Cross-sectional Transmission Electron Microscopy Research Articles

Sharp interface of undoped Ge/SiGe quantum well grown by ultrahigh vacuum chemical vapor deposition

An undoped Ge/SiGe quantum well has been grown by ultrahigh vacuum chemical vapor deposition, and the sharp interface with a characteristic length of 0.6 nm is confirmed by cross-sectional transmission electron microscopy and electron energy loss spectroscopy. In addition, a 2D hole gas with a high mobility of up to 4.6 × 105 cm2/V s is achieved in the Hall-bar shaped field effect transistor, showing a low percolation density of 8.7 × 1010 cm−2, a light hole effective mass of 0.071 m0, and a high effective g-factor of 11.3. These favorable properties confirm the benefits of high-quality interface, which has promising applications in the field of qubits.

Rapid phase formation and interfacial reaction of YBa2Cu3O7−δ coated conducts by using high-rate crystallization of fluorine-free chemical solution

In the present work, the high-rate epitaxial growth for superconducting YBa2Cu3O7-δ (YBCO) films on the substrates of LaMnO3(LMO)/IBAD-MgO/Y2O3/Al2O3/Hastelloy is achieved by fluorine-free metal organic deposition (FF-MOD) by rapidly switching the partial oxygen pressure (pO2) at a fixed temperature. Further investigation into the effects of temperature and switching pO2 on the phase transition of YBCO elucidates that the growth mode and microstructural characteristics of the studied films. BaMnO3 heterogeneous particles are observed in localized regions in the films by cross-sectional transmission electron microscopy, which may be generated by the interfacial reactions between the substrate and transient liquid phases. The present work implies the feasibility of FF-MOD for the rapid growth of YBCO films on LMO buffered metallic substrates, regardless of interfacial reactions during the preparation.

Chemical vapor deposition of sp2-boron nitride on mechanically polished pyrolytic boron nitride substrates

Nanocrystalline sp2-boron nitride (BN) thin films have been grown for 2 and 4.5 h on mechanically polished polycrystalline pyrolytic boron nitride substrates at 1060 °C via chemical vapor deposition using diborane (B2H6) and ammonia (NH3) as the B- and N sources, respectively. The use of an N/B gas phase ratio of 200 resulted in sp2-BN thin films having a stoichiometric surface chemistry, as determined by x-ray photoelectron spectroscopy. Cross-sectional transmission electron microscopy revealed the initial growth of ∼4 nm thick partially ordered [0001] sp2-BN layers regardless of the crystallographic orientation of the sets of layers in the substrate. A transition zone then formed within the deposited films that contained randomly oriented polycrystalline grains and that culminated in the formation of protrusions. Atomic force microscopy verified an increase in roughening of the surfaces of the films with an increase in growth time and the associated thickness.

Multiple layers of aluminum silicate and silicon dioxide deposited by room-temperature atomic layer deposition for enhanced cation sorption

Multiple layers of aluminum silicate and SiO2 are deposited at room temperature (RT) by repeating a combination of aluminum-silicate atomic layer deposition (ALD) and a plural number of SiO2 ALDs. The RT-ALD is performed by using precursors of tris(dimethylamino)silane and trimethyl aluminum (TMA). The oxidizing gas is plasma excited humidified Ar. A cross-sectional transmission electron microscope and angle resolved x-ray photoelectron spectroscopy suggest that aluminum from the TMA molecule is diffused to SiO2 to form an aluminum silicate layer with the Al rich surface at RT. The deposited film shows an enhanced Na sorption in an NaCl aqueous solution of 10 mM, compared with the pure aluminum silicate layer without the SiO2 layer. It is assumed that the bond formation of Al–O–Al in the aluminum silicate film is suppressed by limiting the thickness of the aluminum silicate layer. Multiple films are also applicable for Cs and K ion sorption. The effects of the multiple layers on the ion sorptivity are discussed in the paper.

Absorption enhancement in visible range from Fano resonant silicon nanoparticle arrays embedded in single crystal Mg:Er:LiNbO3 synthesized by direct ion implantation

Fano resonant Si nanoparticles (NPs) are synthesized in single-crystal Mg:Er:LiNbO3 using ion implantation and subsequent thermal annealing. The structural and optical properties of the Si NPs embedded in the crystal have been investigated. Spherical particles with radius of about 60 nm are observed by cross-sectional transmission electron microscope, while ion beam analysis are used to characterize the NPs formation process. The absorption of the Mg:Er:LiNbO3 crystals have been enhanced significantly due to the embedded Si NPs, which are induced by the Fano resonance effect in the visible light wavelength band. Periodic structures of spherical Si particles model is proposed and analyzed using the Mie theory to study the optical response features and local fields. As a result, numerical simulations demonstrate that periodicities of the array of Si NPs can yield narrow resonant peaks connected with multiple light scattering by the NPs and displaying a Fano-type resonant profile. The wavelengths of the absorption peak show clear red shift with increasing the radius of NPs and the peak intensity can be enhanced by decreasing the array period. This work opens an avenue to modulate the optical filed by embedding Fano resonant Si NPs for potential application in optical devices.

