M. De Graef

Microstructural characterization of α‐GaN films grown on sapphire by organometallic vapor phase epitaxy

Microstructure of α‐GaN films grown by organometallic vapor phase epitaxy on sapphire substrates using low temperature AlN (or GaN) buffer layers has been studied by transmission electron microscopy. The defects which penetrate the GaN films are predominantly perfect edge dislocations with Burgers vectors of the 1/3〈1120〉 type, lying along the [0001] growth direction. The main sources of threading dislocations are the low angle grain boundaries, formed during coalescence of islands at the initial stages of GaN growth. The grain sizes range from 50 to 500 nm, with in‐plane misorientations of less than 3°. The nature of these threading dislocations suggests that the defect density would not likely decrease appreciably at increasing film thickness, and the suppression of these dislocations could be more difficult.

A new symmetrized solution for phase retrieval using the transport of intensity equation.

We propose a novel symmetrization method for solving the transport of intensity equation (TIE) using fast Fourier transforms for situations where the input images may or may not exhibit spatial periodicity. The method is derived from the analysis of intensity conservation law and the internal symmetry of the TIE, and is illustrated for both a computational and an experimental data set.

Demagnetization factors for elliptic cylinders

The magnetometric (volume averaged) demagnetization factors for cylinders with elliptical cross section are computed using a Fourier-space approach and compared with similar results obtained with a different treatment. The demagnetization factors are given as a series expansion in the eccentricity � of the elliptical cross section, where the terms up to order � 10 are given explicitly as a function of the cylinder aspect ratio. Other simplified expressions, valid in restricted regimes, are also given. Two different series expansions, obtained previously and valid in particular combinations of shape parameters, are recalled and compared with the new results. After the computation of the magnetostatic and exchange-energy terms associated with a vortex closure-domain state in the elliptic cylinder, the single-domain limit, or the critical size below which the structure can support quasi-uniform magnetization, is derived and discussed.

Growth and characterization of (111) and (001) oriented MgO films on (001) GaAs

The effects of substrate preparation on the structure and orientation of MgO films grown on (001) GaAs using pulsed laser deposition has been investigated. Textured MgO films displaying a (111)MgO∥(001)GaAs orientation relation with x‐ray rocking curve full width at half maximum (FWHM) values as low as 1.8° were obtained in cases where the native GaAs surface oxide was only partially desorbed prior to growth. Reflection high‐energy electron diffraction, transmission electron microscopy (TEM), and x‐ray pole figure analysis of these films reveals a preferential orientation within the plane of the substrate: [110]MgO∥[110]GaAs and [112]MgO∥[110]GaAs. An interfacial layer (∼5 nm thick) was observed in high resolution TEM analysis, and was attributed to a remnant native GaAs oxide layer. Complete desorption of the native GaAs oxide at ∼600 °C in vacuum prior to MgO growth led to significant surface roughening due to Langmuir evaporation, and resulted in randomly oriented polycrystalline MgO films. Growth o...

Magnetic properties of carbon‐coated, ferromagnetic nanoparticles produced by a carbon‐arc method

The Kratschmer–Huffman carbon‐arc method of preparing fullerenes has been used to generate carbon‐coated transition metal (TM) and TM‐carbide nanocrystallites. The magnetic nanocrystallites were extracted from the soot with a magnetic gradient field technique. For TM=Co the majority of nanocrystals exist as nominally spherical particles, 0.5–5 nm in radius. Hysteretic and temperature‐dependent magnetic response, in randomly and magnetically aligned powder samples frozen in epoxy, correspond to fine particle magnetism associated with monodomain TM particles. The magnetization exhibits a unique functional dependence on H/T, and hysteresis below a blocking temperature TB. Below TB, the temperature dependence of the coercivity can be expressed as Hc=Hc0[1−(T/TB)1/2], where Hc0 is the 0 K coercivity.

Diffraction contrast STEM of dislocations: imaging and simulations.

The application of scanning transmission electron microscopy (STEM) to crystalline defect analysis has been extended to dislocations. The present contribution highlights the use of STEM on two oppositely signed sets of near-screw dislocations in hcp α-Ti with 6wt% Al in solid solution. In addition to common systematic row diffraction conditions, other configurations such as zone axis and 3g imaging are explored, and appear to be very useful not only for defect analysis, but for general defect observation. It is demonstrated that conventional TEM rules for diffraction contrast such as g·b and g·R are applicable in STEM. Experimental and computational micrographs of dislocations imaged in the aforementioned modes are presented.

