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Full-Text Articles in Physics
Transient Lattice Deformations Of Crystals Studied By Means Of Ultrafast Time-Resolved X-Ray And Electron Diffraction, Runze Li, Kyle Sundqvist, Jie Chen, H. E. Elsayed-Ali, Jie Zhang, Peter M. Rentzepis
Transient Lattice Deformations Of Crystals Studied By Means Of Ultrafast Time-Resolved X-Ray And Electron Diffraction, Runze Li, Kyle Sundqvist, Jie Chen, H. E. Elsayed-Ali, Jie Zhang, Peter M. Rentzepis
Electrical & Computer Engineering Faculty Publications
Ultrafast lattice deformation of tens to hundreds of nanometer thick metallic crystals, after femtosecond laser excitation, was measured directly using 8.04 keV subpicosecond x-ray and 59 keV femtosecond electron pulses. Coherent phonons were generated in both single crystal and polycrystalline films. Lattice compression was observed within the first few picoseconds after laser irradiation in single crystal aluminum, which was attributed to the generation of a blast force and the propagation of elastic waves. The different time scales of lattice heating for tens and hundreds nanometer thick films are clearly distinguished by electron and x-ray pulse diffraction. The electron and lattice …
Reflection High-Energy Electron Diffraction Studies Of Indium Phosphide (100) And Growth On Indium And Indium Nitride On Silicon (100), Mohamed Abd-Elsattar Hafez
Reflection High-Energy Electron Diffraction Studies Of Indium Phosphide (100) And Growth On Indium And Indium Nitride On Silicon (100), Mohamed Abd-Elsattar Hafez
Electrical & Computer Engineering Theses & Dissertations
Study of the effects of atomic hydrogen exposure on structure and morphology of semiconductor surfaces is important for fundamental properties and applications. In this dissertation, the electron yield of a hydrogen-cleaned indium phosphide (InP) surface was measured and correlated to the development of the surface morphology, which was monitored by in situ reflection high-energy electron diffraction (RHEED). Atomic hydrogen treatment produced a clean, well-ordered, and (2x4)-reconstructed InP(100) surface. The quantum efficiency, after activation to negative electron affinity, and the secondary electron emission were shown to increase with hydrogen cleaning time. RHEED patterns of low-index InP(100) surface were modified by the …
Growth Of Ge Quantum Dots On Si(100)-(2×1) By Pulsed Laser Deposition, M. S. Hegazy, H. E. Elsayed-Ali
Growth Of Ge Quantum Dots On Si(100)-(2×1) By Pulsed Laser Deposition, M. S. Hegazy, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
Self-assembled germanium quantum dots (QDs) were grown on Si(100)-(2×1) by pulsed laser deposition. In situ reflection-high energy electron diffraction (RHEED) and postdeposition atomic force microscopy are used to study the growth of the QDs. Several films of different thicknesses were grown at a substrate temperature of 400 °C using a Q-switched Nd:yttrium aluminum garnet laser (λ= 1064 nm, 40 ns pulse width, 23 J/cm 2 fluence, and 10 Hz repetition rate). At low film thicknesses, hut clusters that are faceted by different planes, depending on their height, are observed after the completion of the wetting layer. With increasing film thickness, …
Self-Assembly Of Ge Quantum Dots On Si(100)- 2×1 By Pulsed Laser Deposition, M. S. Hegazy, H. E. Elsayed-Ali
Self-Assembly Of Ge Quantum Dots On Si(100)- 2×1 By Pulsed Laser Deposition, M. S. Hegazy, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
Self-assembled Ge quantum dots are grown on Si(100)- 2×1 by pulsed laser deposition. The growth is studied by in situ reflection high-energy electron diffraction and postdeposition atomic force microscopy. After the completion of the wetting layer, transient hut clusters, faceted by different planes, are observed. When the height of these clusters exceeded a certain value, the facets developed into {305} planes. Some of these huts become {305}-faceted pyramids as the film mean thickness was increased. With further thickness increase, dome clusters developed on the expense of these pyramids. © 2005 American Institute of Physics. [DOI: 10.1063/1.1949285]
Condensation On (002) Graphite Of Liquid Bismuth Far Below Its Bulk Melting Point, M. K. Zayed, H. E. Elsayed-Ali
Condensation On (002) Graphite Of Liquid Bismuth Far Below Its Bulk Melting Point, M. K. Zayed, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
Condensation of thermally evaporated Bi on (002) graphite, at temperatures of 300-523K, was studied using in situ reflection high-energy electron diffraction (RHEED) and room temperature ex situ atomic force microscopy (AFM). For deposition at temperatures below 415±5K, transmission RHEED patterns of Bi appeared at an average thickness of ∼0.5 monolayer (ML). AFM images showed that the film consisted of crystallites in the shape of triangular step pyramids with step heights corresponding to single and double Bi layers in the [111] direction. This morphology indicates crystallization from the vapor. For deposition at higher temperatures, diffuse RHEED patterns appeared independent of the …
Atomic Hydrogen Cleaning Of Inp(100): Electron Yield And Surface Morphology Of Negative Electron Affinity Activated Surfaces, M. A. Hafez, H. E. Elsayed-Ali
Atomic Hydrogen Cleaning Of Inp(100): Electron Yield And Surface Morphology Of Negative Electron Affinity Activated Surfaces, M. A. Hafez, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
Atomic hydrogen cleaning of the InP(100) surface has been investigated using quantitative reflection high-energy electron diffraction. The quantum efficiency of the surface when activated to negative electron affinity was correlated with surface morphology. The electron diffraction patterns showed that hydrogen cleaning is effective in removing surface contaminants, leaving a clean, ordered, and (2×4)-reconstructed surface. After activation to negative electron affinity, a quantum efficiency of ∼6% was produced in response to photoactivation at 632 nm. Secondary electron emission from the hydrogen-cleaned InP(100)-(2×4) surface was measured and correlated to the quantum efficiency. The morphology of the vicinal InP(100) surface was investigated using …