Chemical imaging of oxide confinement layers in VCSELs

The authors have studied the lateral oxidation of layers buried in vertical-cavity surface-emitting lasers using cross-sectional scanning transmission electron microscopy coupled with electron energy loss spectroscopy. Chemical maps and composition profiles of the oxidized AlO x layers have been produced. The sensitivity is such that trace compositions of a few % As and Ga can be detected in the AlO x with a spatial resolution of a few nanometers on the recorded chemical maps. To demonstrate the performance of the mapping technique, we compare results from an area in the vertical-cavity surface-emitting laser (VCSEL) which is pure to an oxidized area. These measurements are performed on a thin sample prepared by the focused ion beam technique within an actual VCSEL, which makes the mapping technique applicable for degradation investigations in devices at different stages of their lifetime. More generally, this measurement method is effective for detailed evaluation of AlO x layers and their fabrication process.

Formation of high density stacking faults in polycrystalline 3C-SiC by vibration-assisted diamond cutting

Revealing the ductile deformation mechanisms of ultra-hard brittle cubic silicon carbide (3C-SiC), as well as their correlations with microstructure evolution, are crucial for facilitating the ductile machinability of the ceramic material. In the present work, we report the formation of highly oriented high density stacking faults accompanied with suppressed amorphization and cracking in polycrystalline 3C-SiC in ultrasonic elliptical vibration-assisted diamond cutting, which contributes to significantly enhanced ductile material removal of the ceramic material compared to ordinary cutting. Specifically, characterizations of Raman spectroscopy on machined surface and cross-sectional transmission electron microscopy on subsurface, as well as molecular dynamics simulations of the two kinds of cutting processes, are jointly performed to elucidate the mechanisms of phase transformation and microstructure evolution that govern the ductile material removal of polycrystalline 3C-SiC under the vibration assistance. In particular, the formation mechanisms of highly oriented high density stacking faults emitted from grain boundaries are revealed. Current findings provide insights into the ductile deformation behavior of hard brittle ceramics enhanced by field-assisted deformation process.

Improvement in perpendicular magnetic anisotropy and its voltage control efficiency in CoFeB/MgO tunnel junctions with Ta/Mo layered adhesion structures

By utilizing Ta/Mo layered adhesion structures, thermally robust perpendicular magnetic anisotropy and voltage-controlled magnetic anisotropy (VCMA) effects were achieved in magnetic tunnel junctions (MTJs) with ultrathin CoFeB films grown on MgO. After annealing at 400 °C, MTJs with Ta/Mo layered adhesion exhibited VCMA coefficients of 48 fJ/Vm. The combination of Ta and Mo improved the crystalline orientation and flatness of the CoFeB/MgO tunneling barrier interfaces, as determined by cross-sectional scanning transmission electron microscopy. Additionally, we demonstrate that the small interdiffusion between Mo and CoFe enables effective scavenging of B from CoFeB by increasing the thickness of the B sink layer without impairing the device performance due to atomic diffusion.

Epitaxial single-crystal hexagonal boron nitride multilayers on Ni (111).

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene1-3, hexagonal boron nitride (hBN)4-6 and transition metal dichalcogenides7,8 have been grown. hBN is considered to be the 'ideal' dielectric for 2D-materials-based field-effect transistors (FETs), offering the potential for extending Moore's law9,10. Although hBN thicker than a monolayer is more desirable as substrate for 2D semiconductors11,12, highly uniform and single-crystal multilayer hBN growth has yet to be demonstrated. Here we report the epitaxial growth of wafer-scale single-crystal trilayer hBN by a chemical vapour deposition (CVD) method. Uniformly aligned hBN islands are found to grow on single-crystal Ni (111) at early stage and finally to coalesce into a single-crystal film. Cross-sectional transmission electron microscopy (TEM) results show that a Ni23B6 interlayer is formed (during cooling) between the single-crystal hBN film and Ni substrate by boron dissolution in Ni. There are epitaxial relationships between hBN and Ni23B6 and between Ni23B6 and Ni. We also find that the hBN film acts as a protective layer that remains intact during catalytic evolution of hydrogen, suggesting continuous single-crystal hBN. This hBN transferred onto the SiO2 (300 nm)/Si wafer acts as a dielectric layer to reduce electron doping from the SiO2 substrate in MoS2 FETs. Our results demonstrate high-qualitysingle-crystal multilayered hBN over large areas, which should open up new pathways for making it a ubiquitous substrate for 2D semiconductors.

