S. J. Lloyd

Observations of nanoindents via cross-sectional transmission electron microscopy: a survey of deformation mechanisms

Examination of cross-sections of nanoindents with the transmission electron microscope has recently become feasible owing to the development of focused ion beam milling of site-specific electron transparent foils. Here, we discuss the development of this technique for the examination of nanoindents and survey the deformation behaviour in a range of single crystal materials with differing resistances to dislocation flow. The principal deformation modes we discuss, in addition to dislocation flow, are phase transformation (silicon and germanium), twinning (gallium arsenide and germanium at 400 °C), lattice rotations (spinel), shear (spinel), lattice rotations (copper) and lattice rotations and densification (TiN/NbN multilayers). The magnitude of the lattice rotation, about the normal to the foil, was measured at different positions under the indents. Indents in a partially recrystallized metallic glass Mg66Ni20Nd14 were also examined. In this case a low-density porous region was formed at the indent tip and evidence of shear bands was also found. Further understanding of indentation deformation will be possible with three-dimensional characterization coupled with modelling which takes account of the variety of competing deformation mechanisms that can occur in addition to dislocation glide. Mapping the lattice rotations will be a particularly useful way to evaluate models of the deformation process.

Deformation under nanoindents in Si, Ge, and GaAs examined through transmission electron microscopy

Cross sections through nanoindents on Si, Ge, and GaAs {001} were examined through transmission electron microscopy. A focused ion beam workstation was used to machine electron transparent windows through the indents. In both Si and Ge there was a transformed zone immediately under the indent composed of amorphous material and a mixture of face-centered-cubic and body-centered cubic crystals. Cracking and dislocation generation were also observed around the transformed zone. In GaAs the dominant deformation mechanism was twinning on the {11} planes. The hardness of these materials is discussed in light of these observations and their macroscopic material properties such as phase transformation pressure.

Deformation under nanoindents in sapphire, spinel and magnesia examined using transmission electron microscopy

Abstract Cross-sections through nanoindentions in the (001) surface of sapphire, spinel and magnesia have been examined in the transmission electron microscope. Electron-transparent sections were prepared using a focused ion beam microscope. All three recognized high-temperature slip systems were observed in sapphire, while spinel deformed in slip bands on the {111} planes. Evidence was obtained for slip on {110} planes in magnesia and the possibility that slip also occurs on {100} planes is discussed as an explanation of its high ratio of hardness to yield stress.

Multilayered materials: a palette for the materials artist

Developments in the understanding of how materials behave enable us to design material structures to display specified properties. We introduce multilayered materials as systems in which new properties, not found in their constituents in bulk form, can emerge. The importance of transmission electron microscopy to determine structure-property relationships in nanoscale multilayers through characterization of their atomic and electronic structure is emphasized. Two examples of technologically useful multilayer systems are considered in more detail: hard coatings made from nitride multilayer films and the new structures and magnetic properties that are found in some metal multilayer systems. Finally, we discuss the future developments that are required to fully exploit the novel properties found in multilayered materials.

Development of non-weak link bulk YBCO grain boundaries for high magnetic field engineering applications

A non-weak link joining technique has been developed for YBCO pseudo-crystals fabricated by seeded peritectic solidification based on the formation of a liquid phase which segregates from the platelet boundaries at temperatures above /spl ap/920/spl deg/C. Electrical and magnetic measurements on these boundaries suggest that their irreversibility field can be as high as 7 T at 77 K in fully oxygenated pseudo-crystals joined along their crystallographic ab-planes which is comparable to the irreversibility behaviour of the adjacent YBCO grains.

Effect of coherency strain on the deformation of InxGa1-xAs superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In x Ga1− x As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3 µm and 1.1 µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.

Nanoscale SNS junction fabrication in superconductor-normal metal bilayers

We have developed a reliable and versatile technique for fabricating SNS junctions in a superconductor-normal metal bilayer using a focused ion beam microscope (FIB) in conjunction with an in-situ resistance measurement technique. This technique offers a simple method for creating multi-junction devices (SQUIDs, 3-terminal devices, arrays) with high integration densities. In this paper we discuss recent results from devices created in Nb-Cu tracks by cutting 50 nm trenches in the top Nb layer to weaken the superconducting coupling. Cuts of depths between 60 and 100% of the Nb thickness yield reproducible junctions with current voltage (I(V)) characteristics in accordance with the resistively-shunted-junction (RSJ) model, characteristic voltage I/sub C/R/sub N//spl sim/50 /spl mu/V at 4.2 K and excellent microwave response. A thorough study has been carried out of the effect on device parameters of varying the Cu layer thickness (0-175 nm). In addition transmission electron microscopy (TEM) studies have been carried out on the device structure. A two-channel model of device operation has been developed and related to the results of I/sup C/R/sub N/(T) measurements (down to 350 mK) on selected devices.

Structural characterization of TiN/NbN multilayers: X-ray diffraction, energy-filtered TEM and Fresnel contrast techniques compared.

Two TiN/NbN multilayers with wavelength 13.6 and 6.15 nm have been characterized by X‐ray diffraction (XRD), Fresnel contrast analysis (FCA) and energy‐filtered transmission electron microscopy (EFTEM). Good agreement between the composition profile obtained by FCA and EFTEM is obtained if the lower resolution of the EFTEM images is taken into account. The relative advantages and disadvantages of the techniques are discussed. Used together the two TEM techniques provide a quantitative characterization that is consistent with, and for some parameters provides more precise values than, that from XRD. The analysis shows that the multilayers have narrow interfaces (< 1 nm) and a composition amplitude close to 95% for the longer wavelength.

Growth of niobium nitride/aluminium nitride trilayers and multilayers

Abstract We examine the epitaxial growth of sputtered-deposited NbN/AIN/NbN trilayers on A- and C-plane sapphire and MgO(001), MgO(110) and MgO(111) substrates using transmission electron microscopy. Epitaxial growth is obtained on all the substrates, but columnar growth associated with the presence of more than one variant in at least one of the layers leads to a rough surface on which it is difficult to grow further layers uniformly. We also report the structure of NbN/AIN multilayers grown on silica. In this case the NbN is polycrystalline and untextured, and the AIN is predominantly amorphous. The layer roughness scales with the NbN grain size which is similar to the layer thickness, and it does not significantly increase near the top of the multilayer stacks. The composition profiles along the layer normal as determined by Fresnel contrast analysis and electron-energy-loss imaging are compared.

Measurement of magnetic domain wall width using energy-filtered Fresnel images

Magnetic domain walls in Nd2Fe14B have been examined using a series of energy‐filtered Fresnel images in the field emission gun transmission electron microscope (FEGTEM). We describe the changes in the intensity distribution of the convergent wall image as a function of defocus, foil thickness and domain wall width. The effect of tilted domain walls and beam convergence on the fringe pattern is also discussed. A comparison of the experimental intensity profile with that from simulations allows the domain wall width to be determined. Measurement of very narrow walls is made possible only by using a relatively thick foil, which necessitates energy‐filtering to allow quantitative comparison with simulations. The magnetic domain wall width in Nd2Fe14B was found to be 3 ± 2 nm.