiation using a 45 keV 28Si focused ion beam with fluences above 1 × 1019 ions cm−2. Nanoscale secondary ion mass spectrometry analysis of the irradiated volumes shows residual 29Si concentration down to 2.3 ± 0.7 ppm and with residual C and O comparable to the background concentration in the unimplanted wafer. After annealing, transmission electron microscopy lattice images confirm the solid phase epitaxial re-crystallization of the as-implanted amorphous enriched volume extending over 200 nm in depth.

Growth and characterization of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

We study the growth and properties of the quaternary-alloy ferromagnetic semiconductor (In0.94−x,Gax,Fe0.06)Sb (x = 5%–30%; the Fe concentration is fixed at 6%) grown by low-temperature molecular beam epitaxy. Reflection high-energy electron diffraction patterns, scanning transmission electron microscopy lattice images, and x-ray diffraction spectra indicate that the (In0.94−x,Gax,Fe0.06)Sb layers have a zinc blende crystal structure without any other second phase. The lattice constant of the (In0.94−x,Gax,Fe0.06)Sb films changes linearly with the Ga concentration x, indicating that Ga atoms substitute In atoms in the zinc-blende structure. We found that the carrier type of (In0.94−x,Gax,Fe0.06)Sb can be systematically controlled by varying x, being n-type when x ≤ 10% and p-type when x ≥ 20%. Characterization studies using magnetic circular dichroism spectroscopy indicate that the (In0.94−x,Gax,Fe0.06)Sb layers have intrinsic ferromagnetism with relatively high Curie temperatures (TC = 40–120 K). The ability to widely control the fundamental material properties (lattice constant, bandgap, carrier type, and magnetic property) of (In0.94−x,Gax,Fe0.06)Sb demonstrated in this work is essential for spintronic device applications.

Te-rich Bi2Te3 thin films by electron−beam deposition: Structural, electrical, optical and thermoelectric properties

Tellurium-rich Bi2Te3 thin films are deposited by electron−beam evaporation technique at 300 K. These as-deposited thin films are further annealed at 100 °C, 200 °C and 300 °C for 1 h at a pressure of 3 × 10−4 Pa. X-ray diffraction (XRD) patterns of as-deposited films distinctly show Te phase along with Bi2Te3. Peak intensity ratio suggests the polycrystalline nature of as-deposited Bi2Te3 films. On vacuum annealing Te-rich Bi2Te3 films exhibit improved crystallinity with a c-axis preferred orientation. In addition, structural features related to Te and Bi2Te3 composition change with Te fraction diminishing on annealing at 300 °C. From Raman spectral studies, the presence of distinct Te-rich regions, predominantly within the interlayers of Bi2Te3, are discerned. Te becomes structurally integrated within the quintuples of Bi2Te3 lattice as intergrown layers. Disordered planar structures, mostly concentrated on crystallite surfaces result from Te accumulations as evidenced in high-resolution transmission electron microscopy lattice images and energy dispersive X-ray spectroscopy mapping. These are consistent with the observations from XRD and Raman studies further confirming Te-rich Bi2Te3 characteristics. Electrical properties of Te-rich Bi2Te3 thin films exhibit n-type semiconductor behaviour. Seebeck coefficient for as-deposited film is ~ 32 μV/K, which increases to ~ 97 μV/K on 200 °C annealing. Resistivity increases from 1.39 × 10−4 Ωcm to 18.76 × 10−4 Ωcm and power factor changes from 7.4×10−4W/K2m to 27.17×10−4W/K2m going through a maximum at 200 °C upon systematic annealing. Te-rich Bi2Te3 thin films annealed at 200 °C exhibit high power factor (∼29×10−4W/K2m) for a wide range of temperature gradients ( ΔT from 30 °C to 165 °C).

Crystallographic polarity measurements in two-terminal GaN nanowire devices by lateral piezoresponse force microscopy∗∗Contribution of an agency of the U.S. government; not subject to copyright.

