Composition and strain relaxation of In x Ga1-x N graded core-shell nanorods.

Two In x Ga1-x N nanorod samples with graded In compositions of x = 0.5-0 (Ga-rich) and x = 0.5-1 (In-rich) grown by molecular beam epitaxy were studied using transmission electron microscopy. The nanorods had a wurtzite crystal structure with growth along [Formula: see text] and core-shell structures with an In-rich core and Ga-rich shell. Energy-dispersive x-ray analysis confirmed grading over the entire compositional range and showed that the axial growth rate was primarily determined by the In flux, and the radial growth rate by the Ga flux. There was no evidence of misfit dislocations due to grading, but the strain due to the lattice mismatch between the In-rich core and Ga-rich shell was relaxed by edge dislocations at the core-shell interface with Burgers vectors [Formula: see text] and [Formula: see text].

Observation of antisite domain boundaries in Cu2ZnSnS4 by atomic-resolution transmission electron microscopy.

Atomic resolution transmission electron microscopy has been used to examine antisite defects in Cu2ZnSnS4 (CZTS) kesterite crystals grown by a hot injection method. High angle annular dark field (HAADF) imaging at sub-0.1 nm resolution, and lower magnification dark field imaging using reflections sensitive to cation ordering, are used to reveal antisite domain boundaries (ADBs). These boundaries, typically 5-20 nm apart, and extending distances of 100 nm or more into the crystals, lie on a variety of planes and have displacements of the type ½[110] or »[201], which translate Sn, Cu and Zn cations into antisite positions. It is shown that some ADBs describe a change in the local stoichiometry by removing planes of S and either Cu or Zn atoms, implying that these boundaries can be electrically charged. The observations also showed a marked increase in cation disorder in regions within 1-2 nm of the grain surfaces suggesting that growth of the ordered crystal takes place at the interface with a disordered shell. It is estimated that the ADBs contribute on average ∼0.1 antisite defect pairs per unit cell. Although this is up to an order of magnitude less than the highest antisite defect densities reported, the presence of high densities of ADBs that may be charged suggests these defects may have a significant influence on the efficiency of CZTS solar cells.

Distinguishing cubic and hexagonal phases within InGaN/GaN microstructures using electron energy loss spectroscopy.

3D InGaN/GaN microstructures grown by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been extensively studied using a range of electron microscopy techniques. The growth of material by MBE has led to the growth of cubic GaN material. The changes in these crystal phases has been investigated by Electron Energy Loss Spectroscopy, where the variations in the fine structure of the N K-edge shows a clear difference allowing the mapping of the phases to take place. GaN layers grown for light emitting devices sometimes have cubic inclusions in the normally hexagonal wurtzite structures, which can influence the device electronic properties. Differences in the fine structure of the N K-edge between cubic and hexagonal material in electron energy loss spectra are used to map cubic and hexagonal regions in a GaN/InGaN microcolumnar device. The method of mapping is explained, and the factors limiting spatial resolution are discussed.

Compositional variations in In(0.5)Ga(0.5)N nanorods grown by molecular beam epitaxy.

The composition of InxGa1 - xN nanorods grown by molecular beam epitaxy with nominal x = 0.5 has been mapped by electron microscopy using Z-contrast imaging and x-ray microanalysis. This shows a coherent and highly strained core-shell structure with a near-atomically sharp boundary between a Ga-rich shell (x âˆ¼ 0.3) and an In-rich core (x âˆ¼ 0.7), which itself has In- and Ga-rich platelets alternating along the growth axis. It is proposed that the shell and core regions are lateral and vertical growth sectors, with the core structure determined by spinodal decomposition.

Diffusion of Mn in ZnO nanowires annealed in air.

We studied diffusion of Mn in ZnO nanowires by means of field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence and Raman scattering spectroscopy. The Mn-diffused samples were prepared by covering synthesized ZnO nanowires with a Mn chip and then annealing at temperatures between 200 and 1000 degrees C for 1 h in air. Microstructural analyses, and photoluminescence and Raman studies revealed that Mn atoms started diffusing in ZnO nanowires at 800 degrees C. The annealing-temperature increase up to 1000 degrees C led to a strong diffusion of Mn in the ZnO host lattice, which caused the blueshift of the ultra-violet emission. Concurrently, recored Raman scattering spectra showed some additional Mn-related modes. The origin of these lines was discussed in detail.

CBED study of grain misorientations in AlGaN epilayers.

