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In this work, a simple two-step method to create tunable self-assembled three-dimensional nanostructure array-like nanoantennas directly on the tip of an optical quartz glass fiber is described. The structures are prepared by using dry etching of the fiber tip. For the etching process, gold nanoparticles fabricated by thermal dewetting technique were used as template. The structures are applied as sensors for label-free analysis of organic substances in ppb range, such as drug residues in liquid solutions. The measurements are carried out by a portable Raman device with an exchangeable sensor head utilizing the SERS-effect. This method allows in situ applications. In order to characterize the SERS cells, para-thiocresol and diclofenac sodium are used as model substances. For optimization of the substrate performance, different thicknesses of the dewetting-layer (6, 9, 12 nm), different etching times for formation of the pillars (6, 8, 10 and 12 min), and different thicknesses of SERS-metallization (25, 50, 75 nm) of gold and silver are compared. In order to show the applicability of the structure on the tip of a fiber, measurements from the upper side and from the underside of the substrate on quartz plates are compared. Reproducible SERS enhancement factors up to 10 7 {10^{7}} were achieved.
In this work, a simple cost effective method to create tunable self-assembled three-dimensional nanostructure array-like nanoantennas on a tip of an optical quartz glass fiber is described. The structures are prepared using lithography-less dry etching. Gold nanoparticles are used as an etching mask using a thin metal-film thermal dewetting technique. The structures are applied as sensor for label-free analysis of organic substances in ppb range, such as drug residues in groundwater. The measurements are carried out by means of a Surface-enhanced Raman scattering (SERS) effect, an exchangeable sensor head, and a portable Raman device. This method allows in situ applications. Parathiocresol is used as a model substance to characterize the SERS cells. For metallization, gold and silver are compared. Reproducible SERS enhancement factors up to 10 7 are evaluated.
Summary form only given, as follow. Ultrafast optical-pump, X-ray diffraction probe experiments are providing new ways to study transient processes including the direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place. Current table-top-terawatt femtosecond laser systems provide an attractive source of few-hundred femtosecond duration bursts of angstrom-scale X-ray radiation with fluxes comparable to standard rotating anode sources. Their compact size enables time resolved structural dynamics to be studied in the small laboratory with temporal resolution better than typical molecular vibrational periods. Ultrafast structural dynamics in crystalline samples are readily studied with such systems and experiments to be discussed in this talk include ultrafast non-thermal solid-to-liquid transition in thin single-crystal Ge-111 films grown on Si-111 substrates; ultrafast non-thermal solid-to-solid transitions in bulk vanadium dioxide from a low temperature insulating phase to a high temperature metallic phase; and harmonic and anharmonic coherent acoustic dynamics in layered Ge-111/Si-111 and bulk GaAs-111 samples. Future improvements in high-average power short-pulse lasers will enable the study of a wider class of materials such as amorphous solids or liquid-phase dynamics of simple molecules, while proposed 4th generation light sources based upon single-pass X-ray free-electron lasers will permit singleshot structural determination of complex biomolecules.
Transient lattice dynamics in fs-laser-excited semiconductors probed by ultrafast X-ray diffraction
(2001)
A study of lattice dynamics in fs-laser-excited crystalline bulk Ge and Ge/Si-heterostructures, using time-resolved x-ray diffraction, was presented. Fast energy transport deep into the bulk of the material, coherent acoustic phonon dynamics, lattice anharmonicity, and vibrational transport across a buried interfaces were observed. Graphs showing time-dependent shifts of the centroid of the measured rocking curves in the Ge-overlayer and in the Si-substrate for different fluences, were presented.
Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium.
Ultrafast X-ray diffraction
(2005)
Using ultrafast, time-resolved, 1.54 angstrom x-ray diffraction, thermal and ultrafast nonthermal melting of germanium, involving passage through nonequilibrium extreme states of matter, was observed. Such ultrafast, optical-pump, x-ray diffraction probe measurements provide a way to study many other transient processes in physics, chemistry, and biology, including direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place.
Ultrafast X-ray diffraction
(2004)
Time-resolved x-ray diffraction with ultrashort ( approximately 300 fs), multi-keV x-ray pulses has been used to study the femtosecond laser-induced solid-to-liquid phase transition in a thin crystalline layer of germanium. Nonthermal melting is observed to take place within 300-500 fs. Following ultrafast melting we observe strong acoustic perturbations evolving on a picosecond time scale.