Lateral piezoresponse force microscopy (L-PFM) is demonstrated as a reliable method for determining the crystallographic polarity of individual, dispersed GaN nanowires that were functional components in electrical test structures. In contrast to PFM measurements of vertically oriented (as-grown) nanowires, where a biased probe tip couples to out-of-plane deformations through the d33 piezoelectic coefficient, the L-PFM measurements in this study were implemented on horizontally oriented nanowires that coupled to shear deformations through the d15 coefficient. L-PFM phase-polarity relationships were determined experimentally using a bulk m-plane GaN sample with a known [0001] direction and further indicated that the sign of the d15 piezoelectric coefficient was negative. L-PFM phase images successfully revealed the in-plane [0001] orientation of self-assembed GaN nanowires as part of a growth polarity study and results were validated against scanning transmission electron microscopy lattice images. Combined characterization of electrical properties and crystallographic polarity was also implemented for two-terminal GaN/Al0.1Ga0.9N/GaN nanowires devices, demonstrating L-PFM measurements as a viable tool for assessing correlations between device rectification and polarization-induced band bending.

Self-formed grain boundary healing layer for highly efficient CH3NH3PbI3 perovskite solar cells

Perovskite solar cells have attracted significant research efforts due to their remarkable performance, with certified power conversion efficiency now reaching 22%. Solution-processed perovskite thin films are polycrystalline, and grain boundaries are thought to be responsible for causing recombination and trapping of charge carriers. Here we report an effective and reproducible way of treating grain boundaries in CH3NH3PbI3 films deposited by means of a Lewis acid–base adduct approach. We show by high-resolution transmission electron microscopy lattice images that adding 6 mol% excess CH3NH3I to the precursor solution resulted in a CH3NH3I layer forming at the grain boundaries. This layer is responsible for suppressing non-radiative recombination and improving hole and electron extraction at the grain boundaries by forming highly ionic-conducting pathways. We report an average power conversion efficiency of 20.1% over 50 cells (best cell at 20.4%) together with significantly reduced current–voltage hysteresis achieved by this grain boundary healing process. The grain boundaries in thin-film perovskite solar cells are responsible for non-radiative carrier recombination, which is deleterious for the optoelectronic performance. Son et al. show how to passivate the grain boundaries by using excess CH3NH3I in the precursor solution, achieving efficiencies of 20.4%.

Growth of Co-doped ZnO nanoparticles by porous alumina assisted sol–gel route: Structural optical and magnetic properties

We report a simple sol–gel method with spin coating to grow Co-doped ZnO nanoparticles (NPs) using porous alumina template. X-ray diffraction analysis confirms the wurtzite hexagonal structure for the Zn1−xCoxO NPs. Transmission electron microscopy (TEM) micrograph reveals clearly the formation of NPs with average diameter in the range 40–50 nm. High resolution TEM lattice images and the electron diffraction (SAED) pattern show that the NPs are single crystalline with lattice plane spacing of 2.47 Å for the (101) plane. UV–Vis absorption spectra show a slight blue shift in the absorption edge due to doping related modification in the band structure. PL spectra show a band edge related UV emission around 378 nm at low temperature, while defect (Zni, OV) related visible emission dominates at room temperature. Room temperature ferromagnetism (RTFM) is observed in Zn1−xCoxO NPs with the moment of 450–750 memu/g. The temperature dependent magnetization (M-T) curve shows high transition temperature (Tc) as 750 K. The observed FM is explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Co ions and Zni, OV related defects. This study provides an effective way to obtain HTFM with lower doping concentration as well as to have control over NP size by alumina pores in the Zn1−xCoxO system.

