@unpublished{TringidesConradMenzeletal.2002, author = {Tringides, Michael C. and Conrad, E. H. and Menzel, Andreas and Voss, Neil R. and Kammler, Martin and Horn-von Hoegen, Michael}, title = {Dynamics of atomic steps on Si(001) studied by temporal LEED spectroscopy [Poster]}, year = {2002}, abstract = {While surface kinetics measurements for single atom diffusion have proliferated, little work exists attempting to measure surface dynamics under equilibrium conditions. We will present our recent work on developing Temporal LEED Spectroscopy (TLS) as a dynamic probe of surfaces capable of kinetics measurements at both high temperatures and surface coverages. By tuning the diffraction to specific structural entities such as steps, islands, etc., and monitoring the diffracted signal as a function of time, dynamics information about these structures can be obtained. As an example we will present TLS results for the dynamics of step fluctuations on Si(001) [1]. We have measured activation energies and identified step-edge diffusion as the rate limiting process for 950K < T < 1130K. Although this result is in apparent disagreement with earlier work, a careful analysis, considering crossover temperatures between different microscopic processes, can reconcile the differences. 1. M. Kammler, et al., Phys. Rev. B (in press).}, language = {en} } @article{KammlerHornvonHoegen, author = {Kammler, Martin and Horn-von Hoegen, Michael}, title = {Transition in growth mode by competing strain relaxation mechanisms}, series = {Applied Physics Letters}, volume = {85}, journal = {Applied Physics Letters}, number = {15}, publisher = {American Institute of Physics}, doi = {10.1063/1.1803914}, pages = {3056 -- 3058}, abstract = {Surfactant mediated epitaxy of Si(1-x)Gex alloys on Si(111) can, besides technological importance, clarify the influence of the lattice mismatch during surfactant mediated heteroepitaxial growth. For low Ge concentration we find an immediate layer-by-layer growth, whereas at high Ge concentration a roughening transition followed by relaxation of the lattice mismatch in a periodic dislocation network is preferred. This behavior can be explained by comparing the dislocation nucleation rate on a smooth surface with the island nucleation rate on a pseudomorphic film.}, language = {en} } @article{SokolowskiTintenBlomeBlumsetal., author = {Sokolowski-Tinten, Klaus and Blome, Christian and Blums, Juris and Dietrich, Clemens and Tarsevitch, Alexander and Horn-von Hoegen, Michael and von der Linde, Dietrich and Cavalleri, Andrea and Kammler, Martin and Uschmann, Ingo and Foerster, Eckhart}, title = {Femtosecond X-ray Diffraction Study of Large Amplitude Coherent Optical Phonons in Laser-Excited Bismuth}, series = {Conference on Lasers and Electro-Optics, 1-6 June 2003, Baltimore, Maryland}, journal = {Conference on Lasers and Electro-Optics, 1-6 June 2003, Baltimore, Maryland}, publisher = {Optical Society of America}, abstract = {Femtosecond time-resolved X-ray diffraction was used to directly study optical phonons in laser-excited Bismuth. A periodic modulation of the diffraction efficiency reveals initially coherent lattice distortions with atomic displacements of nearly 10\% of the nearest- neighbor-distance.}, language = {en} } @article{WallKrenzerWippermannetal., author = {Wall, Simone and Krenzer, Boris and Wippermann, Stefan and Sanna, Simone and Klasing, Friedrich and Hanisch-Blicharski, Anja and Kammler, Martin and Schmidt, Wolf Gero and Horn-von Hoegen, Michael}, title = {Atomistic picture of charge density wave formation at surfaces}, series = {Physical review letters}, volume = {109}, journal = {Physical review letters}, number = {18}, publisher = {APS}, doi = {10.1103/PhysRevLett.109.186101}, abstract = {We used ultrafast electron diffraction and density-functional theory calculations to gain insight into the charge density wave (CDW) formation on In/Si(111). Weak excitation by a femtosecond-laser pulse results in the melting of the CDW. The immediate freezing is hindered by a barrier for the motion of atoms during the phase transition: The melted CDW constitutes a long-lived, supercooled phase and is strong evidence for a first-order transition. The freezing into the CDW is triggered by preexisting adsorbates. Starting at these condensation nuclei, the CDW expands one dimensionally on the In/Si(111) surface, with a constant velocity of more than 80 m/s.}, language = {en} } @article{FriggeKalusKlasingetal., author = {Frigge, Tim and Kalus, Annika and Klasing, Friedrich and Kammler, Martin and Hanisch-Blicharski, Anja and Horn-von Hoegen, Michael}, title = {Nanoscale Heat Transport in Self-Organized Ge Clusters on Si(001)}, series = {MRS Online Proceedings Library}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/opl.2013.148}, pages = {45 -- 50}, abstract = {Ultrafast time resolved transmission electron diffraction (TED) in a reflection geometry was used to study the cooling behavior of self-organized, well defined nanoscale germanium hut and dome clusters on Si(001). The clusters were heated in a pump-probe scheme by fs-laser pulses. The resulting transient temperature rise was then determined from the drop in diffraction intensity caused by the Debye-Waller effect. From a cooling time of τ=177 ps we estimated a strongly reduced heat transfer compared with homogeneous films of equivalent thickness.