Yoosuf N. Picard

Electron channeling contrast imaging of atomic steps and threading dislocations in 4H-SiC

Direct imaging of atomic step morphologies and individual threading dislocations in on-axis epitaxial 4H-SiC surfaces is presented. Topographically sensitive electron images of the crystalline surfaces were obtained through forescattered electron detection inside a conventional scanning electron microscope. This technique, termed electron channeling contrast imaging (ECCI), has been utilized to reveal the configuration of highly stepped, homoepitaxial 4H-SiC films grown on 4H-SiC mesa structures. Individual threading dislocations have been consistently imaged at the core of spiral atomic step morphologies located on the 4H-SiC surfaces. The ability of ECCI to image atomic steps was verified by atomic force microscopy.

Pulsed laser ignition of reactive multilayer films

Nanostructured Al∕Pt multilayer films were ignited by single pulse irradiation from a Ti:sapphire femtosecond laser system. Critical ignition fluences (0.9–22J∕cm2) required to initiate a self-propagating reaction were quantified for different multilayer designs. Multilayers with smaller bilayer thickness required relatively lower fluence for ignition. Ignition threshold fluence was also found to be 1.4–3.6 times higher for Al-capped multilayers than for Pt-capped multilayers. Ablation threshold fluences were measured for Al (860±70mJ∕cm2) and Pt (540±50mJ∕cm2) and related to the observed difference in ignition fluences for Al- and Pt-capped multilayers.

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

Impact of Joule heating on the microstructure of nanoscale TiO2 resistive switching devices

The microstructure of TiO2 functional layers in nanoscale resistive switching devices was analyzed using Scanning Electron and Transmission Electron Microscopies (SEM and TEM). The TiO2 layers in as-fabricated devices were amorphous with very weak lattice fringes in High Resolution TEM. After electroformation with low power dissipation (PDIS < 0.4 mW), the microstructural changes in the TiO2 layer were limited to an area approximately 75∼100 nm in radius indicating that the current path and Joule heating were localized. Since the reset power (≈2.4 mW) was greater than the electroformation power, switching cycles resulted in an increased area of the TiO2 affected zone and more morphological changes to the Pt electrodes and functional layers. Electroformation under large power dissipation (15 mW) led to massive redistribution of Pt, including shorting of electrodes through the oxide layer. Modeling temperature distribution in the devices found maximum temperature to be strongly dependent on the power dissip...

Direct observation of basal-plane to threading-edge dislocation conversion in 4H-SiC epitaxy

The propagation behavior of basal plane dislocations from off-oriented 4H-SiC substrates into homoepitaxial layers has been investigated using transmission electron microscopy (TEM), secondary electron microscopy (SEM), and chemical etching. Cross-sectional TEM shows that basal plane dislocations in the substrate are dissociated into pairs of partial dislocations separated by a stacking fault with a width of about 40 nm. Near the substrate/epilayer interface, where most of the basal plane dislocations convert to threading edge dislocations, the two partials constrict before converting. Threading edge segments are inclined by about 20° from the c-axis toward the down-step direction. It is concluded that the critical and limiting step of the dislocation conversion process is constriction of the dissociated partials. Growth surface morphology at the emergence point of the basal plane dislocation was imaged using SEM and is thought to play an important role in the constriction.

Diffraction contrast and Bragg reflection determination in forescattered electron channeling contrast images of threading screw dislocations in 4H-SiC

The experimental diffraction parameters and contrast features associated with electron channeling contrast imaging (ECCI) are determined by the study of threading screw dislocations (TSDs) in 4H-SiC. The images are recorded by forescatter diode detectors mounted on a commercial electron backscatter diffraction system. ECCI of TSDs penetrating the (0001) surface reveals dark-to-light contrast, the direction of which depends on the acting Bragg reflection, the deviation from the Bragg condition, and the dislocation Burgers vector. Burgers vector identification is confirmed through observations of the rotational direction of atomic step spirals associated with various screw dislocations. Dark “twin-lobed” features are observed when the incoming electron beam exactly satisfies the Bragg condition for various Bragg reflections. The experimentally observed behavior of channeling contrast features for screw dislocations in this study is found to be consistent with transmission electron microscopy diffraction con...

Theory of dynamical electron channeling contrast images of near-surface crystal defects

This paper describes the dynamical simulation of electron channeling contrast images (ECCIs) of dislocations. The approach utilizes both the Bloch wave formalism and the scattering matrix formalism to generate electron channeling patterns (ECPs). The latter formalism is then adapted to include the effect of lattice defects on the back-scattered electron yield, resulting in a computational algorithm for the simulation of ECCIs. Dislocations of known line direction and Burgers vector are imaged experimentally by ECCI and match well with simulated ECCIs for various channeling conditions. Experiment/simulation comparisons for ECPs and ECCIs are demonstrated for metals (Al), semiconductors (Si), and ceramics (SrTiO₃).

Nanosecond laser induced ignition thresholds and reaction velocities of energetic bimetallic nanolaminates

Thresholds for optically igniting self-propagating reactions are quantified for energetic Ni/Ti, Co/Al, and Al/Pt nanolaminates, where smaller enthalpy material pairs required larger laser ignition fluences. The threshold fluences (J/cm2) for ignition by 30 ns laser pulses focused to ∼8 μm spot size varied from 720 to 15 000 J/cm2 for Ni/Ti, 8.6 to 380 J/cm2 for Co/Al, and 3.2 to 27 J/cm2 for Al/Pt. Conversely, smaller enthalpy nanolaminates exhibited reduced steady-state propagation speeds ranging from 0.05 to 0.9 m/s for Ni/Ti, 0.6 to 8.5 m/s for Co/Al, and 24 to 73 m/s for Al/Pt. Increasing the laser spot diameter tenfold reduced the ignition threshold fluence by as much as two orders of magnitude.

Rapid misfit dislocation characterization in heteroepitaxial III-V/Si thin films by electron channeling contrast imaging

Electron channeling contrast imaging (ECCI) is used to characterize misfit dislocations in heteroepitaxial layers of GaP grown on Si(100) substrates. Electron channeling patterns serve as a guide to tilt and rotate sample orientation so that imaging can occur under specific diffraction conditions. This leads to the selective contrast of misfit dislocations depending on imaging conditions, confirmed by dynamical simulations, similar to using standard invisibility criteria in transmission electron microscopy (TEM). The onset and evolution of misfit dislocations in GaP films with varying thicknesses (30 to 250 nm) are studied. This application simultaneously reveals interesting information about misfit dislocations in GaP/Si layers and demonstrates a specific measurement for which ECCI is preferable versus traditional plan-view TEM.

Simulation and analysis of electron channeling contrast images of threading screw dislocations in 4H-SiC

Using methods developed for modeling diffraction contrast of extended defects in thin foils, electron intensity profiles are simulated and found to qualitatively match channeling contrast of threading screw dislocations (TSDs) experimentally recorded by electron channeling contrast imaging (ECCI) using scanning electron microscopy. Plan-view images of TSDs axially penetrating (0001) 4H-SiC surfaces were computed using the Sturkey scattering matrix approach incorporating surface relaxation effects. Simulated diffraction contrast of the TSD allows identification of these threading defects as well as facilitates the determination of the dislocation Burgers vector. The directionality of TSD contrast features, simulated for various Bragg reflections and deviation parameters, is consistent with both ECCI and diffraction contrast imaging by transmission electron microscopy. Topographically enhanced imaging of atomic step spirals, generated by the TSDs, provides a direct determination of the TSD Burgers vector, w...