Evolution of room temperature ferromagnetism with increasing 1D growth in Ni-doped ZnO nanostructures

Zn1−xNixO (x = 0, 0.03, and 0.5) 1D nanostructures showing room temperature ferromagnetism (RT FM) with high moment have been synthesized by a solvothermal route. X-ray diffraction, transmission electron microscopy (TEM), energy dispersive X-ray spectrum (EDS) and X-ray photoelectron spectroscopy (XPS) analysis reveal the growth of single phase wurtzite structure Zn1−xNixO NRs of diameter 60–70 nm and length of 0.4–0.6 μm with the successful incorporation of Ni ions inside ZnO matrix. High resolution TEM lattice images show that all the NRs are single crystalline with a d-spacing of 2.57 Å with c-axis growth. Room temperature magnetic measurements exhibit strong ferromagnetic characteristic with magnetic moment of 1.13 emu/g, coercivity of 150 G. Photoluminescence (PL) spectra exhibit near band edge UV emission as well as defect related visible emission which is expected to play a significant role in the FM ordering, also PL spectra reveal slight band edge modification due to doping effect. Systemic structural, magnetic, and optical properties reveal that both the nature of the defects as well as Ni2+ ions are significant ingredients to attain FM characteristics with high moment and ordering temperature in the 1-dimensional ZnO NRs. Magnetic interaction is analysed using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Ni ions, Zn Vacancy and O iterstitial related defects.

Growth of crystalline Al2O3 via thermal atomic layer deposition: Nanomaterial phase stabilization

We report the growth of crystalline Al2O3 thin films deposited by thermal Atomic Layer Deposition (ALD) at 200 °C, which up to now has always resulted in the amorphous phase. The 5 nm thick films were deposited on Ga2O3, ZnO, and Si nanowire substrates 100 nm or less in diameter. The crystalline nature of the Al2O3 thin film coating was confirmed using Transmission Electron Microscopy (TEM), including high-resolution TEM lattice imaging, selected area diffraction, and energy filtered TEM. Al2O3 coatings on nanowires with diameters of 10 nm or less formed a fully crystalline phase, while those with diameters in the 20–25 nm range resulted in a partially crystalline coating, and those with diameters in excess of 50 nm were fully amorphous. We suggest that the amorphous Al2O3 phase becomes metastable with respect to a crystalline alumina polymorph, due to the nanometer size scale of the film/substrate combination. Since ALD Al2O3 films are widely used as protective barriers, dielectric layers, as well as potential coatings in energy materials, these findings may have important implications.

Simple chemical synthesis of porous carbon spheres and its improved field emission by water vapor adsorption

A very simple solvothermal approach for synthesizing porous carbon microspheres is reported here. The effect of water adsorption on the field emission properties of such samples was investigated experimentally as well as from first principle calculation. The as-synthesized microspheres were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (TEM), atomic force microscopy and Fourier transformed infrared (FTIR) spectroscopy. XRD patterns as well as TEM lattice images confirmed that the spheres were graphitic in nature. From the microscopic analyses it was seen that the as-synthesized porous carbon spheres were actually composed of few layer thick carbon nanosheets. FTIR confirmed the adsorption of water in the sample. It has been found that samples with adsorbed water can act as a better field emitter compared to the pure ones. A first principle study showed that although no development of H2O-induced impurity states close to the Fermi level was observed due to water adsorption, there was a pronounced upward shift in the Fermi level, i.e. a decrease of work function, which is believed to be the main reason for the observed improved field emission performance.

Tuning the size of aluminum oxide nanoparticles synthesized by laser ablation in water using physical and chemical approaches

