Cross-sectional Transmission Electron Microscopy Specimens Research Articles

Rapid Synthesis of Semipolar ZnO Nanorod Arrays on M-Sapphire by Microwave-Assisted Chemical Bath Deposition

ZnO is a wide-band gap (3.37 eV) II–VI semiconductor with large exciton binding energy of 60 meV which is larger than other materials commonly used for light emission devices, such as ZnSe (22 meV) and GaN (25 meV). Especially ZnO nanorods due to their unique physical and chemical properties have many important applications for devices, such as transparent electronics, sensors, detectors, etc. It is well known that wurtzite ZnO in c-plane orientation has strong polarization which significantly reduces light-emitting efficiency. Though the polarization is eliminated for nonpolar ZnO, it is difficult to grow nonpolar ZnO nanorods as they are normally grown along c-axis normal to the substrate surface. Polarization can be greatly reduced for semipolar ZnO which is likely to be obtained if nanorods can be grown with c-axis in an inclined angle with substrate surface; however, well-aligned semipolar oriented ZnO nanorods have been rarely reported. Here, we report the first observations of growth of epi-ZnO nanorods with semipolar orientation on m-sapphire by microwave-assisted chemical bath deposition (CBD) with a fast growth rate. ZnO nanorods have been widely grown by CBD which is a simple, low cost and low temperature process but with a low growth rate generally in the range of 5-10 nm/min. Microwave-assisted CBD is a newly developed technique because of microwave can directly transmit its energy to dielectric medium by radiation, such that quick and uniform heating without much energy loss caused by conduction is feasible for aqueous solution. In recent years, microwave heating has been demonstrated to have much higher ZnO nucleation and growth rates than with conventional heating. Furthermore, a several microns thick epitaxial c-plane ZnO film can be deposited on MgAl2O4 (111) substrate in just a few minutes by microwave assisted CBD. In this study, 1 x 1 cm2 m-sapphire substrates were seeded with semipolar ZnO at a temperature between 500-700 °C by using mist chemical vapor deposition. The seeded substrate was then placed in a 90 °C aqueous solution for microwave-assisted CBD of ZnO nanorods. The solution was pre-formulated at room temperature with 0.0375 mol/L zinc nitrate hexahydrate and 0.0375 mol/L hexamethylenetetramine. The microwave power can be automatically tuned to maintain the solution at a constant temperature during CBD. Microstructural characterization was performed with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) to understand the growth process. Cross-sectional TEM specimens were prepared with focused ion beam. Photoluminescence (PL) spectroscopy was performed with He-Cd laser for measurements of optical properties. For ZnO nanorods formed after growth for 10 minutes at 90°C, SEM in Fig. 1 shows that tilted hexagonal columns of ZnO with diameter about 100-200 nm are formed as a well-aligned and inclined array on the sapphire. Figure 2 shows a typical bright field TEM micrograph in cross-section view, illustrating that the ZnO nanorods grown on the 30 nm thick seed layer have an inclined angle about 30° with m-sapphire surface and an average length about 500 nm which gives a growth rate of 50 nm/min and an aspect ratio of 2.5-5 for the nanorods. Furthermore, high-resolution TEM and atomic-resolution annular dark field STEM micrographs with FFT patterns of sapphire and ZnO as shown in Fig. 2 reveal that the semipolar ZnO seed layer has an epitaxial relationship with m-sapphire as ZnO[1-100] || sapphire[11-20] and ZnO(0001)|| sapphire(1-10-4). Hence, the c-plane of ZnO seed has a 30° tilt away from the surface normal of the sapphire, resulting in the following growth of nanorods along the c-axis direction. As a result, the inclined nanorods has a semipolar orientation of (-1-122) || sapphire m-plane. It should be pointed out that ZnO (-1-122) reflection is hardly seen in XRD as its interplanar spacing is very close to that of sapphire (3-300). A typical PL spectrum of the ZnO nanorods measured at 10K is shown in Fig. 3 from which a strong narrow near band emission (NBE) peak centered at 3.31 eV is clearly seen with and a weak deep-level emission peak at around 2.17 eV, implying that the nanorods have a reasonably good quality. Keywords: ZnO, nanorod, chemical bath deposition microwave Figure 1