On the computation of the demagnetization tensor field for an arbitrary particle shape using a Fourier space approach

A method is presented to compute the demagnetization tensor field for uniformly magnetized particles of arbitrary shape. By means of a Fourier space approach it is possible to compute analytically the Fourier representation of the demagnetization tensor field for a given shape. Then, specifying the direction of the uniform magnetization, the demagnetizing field and the magnetostatic energy associated with the particle can be evaluated. In some particular cases, the real space representation is computable analytically. In general, a numerical inverse fast Fourier transform is required to perform the inversion. As an example, the demagnetization tensor field for the tetrahedron will be given.

The evolution of metastable Bƒ borides in a TiAlB alloy

Additions of boron to titanium aluminides in the α2 to γ composition range leads to the formation of a variety of boride particles. Information available on the ternary phase diagram indicates that the equilibrium borides are TiB(B27), Ti3B4(D7b) and TiB2(C32). Conventional and high resolution transmission microscopy of the borides produced in a dilute Ti-40.97Al-0.97B alloy revealed the presence of numerous monoboride particles with the Bƒ(CrB-type) structure. This metastable form of TiB evolves during solidification, nucleating on preexisting particles of the same compound with the normal B27 structure. In addition, the Bƒ particles contain faults equivalent to nanoscale intergrowths of the D7b and C32 structures. The evolution of the boride phases in this system was analyzed in terms of crystallographic relationships elicited from structural models, all of which are based on th same trigonal building block of 6 metal atoms around each B atom. A Mie potential calculation was used to assess the relative stability of the monoboride forms. It is proposed that the Bƒ structure arises in response to a change in the melt chemistry which would produce Ti3B4 or TiB2 under equilibrium conditions. The preferential formation of Bƒ is ascribed to its closer crystallographic relationship with the B27 structure, which facilitates its nucleation on existing TiB crystals.

Nucleation of misfit and threading dislocations during epitaxial growth of GaSb on GaAs(001) substrates

The initial stages of molecular beam epitaxy of GaSb on highly mismatched GaAs(001) substrates were investigated. Transmission electron microscopy was used to analyze the defect structure in GaSb islands and at their interfaces with GaAs(001) at different stages of growth. Based on experimental observations, we propose that the semiperiodic net of 90° misfit dislocations at the GaSb/GaAs(001) interface nucleate homogeneously at the leading edges of advancing {111} planes. After nucleation, they glide inwards along the interface plane to reach their equilibrium position. Threading dislocations in GaSb layers were directly correlated with the misfit dislocation net. We demonstrate that there are no threading defects in GaSb islands when their interfaces consist solely of 90° misfit dislocations, and that threading dislocations in the GaSb epilayer are all associated with minority 60° misfit dislocations nucleated in growing islands. The number of threading dislocations per unit area of the GaSb film is found to be independent of GaSb coverage, indicating that island coalescence does not substantially increase the number of 60° dislocations.

A Model Based Iterative Reconstruction Algorithm For High Angle Annular Dark Field-Scanning Transmission Electron Microscope (HAADF-STEM) Tomography

High angle annular dark field (HAADF)-scanning transmission electron microscope (STEM) data is increasingly being used in the physical sciences to research materials in 3D because it reduces the effects of Bragg diffraction seen in bright field TEM data. Typically, tomographic reconstructions are performed by directly applying either filtered back projection (FBP) or the simultaneous iterative reconstruction technique (SIRT) to the data. Since HAADF-STEM tomography is a limited angle tomography modality with low signal to noise ratio, these methods can result in significant artifacts in the reconstructed volume. In this paper, we develop a model based iterative reconstruction algorithm for HAADF-STEM tomography. We combine a model for image formation in HAADF-STEM tomography along with a prior model to formulate the tomographic reconstruction as a maximum a posteriori probability (MAP) estimation problem. Our formulation also accounts for certain missing measurements by treating them as nuisance parameters in the MAP estimation framework. We adapt the iterative coordinate descent algorithm to develop an efficient method to minimize the corresponding MAP cost function. Reconstructions of simulated as well as experimental data sets show results that are superior to FBP and SIRT reconstructions, significantly suppressing artifacts and enhancing contrast.