Colloidal solution of nano-sized spherical Al2O3 particles were produced by nanosecond laser ablation upon irradiation on a corundum target in a distilled water environment. The effects of target inclination along the direction of laser irradiation and defocusing of the laser beam have been investigated in this study. The effect of the pH of the aqueous solution has also been studied. Synthesized particles were analyzed using transmission electron microscopy (TEM) to investigate particle shape and size distributions. Ablated nanoparticles (NPs) were spherical in shape, with the average particle size ranging from 8 to 18nm in different operating conditions. Target inclination resulted in a decrease in the average particle size. Laser defocusing at the same power and thus with reduced fluence caused a decrease in the average size and standard deviation (SD), whereas defocusing that maintained the same fluence caused the reverse effect. Phase identification of NPs performed with high resolution TEM lattice images and fast Fourier transform indicated both a metastable γ-Al2O3 phase and a stable α-Al2O3 phase. X-ray diffraction analysis was also performed, which showed peaks of both α-Al2O3 and γ-Al2O3 with the presence of α- and γ-AlO(OH) polymorphs in acidic and alkaline solution, respectively. Surface conditions of the ablated particles representing the acidic and alkaline conditions were found to have a significant influence on both the size and crystallographic phase, which indicates it may be possible to induce size and phase transitions by changing the surface chemistry.

Sol−Gel Growth of Hexagonal Faceted ZnO Prism Quantum Dots with Polar Surfaces for Enhanced Photocatalytic Activity

The hexagonal faceted ZnO quantum dots (QDs) about 3-4 nm have been prepared via a sol-gel route by using oleic acid (OA) as the capping agent. It is revealed by electron diffraction patterns and high resolution transmission electron microscopy lattice images that the profile surfaces of the highly crystalline ZnO QDs are mainly composed of {100} planes, with the Zn-terminated (001) faces and the opposite (001) faces presented as polar planes. Compared with spherical ZnO QDs, the hexagonal faceted ZnO QDs show enhanced photocatalytic activity for photocatalytic decomposition of methylene blue. A mechanism for the enhanced photocatalytic activity of the hexagonal faceted ZnO QDs for degradation of methylene blue is proposed. In addition to the large specific surface areas due to small size and high crystalline, the enhanced photocatalytic activity can mainly be ascribed to the special hexagonal morphology. The Zn-terminated (001) and O-terminated (001) polar faces are facile to adsorb oxygen molecules and OH(-) ions, resulting in a greater production rate of H(2)O(2) and OH(*) radicals, hence promoting the photocatalysis reaction. The synthesized hexagonal-shaped ZnO QDs with high photocatalytic efficiency will find widespread potential applications in environmental and biological fields.

Low-temperature growth of (2 [formula omitted] 0) ZnO nanofilm on NaCl (0 0 1) surface by ion beam sputtering

ZnO nanofilm of the (2 1 ¯ 1 ¯ 0) surface was prepared by ion beam sputtering deposition. The nanofilm was prepared on NaCl (0 0 1) surface at 200 °C to produce nearly pure (2 1 ¯ 1 ¯ 0) ZnO texture and the orientation relationship was determined and the interface discussed. Transmission electron microscopy lattice images were used to find the interface formed between ZnO nanocrystals. The ZnO nanocrystals coalesced to form a straight (0 1 ¯ 1 ¯ 2) interface. The photoluminescence spectrum from the (2 1 ¯ 1 ¯ 0) ZnO surface showed only a near-band-edge UV emission peak.

Small, highly oriented Ru grains in intermediate layer realized through suppressing relaxation of low-angle grain boundaries for perpendicular recording media

Through analyzing the growth mechanism of the Ru layer in a nonmagnetic intermediate layer (NMIL) for perpendicular magnetic recording media, a concept for the NMIL is proposed in order to realize a recording layer of small, highly c-plane oriented grains with no intergranular exchange coupling. It was found that (1) fast Fourier transform analysis of plan-view transmission electron microscopy lattice images of Ru layers revealed that hexagonal close packed Ru grains in a c-plane oriented film readily coalesce with each other due to the disappearance of low-angle tilt boundaries. (2) A promising candidate for a NMIL consists of three individual epitaxially grown functional layers: a large-grain seed layer with a highly oriented sheet texture, a first interlayer of small grains, and a second interlayer of nonmagnetic grains isolated by a segregated oxide. (3) The Ru–SiO2/Ru/Mg NMIL based on the proposed concept exhibited small (diameter: 4.8 nm) Ru grains while retaining a narrow orientation distribution of 4.1°.