Summary form only given. Ultrashort x-ray pulses offer a unique combination of atomic-scale spatial and temporal resolution, which permits direct measurements of structural transients on an ultrafast time scale. Using time-resolved X-ray diffraction with femtosecond, multi-keV X-ray pulses we have studied transient lattice dynamics in optically excited semiconductors.
Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond time scale. In order to study such ultra-fast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Reflection high energy electron diffraction (RHEED) with grazing incident electrons ensures surface sensitivity for the probing electron pulses. Utilizing the Debye-Waller effect, we studied the cooling of vibrational excitations in monolayer adsorbate systems or the nanoscale heat transport from an ultra-thin film through a hetero-interface on the lower ps-time scale. The relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated with the In-induced (8×2) reconstruction on Si(111). This surface exhibits a Peierls-like phase transition at 100K from a (8×2) ground state to (4×1) excited state. Upon excitation by a fs-laser pulse, this structural phase transition is driven into an excited (4×1) state at a sample temperature of 20K. Relaxation into the (8×2) ground state occurs after more than 150 ps.
We show that templating a Si surface with a focused beam of Si2+ or Si+ ions can create suitable nucleation sites for the subsequent growth of self-assembled Ge quantum dots by chemical vapor deposition. To determine the mechanism of patterning we use atomic force microscopy to show that, similar to Ga+ patterning, the formation of a surface pit is required to enable control over Ge quantum dot locations. We find that relatively high implantation doses are required to achieve patterning, and these doses lead to amorphization of the substrate. We assess the degree to which the substrate crystallinity can be recovered by subsequent processing. Using in situ transmission electron microscopy heating experiments we find that recrystallization is possible at the growth temperature of the Ge quantum dots, but defects remain that follow the pattern of the initial implantation. We discuss the formation mechanism of the defects and the benefits of using Si ions for patterning both defects and quantum dots on Si substrates.
Microstructural changes in silicon induced by patterning with focused ion beams of Ga, Si and Au
(2013)
We use focused beams of Ga(+), Au(+) and Si(++) ions to induce local microstructural changes in single crystal silicon. The ions were delivered as single spot pulses into thin Si membranes that could subsequently be imaged and annealed in situ in a transmission electron microscope. For each ion, the focused ion beam implantation created an array of amorphous regions in the crystalline membrane. Annealing causes solid phase epitaxial regrowth to take place, but we show that the resulting microstructure depends on the ion species. For Ga(+) and Au(+), precipitates remain after recrystallization, while for Si(++), dislocation loops form around the periphery of each implanted spot. We attribute these loops to defects formed during solid phase epitaxial regrowth, with controlled placement of the loops possible.
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.
From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time of the random atomic motion resulting from the electronic relaxation after short pulse laser excitation. The surface sensitive reflection high energy electron diffraction and transmission electron diffraction yield a time constant of about 12 ps and 3 ps, respectively. The different energy transfer rates indicate relatively weak coupling between bulk and surface vibrational modes. (C) 2014 AIP Publishing LLC.
Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts
(2013)
We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter for the compensation of the velocity mismatch of electrons and light. The laser pulses with tilted fronts were characterized by a spatially resolving cross correlator. The response of the surface upon excitation was observed to be less than 2 ps.
The chemical vapor deposition (CVD) of Si and Ge on Si(0 0 1) substrates covered by 1 monolayer of Ga has been studied. Ga modifies surface reactivity and surface atom mobility. We show that islanding occurs during Ga-mediated CVD even in the absence of strain, allowing for the formation of islands without a wetting layer or with a thinner wetting layer. The morphology of the grown layer is highly dependent on kinetic parameters, i.e. growth temperature and growth pressure, that control Si and Ge deposition and Ga desorption. Si Ga-mediated CVD promotes the formation of self-assembled epitaxial Si islands exhibiting {1 1 3} facets. Ge islands grown by Ga-mediated CVD present reduced lateral sizes and an increased surface density compared to non Ga-mediated CVD. The growth morphology (particularly island size and density) can be understood by considering a qualitative model in which atoms are deposited only on sites whose dangling bonds are not passivated by Ga. We suggest that modification of surface reactivity during surfactant-mediated CVD can thus provide a new degree of freedom for quantum dot fabrication.
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.
