J. S. Barnard

Electron-beam-induced strain within InGaN quantum wells: False indium “cluster” detection in the transmission electron microscope

InGaN quantum wells have been found to be extremely sensitive to exposure to the electron beam in the transmission electron microscope (TEM). High-resolution TEM images acquired immediately after first irradiating a region of quantum well indicates no gross fluctuations of indium content in the InGaN alloy. During only a brief period of irradiation, inhomogeneous strain is introduced in the material due to electron beam damage. This strain is very similar to that expected from genuine nanometer-scale indium composition fluctuations which suggests there is the possibility of falsely detecting indium-rich “clusters” in a homogeneous quantum well.

Optical and microstructural studies of InGaN/GaN single-quantum-well structures

We have studied the low-temperature (T=6K) optical properties of a series of InGaN∕GaN single-quantum-well structures with varying indium fractions. With increasing indium fraction the peak emission moves to lower energy and the strength of the exciton–longitudinal-optical (LO)-phonon coupling increases. The Huang–Rhys factor extracted from the Fabry–Perot interference-free photoluminescence spectra has been compared with the results of a model calculation, yielding a value of approximately 2nm for the in-plane localization length scale of carriers. We have found reasonable agreement between this length scale and the in-plane extent of well-width fluctuations observed in scanning transmission electron microscopy high-angle annular dark-field images. High-resolution transmission electron microscopy images taken with a short exposure time and a low electron flux have not revealed any evidence of gross indium fluctuations within our InGaN quantum wells. These images could not, however, rule out the possible ...

Determination of the indium content and layer thicknesses in InGaN/GaN quantum wells by x-ray scattering

We have determined the indium content and the layer thicknesses in an InGaN epilayer and InGaN/GaN quantum well structures by high-resolution x-ray diffraction (XRD) using the (002) reflection. The thickness of the total repeat (an InGaN well plus a GaN barrier) in the superlattice is easily determined from the spacing between the satellite peaks in an omega/2theta scan. Measurement of the individual thickness of InGaN and GaN layers and the indium content is less straightforward, since for multilayer structures the peak positions are influenced by both the indium content and the thickness ratio of the GaN to the InGaN layer. Thus, several different models may give reasonable fits to data collected over a limited range (about 1° omega/2theta either side of the (002)) showing only lower-order (−3 to +3) satellite peaks. Whenever possible, we have collected data over a wide range (about 4° omega/2theta) and determined the thickness ratio by examination of the relative intensities of weak higher-order satell...

On the origin of threading dislocations in GaN films

A series of GaN films were grown by metalorganic vapor phase epitaxy on nitrided sapphire using an initial annealed low-temperature nucleation layer (LT-NL), without employing any conventional threading dislocation (TD) reduction methods. Film thicknesses ranging from the LT-NL to 500 nm were used. The island network morphology was investigated at each growth stage using atomic force microscopy. Data from cathodoluminescence studies showed initially uniform luminescence, followed by the gradual development of bright (low TD) regions which had lateral sizes different from the island sizes at all times and which continued to increase in size after coalescence. The formation of low-energy arrays of a-type TDs also continued after island coalescence. X-ray diffraction, transmission electron microscopy (TEM) and AFM data indicated that the highest (a+c)-type TD densities were found in the LT-NL, but subsequently decreased due to TD loop formation (promoted by island facets) and reaction to produce a-type TDs. ...

Organization of Nanoparticles in Polymer Brushes

We have demonstrated a facile infiltration process, in which gold nanoparticles are assembled into block copolymer brushes. After solvent annealing, the polymer-covered nanoparticles are either sequestered into the corresponding block copolymer domain or expulsed from the brush, depending on the shell density of the nanoparticles.

High-angle triple-axis specimen holder for three-dimensional diffraction contrast imaging in transmission electron microscopy.