}, language = {en} } @article{MoellenbeckHanischBlicharskiSchneideretal., author = {M{\"o}llenbeck, Simone and Hanisch-Blicharski, Anja and Schneider, Paul and Ligges, Manuel and Zhou, Ping and Kammler, Martin and Krenzer, Boris and Horn-von Hoegen, Michael}, title = {Ultra-fast Time-Resolved Electron Diffraction of Strongly Driven Phase Transitions on Silicon Surfaces}, series = {MRS Online Proceedings Library}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/PROC-1230-MM03-09}, abstract = {The dynamics of strongly driven phase transitions at surfaces are studied by ultra-fast time-resolved reflection high energy electron diffraction. The surfaces are excited by an intense fs-laser pulse (pump) and probed by an ultra-short electron pulse with variable time delay. The order-disorder phase transition from a c(4 {\~A}— 2) to a(2 {\~A}— 1) of the bare Si(001) surface shows a transient decrease of the intensity of the c(4 {\~A}— 2) spots which recovers on a time scale of a few hundred picoseconds indicating the excitation of the phase transition. On Si(111) a monolayer of Indium induces a (4 {\~A}— 1) reconstruction which undergoes a Peierls like phase transition to a (8 {\~A}— "2") reconstruction below 100 K. Upon laser excitation with a fluence of 3.4 mJ/cm 2 at a temperature of 72 K the phase transition was strongly driven. The (8 {\~A}— "2")-diffraction spots instantaneously disappears, while the intensity of the (4 {\~A}— 1)-spots increases. This increase of the (4 {\~A}— 1) spot intensity excludes an explanation by the Debye-Waller-Effect and is evidence for a true structural phase transition at a surface. {\^A}© 2010 Materials Research Society.}, language = {en} } @article{LuNicoulShymanovichetal., author = {Lu, Wei and Nicoul, Matthieu and Shymanovich, Uladzimir and Tarsevitch, Alexander and Kammler, Martin and Horn-von Hoegen, Michael and von der Linde, Dietrich and Sokolowski-Tinten, Klaus}, title = {Extreme Phonon Softening in Laser-excited Bismuth - Towards an Inverse Peierls-transition}, series = {MRS Online Proceedings Library}, volume = {1230}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/PROC-1230-MM03-05}, abstract = {Large amplitude coherent optical phonons have been investigated in laser-excited Bismuth by means of femtosecond time-resolved X-ray diffraction. For absorbed laser fluences above 2 mJ/cm2, the experimental data reveal an extreme softening of the excited A1g-mode down to frequencies of about 1 THz, only 1/3 of the unperturbed A1g-frequency. At even stronger excitation the measured diffraction signals no longer exhibit an oscillatory behavior presenting strong indication that upon intense laser-excitation the Peierls-distortion, which defines the equilibrium structure of Bismuth, can be transiently reversed.}, language = {en} } @article{HanischBlicharskiKrenzerMoellenbecketal., author = {Hanisch-Blicharski, Anja and Krenzer, Boris and M{\"o}llenbeck, Simone and Ligges, Manuel and Zhou, Ping and Kammler, Martin and Horn-von Hoegen, Michael}, title = {Transient Cooling of Ultrathin Epitaxial Bi(111)-Films on Si(111) Upon Femtosecond Laser Excitation Studied by Ultrafast Reflection High Energy Electron Diffraction}, series = {MRS Online Proceedings Library}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/PROC-1172-T04-08}, pages = {1 -- 6}, abstract = {With time resolved ultrafast electron diffraction the cooling process across the interface between a thin film and the underlying substrate was studied after excitation with short laser pulses. From the exponential decay of the surface temperature evolution a thermal boundary conductance of 1430 W/(cm2K) is determined for a 9.7 nm thin Bi(111) film on Si(111). A linear dependence between laser fluence and initial temperature rise was measured for film-thicknesses between 2.5 nm and 34.5 nm. The ratio of initial temperature rise and laser fluence for different film-thicknesses is compared to a model taking multilayer optics into account. The data agree well with this model.}, language = {en} } @article{AlFalouKammlerHornvonHoegen, author = {Al-Falou, A. A. and Kammler, Martin and Horn-von Hoegen, Michael}, title = {Strain state analysis of hetero-epitaxial systems}, series = {Europhysics Letters}, volume = {69}, journal = {Europhysics Letters}, number = {4}, publisher = {IOP}, doi = {10.1209/epl/i2004-10399-6}, pages = {570 -- 576}, abstract = {We present a new method to analyze the strain state of epitaxial hetero structures by high-resolution spot profile analysis low-energy electron diffraction. From the variation of the spot profiles with the vertical scattering vector we determine the hetero film roughness, the change of interlayer spacing due to tetragonal distortion, and the variation of the interlayer distance due to partial relaxation of the strained rough film. The practical implementation of this method is simple and can be used to determine the onset of strain-relieving defects during the growth process.}, language = {en} } @article{KammlerHornvonHoegen, author = {Kammler, Martin and Horn-von Hoegen, Michael}, title = {Low energy electron diffraction of epitaxial growth of bismuth on Si(111)}, series = {Surface Science}, volume = {576}, journal = {Surface Science}, number = {1-3}, publisher = {Elsevier}, doi = {10.1016/j.susc.2004.11.033}, pages = {56 -- 60}, abstract = {The epitaxial growth of Bi on Si(1 1 1) studied by spot profile analyzing low energy electron diffraction shows for low coverage rotationally disordered Bi cluster with preferred orientations following the threefold symmetry of the Si substrate. With further deposition the Bi cluster coalesces and the surface orientation changes from the pseudo cubic Bi(1 1 0) surface orientation of the Bi cluster into the hexagonal Bi(1 1 1) surface of the resulting Bi film.}, language = {en} }