108 research outputs found
Surface Morphology and Strain Relief in Surfactant Mediated Growth of Germanium on Silicon (111)
The growth of Ge on Si is strongly modified by adsorbates called surfactants. The relevance of the stress on surface morphology and the growth mode of Ge on Si(111) is presented in a detailed in situ study by high resolution low energy electron diffraction (LEED) during the deposition. The change from islanding to layer-by-layer growth mode is seen in the oscillatory intensity behaviour of the 00-spot. As a strain relief mechanism, the Ge-film forms a microscopic rough surface of small triangular and defect-free pyramids in the pseudomorphic growth regime up to 8 monolayers. As soon as the pyramids are completed and start to coalesce, strain relieving defects are created at their base, finally arranging to the dislocation network. Without the driving force for the micro-roughness, the stress, the surface flattens again showing a much larger terrace length. The formation process of the dislocation network results in a spot splitting in LEED, since the periodic dislocations at the interface give rise to elastic deformation of the surface. Surprisingly the Ge-film is relaxed to 70% immediately after 8 monolayers of coverage, which is attributed to the micro rough surface morphology, providing innumerous nucleation sites for dislocation
Violation of Boltzmann Equipartition Theorem in Angular Phonon Phase Space Slows down Nanoscale Heat Transfer in Ultrathin Heterofilms
Heat transfer through heterointerfaces is intrinsically hampered by a thermal boundary resistance originating from the discontinuity of the elastic properties. Here, we show that with shrinking dimensions the heat flow from an ultrathin epitaxial film through atomically flat interfaces into a single crystalline substrate is significantly reduced due to violation of Boltzmann equipartition theorem in the angular phonon phase space. For films thinner than the phonons mean free path, we find phonons trapped in the film by total internal reflection, thus suppressing heat transfer. Repopulation of those phonon states, which can escape the film through the interface by transmission and refraction, becomes the bottleneck for cooling. The resulting nonequipartition in the angular phonon phase space slows down the cooling by more than a factor of 2 compared to films governed by phonons diffuse scattering. These allow tailoring of the thermal interface conductance via manipulation of the interface
Electron phonon coupling in ultrathin Pb films on Si(111): Where the heck is the energy?
In this work, we study the heat transfer from electron to phonon system
within a five monolayer thin epitaxial Pb film on Si(111) upon fs-laser
excitation. The response of the electron system is determined using
time-resolved photoelectron spectroscopy while the lattice excitation is
measured by means of the Debye-Waller effect in time-resolved reflection
high-energy electron diffraction. The electrons lose their heat within 0.5 ps
while the lattice temperature rises slowly in 3.5 to 8 ps, leaving a gap of 3-7
ps. We propose that the hidden energy is transiently stored in high-frequency
phonon modes for which diffraction is insensitive and which are excited in 0.5
ps. Within a three-temperature model we use three heat baths, namely electrons,
high-frequency and low-frequency phonon modes to simulate the observations. The
excitation of low-frequency acoustic phonons, i.e., thermalization of the
lattice is facilitated through anharmonic phonon-phonon interaction
Non-Equilibrium Pathways for Excitation of Bulk and Surface Phonons through Anharmonic Coupling
Upon impulsive optical excitation of solid-state materials, the
non-equilibrium flow of energy from the excited electronic system to the
lattice degrees of freedom typically happens in a few picoseconds. Here we
identified the surface of thin Bi films grown on Si(001) as an additional
subsystem which is excited much slower on a 100 ps timescale that is caused by
decoupling due to mismatched phonon dispersions relations of bulk and surface.
Anharmonic coupling among the phonon systems provides pathways for excitations
which exhibits a 1/T-dependence causing a speed-up of surface excitation at
higher temperatures. A quantitative justification is provided by phonon Umklapp
processes from lattice thermal conductivity of the Bi bulk. Three-temperature
model simulations reveal a pronounced non-equilibrium situation up to
nanoseconds: initially, the surface is colder than the bulk, that situation is
then inverted during cooling and the surface feeds energy back into the bulk
phonon system
Selecting a single orientation for millimeter sized graphene sheets
We have used Low Energy Electron Microscopy (LEEM) and Photo Emission
Electron Microscopy (PEEM) to study and improve the quality of graphene films
grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated
temperature already yields graphene sheets that are uniform and of monatomic
thickness. Besides domains that are aligned with respect to the substrate,
other rotational variants grow. Cyclic growth exploiting the faster growth and
etch rates of the rotational variants, yields films that are 99 % composed of
aligned domains. Precovering the substrate with a high density of graphene
nuclei prior to CVD yields pure films of aligned domains extending over
millimeters. Such films can be used to prepare cluster-graphene hybrid
materials for catalysis or nanomagnetism and can potentially be combined with
lift-off techniques to yield high-quality, graphene based electronic devices
Electron and lattice structure of ultra thin Ag films on Si(111) and Si(001)
We studied the low temperature (T<130K) growth of Ag on Si(001) and Si(111)
flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic
films. The band structure and morphology of the Ag overlayers have been
investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a
(root3xroot3)R30^o LEED structure for Ag films has been observed after
deposition of 2-6 ML Ag onto a Si(111)(root3xroot3)R30^o Ag surface at low
temperatures. XPS investigations showed that these films are solid, and UPS
measurements indicate that they are metallic. However, after closer STM studies
we found that these films consists of sharp Ag islands and (root3xroot3)R30^o
Ag flat terraces in between. On Si(001) the low-temperature deposition yields
an epitaxial growth of Ag on clean Si(001)2x1 with a twinned Ag(111) structure
at coverages as low as 10 ML. Furthermore the conductivity of few monolayer Ag
films on Si(100) surfaces has been studied as a function of temperature (40-300
K).Comment: 12 pages, 9 figure
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