We apply ultrafast time‐resolved X‐ray diffraction to directly study coherent acoustic and optical phonons in laser‐excited materials. In Au the ps acoustic response has been investigated with the particular goal to clarify the interplay of electronic and thermal pressure contributions. In Bi the extreme softening of the coherently excited A 1 g optical phonon presents strong indication that upon intense laser‐excitation the Peierls‐transition which determines the equilibrium structure of Bi can be reversed. In FeRh we studied the lattice response after a fast laser‐induced anti‐ferromagnetic to ferromagnetic phase transition.
The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions.
Bi(111) films grown on Si(111) at room temperature show a significantly higher roughness compared to Bi films grown on Si(100) utilizing a kinetic pathway based on a low-temperature process. Isochronal annealing steps of 3 min duration each with temperatures up to 200 °C cause a relaxation of the Bi films' lattice parameter toward the Bi bulk value and yield an atomically flat Bi surface. Driving force for the relaxation and surface reordering is the magic mismatch of 11 Bi atoms to 13 Si atoms that emerges at annealing temperatures above 150 °C and reduces the remaining strain to less than 0.2%.
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.
Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.
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 × 2) to a(2 × 1) of the bare Si(001) surface shows a transient decrease of the intensity of the c(4 × 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 × 1) reconstruction which undergoes a Peierls like phase transition to a (8 × "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 × "2")-diffraction spots instantaneously disappears, while the intensity of the (4 × 1)-spots increases. This increase of the (4 × 1) spot intensity excludes an explanation by the Debye-Waller-Effect and is evidence for a true structural phase transition at a surface. © 2010 Materials Research Society.
The transient temperature evolution of ultrathin bismuth films, epitaxially grown on a silicon single crystal, upon femtosecond laser excitation is studied by time-resolved electron diffraction. The exponential decay of the film temperature is explained by phonon reflection at the interface, which results in a strongly reduced thermal conduction in the cross plane of the layered system. The thermal boundary conductance is found to be as low as 1273 W/(K cm2). Model calculations, including phonon confinement effects, explain the linear relationship between the observed film-temperature decay constant and the film thickness. Even for 2.5 nm thin films the phonon transmission probability across the interface is given by bulk properties. Our simulations show that phonon confinement effects are negligible for bismuth-film thicknesses larger than 1 nm.
We investigate the fundamental mechanism by which self-assembled Ge islands can be nucleated at specific sites on Si(001) using ultra-low-dose focused ion beam (FIB) pre-patterning. Island nucleation is controlled by a nanotopography that forms after the implantation of Ga ions during subsequent thermal annealing of the substrate. This nanotopography evolves during the annealing stage, changing from a nanoscale annular depression associated with each focused ion beam spot to a nanoscale pit, and eventually disappearing (planarizing). The correspondence of Ge quantum dot nucleation sites to the focused ion beam features requires a growth surface upon which the nanotopography is preserved. A further key observation is that the Ge wetting layer thickness is reduced in patterned regions, allowing the formation of islands on the templated regions without nucleation elsewhere. These results provide routes to the greatly enhanced design and control of quantum dot distributions and dimensions.
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.
The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation.
We analyze the nucleation of dislocations in silicon at spatially localized stress fields generated by silicon nitride pads having a high intrinsic stress. The nucleation and final configuration of the dislocations were studied using hot-stage transmission electron microscopy and were compared with dislocation simulations based on calculations of the stress fields around the pads. We find that the simulated configurations match well with the experimental data, and we show that the dislocation configuration can be controlled by the pad size.
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.
Femtosecond X-ray measurement of coherent lattice vibrations near the Lindemann stability limit
(2003)
The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10(-14) to 10(-12) s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance-near the Lindemann limit-leads to a subsequent loss of long-range order, which is most probably due to melting of the material.
For a characterization of interface and “bulk” properties of Ge films grown on Si(111) by Sb surfactant-mediated epitaxy, grazing incidence x-ray diffraction and transmission electron microscopy have been used. The interface roughness, defect structure, and strain state have been investigated in dependence of film thickness and growth temperature. For all growth parameters, atomically smooth interfaces are observed. For thin Ge layers, about 75% of the strain induced by the lattice mismatch is relaxed by misfit dislocations at the Ge/Si interface. Only a slight increase of the degree of relaxation is found for thicker films. At growth temperatures below about 600 °C, the formation of twins is observed, which can be avoided at higher temperatures.
