John W Steeds

The symmetry of electron diffraction zone axis patterns

The convergent beam and bend extinction contour techniques of electron microscopy are capable of providing much more information than can be obtained from conventional diffraction patterns and it is the objective of this work to examine the symmetry properties of each of these patterns. The diffraction of fast electrons by a thin parallelsided slab has been studied by group theory and by a graphical construction. We find that the pattern symmetries may be described by thirty-one diffraction groups and that each of these diffraction groups is isomorphic to one of the point groups of diperiodic plane figures and to one of the thirty-one Shubnikov groups of coloured plane figures. A graphical representation of each diffraction group is given, together with tables showing how the diffraction groups are related to the specimen point groups and under certain assumptions to the crystal point groups. These tables assume the symmetric Laue condition and ignore the presence of irreducible lattice translations normal to the slab. By using the tables, crystal point groups can be obtained from convergent beam or bend contour patterns. The method is demonstrated by experiments on several materials, but particularly on germanium and gallium-arsenide specimens since the similarity of these materials exemplifies the sensitivity of the technique.

Determination of lattice polarity for growth of GaN bulk single crystals and epitaxial layers

The polarity of the lattice of bulk single GaN crystals and the polarity of homoepitaxial and heteroepitaxial‐on‐sapphire GaN thin films has been studied using convergent beam electron diffraction. Diffraction patterns obtained at 200 kV for the 〈1–100〉 projection of GaN were matched with calculated patterns. The lattice orientations of two commonly observed bulk single‐crystal facets were identified. It is shown that the smooth facets in single crystals correspond to the (0001), Ga‐terminated, lattice planes, whereas the rough facets correspond to the (0001), N‐terminated, planes. It is also shown that metalorganic chemical vapor deposition epitaxy retains the polarity of the substrate, i.e., no inversion boundaries were observed. Heteroepitaxy on sapphire is shown to grow in the (0001), Ga‐terminated orientation.

Characterization of dislocations in GaN by transmission electron diffraction and microscopy techniques

A combination of transmission electron microscopy imaging and diffraction techniques is used to characterize crystal defects in homoepitaxial GaN thin films. The Burgers vectors of dislocations is established by combining large‐angle convergent beam electron diffraction and conventional diffraction contrast techniques. It is shown that dislocations with Burgers vectors c, a, and c+a are present. Evidence is presented that dislocation segments lying in the interfacial plane are dissociated on a fine scale. The significance of the observations for understanding homoepitaxial growth of GaN is discussed.

Microdiffraction as a tool for crystal structure identification and determination

Abstract Microdiffraction patterns obtained with a focused and nearly parallel incoherent electron beam allow one to obtain the crystal system and the Bravais lattice and to reveal the presence of glide planes and screw axes of a specimen. These crystallographic features are easily and reliably obtained, in a systematic way, from a few patterns by means of tables and theoretical patterns established for each crystal system. They lead to an extinction symbol which is in agreement with a small number of possible space groups. Usually, these possible space groups belong to different point groups which may be distinguished by study of the intensity distributions in the diffraction patterns. Consequently the space group or a small sub-set of consistent space groups can be deduced. The technique is therefore very well adapted to phase identification. It can be used as a routine technique and applied to most specimens and especially to small particles, and this constitutes its main advantage. It is particularly valuable with specimens which give poor convergent beam electron diffraction (CBED) patterns. It can also be used in connection with CBED.

Higher order Laue zone effects in electron diffraction and their use in lattice parameter determination

By fitting small probe-forming lenses into a conventional electron microscope, we have been able to observe higher order Laue zone (h.o.l.z.) diffraction effects from high symmetry zone axes of a wide variety of materials. Cooling the specimen with liquid nitrogen both greatly reduces the contamination rate and increases the visibility of the h.o.l.z. lines. An interpretation of these lines is given in terms of the dispersion surface construction and conditions for the visibility of h.o.l.z. effects are deduced. A theory from which numerical solutions have been obtained is outlined. Using h.o.l.z. lines, we can deduce the microscope operating voltage or the lattice parameter of the specimen to approximately one part in a thousand; relative changes can be measured about five times more precisely. The spatial resolution of the technique is approximately 10 nm. Strain gradients within the illuminated area can produce fringe patterns.

