Time-resolved Reflection High-energy Electron Diffraction Research Articles

Ultrafast electron diffraction from a Bi(111) surface: Impulsive lattice excitation and Debye-Waller analysis at large momentum transfer.

The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye–Waller analysis at large momentum transfer of 9.3 Å−1 ≤ Δ k ≤ 21.8 Å−1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm2 incident pump fluence. The observed time constants τint of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τint is caused by the nonlinearity of the exponential function in the Debye–Waller factor and has to be taken into account for an intensity drop ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τT of vibrational excitation of the surface lattice of 12 ± 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye–Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface.

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(001)

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(001)

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(0 0 1)

The surface phase transition on GaAs(001) was investigated in real time by core-level photoelectron spectroscopy and reflection high-energy electron diffraction (RHEED). The time dependence of the relative surface band bending was obtained from the deconvolution of core-level photoelectron spectra. We found that band bending on the 4 × 2 surface increases 0.06eV from that of the 2 × 4 surface and that the time dependence of relative surface band bending is in good agreement with that of core-level photoelectron intensities but not with the RHEED intensities. This indicates that the long-range ordering of surface superstructures and microscopic surface morphology does not influence surface band bending, whereas the termination atoms, As or Ga, does.

Temperature-gradient epitaxy under in situ growth mode diagnostics by scanning reflection high-energy electron diffraction

We have developed a parallel film growth method on a temperature-gradient substrate to quickly control and optimize the film growth mode. A continuous-wave neodymium-doped yttrium–aluminum–garnet laser heating was used to achieve a stable temperature gradient covering a 300 °C range of temperatures over a distance of 11 mm. The growth mode was determined by time-resolved scanning reflection high-energy electron diffraction. Transition from layer-by-layer to step-flow growth by the deposition temperature was observed during La0.5Sr0.5MnO3 film growth on a single SrTiO3 substrate, proving a powerful tool not only for investigating the growth dynamics but also for seeking the optimized deposition conditions in one run of experiment.

Time-resolved electron diffraction study of the Ge( [formula omitted])-(2×1)–(1×1) phase transition

The dynamics of the Ge(1 0 0)-(2×1)–(1×1) phase transition has been investigated by 100-ps time-resolved reflection high-energy electron diffraction (RHEED). A laser pulse heats the surface while a synchronized electron pulse is used to obtain a diffraction pattern of the surface. For slow heating, on a heated stage, the Ge(1 0 0)-(2×1) reconstruction is observed to lose its long-range order between 900 and 1000 K. For heating with 100-ps laser pulses, time-resolved electron diffraction shows that the Ge(1 0 0)-(2×1) reconstructed surface starts to disorder at 1027±44 K, 127±44 K above the onset temperature of 900 K for the disordering of Ge(1 0 0)-(2×1) observed for slow heating. The overheating of the Ge(1 0 0)-(2×1) structure by ultrafast laser pulses is indicative of the first-order nature of the transition, and provides support for the conclusion that the mechanism of the Ge(1 0 0)-(2×1)–(1×1) phase transition involves (2×1) domain wall proliferation instead of dimer break-up. RHEED intensity and profile analysis under slow heating shows increased surface vacancy density and roughness with temperature. The onset of the increase in surface roughness, as measured by RHEED profile width, correlates with the Ge(1 0 0)-(2×1)–(1×1) phase transition.

Time-resolved structural study of low-index surfaces of germanium near its bulk melting temperature

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 ${T}_{m}+134\ifmmode\pm\else\textpm\fi{}40\mathrm{K}$ 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 uncertainty in the time-resolved experiments, we are unable to accurately identify the melting point of Ge(100) and Ge(110) when heated with a 100-ps laser pulse. The results, however, favor the lack of surface superheating of Ge(100) and, to some extent, Ge(110). The superheating of the incomplete melting state of Ge(111) could be due to the metallization of the top germanium bilayer and its interaction with the solid underneath causing an energy barrier sufficient to allow for transient surface superheating.

Growth mode mapping of SrTiO3 epitaxy

We have mapped the growth mode of homoepitaxial SrTiO3 thin films as a function of deposition rate and substrate temperature during pulsed laser deposition. The transition from layer by layer growth to step flow growth was mapped by making 260 depositions, 3 monolayers each, on a single substrate. The growth mode was determined by time-resolved reflection high-energy electron diffraction. An atomically smooth surface was regenerated after each deposition by annealing the sample at temperatures above 1200 °C. The depositions were performed at an oxygen pressure of 10−6 Torr and covered a temperature range from 900 to 1380 °C. The effective activation energies of surface migration on Ti- and Sr-terminated surfaces were determined from the mapping results.