Growth of nanofibrous barium carbonate on calcium carbonate seeds

Fibrous barium carbonate (BaCO 3/witherite) crystals 50–100 nm in diameter and several microns in length were grown on calcium carbonate (CaCO 3) seeds at temperatures as low as 4 °C. The BaCO 3 fibers were deposited onto calcite rhombs or CaCO 3 films using the polymer-induced liquid-precursor (PILP) process, which was induced with the sodium salt of polyacrylic acid (PAA). The structure and morphology of the resultant fibers were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and polarized light microscopy (PLM). Fibers were successfully grown on calcite seeds of various morphologies, with a range of barium concentrations, and PAA molecular weight and concentration. Two categories of fibers were grown: straight and twisted. Both types of fibers displayed single-crystalline SAED diffraction patterns, but after examining high-resolution TEM lattice images, it was revealed that the fibers were in fact made up of nanocrystalline domains. We postulate that these nanocrystalline domains are well aligned due to a singular nucleation event (i.e., each fiber propagates from a single nucleation event on the seed crystal) with the nanocrystalline domains resulting from stresses caused by dehydration during crystallization of the highly hydrated precursor phase. These BaCO 3 fibers grown on calcite substrates further illustrate the robustness and non-specificity of the PILP process.

Characterization of polycrystalline CuInSe 2 thin films deposited by sputtering and evaporation as a function of composition

In this study, a series of polycrystalline single-layer CulnSe 2 thin films with Cu/In atomic ratios between 1.7 and 0.5 were grown by using a physical vapor deposition system Cu, In sputtering, and Se evaporation. The variation in the microstructure (surface morphology, grain size, and defects) of the CulnSe 2 thin films on gradually changing the stoichiometry from Cu-rich to Cu-poor was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In general, Cu-rich thin films showed large grains with relatively low defect densities, while Cu-poor thin films exhibited highly planar defects, twins and stacking faults, with small grains. Polycrystalline bi-layer CulnSe 2 thin films were also grown by depositing a Cu-poor layer on the top of a Cu-rich layer. The bi-layer Cu-poor thin films exhibited large grains (>3 μm). High-resolution TEM lattice image shows a thin coherent layer (∼20 nm) at the both sides of the grain boundary for a slightly Cu-poor film. TEM energy dispersive X-ray spectroscopy (EDX) analyses indicate that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-poor film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse. The surface morphologies and the grain boundaries of the polycrystalline CuInSe 2 could provide an understanding of the growth behavior of CuInSe 2 and the performance of CuInSe 2 devices.

Direct Observation of Multilayer Adsorption on Alumina Grain Boundaries

Grain‐boundary films 0.6 nm in size have been observed on the grain boundaries of neodymia (Nd2O3)‐doped alumina (α‐Al2O3) sintered at 1800°C. Direct observation by high‐angle annular dark‐field imaging in the aberration‐corrected scanning transmission electron microscope shows that this type of grain‐boundary structure is the result of multilayer adsorption. Neodymium cations adsorb onto the faces of each of the two grains that comprise the grain boundary by substituting for aluminum cations. The positions of these cations are slightly distorted relative to the perfect lattice, and a third atomic layer in the core of the grain‐boundary resides between these two layers. The measurements also confirm that the thickness deduced from high‐resolution transmission electron microscopy lattice images are accurate.

