Martin Kammler

$Research paper thumbnail of Femtosecond time-resolved X-ray diffraction study of ultrafast structural dynamics in laser-excited solids$

Femtosecond time-resolved X-ray diffraction study of ultrafast structural dynamics in laser-excited solids

The Thirteenth International Conference on Ultrafast Phenomena, 2002

Time-resolved x-ray diffraction with sub-300 fs, multi-keV x-ray pulses was used to study ultrafa... more

$Research paper thumbnail of Time-resolved x-ray diffraction study of ultrafast acoustic phonon dynamics in Ge/Si-heterostructures$

Time-resolved x-ray diffraction study of ultrafast acoustic phonon dynamics in Ge/Si-heterostructures

Springer Series in Chemical Physics, 2001

Using time-resolved x-ray diffraction the ultafast strain dynamics in fslaserexcited Ge/Si-hetero... more

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$Research paper thumbnail of Measurement of harmonic and anharmonic semi-conductor-lattice dynamics with ultrafast x-ray diffraction$

Measurement of harmonic and anharmonic semi-conductor-lattice dynamics with ultrafast x-ray diffraction

ABSTRACT Many fundamental processes in solids, such as heat transport and phase transitions invol... more ABSTRACT Many fundamental processes in solids, such as heat transport and phase transitions involve atomic movement on ultrafast time-scales, but cannot be directly measured with visible light. Using ultrafast x-ray diffraction, atomic motion can be measured on its natural time-scale. Photo-excitation of bulk and layered semiconductors was achieved with 800-nm wavelength femtosecond laser pulses. Optical excitation triggers a chain of relaxation processes, causing rapid lattice heating, generation and propagation of stress/strain waves into the bulk and eventual anharmonic decay of coherent acoustic phonons into the thermal bath of the solid. While the first steps of carrier relaxation, excitation and decay of Raman-active optical phonons have been in the past characterized optically at surfaces, the bulk acoustic dynamics at longer times has not. We measure length scales of lattice excitation, thereby detecting ultrafast heat diffusion via fast electrons, coherent acoustic pulse propagation into the bulk or across buried interfaces and anharmonic damping of non-equilibrium acoustic phonons. The measurement is performed with sensitivity on lattice spacing of 10 femtometers (i.e., 3 nuclear diameters).

B8.2 Tunable Nanopillars as Surface Enhanced Raman Scattering (SERS) Active Structure for Optical Quartz Glass Fiber

In this work, a simple cost effective method to create tunable self-assembled three-dimensional n... more 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.

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$Research paper thumbnail of Comparing ultrafast surface and bulk heating using time-resolved electron diffraction$

Comparing ultrafast surface and bulk heating using time-resolved electron diffraction

Applied Physics Letters, 2014

From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time ... more 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.

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$Research paper thumbnail of Time-dependent phenomena on surfaces measured from diffraction intensity equilibrium fluctuations?$

Time-dependent phenomena on surfaces measured from diffraction intensity equilibrium fluctuations?

Time-dependent equilibrium phenomena in surface overlayers can be studied by measuring the time a... more Time-dependent equilibrium phenomena in surface overlayers can be studied by measuring the time autocorrelation of thermally generated equilibrium fluctuations. Surface diffusion at equilibrium and dynamic critical phenomena can be analyzed in such experiments. The potential use of diffraction intensity fluctuations is examined by considering the role of incoherent summation over the finite beam size, statistical noise and signal collection time. In diffraction experiments, where the incident current probe density is kept constant, the fluctuations related to the physical process and statistical noise increase as N^1/2, where N=(L_b/L_c)^2 is the number of incoherent regions (Lb is the beam size and Lc is the coherence length of the diffractometer). In deciding the optimal beam size Lb a compromise should be reached between maximizing relative fluctuations but without increasing statistical fluctuations so that no impractically long collection times are needed. Different surface dif...

$Research paper thumbnail of Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts$

Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts

EPJ Web of Conferences, 2013

We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter fo... more 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.

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Tunable nanopillar array on a quartz-fiber tip for surface enhanced Raman scattering (SERS) detection

tm - Technisches Messen, 2021

In this work, a simple two-step method to create tunable self-assembled three-dimensional nanostr... more 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 SER...