Time-resolved structural study of the Ge(111) high-temperature phase transition

The dynamic behavior of the Ge(111) high-temperature phase transition has been investigated using 100 ps time-resolved reflection high-energy electron diffraction (RHEED). A laser pulse heats the surface while a synchronized electron pulse is used to obtain the surface diffraction pattern at some time during and after the heating laser pulse. For slow heating, the RHEED results show that the high-temperature phase transition spreads from ∼1020 K to ∼1070 K. For 100 ps laser heating, the time-resolved RHEED measurements show that the phase transition occurs in the transient temperature range from 1083±23 K to 1138±32 K. The time-resolved RHEED results are in agreement with incomplete melting in which only the topmost layer melts during the phase transition. Thus, overheating of the topmost bilayer beyond the onset temperature of incomplete melting by 63±23 K was observed for 100 ps laser heating.

Time-resolved reflection high-energy electron diffraction study of theGe(111)−c(2×8)–(1×1)phase transition

The dynamics of the $\mathrm{Ge}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)--(1\ifmmode\times\else\texttimes\fi{}1)$ 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 $\mathrm{Ge}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)$ start to disorder at \ensuremath{\sim}510 K and are converted to a disordered adatom arrangement at 573 K. For heating with 100-ps laser pulses, the $\mathrm{Ge}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)$ reconstructed adatom arrangement starts to disorder at $584\ifmmode\pm\else\textpm\fi{}16\mathrm{K},$ well above the onset temperature of \ensuremath{\sim}510 K for the disordering of $\mathrm{Ge}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)$ observed for slow heating.

Initial Stage of Nitridation of GaAs(001): Atomic Scale View

The nitridation process of an As-terminated GaAs(001)-β2(2×4) surface was studied by time-resolved reflection high-energy electron diffraction (RHEED) and in situ scanning tunneling microscopy (STM). On the intermediate nitrided surface, a (3×4)-ordered phase was formed by the rearrangement of the As-dimer pairs at 520°C. This phase reverted back to the (2×4)-ordered phase below 400°C (reverse reconstruction by the re-adsorption of As atoms). We propose a model to explain this behavior.

Atomic-scale nature of the (3×3)-ordered GaAs(001):N surface prepared by plasma-assisted molecular-beam epitaxy

In situ scanning tunneling microscopy and time-resolved reflection high-energy electron diffraction measurements were performed to study the nitridation process of the As-terminated GaAs(001)-(2×4) surface by using electron cyclotron resonance plasma-assisted molecular-beam epitaxy. We report the real-space atomic structure of the coherently strained (3×3)-ordered GaN monolayer on GaAs(001) after a limited-exposure nitridation process and the atomically smooth morphology of this nitrided surface. The unique (3×3) phase is found consisting of nitrogen dimers and a regular array of missing nitrogen rows in both [1̄10] and [110] directions.

Heterointerface Control and Epitaxial Growth of 3C-SiC on Si by Gas Source Molecular Beam Epitaxy

Heterointerface modification and epitaxial growth of 3C-SiC on Si by gas source molecular beam epitaxy (MBE) are surveyed. A Si surface was carbonized by the use of C 2 H 2 , thermal cracking of C 3 H 8 and dimethylgermane (CH 3 ) 2 GeH 2 (DMGe) to chemically convert the surface region into single crystalline 3C-SiC prior to crystal growth. It was found that a Si surface can be carbonized reproducibly by the use of hydrocarbon radicals at a temperature as low as 750 °C. The initial stage of carbonization is discussed based on the time-resolved reflection high-energy electron diffraction analysis. Low-temperature heterointerface modification by DMGe is described. As an advanced epitaxial growth, atomic-level control in SiC crystal growth by gas source MBE is given. Crystallinity and surface morphology of low-temperature 3C-SiC homoepitaxy on a carbonized layer is presented.

Hetero-interface control and atomic layer epitaxy of SiC

Hetero-interface control in an atomic level and subsequent heteroepitaxial growth of cubic(3C)–SiC on Si were achieved using hydrocarbon radicals in gas source molecular beam epitaxy. The number of methyl (CH 3) radicals was estimated quantitatively by a threshold-ionization quadrupole mass spectroscopy method. The effects of CH 3 radicals on carbonization of a Si surface were discussed relating with the change of surface superstructures revealed by a time-resolved reflection high-energy electron diffraction analysis. Heteroepitaxial growth of SiC on Si was achieved by a combination of carbonization of a Si surface and subsequent crystal growth using hydrocarbon radicals and disilane (Si 2H 6). A 3C–SiC (3×2) reconstruction can be observed at a suitable flow rate. At a typical flow rate of CH 3 radicals, the growth rate was kept constant regardless of the Si 2H 6 flow rate, which implies an atomic level control of crystal growth using a layer-by-layer growth mode.