Correlative Magnetic Imaging of Heat-Assisted Magnetic Recording Media in Cross Section Using Lorentz TEM and MFM

In order to increase the storage density of hard disk drives, a detailed understanding of the magnetic structure of the granular magnetic layer is essential. Here, we demonstrate an experimental procedure of imaging recorded bits on heat-assisted magnetic recording (HAMR) media in cross section using Lorentz transmission electron microscopy (TEM). With magnetic force microscopy and focused ion beam (FIB), we successfully targeted a single track to prepare cross-sectional TEM specimens. Then, we characterized the magnetic structure of bits with their precise location and orientation using Fresnel mode of Lorentz TEM. This method can promote understanding of the correlation between bits and their material structure in HAMR media to design better the magnetic layer.

Electron Microscopy of the Tin-oxide Nanolayer Formed on the Surface of Sn-Ag-Cu Alloys

Sn-Ag-Cu alloy used in the present study is commercial Sn-3.0Ag-0.5Cu solder ball alloys with a diameter of 400 and 300 μm which were long term atmospheric oxidized for about 6 years (specimen-1) and under high temperature/humidity at 85°C and relative humidity of 85% for 2140 h, respectively. Morphologies and nanostructure of the oxide nanolayers formed on the surface of Sn-Ag-Cu alloys were studied from the interface of the oxide film and the tin substrate by transmission electron microscopy (TEM) to verify the oxidation mechanism. Cross-sectional TEM specimens were prepared using a focused-ion-beam (FIB) micro-sampling technique. Before the FIB fabrication, the specimen surface was coated with carbon (C) and tungsten (W) films. Inhomogeneous thickness of tin-oxide nanolayer formed on specimen-1 and specimen-2 were fluctuated between 20-40 nm and 40-50 nm, respectively. The nanolayer on specimen-1, however, consists of polycrystalline SnO and SnO2, whereas the one on the specimen-2 comprises of polycrystalline SnO2. High resolution (HRTEM) image and fast Fourier transformation (FFT) spectra corresponding to the interface and the substrate areas have confirmed those results. The results verify that at very long atmospheric oxidation Sn was gradually oxidized to be SnO (Sn2+) and then SnO2 (Sn4+), in which SnO is present at the region closed to interface between Sn-substrate and the tin-oxide layer. At high temperature oxidation, however, Sn was completely oxidized to be SnO2.

Cross-Sectional Transmission Electron Microscopy Specimen Preparation Technique by Backside Ar Ion Milling

Cross-Sectional Transmission Electron Microscopy Specimen Preparation Technique by Backside Ar Ion Milling

Electron microscopy observations of the spinel-forming reaction using MgO nanocubes on Al2O3 substrates

The morphology evolution and associated topotactic relations between MgO nanocubes deposited on electron-transparent Al2O3 substrates were monitored after repeated high-temperature ex situ heat treatments. Owing to the well-defined morphology of MgO smoke cubes and flat basal-plane-oriented substrate, the initial orientation relationship is constrained to be {100}MgO || (0001)sapphire. In this geometry, only one rotational degree of freedom is allowed for MgO particles, and hence, a full set of coincident site lattices are formed, providing the opportunity to examine thermodynamic and kinetic processes and track competing surface and bulk ion-diffusion mechanisms during spinel formation. Crystallographic orientation relationships (ORs) between the sapphire (Al2O3) substrate, the magnesia (MgO) smoke nanoparticles, and the MgAl2O4 spinel reaction products were studied before and after annealing in the temperature range between 1000 and 1100 °C. The ORs adopted between the different pairs of materials were studied using single (0001)-oriented sapphire crystals pre-thinned for transmission electron microscopy (TEM) observations; the spinel/sapphire interface was further investigated on cross-section TEM specimens prepared from bulk samples using the focused ion-beam technique. At temperatures below ~1050 °C, the prevailing OR is \( \left\langle {110} \right\rangle \cdot \left\{ {111} \right\}_{\text{spinel}} //\left\langle {10\bar{1}0} \right\rangle \cdot \left( {0001} \right)_{\text{sapphire}} , \) whereas above that temperature \( \left\langle {110} \right\rangle \cdot \left\{ {100} \right\}_{\text{spinel}} //\left\langle {10\bar{1}0} \right\rangle \cdot \left( {0001} \right)_{\text{sapphire}} \) becomes more common. With the increasing temperature also the morphology of the spinel product is transformed from hexahedral to octahedral. The different ORs and microstructures appear to depend on the reaction temperature and result in different dominating diffusion mechanisms.

