Alexander V. Kvit

Dislocation-Driven Nanowire Growth and Eshelby Twist

Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations with a dislocation Burgers vector along the 〈110〉 directions having an estimated magnitude of 6 ± 2 angstroms for the screw component. The results confirm the Eshelby theory of dislocations, and the proposed nanowire growth mechanism could be general to many materials.

Electron scattering mechanisms in GZO films grown on a-sapphire substrates by plasma-enhanced molecular beam epitaxy

We report on the mechanisms governing electron transport using a comprehensive set of ZnO layers heavily doped with Ga (GZO) grown by plasma-enhanced molecular-beam epitaxy on a-plane sapphire substrates with varying oxygen-to-metal ratios and Ga fluxes. The analyses were conducted by temperature dependent Hall measurements which were supported by microstructural investigations as well. Highly degenerate GZO layers with n > 5 × 1020 cm−3 grown under metal-rich conditions (reactive oxygen-to-metal ratio <1) show relatively larger grains (∼20–25 nm by x-ray diffraction) with low-angle boundaries parallel to the polar c-direction. For highly conductive GZO layers, ionized-impurity scattering with almost no compensation is the dominant mechanism limiting the mobility in the temperature range from 15 to 330 K and the grain-boundary scattering governed by quantum-mechanical tunnelling is negligible. However, due to the polar nature of ZnO having high crystalline quality, polar optical phonon scattering cannot b...

A freestanding cellulose nanofibril–reduced graphene oxide–molybdenum oxynitride aerogel film electrode for all-solid-state supercapacitors with ultrahigh energy density

A free-standing, lightweight, highly porous, and highly flexible cellulose nanofibril (CNF)–reduced graphene oxide (RGO)–molybdenum oxynitride (MoOxNy) aerogel film electrode was synthesized by freeze-drying a CNF/GO/MoO3 aqueous dispersion followed by subsequent in situ hydrazine reduction. Due to the partial reduction and nitrogen doping of the MoO3 nanobelts and the highly open and continuous porous structure, the free-standing CNF/RGO/MoOxNy aerogel film electrode delivered an outstanding specific capacitance of 680 F g−1 in an aqueous electrolyte and 518 F g−1 in an ionic liquid electrolyte in a three-electrode configuration at a current density of 1.0 A g−1. Furthermore, highly flexible and all-solid-state supercapacitors using CNF/RGO/MoOxNy as electrodes and either a hydrogel or ionogel as the electrolyte were fabricated and both types of supercapacitors demonstrated high specific capacitance and excellent long-term cycling stability. Remarkably, the solid-state supercapacitor devices fabricated using an ionogel electrolyte demonstrated an energy density of 114 W h kg−1 (corresponding to a volumetric energy density of 18.8 W h L−1), which is among the highest values achieved for any type of solid-state supercapacitor and is even comparable to the energy densities of Li-ion batteries. Therefore, this study provides a simple and cost-efficient method for fabricating flexible energy storage devices with excellent electrochemical performance.

Donor behavior of Sb in ZnO

Electrical behavior of Sb in ZnO:Sb layers doped in a wide concentration range was studied using temperature dependent Hall effect measurements. The layers were grown by plasma-enhanced molecular beam epitaxy, and the Sb concentration was changed by varying the Sb flux, resulting in electron concentrations in the range of 1016 to nearly 1020 cm−3. Upon annealing, the electron concentration increased slightly and more notable was that the electron mobility significantly improved, reaching a room-temperature value of 110 cm2/V s and a low-temperature value of 145 cm2/V s, close to the maximum of ∼155 cm2/V s set by ionized impurity scattering. Hall data and structural data suggest that Sb predominantly occupies Zn sublattice positions and acts as a shallow donor in the whole concentration range studied. In the layers with high Sb content (∼1 at. %), acceptor-type compensating defects (possibly Sb on oxygen sites and/or point-defect complexes involving SbO) are formed. The increase of electron concentration ...