Reflection High-Energy Electron Diffraction Image Analysis during Molecular Beam Epitaxial Growth of InAs/AlGaAsSb Deep Quantum Well Structures

We have investigated the lattice strain relaxation during the initial stage of Alx Ga1-x AsSb (0<x<0.5) growth on GaAs (100) substrates by molecular beam epitaxy (MBE). Using time-resolved reflection high-energy electron diffraction (RHEED) image analysis, we have found that AlGaAsSb on a GaAs surface quickly relaxes within 3 to 7 monolayers (MLs). We have also observed that two-dimensional (2D) growth occurs after deposition of 20MLs of GaAsSb on GaAs surfaces. Furthermore, we have confirmed the InAs grows on AlGaAsSb two-dimensionally because AlGaAsSb is lattice matched to InAs. With a total thickness of less of than 700 nm in the InAs/AlGaAsSb deep quantum wells (DQWs), we have achieved very high electron mobilities of more than 32,000 cm2/(V·s) at room temperature.

Time-Resolved Reflection High-Energy Electron Diffraction Analysis in Initial Stage of 3C-SiC Growth on Si(001) by Gas Source Molecular Beam Epitaxy

Initial stages of 3C-SiC growth on Si using cracked C3H8 in gas source molecular beam epitaxy were studied using in situ reflection high-energy electron diffraction. Starting from a Si (001) clean surface showing a (2×1) structure, the surface structure changed in the order of Si(2×1) → structure of mixed Si(2×1) and Si c(4×4) → 3C-SiC with a continuous supply of cracked C3H8. An activation energy of 46.9 kcal/mol was obtained in the initial stage of 3C-SiC growth. After the appearance of 3C-SiC diffraction spots, island structures of 3C-SiC nuclei were observed using a high-resolution scanning electron microscope. Based on these results, growth mechanisms in the 3C-SiC/Si heteroepitaxial system are discussed.

Time-resolved reflection high energy electron diffraction analysis for atomic layer depositions of GaSb by molecular beam epitaxy

This paper describes the growth kinetics of GaSb under atomic layer depositions by molecular beam epitaxy (MBE). In order to understand the growth kinetics, the reconstruction changes were related with the shift of surface composition using a time-resolved reflection high-energy electron diffraction (RHEED) technique. This understanding was applied to keep the growth condition at the optimal by monitoring the RHEED oscillation waveform.

Superheating of Bi(0001).

Superheating of Bi(0001) by about 90 K above the bulk melting temperature is observed using time-resolved reflection high-energy electron diffraction with \ensuremath{\sim}200-ps time resolution. Larger temperature excursions above the bulk melting temperature result in melting accompanied by irreversible laser damage to the surface. The rhombohedral structure of Bi leads to very different behavior of material parameters upon melting as compared with fcc metals. Therefore, our observation of the superheating of Bi(0001), together with the previously observed superheating of Pb(111), suggests the generality of the superheating phenomenon.

Time-resolved structural study of Pb(100).

Time-resolved reflection high-energy electron diffraction is used to study the structural behavior of Pb(100) subjected to 200 ps laser pulses. Analysis of the electron diffraction intensity indicates that residual order is present on the surface at a temperature of 615\ifmmode\pm\else\textpm\fi{}20 K, where the bulk melting temperature of Pb is 600.7 K. This implies that the disordered film that forms on Pb(100) is stable up to the bulk melting temperature and above.

Structures of Co/Ag metallic superlattices grown by molecular beam epitaxy

Co/Ag metallic superlattices were epitaxially grown onto Ag buffer layers pre-deposited on (0001)sapphire or (111)Si substrates by molecular beam epitaxy. In-situ time-resolved reflection high-energy electron diffraction observations during superlattice growth revealed the structures of Ag and Co layers. Ag layers had an FCC structure with growth direction 〈111〉 FCC. On the other hand, the structure of Co layers on Ag layers was initially close-packed with random stacking normal to the close-packed plane and after about 7 monolayer growth it was changed to FCC at temperatures below 323 K and to HCP at above 323 K. Interface roughness was deduced from the period of time when both patterns from Co and from Ag were being observed. Ex-situ X-ray diffraction (XRD) measurements and high-resolution transmission electron microscopy observations disclosed the atomic-level structures, and showed that the interfaces between Co layers and Ag layers were wavy at 273 K and the Co layers grown at 373 K discontinuous. A wave-model and an island-model were proposed to interpret the experimental XRD profiles. XRD with 4-circle diffractometer revealed that Ag layers on sapphire had a local lattice distortion, but that Ag on Si did not.

Superheating of Pb(111).

Time-resolved reflection high-energy electron diffraction is used to study the laser-induced superheating of Pb(111). For a heating laser pulse width of \ensuremath{\sim}180 ps, two regimes are observed depending upon the incident peak laser intensity. In the first regime, the time-resolved electron diffraction intensity indicates a transient superheating of the surface followed by cooling below the bulk melting temperature. In the second regime, which occurs at higher peak laser intensities, superheating is followed by melting. Superheating up to 120 K above the bulk melting temperature is observed.