Microscopic investigation of Cu-induced crystallization of amorphous carbon at low temperatures

Graphene or graphene-like carbon-reinforced copper matrix composites have gained extensive application prospects in aerospace and electronic fields due to their high electrical, thermal conductivity and excellent machining performance. The preparation of Cu/graphene composites at a relatively low temperature is highly demanded for practical applications. Herein, we develop an approach for synthesizing few-layer graphene (FLG) by contacting amorphous carbon (a-C) with Cu at a low annealing temperature of 300 °C. The mechanism of such low-temperature growth of graphene was discussed by the combined experimental methods of scanning electron microscopy, Auger electron spectroscopy, cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. It is found that the formation of FLG is mediated by the grain-boundary (GB) diffusion, interfacial diffusion and surface diffusion of carbon atoms simultaneously. The high density of Cu GBs can provide a fast diffusion path for “free” C atoms in the a-C sublayer, and the graphene layer was formed at the Cu GBs, at the Cu/a-C interface and on top of the Cu layer where carbon atoms are present. The Cu-induced growth of graphene at low temperatures may provide a promising support for synthesizing Cu/graphene composites in the future.

Transmission Electron Microscopy Study on the Effect of Thermal and Electrical Stimuli on Ge2Te3 Based Memristor Devices

Memristor devices fabricated using the chalcogenide Ge2Te3 phase change thin films in a metal-insulator-metal structure are characterized using thermal and electrical stimuli in this study. Once the thermal and electrical stimuli are applied, cross-sectional transmission electron microscopy (TEM) and X-ray energy-dispersive spectroscopy (XEDS) analyses are performed to determine structural and compositional changes in the devices. Electrical measurements on these devices showed a need for increasing compliance current between cycles to initiate switching from low resistance state (LRS) to high resistance state (HRS). The measured resistance in HRS also exhibited a steady decrease with increase in the compliance current. High resolution TEM studies on devices in HRS showed the presence of residual crystalline phase at the top-electrode/dielectric interface, which may explain the observed dependence on compliance current. XEDS study revealed diffusion related processes at dielectric-electrode interface characterized, by the separation of Ge2Te3 into Ge- and Te- enriched interfacial layers. This was also accompanied by spikes in O level at these regions. Furthermore, in-situ heating experiments on as-grown thin films revealed a deleterious effect of Ti adhesive layer, wherein the in-diffusion of Ti leads to further degradation of the dielectric layer. This experimental physics-based study shows that the large HRS/LRS ratio below the current compliance limit of 1 mA and the ability to control the HRS and LRS by varying the compliance current are attractive for memristor and neuromorphic computing applications.

Growth of self-assembled nanovoids embedded AlN layer on a low- temperature buffer by metal organic chemical vapor deposition

In this study, we demonstrate a high-quality aluminum nitride (AlN) film with a thickness of 2.5 μm grown on 4H-SiC substrate using a three-dimensional (3D) buffer via high-temperature metal-organic chemical vapor deposition. For the buffer layer, 3D structure morphology without polycrystals was selected by the temperature change from 850 to 1350°C. The main layer was grown on the 3D buffer, and self-assembled nanovoids were formed, enabling the growth of a high-quality AlN film without cracks. However, many cracks occurred in the sample without a buffer. The full width at half maximum of the X-ray rocking curve was 144/368 arcsec for (002)/(102) planes, respectively. Cross-sectional transmission electron microscopy showed nanovoids and a rapid defect reduction effect at the interface of the buffer layer.

Identification of Interface Structure for a Topological CoS2 Single Crystal in Oxygen Evolution Reaction with High Intrinsic Reactivity.