A Cross-Sectional TEM Specimen of a Multilayer Thin Film Prepared Using the FIB Technique

A focused ion beam (FIB) mill equipped with a microsampling (MS) unit and combined with transmission electron microscopy (TEM)/scanning TEM-energy dispersive x-ray spectroscopy (STEM-EDXS) is a powerful tool for studies of the functional advanced materials. For the studies, the specimen must be prepared as a thin foil which is tranparent to the electron beam. Focused ion beam is very effective method for fabricating TEM specimen of the cross-sectional thin film with the “lift-out” technique using a tungsten (W)-needle probe as a micromanipulator. A multilayer film of MgB2/Ni deposited on a Si (001) substrate prepared by FIB-MS technique is presented. Before FIB fabrication, the surface of the multilayer film was coated with W-film to prevent the surface from bombardment by the ion beam. A bright field (BF)-STEM image of the multilayer film related to two-dimensional (2D) elemental mapping clearly showed the presence of MgB2-and Ni-nanolayers. The measured experimental spacing between Ni-nanolayers was comparable with the actual specimen design, but the thickness of Ni-nanolayer was not. Unexpected nanostructures of the formation of SiO2 film on the substrate surface and holes within the film were observed.

Formation of omega and Ti2Ni phases in a residual Ti–Ni–Ti multilayer between stainless steel and zirconia after bonding at 1173 K

Stainless steel (316L) and 8 mol pct Y2O3-stabilized zirconia (8Y-ZrO2) were bonded using a Ti–Ni–Ti multilayer at 1173 K (900 °C) for 1 h. Cross-sectional transmission electron microscopy specimens were prepared by an innovative focused ion beam plus lift-out technique. In addition to acicular α-Ti, the dissolution of Fe, Cr, and Ni diffusing outwards from 316L into β-Ti led to the precipitation of the omega (ω) phase with different variants in the residual Ti foil between 316L and Ni. The ω-phase was not found in the residual Ti foil between Ni and 8Y-ZrO2, while Ti2Ni precipitates were precipitated in some α-Ti grains owing to the exclusion of Ni from the β-Ti.

Oxidation and reduction behavior of Ni and NiO layers sputter deposited onto yttrium-stabilized zirconia single crystals

Thin single-crystal yttrium-stabilized zirconia (YSZ) substrate was prepared by indentation fracture and mechanical polishing. The specimen was analyzed in detail by transmission electron microscopy (TEM). The (110) edge surface was faceted, in contrast to the smooth (001) edge surface, and the facet surfaces were identified as {111}-type planes. Good cross-sectional TEM specimens comprised of crystalline Ni and NiO layers deposited on YSZ edge surface could be prepared by sputtering of a Ni support grid using Ar + ion milling and subsequent re-deposiotion on the smooth (001) fracture surface of the YSZ specimen. The epitaxial growth of a pure Ni layer on the YSZ edge planes occurred during ion milling in vacuum. However, subsequent ion milling of the specimen after exposure in air for several minutes resulted in the formation of a NiO layer on top of the first Ni layer. Reduction of the NiO layer was confirmed by electron energy-loss spectroscopy after annealing at 973 K in a vacuum of 1.2 × 10 −5 Pa. This Ni layer was re-oxidized upon annealing in air at 1073 K for 1 h. The deposition behavior of the Ni and NiO layers was discussed on the basis of the surface oxidation of Ni layer.