Acceleration Element For Femtosecond Electron Pulse Compression, Bao-Liang Qian, Hani E. Elsayed-Ali
Acceleration Element For Femtosecond Electron Pulse Compression, Bao-Liang Qian, Hani E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
An acceleration element is proposed for compressing the electron pulse duration in a femtosecond photoelectron gun. The element is a compact metal cavity with curved-shaped walls. An external voltage is applied to the cavity where a special electric field forms in such a way that the slow electrons in the electron pulse front are accelerated more than the fast electrons, and consequently the electron pulse duration will be compressed. The distribution of the electric field inside the acceleration cavity is analyzed for the geometry of the cavity. The electron dynamics in this acceleration cavity is also investigated numerically. Numerical results …
Electron Pulse Broadening Due To Space Charge Effects In A Photoelectron Gun For Electron Diffraction And Streak Camera Systems, Bao-Liang Qian, Hani E. Elsayed-Ali
Electron Pulse Broadening Due To Space Charge Effects In A Photoelectron Gun For Electron Diffraction And Streak Camera Systems, Bao-Liang Qian, Hani E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead …
Time-Resolved Structural Study Of Low-Index Surfaces Of Germanium Near Its Bulk Melting Temperature, Xinglin Zeng, H. E. Elsayed-Ali
Time-Resolved Structural Study Of Low-Index Surfaces Of Germanium Near Its Bulk Melting Temperature, Xinglin Zeng, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The structure of the low-index surfaces of germanium near its bulk melting temperature is investigated using 100-ps time-resolved reflection high-energy electron diffraction. The surface is heated by 100-ps laser pulses while a synchronized electron beam probes the structure. Ge(111)was observed to remain in its incomplete melting structure up to at least Tm + 134 ± 40 K when heated by a 100-ps laser pulse. Both the Ge(100) and Ge(110) surfaces are observed to melt near the bulk melting temperature when heated with 100-ps laser pulses. Because of the low-diffraction intensity-to-background ratio at high temperatures and because of the temperature …
Time-Resolved Reflection High-Energy Electron Diffraction Study Of The Ge(111)-C(2×8)-(1×1) Phase Transition, Xinglin Zeng, Bo Lin, Ibrahim El-Kholy, Hani E. Elsayed-Ali
Time-Resolved Reflection High-Energy Electron Diffraction Study Of The Ge(111)-C(2×8)-(1×1) Phase Transition, Xinglin Zeng, Bo Lin, Ibrahim El-Kholy, Hani E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The dynamics of the Ge(111)-c(2×8)-(1×1) phase transition is investigated by 100-ps time-resolved reflection high-energy electron diffraction. A laser pulse heats the surface while a synchronized electron pulse is used to obtain the surface diffraction pattern. Slow heating shows that the adatoms in Ge(111)-c(2×8) start to disorder at ∼510 K and are converted to a disordered adatom arrangement at 573 K. For heating with 100-ps laser pulses, the Ge(111)-c(2×8) reconstructed adatom arrangement starts to disorder at 584±16K, well above the onset temperature of ∼510 K for the disordering of Ge(111)-c(2×8) observed for slow …
Temperature Dependence Of Step Density On Vicinal Pb(111), Z. H. Zhang, H. E. Elsayed-Ali
Temperature Dependence Of Step Density On Vicinal Pb(111), Z. H. Zhang, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The temperature dependence of step density on the vicinal Pb(111) surface is investigated using reflection high-energy electron diffraction. When the temperature is increased from 323 to 590 K. the average terrace width and the average string length at the step edge decrease from 85±25 to 37±16 Å and from 220±33 to 25±8 Å, respectively. Thermal step collapse on the Pb(111) surface near its bulk melting temperature is not observed. Above 530±7 K, the change in the string length at the step edge with temperature becomes small, and the intensity of the (00) beam is significantly decreased. We conclude that partial …
Surface Morphology Of Laser-Superheated Pb(111) And Pb(100), Z. H. Zhang, Bo Lin, X. L. Zeng, H. E. Elsayed-Ali
Surface Morphology Of Laser-Superheated Pb(111) And Pb(100), Z. H. Zhang, Bo Lin, X. L. Zeng, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The surface step density on the vicinal Pb(111) and the surface vacancy density on Pb(100) after laser superheating and melting are investigated using reflection high-energy electron diffraction. With ∼100-ps laser pulses, Pb(111) surface superheating does not significantly change the density of the steps and step-edge roughness. However, after laser surface melting, the average terrace width and the string length at the step edge become as large as those at room temperature. The average terrace width at 573 K changes from 38±15 to 64±19 Å after laser surface melting, while the average string length at the step edge changes from 90±14 …
Reflection High-Energy Electron-Diffraction Study Of Melting And Solidification Of Pb On Graphite, Z. H. Zhang, P. Kulatunga, H. E. Elsayed-Ali
Reflection High-Energy Electron-Diffraction Study Of Melting And Solidification Of Pb On Graphite, Z. H. Zhang, P. Kulatunga, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The melting and solidification of Pb thin films on pyrolytic graphite are investigated in situ by reflection high-energy electron diffraction. Thin films with thicknesses of 4-150 monolayers are investigated. The surface morphology of the thin films were studied by scanning electron microscopy. Superheating of the Pb thin films by 4±2 to 12±2 K is observed from diffraction intensity measurements. Upon cooling the substrate, the Pb on graphite is seen to supercool by ∼69±4 K.