We compare in-situ electron microscopy observations of Ge island growth on Si surfaces modulated by different techniques. Si(001) substrates were patterned either using a focused Ga ion beam or lithographically to produce similar feature sizes. In the case of the focused-ion-beam patterned substrates, chemical or strain effects caused by low Ga doses control the positions at which islands nucleate. Nucleation on topographically modulated substrates, with the pattern produced either by high-dose sputtering or lithographically, is controlled by the nature of the side walls of topographic features. The two patterning approaches have different benefits in creating arrays of islands for device applications.
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.
In this paper we discuss how finite data sets influence experimental measurements of the autocovariance function. Autocovariance estimators are biased, meaning that the expectation value for any measured autocovariance function is not identical to the actual autocovariance function. In this work we show that the measured autocovariance function for a finite length time series must become negative for some lag times. We derive analytic corrections to these finite time errors for different types of correlated random sequences. Our results explain the apparent anticorrelated noise observed in experimental observations.
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.
We describe two new approaches to the patterning of Si(1 0 0) surfaces for controlled nucleation of heteroepitaxial Ge semiconductor clusters. In the first method, a Ga ' -focused ion beam in situ to the growth chamber is used to create local regions of enhanced Ga ' concentration and surface topography. It is shown that at low ion doses (Â/10 14 cm (2), implanted Ga causes local nucleation of Ge clusters upon the implanted region. At higher doses (]/10 15 cm (2), a complex surface topography localizes nucleation of Ge clusters. This approach can be used to seed complex patterns of Ge clusters with diameters of tens of nanometers and positional accuracy of sub-100 nm. The second method employs self-assembly of complex strain-stabilized ''quantum quadruplet'' and ''quantum fortress'' structures, whereby cooperative island nucleation around shallow strain-relieving pits is identified during Ge x Si 1(x /Si(1 0 0) heteroepitaxy. These configurations are kinetically limited structures that exist over a range of compositions, growth temperatures, and growth rates, but which are destabilized by strain relaxation (e.g. by the introduction of misfit dislocations) and by growth conditions which provide high adatom surface mobilities. Both methods have broad potential application to nanoelectronic device architectures. 2003 Elsevier Science B.V. All rights reserved.
Au induced faceting of vicinal Si(1 1 1) has been studied during adsorption at elevated temperature by spot profile analyzing of low energy electron diffraction and after quenching to room temperature by scanning tunneling microscopy. On the surfaces inclined towards five different types of facets form with increasing Au coverage at adsorption temperatures Tads below 800 °C. They are (4 4 3), (7 7 5), (5 5 3), a stepped (2 2 1), and the (3 3 1) facets. Atomic models for the (5 5 3) and (7 7 5) facet planes are proposed on the basis of high resolution STM images. At C we found the formation of an ordered step train which covers the entire surface. With further increasing Au coverage the stepped surface decomposes again into (1 1 1) terraces and step bunches. Driving force is the formation of the Si(1 1 1)-(5×2)–Au reconstruction.
We present the results of kinetics experiments on quantum-size-effects (QSE) Pb islands formed on Si(111). We have looked at the evolution of seven-layer Pb islands from five-layer Pb islands due to the transport of Pb atoms from the interisland region, overcoming a barrier at the island edges, and moving to the island tops. By analyzing low-energy electron-diffraction and scanning-tunneling microscopy data, we have estimated the barrier for layer formation in this system to be ∼0.32eV. This energy is much larger than that expected from QSE electronic contributions alone.
We present equilibrium measurements of the dynamics of steps on Si(001) using temporal electron-diffraction spectroscopy. Activation energies and the rate limiting kinetics are identified for 950K<~T<~1130K. Unlike previous studies at higher temperatures, we can exclude evaporation and condensation of atoms or dimers from the step edges as the rate limiting process in this temperature regime. The possible reason for this difference is discussed in terms of a crossover from different rate-limiting kinetics.
We demonstrate that the nucleation sites of nanoscale, self-assembled Ge islands on Si(001) can be controlled by patterning the Si surface in situ with a focused ion beam. At low doses of 6000 Ga+ ions per <100 nm spot, the selective growth is achieved without modifying the initial surface topography. At larger doses, topographic effects produced by sputtering and redeposition control the selective nucleation sites. Islands grown on irradiated spots are smaller with higher aspect ratio than islands grown on clean Si(001), suggesting a strong surfactant effect of Ga.