In situ and ex situ transmission electron microscopy investigation of Cu–Al–Cu–Ti reactive metallic multilayer coatings

Metallic multilayers of Cu/Al/Ti composition were studied by transmission electron microscopy (TEM) and plasmon energy-loss mapping as prototypes of nanoscale reactive multilayer systems with exothermic alloy formation in oxygen-free conditions. The selection and arrangement of alloy phases by the system during ex situ and in situ heating experiments were found to depend not only on temperature but strongly on the initial volume ratios of metals, and to a lesser degree on the dimensionality of the reactive sample. Here, a two-dimensional sample was represented by ex situ heating of the full multilayer structure, a one-dimensional sample refers to in situ heating of thin cross-sectional TEM specimens, while a zero-dimensional sample (or metallic dot-array) was obtained after cutting thin pillars using focused ion beams. Lamellar self-organized alternation between Heusler phase and Cu9Al4 was found.

Transmission Electron Microscopy Analysis of Mo–W–S–Se Film Sliding Contact Obtained by Using Focused Ion Beam Microscope and In Situ Microtribometer

To better understand the fundamentals of solid lubrication, microstructural analyses on the wear scar surface and contact interface of Mo–W–S–Se composite films produced by pulsed laser deposition were completed. Focused ion beam (FIB), transmission electron microscopy (TEM), and X-ray energy dispersive spectroscopy were employed to study the cross-sectional microstructure and chemistry of wear scars. In particular, a novel microtribometer was built for in situ tribological measurements within a FIB microscope. The sliding tip was welded in contact to the wear scar surface on the film under load by re-deposition of sputtering materials from the FIB cut of the tip. Using this technique, cross-sectional TEM specimens were prepared precisely at the contact point without tip/film separation. Here, the in situ FIB microtribometer is critically important for retaining the microstructure of lubricant films as formed at the sliding contact interface between the tip and film without separation. It provides the unique ability to stop sliding, section the contact, and reveal microstructural changes to that contact without disrupting the sliding interface. The cross-sectional TEM measurements were performed on the sliding contact interface for both the regions in contact and just past contact, and both the reorientation and recrystallization of lubricant films were revealed.

A cross-sectional transmission electron microscopy study of iron recovered from a laser-heated diamond anvil cell

This paper discusses the use of a focused ion beam for the preparation of cross-sectional transmission electron microscopy specimens from small samples, typically 150 micron in diameter, that have been recovered from a laser-heated diamond anvil cell. We present preliminary observations of iron melted with helium as the pressure-transmitting medium to pressures near 4 GPa. The project is designed to investigate entrapment mechanisms of the inert gases in metals and silicates at high pressure and temperatures.

Effects of surface relaxation on convergent‐beam electron diffraction analysis of stress in silicon

Convergent-beam electron diffraction on cross-sectional transmission electron microscopy specimens can map strains in the silicon substrate of microelectronics devices with high spatial resolution. However, at shallow depths below the interface, most of the diffraction lines within a convergent-beam electron diffraction pattern are split, rendering pattern interpretation impossible in the classic way. The splitting effect was systematically analysed for a variety of materials, and the same qualitative behaviour that can be explained by stress relaxation at the surfaces of the thin transmission electron microscopy specimen was observed. The effects of surface relaxation are modelled by finite elements simulations. The results predict well the experimental magnitude of the splitting for a variety of diffraction lines at different positions below the interface, but fail to simulate the intensity of the secondary lines. Possible reasons for such discrepancies are discussed and assessed.

Nanoscale strain analysis of strained-Si metal-oxide-semiconductor field effect transistors by large angle convergent-beam electron diffraction

In this letter we report the characterization of local compressive strain in p-type strained-silicon channel metal-oxide-semiconductor field effect transistors by large angle convergent-beam electron diffraction. The compressive strain was induced into the channel region of gate length of 80nm strained-Si p-type metal-oxide-semiconductor field effect transistor by Ge preamorphization implantation for source/drain extension. A method to distinguish between compressive strain and shear strain in the cross-sectional transmission electron microscopy specimens is proposed.