Convergent Beam Electron Diffraction

Publisher Summary This chapter discusses convergent beam electron diffraction (CBED). For the purpose of an introduction to CBED, a typical transmission electron microscope (TEM) sample of crystalline material with thickness varying from 20 nm to opacity that is subject to a reasonable amount of bending is considered. If an aperture is inserted into a plane of the microscope that is conjugate with the specimen plane, under conditions of parallel illumination, a selected-area diffraction pattern can be obtained by operation of the appropriate switch. Such a pattern is composed of a set of discrete points that are created by diffracted beams caused by Bragg reflection from the selected region of the crystal lattice. While these patterns are very useful for measuring the angles between diffraction planes and their relative spacing and for recording diffuse scattering caused by disorder in the specimen, the intensities are essentially meaningless. This situation exists because of the nature of electron diffraction and the characteristics of typical TEM samples.

Observation of coreless dislocations in α-GaN

Abstract Large-angle convergent beam electron diffraction is used to show that hollow tubes, 5–25 nm in diameter, observed in α-GaN grown on sapphire by metalorganic chemical vapor deposition, contain screw dislocations with Burgers vectors c (0.52 nm). It is shown that these coreless dislocations are not in an equilibrium state but may arise by the trapping of dislocations at pinholes at an early stage of GaN growth. Although pinholes may contain a , c and c + a dislocations, it is argued that only those containing c dislocations can survive to generate nanopipes of constant cross-section along the [0 0 0 1] axis, as observed experimentally.

Luminescence studies of individual dislocations in II-VI (ZnSe) and III-V (InP) semiconductors

Results are presented of high-resolution luminescence studies from individual dislocations and related defects in ZnSe and InP performed in a transmission electron microscope. In the case of ZnSe unusual luminescence bands (Y at 2.60 eV and S at 2.52 eV) originally observed in photoluminescence studies are attributed to dislocations. In some instances, complete quenching of the excitonic transitions was observed to correlate with the presence of Y emission from complex dislocation tangles. In the case of individual screw dislocations this quenching of the exciton luminescence was found to be variable; for example reduction of the exciton signal was not always observed. For InP, donor-exciton-related transitions were quenched at individual screw dislocations. Donor-acceptor pair/free-to-bound and deep level (band C) transitions were unaffected. For the case of InP, unlike ZnSe, no dislocation-related luminescence was observed within the system detection limit (0.7-4.0 eV).

Field emission from chemical vapor deposited diamond and diamond-like carbon films: Investigations of surface damage and conduction mechanisms

Field emission properties of undoped chemical vapor deposited diamond and diamond-like carbon films have been measured for a variety of different deposition conditions. The nature and appearance of the damage site after testing has been investigated with scanning electron microscopy and laser Raman mapping. These observations, together with the mathematical form of the observed current–voltage relations, are correlated with the conductivity of the film. The results are consistent with a model for the overall emission current that combines conduction mechanisms through the bulk of the film with Fowler–Nordheim tunneling.

Broken hexagonal symmetry in the locked-in state of 2Ha-TaSe2 and the discommensurate microstructure of its incommensurate CDW states

Discommensuration arrays have been seen directly in the electron microscope for the first time in a charge-density-wave-bearing material. The material, 2H-TaSe2, is also discovered to be orthorhombic in its commensurate phase (projected space group Cmcm) and not hexagonal. This leads to a natural description of the occurrence of the incommensurate stripe phase, observed only on warming. It likewise is orthorhombic. A new double-honeycomb discommensurate model is supported for the symmetric incommensurate phase. The remarkable dynamics and inhomogeneity in this wealth of microstructure is of much relevance to the new generation of fluctuation and transition theory papers on these and related incommensurate systems.