Electron tomography requires a wide angular range of specimen-tilt for a reliable three-dimensional (3D) reconstruction. Although specimen holders are commercially available for tomography, they have several limitations, including tilting capability in only one or two axes at most, e.g. tilt-rotate. For amorphous specimens, the image contrast depends on mass and thickness only and the single-tilt holder is adequate for most tomographic image acquisitions. On the other hand, for crystalline materials where image contrast is strongly dependent on diffraction conditions, current commercially available tomography holders are inadequate, because they lack tilt capability in all three orthogonal axes needed to maintain a constant diffraction condition over the whole tilt range. We have developed a high-angle triple-axis (HATA) tomography specimen holder capable of high-angle tilting for the primary horizontal axis with tilting capability in the other (orthogonal) horizontal and vertical axes. This allows the user to trim the specimen tilt to obtain the desired diffraction condition over the whole tilt range of the tomography series. To demonstrate its capabilities, we have used this triple-axis tomography holder with a dual-axis tilt series (the specimen was rotated by 90° ex-situ between series) to obtain tomographic reconstructions of dislocation arrangements in plastically deformed austenitic steel foils.

Atom probe tomography assessment of the impact of electron beam exposure on InxGa1−xN/GaN quantum wells

This study addresses the ongoing debate concerning the distribution of indium in InxGa1−xN quantum wells (QWs) using a combination of atom probe tomography (APT) and transmission electron microscopy (TEM). APT analysis of InxGa1−xN QWs, which had been exposed to the electron beam in a TEM, revealed an inhomogeneous indium distribution which was not observed in a control sample which had not been exposed to the electron beam. These data validate the effectiveness of APT in detecting subtle compositional inhomogeneities in the nitrides.

Revealing all types of threading dislocations in GaN with improved contrast in a single plan view image

In this letter, we demonstrate a transmission electron microscope based technique which reveals all types of threading dislocations (TDs) in GaN with high contrast over a relatively large area, even if the specimen is bent. This method uses a bright-field image with the crystal oriented at the ⟨1–21–3⟩ zone axis, taken using multi-beam diffraction conditions. Such an image reveals all screw, edge, and mixed types of threading dislocations. The multi-beam imaging technique described here for GaN is more generally applicable to counting the total dislocation density in a wide range of materials and structures.

Three-dimensional analysis of dislocation networks in GaN using weak-beam dark-field electron tomography

We have developed a new method of tomographically reconstructing extended three-dimensional dislocation networks using weak-beam dark-field (WBDF) imaging in a TEM. A series of WBDF images is recorded every few degrees over a large tilt range, while ensuring that the dark-field reflection used for imaging maintains a constant deviation parameter. With suitable filtering of the WBDF images prior to tomographic reconstruction, the three-dimensional distribution of dislocations is reproduced with high fidelity and high spatial resolution. The success of this approach is demonstrated for heteroepitaxial Mg-doped GaN films. The fidelity of the tomographic reconstruction varies with the dislocation line-vector and elastic anisotropy of the material.

Dislocation tomography made easy: a reconstruction from ADF STEM images obtained using automated image shift correction

After previous work producing a successful 3D tomographic reconstruction of dislocations in GaN from conventional weak-beam dark-field (WBDF) images, we have reconstructed a cascade of dislocations in deformed and annealed silicon to a comparable standard using the more experimentally straightforward technique of STEM annular dark-field imaging (STEM ADF). In this mode, image contrast was much more consistent over the specimen tilt range than in conventional weak-beam dark-field imaging. Automatic acquisition software could thus restore the correct dislocation array to the field of view at each tilt angle, though manual focusing was still required. Reconstruction was carried out by sequential iterative reconstruction technique using FEIs Inspect3D software. Dislocations were distributed non-uniformly along cascades, with sparse areas between denser clumps in which individual dislocations of in-plane image width 24 nm could be distinguished in images and reconstruction. Denser areas showed more complicated stacking-fault contrast, hampering tomographic reconstruction. The general three-dimensional form of the denser areas was reproduced well, showing the dislocation array to be planar and not parallel to the foil surfaces.