Nanostructural characterization of YBCO films on metal tape with textured buffer layer fabricated by pulsed-laser deposition

Thick YBa2Cu3O7-x (YBCO) films with high critical current density (Jc) values were deposited by pulsed-laser deposition (PLD) on Hastelloy with a textured CeO2/Gd2Zr2O7 buffer layer. Both cross-sectional and plan-view TEM specimens of the YBCO films were prepared, and then the nanostructural characterization of the films was performed by transmission electron microscopy (TEM). The YBCO films less than 1 μm thick were predominantly composed of c-axis-oriented grains, however, many a-axis-oriented grains, which grew larger with the increase of the thickness of the YBCO film, were formed beyond about 1 μm from the CeO2 interface. We found Y2O3 and copper oxides between a- and c-axes-oriented grains. In particular, Y2O3 grains were formed between the {001} plane of an a-axis-oriented grain and the {100} or {010} plane of a c-axis-oriented grain. The orientation relationships between Y2O3 and YBCO are found to be; (001)YBCO//(001)Y2O3 and (100)YBCO//(110)Y2O3. In addition, we also found gaps between YBCO grains. Since a-axis-oriented grain growth and the formation of Y2O3, copper oxides and the gaps are considered to reduce the J c values of the YBCO film, it is important to determine the optimum process conditions to suppress the nucleation of a-axis-oriented grains, impurity oxides and gaps.

Threading dislocations in Ba0.7Sr0.3TiO3/La0.7Sr0.3MnO3 epitaxial films grown on (001) LaAlO3 substrate

The microstructures of Ba0.7Sr0.3TiO3/La0.7Sr0.3MnO3 (BST/LSMO) thin films epitaxially grown on (001) LaAlO3 substrate by pulsed laser deposition were investigated by transmission electron microscopy (TEM) and related techniques. Both cross-sectional and plan-view TEM specimens were studied. The predominant defects in the films are a high density of threading dislocations (TDs) perpendicular or quasi-perpendicular to the substrate surface. Diffraction contrast and high-resolution transmission electron microscopy (HRTEM) techniques reveal that the TDs are mostly pure-edge-type with Burgers vectors b=<100> or <110>. There are a few TDs of screw character, and convergent beam electron diffraction (CBED) analysis shows the screw component of their Burgers vectors is ±[001]. The formation mechanisms of the TDs are discussed.

Cross-sectional TEM observation of iron–zinc intermetallic Γ and Γ 1 phases in commercial galvannealed IF steel sheets

The Γ phase and Γ 1 phase in commercial galvannealed interstitial free (IF) steel sheets were studied by transmission electron microscopy (TEM). The cross-sectional TEM specimen was prepared using focused ion beam (FIB) technique which was also introduced briefly. The results show that there exist superlattice reflections in the selected diffraction patterns of both Γ phase and Γ 1 phase, which indicate that both of the Γ phase and Γ 1 phase are ordered intermetallic compounds. A Γ phase cell can be regarded as a α-Fe cell, which enlarges lattice constant three times in the x, y, z directions and keeps the same crystal structure with the Zn atoms occupying some positions of Fe atoms. A Γ 1 phase can be formed by doubling the Γ phase cell in the x, y, z directions, while the crystal structure transforms from BCC to FCC.

Electron diffraction study on iron–zinc Γ 1 intermetallic phase in a galvannealed IF steel sheet

A series map of selected area electron diffraction patterns(SADPs) of Γ 1 phase in commercial galvannealed IF steel sheet were investigated by transmission electron microscopy(TEM). The cross-sectional TEM specimen was prepared using focused ion beam (FIB) technique which was also introduced briefly. The results show that there exist superlattice reflections in the SADPs of Γ 1 phase which indicate that Γ 1 phase is a kind of ordered intermetallic compound with twice times period. It is different from the six times period structure of Γ 1 single phase specimen which was prepared by special method.