Microstructure and mechanical properties of super-strong nanocrystalline tungsten processed by high-pressure torsion

Fully dense nanocrystalline tungsten (nc-W) with extremely high strength (∼3.0 GPa under quasi-static compression and ∼4.0 GPa under dynamic compression) has been obtained by high-pressure torsion (HPT) at low temperature (500 °C). The nanocrystalline microstructure is revealed by transmission electron microscopy (TEM). The grain boundaries (GBs) are mostly of the large-angle type. High-resolution TEM (lattice images) suggests that the GBs are clean and well defined (atomically sharp). GBs are non-equilibrium and of a high-energy nature. Edge dislocations are present within the grains. The authors hypothesize that these edge dislocations, combined with a depleted impurity concentrations along pre-existing GBs, contribute to enhance the ductility of nc-W. Under dynamic compression, the specimens exhibit localized shearing followed by cracking and subsequent failure, similar to their ultrafine-grain (UFG) counterparts processed by equal-channel angular pressing plus cold rolling, and to many other body-centered cubic metals with UFG/nanocrystalline microstructures. The shear band width in the HPT-processed nc-W is much smaller (shear band width <5 μm) than that observed in the UFG counterparts (shear band width ∼40 μm).

Effect of Bi surfactant on atomic ordering of GaAsSb

The addition of small quantities of a Bi surfactant is shown to have dramatic effects on the microstructure of GaAsSb alloys grown by organometallic vapor phase epitaxy (OMVPE). Epilayers grown without Bi show weak atomic ordering in plan-view selective area electron diffraction measurements, with periodicity of three or six times the [110] lattice spacing as previously reported. The addition of Bi at a ratio of 1% Bi∕Ga to the gas flow results in the appearance of strong CuAu and chalcopyrite ordering, as determined from electron diffraction measurements in both undoped, and heavily carbon doped layers. High-resolution, transmission electron microscopy lattice images clearly show the coexistence of {100}, {210} ordered and disordered structures with domain sizes of ∼10–20nm. Photoluminescence shows no band gap changes in GaAsSb samples with and without {100} and {210} ordering.

Microstructural Observations in Barium Calcium Magnesium Niobate

Microstructural studies on (1 −x)Ba(Mg1/3Nb2/3)O3–xCa(Mg1/3Nb2/3)O3 (BCMN) complex perovskite compounds, which are mixtures of Ba(Mg1/3Nb2/3)O3 (BMN) and Ca(Mg1/3Nb2/3)O3 (CMN), were conducted using scanning electron microscopy, transmission electron microscopy, and X‐ray diffractometry. Pure BMN and CMN both have a 1:2 ordered structure, via the chemical ordering of B‐site cations; however, the tilting of oxygen octahedra is involved in pure CMN, whose structure has a 1:2 ordered monoclinic unit cell that is characterized by (±1/6,±1/6,±1/6)‐type superlattice reflections in electron diffraction patterns along the [110] zone axis that is based on a simple cubic perovskite. Studies of the morphologic differences have indicated two types of inhomogeneities in a mixture of the BCMN system: (i) a rather large‐scale segregation (i.e., grain sizes of several micrometers), where the grains are separated compositionally as being barium‐rich or calcium‐rich, and (ii) fine‐scale lamellar‐type segregations 20 nm wide and 200 nm long. The segregation that is caused by Ba and Ca ions can be identified by the difference of superlattice modulations from high‐resolution transmission electron microscopy lattice images.

A Stress Relief Method to Control Warping of Focused Ion Beam Prepared Membranes for Transmission Electron Microscopy

The continued decrease in microelectronic feature dimensions has led to a reliance on the focused ion beam (FIB) for site-specific transmission electron microscopy (TEM) specimen preparation. To maximize the capabilities of the FIB, methods must be developed to consistently produce specimens thin enough to generate TEM lattice images. The limiting factor in producing quality TEM specimens by either the traditional or lift-out method is the final thickness of the specimen.The FIB is used to prepare TEM specimens by removing the bulk material that surrounds a desired feature by sputtering with a focused gallium ion beam. Successively lower beam currents are used to sputter away material until an electron transparent membrane (-0.2 μm) containing the desired feature remains. For a 300 keV TEM, lattice imaging of silicon requires additional membrane thinning to less than 0.05 μm.The loss of rigidity during the thinning process makes the membrane highly prone to deformation due to residual stresses, linear expansion, and ion beam interaction.