Large angle convergent beam electron diffraction (LACBED) has been used to examine AlGaN epilayers grown by facet-controlled epitaxial lateral overgrowth on GaN/(0001) sapphire substrates in prototype UV laser structures. The substrates, defined by masks with seed openings along a <10-10> stripe direction, had GaN seed columns with {11-22} surfaces. Studies were carried out on cross-sectional samples cut perpendicular to the stripe axis. An LACBED analysis of the orientation of (000 2) planes, and of the (11-20) planes parallel to the stripe axis, revealed that the AlGaN wings were both rotated by angles of 1-2 x 10(-2)radians about the 10-10 stripe axis with respect to the underlying GaN, and distorted due to misfit strains. It is shown that the results are consistent with the observed structure of the AlGaN/GaN and the wing/wing boundaries, and with a new model for the generation of a-type misfit dislocations at the AlGaN/GaN interface.

Electron holography studies of the charge on dislocations in GaN.

Off-axis electron holography in a transmission electron microscope is used to examine the charge on threading edge dislocations in n-GaN (0001). It is shown that the crystal inner potential is reduced within 10 nm of the dislocation consistent with a negatively charged core. The results can be explained by a simple unscreened potential due to a core charge of about 4 x 10(7) electrons cm (-1). The origin of this charge is discussed. The application of the method to other types of dislocation is also considered.

Direct observation of piezoelectric fields in GaN/ InGaN/GaN strained quantum wells

Off-axis electron holography is used to examine a single thin InGaN quantum well in GaN viewed in cross-section. The results show a phase offset across the well, which, under weakly diffracting conditions, is an approximately linear function of specimen thickness. This phase offset is ascribed to a change AV0 in the specimen mean inner potential V0 caused by a piezoelectric field induced by misfit strains in the InGaN layer. This paper examines the dependence of the phase offset on the diffracting conditions and on thin foil relaxation effects. It is shown that relaxation is negligible for the film thicknesses involved. Using a range of weakly diffracting conditions, the phase offset is measured as deltaV0/V0 = 0.042+/-0.012. Zone axis convergent beam electron diffraction patterns were taken and compared to simulations to determine the crystal polarity, showing the magnitude of the inner potential increased in the [0001] direction. By using dark-field displacement fringes to measure the InGaN layer thickness, and recent estimates of V0, the magnitude of the piezoelectric field is determined. This paper assesses the accuracy and limitations of electron holography for the studies of electric fields in other GaN structures.

Analysis of grain boundary dislocations by large angle convergent beam electron diffraction.

Large angle convergent beam electron diffraction (LACBED) is used to analyse secondary dislocations in sigma3 and sigma9 grain boundaries in silicon. By selecting reflections from crystal planes common to the adjoining grains, LACBED images are insensitive to the boundaries except where dislocations are present. The dislocation images are closely similar to those for dislocations in single crystals and can be analysed by standard Cherns-Preston rules. It is shown that, for both boundaries, sufficient common reflections can be selected for a complete analysis, and that dislocations can be analysed assuming integer values of g x b, implying that the Burgers vectors are Displacement Shift Complete (DSC) lattice vectors. For both sigma3 and sigma9 boundaries, DSC dislocations are identified which are specific to these boundaries. The experimental conditions for the analysis of grain boundaries are explained, and the extension of the method to other coincidence boundaries is discussed.

Space-group determination of human tooth-enamel crystals.

In the present work, we have determined the space group of human tooth-enamel crystals using--for the first time for a biological crystal--convergent beam electron diffraction (CBED). The symmetries observed in the different patterns we have obtained lead us to the P6(3)/m hydroxyapatite space group. Disorder, most likely situated in the columns formed by the hydroxyl ions of the crystals, is suggested as a cause of weak intensity in the otherwise forbidden 000l (l odd) reflections and low visibility of first-order Laue zone (FOLZ) reflections in the CBED pattern from crystals oriented along the [0001] zone axis. A monoclinic phase was not observed.

Convergent beam diffraction studies of interfaces, defects, and multilayers.

This paper explains how the convergent beam electron diffraction (CBED) and large angle convergent beam (LACBED) techniques can be used to study crystal defects, bicrystals, and multilayers. It is shown how the LACBED technique in particular can be used to derive the magnitude and sign of the Burgers vectors of dislocations and displacements at stacking faults. For bicrystals and multilayers examined in plan-view, LACBED gives the rocking curve for a chosen reflection. This enables layer strains to be measured to approximately 0.1% and composition profiles derived for both periodic and aperiodic structures. It is shown that a simple kinematic approach can be used to interpret the essential results in most cases.