Study of the near-surface of hot- and cold-rolled AlMg0.5 aluminium alloy

Rolling is known to alter the surface properties of aluminium alloys and to introduce disturbed near-surface microcrystalline layers. The near-surfaces of mostly higher alloyed materials were investigated by various techniques, often combined with a study of their electrochemical behaviour. Cross-sectional transmission electron microscopy (TEM), after ion milling or ultramicrotomy, indicated the presence of disturbed layers characterized by a refined grain structure, rolled-in oxide particles and a fine distribution of intermetallics. Those rolled-in oxide particles reduce the total reflectance of rolled Al alloys. Furthermore, various depth profiling techniques, such as AES, XPS, SIMS and qualitative glow discharge optical emission spectroscopy (GD-OES) have been used to study the in-depth behaviour of specific elements of rolled Al alloys. Here, the surface and near-surface of AlMg0.5 (a commercially pure rolled Al alloy with addition of 0.5 wt.% Mg) after hot and cold rolling, and with and without additional annealing is studied with complementary analytical techniques. Focused ion beam thinning is introduced as a new method for preparing cross-sectional TEM specimens of Al surfaces. Analytical cross-sectional TEM is used to investigate the microstructure and composition. Measuring the total reflectance of progressively etched samples is used as an optical depth profiling method to derive the thickness of disturbed near-surface layers. Quantitative r.f. GD-OES depth profiling is introduced to study the in-depth behaviour of alloying elements, as well as the incorporation of impurity elements within the disturbed layer. The GD-OES depth profiles, total reflectance and cross-sectional TEM analyses are correlated with SEM/energy-dispersive x-ray observations in GD-OES craters. Copyright © 2005 John Wiley & Sons, Ltd.

Studies of lattice mismatch and threading dislocations in GaAs/Si grown by MBE

GaAs/Si(001) thin films with six Si interlayers grown by molecular beam epitaxy (MBE) were examined by transmission electron microscopy (TEM). The interface structure of the films was investigated by using both plan-view and cross-sectional TEM specimens. It was seen that the films have a smooth interface associating with misfit and threading dislocations between the GaAs epilayer and the Si substrate. It was also observed that the Si interlayers had an undulated shape reducing the threading dislocations in the structure. It was obtained that the strain relaxation at the interface occurs by misfit and threading dislocations. The g · b contrast experiments displayed 60 ∘-type misfit dislocations with a/2 〈110〉{111} slip system in the structure. Electron diffraction experiments were also performed to find out the reduction of the lattice mismatch and the strain left in the structure using the cross-sectional GaAs/Si(001) samples.

Interfacial reactions of Ti/n-GaN contacts at elevated temperature

Interfacial reactions in Ti/GaN contacts have been studied using conventional and high-resolution transmission electron microscopy (TEM), energy-filtered TEM (EFTEM), and x-ray diffraction. The thin film contacts were fabricated by evaporating Ti on n-GaN and subsequent rapid thermal annealing in argon. An x-ray result shows that the as-deposited Ti on GaN is an epitaxial hcp phase, whereas a fcc phase was identified from the as-deposited metal layer in cross-sectional TEM specimens. This phenomenon is interpreted as the transformation of hcp-Ti phase to fcc-TiHx phase by hydrogen incorporation during TEM specimen thinning. At elevated temperature, first gallium and then nitrogen diffused into the metal layer. The reaction front moved into the Ti layer, and after annealing at 700 °C many voids were formed along the Ti/GaN interface at the GaN side. A sequence of phases GaN/TiN/Ti2GaN/Ti3Ga/Ti was identified in annealed contacts, and corresponds to the expected diffusion path. The ternary phase Ti2GaN was confirmed with electron diffraction and EFTEM. A planar TiN layer, which formed in direct contact to the GaN, presumably governs the electrical properties of the alloyed contact.