Hexagonal-based pyramid void defects in GaN and InGaN

We report a void defect in gallium nitride (GaN) and InGaN, revealed by aberration-corrected scanning transmission electron microscopy (STEM). The voids are pyramids with symmetric hexagonal {0001} base facets and {101¯1} side facets. Each pyramid void has a dislocation at the peak of the pyramid, which continues up along the [0001] growth direction to the surface. Some of the dislocations are hexagonal open core screw dislocations with {101¯0} side facets, varying lateral widths, and varying degrees of hexagonal symmetry. STEM electron energy loss spectroscopy spectrum imaging showed a large C concentration inside the void and on the void surfaces. There is also a larger C concentration in the GaN (or InGaN) below the void than above the void. We propose that inadvertent carbon deposition during metal organic chemical vapor deposition growth acts as a mask, stopping the GaN deposition locally, which in combination with lateral overgrowth, creates a void. Subsequent layers of GaN deposited around the C co...

Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threadin...

Nanoscratch behaviour, structure and nanoindentation of multilayer TiN/CrN coatings

The nanoscratch behaviour of TiN/CrN nanolaminates has been investigated in the present study. The critical load (LcL) during the loading process characterises the fracture resistance of the coating, whereas that during unloading (LcU), characterises the adhesion strength between the coating and the substrate. For all coatings, the nanoscratch profiles indicate three distinct regimes: elastic deformation, elastic?plastic deformation and delamination with material removal. Multilayer coatings show significantly higher critical loads than monolayer coatings. SEM characterisation indicates the cracking mechanisms are different for monolayer and multilayer coatings explaining why the multilayer coatings withstand higher critical loads.

Optimization of ZnO:Ga properties for application as a transparent conducting oxide in InGaN based light emitting diodes

We report on the effects of substrate temperature and surface morphology of p-GaN templates on the properties of ZnO:Ga (GZO) layers grown by plasma-assisted molecular beam epitaxy. Substrate temperature varying from 200 °C to 450°C was found to have only a moderate effect on the electrical properties of GZO films but it greatly affects the surface morphology of the GZO films. The surface morphology and growth mode of GZO were also found to be considerably affected by the surface morphology of underlying p-GaN templates. On p-GaN templates with a smooth surface (RMS = 0.4 nm) featured by clear atomic steps, GZO layers grew in 2D growth mode and exhibited smooth surfaces with RMS roughness of 2 nm. In contrast, on p-GaN without clear atomic steps but having comparable surface roughness of 0.6 nm, GZO layers grew in 3D growth mode and exhibited rough surface (RMS roughness of ~17.0-20.0 nm). The results of surface roughness are consistent with those from TEM measurements. The lowest resistivity of ~2.3×10-4 Ω·cm for as-grown GZO layers has been achieved at substrate temperature of 350°C, while the data for 2D GZO layers was affected by a parallel conduction channel from underneath GaN and require further studies. Although the differences in electrical properties and surface morphology existed, the GZO layers grown on different p-GaN templates showed optical transparency higher than 90% in the visible spectral range. The performance of 3D GZO layers as p-electrode was tested in InGaN light emitting diodes.

Vertical composition variation in nominally uniform InGaN layers revealed by aberration-corrected STEM imaging

We have found composition variations along the growth direction within regions of nominally constant indium composition in InGaN light emitting diode structures grown by metal-organic chemical vapor deposition using atomic resolution Z-contrast imaging in a scanning transmission electron microscope (STEM). Within 60 nm thick nominally In0.01Ga0.99N layers, we found periodic enhancements in the indium concentration into 4 bands separated by 11 nm. Energy dispersive spectroscopy spectrum imaging confirmed that the higher intensity in the high angle annular dark field (HAADF) Z-contrast STEM images was in fact caused by locally higher indium concentration. We observed no lateral indium composition fluctuations.

Bayesian Statistical Model for Imaging of Single La Vacancies in LaMnO3

Previous simulation studies have demonstrated that single La vacancies can be detected using high precision quantitative high-angle annular dark-field (HAADF) Z-contrast scanning transmission electron microscopy (STEM) [1] by both the reduction in scattered intensity created by the missing atom and the distortion of the surrounding atom positions [2]. We have now experimentally detected single La vacancies on nominally stoichiometric LaMnO3 films grown on DyScO3 substrate by molecular beam epitaxy with ~1% control of cation stoichiometry. The vacancy depth is determined from channeling behavior of the electron probe, quantified by a Bayesian statistical model that compares experiments to a library of simulations