Directed Self-Assembly of Ge Quantum Dots Using Focused Si2+ Ion Beam Patterning

Scientific Reports

We show that templating a Si surface with a focused beam of Si 2+ or Si + ions can create suitabl... more We show that templating a Si surface with a focused beam of Si 2+ 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.

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Method of forming quantum dots at predetermined positions on a substrate

Dynamics of steps on Si(001) studied by temporal LEED [poster]

ABSTRACT While surface kinetics measurements for single atom diffusion have proliferated, little ... more 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 &lt; T &lt; 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).

Dynamics of atomic steps on Si(001) studied by temporal LEED spectroscopy

ABSTRACT While surface kinetics measurements for single atom diffusion have proliferated, little ... more 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 &lt; T &lt; 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).

Growth kinetics of Ge islands during Ga-surfactant-mediated ultrahigh vacuum chemical vapor deposition on Si(001)

Phys Rev B, 2004

ABSTRACT The surfactant effect of Ga pre-deposited prior to ultrahigh vacuum (UHV) chemical vapor... more ABSTRACT The surfactant effect of Ga pre-deposited prior to ultrahigh vacuum (UHV) chemical vapor deposition (CVD) of Ge on Si(001) is investigated using in situ transmission electron microscopy. Island shape, nucleation and growth kinetics, and the influence of growth temperature and pressure are studied. When 1 monolayer of Ga covers the surface, Ge island nucleation occurs after a 5–10× longer growth time than without Ga for the same growth temperature and pressure, and the island density then varies with time in a more complex way than for Ga-free growth. The islands formed have a different shape and aspect ratio compared to conventional hut and dome clusters, and the critical lateral size for dislocation nucleation is reduced. Ex situ medium energy ion scattering and x-ray photoelectron scattering measurements of the total amount of Ge deposited before island nucleation demonstrate that the Ga layer decreases the surface reactivity and inhibits Ge growth. We discuss a simple model in which the surfactant-mediated growth kinetics are principally driven by the balance between Ge deposition and Ga desorption. We show that Ga surfactant-mediated UHV CVD can be used to produce Ge quantum dots with a higher surface density and a thinner wetting layer than is possible using conventional CVD.

Dynamics of steps on Si(001) studied by temporal LEED

While surface kinetics measurements for single atom diffusion have proliferated, little work exis... more 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

Ge p-MOSFETs Compatible with Si CMOS-Technology

ABSTRACT First Page of the Article

Dynamics of steps on Si(001) studied by temporal LEED [poster]

While surface kinetics measurements for single atom diffusion have proliferated, little work exis... more 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. K...

Dynamics of atomic steps on Si(001) studied by temporal LEED spectroscopy

ABSTRACT While surface kinetics measurements for single atom diffusion have proliferated, little ... more 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 &lt; T &lt; 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).

Precision placement of heteroepitaxial semiconductor quantum dots

Materials Science and Engineering: B, 2003

We describe two new approaches to the patterning of Si(1 0 0) surfaces for controlled nucleation ... more 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.

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In-Situ Focused Ion Beam Micropatterning of Ge Islands

Microscopy and Microanalysis, Aug 1, 2003

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$Research paper thumbnail of Ultrafast Nonthermal Melting of Femtosecond-laser-irradiated Germanium Measured by Ultrafast X-ray Diffraction$

Ultrafast Nonthermal Melting of Femtosecond-laser-irradiated Germanium Measured by Ultrafast X-ray Diffraction

ABSTRACT Many fundamental processes in nature, such as chemical reactions and phase transitions i... more ABSTRACT Many fundamental processes in nature, such as chemical reactions and phase transitions involve rearrangement of the constituent atoms. Such changes cannot be directly measured with visible light and occur transiently on time-scales comparable with the natural oscillation periods of atoms (femtoseconds to picoseconds). We present experiments on Ge-111 films irradiated with 100-fs 800-nm light pulses and probed by a time-delayed sub-ps x-ray pulse at 8 keV. After excitation of a dense electron-hole plasma within the 160-nm film, the ultrafast pathway of lattice disordering is measured as a function laser fluence. For close-to-threshold irradiation, inhomogeneous thermal melting, occurring via surface nucleation of the liquid phase and growth into the bulk determines the dynamics of the solid-to-liquid phase transition. For excitation at about twice the melting threshold, a nonthermal pathway, starting from a highly excited cold solid and relaxing into a hot liquid causes ultrafast melting. At long time delays, nearly perfect recrystallization of a (111) crystal is observed.

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