Transition metal chalcogenides such as CoS2 have been reported as competitive catalysts for oxygen evolution reaction. It has been well confirmed that surface modification is inevitable in such a process, with the formation of different re-constructed oxide layers. However, which oxide species should be responsible for the optimized catalytic efficiencies and the detailed interface structure between the modified layer and precatalyst remain controversial. Here, a topological CoS2 single crystal with a well-defined exposed surface is used as a model catalyst, which makes the direct investigation of the interface structure possible. Cross-sectional transmission electron microscopy of the sample reveals the formation of a 2 nm thickness Co3O4 layer that grows epitaxially on the CoS2 surface. Thick CoO pieces are also observed and are loosely attached to the bulk crystal. The compact Co3O4 interface structure can result in the fast electron transfer from adsorbed O species to the bulk crystal compared with CoO pieces as evidenced by the electrochemical impedance measurements. This leads to the competitive apparent and intrinsic reactivity of the crystal despite the low surface geometric area. These findings are helpful for the understanding of catalytic origins of transition metal chalcogenides and the designing of high-performance catalysts with interface-phase engineering.

Deposition of silicon oxide films on silicon using HelixJet - an atmospheric-pressure plasma jet process below 100 °C

Silicon oxide films are widely applied for their superior dielectric, chemical and mechanic properties as well as for their resistance against reactive chemicals. Simultaneously, there is an increasing number of applications which demand a low deposition temperature. In this work, we compare the material properties of SiOx layers deposited on silicon at ca. 70 °C by HelixJet, an atmospheric-pressure plasma jet deposition (PA) using double helix electrodes, with those of SiO2 layers thermally grown or deposited by plasma-enhanced chemical vapour deposition. According to cross-sectional transmission electron microscopy, the interface between the PA oxide and the Si substrate is, similar to the case of thermal oxide, smooth with no apparent defects. In addition, the electrically active interface defect density is only one order of magnitude higher than that of the interface between thermal oxide and Si. In contrast, the bulk of the PA oxide significantly differs from that of thermal oxide. Right after deposition, the PA oxide contains carbon and hydrogen in a concentration of several at%, and the SiO2 network comprises several active centres. After furnace annealing, the formation of the SiO2 network is completed and the optical and electrical properties of the PA oxide converge to that of thermal oxide.

Kinetically Controlled Epitaxial Growth of Fe3GeTe2 van der Waals Ferromagnetic Films

We demonstrate that kinetics play an important role in the epitaxial growth of Fe$_3$GeTe$_2$ (FGT) van der Waals (vdW) ferromagnetic films by molecular beam epitaxy. By varying the deposition rate, we control the formation or suppression of an initial tellurium-deficient non-van der Waals phase (Fe$_3$Ge$_2$) prior to realizing epitaxial growth of the vdW FGT phase. Using cross-sectional scanning transmission electron microscopy and scanning tunneling microscopy, we optimize the FGT films to have atomically smooth surfaces and abrupt interfaces with the Ge(111) substrate. The magnetic properties of our high quality material are confirmed through magneto-optic, magnetotransport, and spin-polarized STM studies. Importantly, this demonstrates how the interplay of energetics and kinetics can help tune the re-evaporation rate of chalcogen atoms and interdiffusion from the underlayer, which paves the way for future studies of van der Waals epitaxy.

Microstructural study of Ni and Ni-20Cr particles plasma sprayed on stainless steel substrate at 300 ˚C

In thermal spray coatings, the bonding and adherence of the spray particles (called splats) to the substrate surface significantly influence the overall quality of the finished coating. In this study, both elemental Ni and Ni-20Cr powders were plasma sprayed onto polished stainless-steel substrates held at 300 ˚C. Splat formation for both feedstock materials was investigated and compared. Detailed studies of the microstructure of single splats as well as the splat-substrate interface were performed using scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The additions of Cr to nickel promoted improved adherence of the splat to the substrate and modified splat formation by improving wettability during spreading. More intriguingly, energy dispersive spectroscopy (EDS) mapping and line scans, performed on TEM cross-sections, revealed splat-substrate inter-diffusion and chemical inter-mixing. Of particular note, a ‘jet’ of iron/chromium-based material, arising presumably from the substrate, was noted to be intimately intermixed into the interior of the nickel splat.

Study on Irradiation Response of Nanocrystalline Phase in Sm-Doping Fluorapatite Glass-Ceramics under He Ion Irradiation.

Two different of Sm-loading fluorapatite (Ca10−2xNaxSmx(PO4)6F2, x = 1 and 2) glass-ceramics were synthesized by a two-step melt sintering method. The samples were irradiated with 50 keV He+ ions with a fluence of 2.6 × 1016 ions/cm2 at 593 K. The irradiation induced microstructural evolution were characterized by grazing incidence X-ray diffraction and cross-sectional transmission electron microscopy. For the smaller Sm-doping samples, no phase transformation is observed. Meanwhile, in the lager Sm-doping samples, the irradiation induced the crystals into smaller nanocrystals. The mechanism of the transformation of the crystalline phase was also analyzed and discussed.

Relationship between the Formation of Magnetic Clusters and Hexagonal Phase of Gold Matrix in AuxFe1-x Nanophase Thin Films.

AuxFe1−x nanophase thin films of different compositions and thicknesses were prepared by co-deposition magnetron sputtering. Complex morpho-structural and magnetic investigations of the films were performed by X-ray Diffraction, cross-section Transmission Electron Microscopy, Selected Area Electron Diffraction, Magneto Optical Kerr Effect, Superconducting Quantum Interference Device magnetometry and Conversion Electron Mössbauer Spectroscopy. It was proven that depending on the preparation conditions, different configurations of defect α-Fe magnetic clusters, i.e., randomly distributed or auto-assembled in lamellar or filiform configurations, can be formed in the Au matrix. A close relationship between the Fe clustering process and the type of the crystalline structure of the Au matrix was underlined, with the stabilization of a hexagonal phase at a composition close to 70 at. % of Au and at optimal thickness. Due to different types of inter-cluster magnetic interactions and spin anisotropies, different types of magnetic order from 2D Ising type to 3D Heisenberg type, as well as superparamagnetic behavior of non-interacting Fe clusters of similar average size, were evidenced.

In Situ Transmission Electron Microscopy Study of Shrinkage Kinetics of CH4N-Molecular-Ion-Implantation-Induced Extended Defects

The thermal stability of end-of-range (EOR) defects formed in a CH4N-molecular-ion-implanted epitaxial silicon (Si) wafer was studied by transmission electron microscopy (TEM) observation. By plan-view TEM observation, we found that the density and size of the CH4N-ion-implantation-induced EOR defects negligibly changed upon heat treatment at temperatures below 1000 °C, whereas the EOR defect density was drastically reduced by heating at 1100 °C. This result suggests that almost all CH4N-ion-implantation-induced EOR defects were sufficiently thermally stable to maintain their size at temperatures below 1000 °C, and that above 1100 °C, most of the EOR defects lost their stability, shrank and finally dissolved. Additionally, by in situ cross-sectional TEM observation during heat treatment, we found a large difference in the shrinkage rates of the EOR defects between at the beginning of heat treatment and the last minute of just before defect disappearance. We found that the EOR defects began to gradually shrank at the beginning of heat treatment (1st stage), and then the shrinkage rate rapidly increased (2nd stage), finally resulting in the dissolution of the defects. The activation energies for the shrinkage of EOR defects in the 1st and 2nd stages (E D-1 and E D-2) were found to be 7.55 ± 1.03 and 4.57 ± 0.32 eV, respectively. The shrinkage behavior in the 1st stage is likely to be due to the thermally activated desorption of C and N species that segregated along the edge of an EOR defect. On the other hand, from the E D-2 value, the shrinkage behavior in the 2nd stage is deduced to be due to the desorption of interstitial Si atoms. These findings suggest that this two-stage shrinkage behavior is peculiar to the EOR defects formed in the CH4N-ion-implanted epitaxial Si wafer, and that the interaction between the EOR defect and the impurities segregated at the edge of the defect affects the thermal robustness of the molecular-ion-implantation-induced EOR defects.

Temperature dependence of capacitance–voltage characteristics of germanium telluride thin films

Temperature-dependent capacitance–voltage and conductance–voltage measurements of 200-nm-thick germanium telluride (GeTe) thin films deposited via direct current magnetron sputtering were investigated. Measurements were performed across a temperature range of 27–357 °C, with a sweep of voltage values from −20 to +20 V at an operating frequency of 1 MHz. An amorphous–crystalline transition temperature (Tc) of 197 °C was found. Below Tc, the GeTe films were highly resistive, and their capacitance was insensitive to the applied bias voltage and temperature. Above Tc, the capacitance showed strong dependence on temperature, owing to the triggering of GeTe crystallization incorporating nucleation and growth processes. Applied bias voltage did not show a profound impact on the GeTe capacitance even after crystallization, but it had an observable impact on the film conductance beyond 250 °C. Moreover, the GeTe thin film capacitance increased by a factor of 13 upon crystallization. Hall-effect measurements revealed that the film conductivity increased upon crystallization, as did the hole concentration and mobility. Cross-sectional transmission electron microscopy demonstrated that the GeTe thin films crystallize via a nucleation-dominated crystallization mechanism.