Papers by Daniele Scopece
Assessing the equilibrium morphologies of self-assembled heteroepitaxial quantum dots requires th... more Assessing the equilibrium morphologies of self-assembled heteroepitaxial quantum dots requires the estimation
of their elastic (volumetric), surface, and edge energy contribution, all of them being shape dependent. Due to the
size and multifaceted morphology of these islands, the estimation of the first term is typically a time-consuming
or complicated task. A general rule to predict it from the sole morphologies would guarantee a precious advantage
in this field. Here we present an interpolating function to fulfill this purpose for the prototypical systems of Ge/Si
and InAs/GaAs. The trend is first extracted from a systematic analysis of realistic shapes observed on (001)
substrates. It is then tested and corroborated for selected vicinal (tilted) substrates. Finally, the deviations due
to intermixing and the underlying wetting layer are quantified. Of fundamental importance in this process is the
identification of a morphological descriptor more accurate than the widely adopted aspect ratio, the limitations
of which are discussed.
We have implemented the potential of Lazic and Thijsse (2012) in LAMMPS and have noticed some cor... more We have implemented the potential of Lazic and Thijsse (2012) in LAMMPS and have noticed some corrections to be applied to this paper. Here we report them point by point for future users.
We show that a relatively simple top-down fabrication can be used to locally deform germanium in
... more We show that a relatively simple top-down fabrication can be used to locally deform germanium in
order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically pre-
dicted to transform germanium from an indirect to a direct gap semiconductor. These values of
strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures
acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers
Germanium is known to become a direct band gap material when subject to a biaxial tensile
strain ... more Germanium is known to become a direct band gap material when subject to a biaxial tensile
strain of 2% (Vogl et al 1993 Phys. Scr. T49B 476) or uniaxial tensile strain of 4% (Aldaghri
et al 2012 J. Appl. Phys. 111 053106). This makes it appealing for the integration of
optoelectronics into current CMOS technology. It is known that the induced strain is highly
dependent on the geometry and composition of the whole system (stressors and substrate),
leaving a large number of variables to the experimenters willing to realize this transition and just
a trial-and-error procedure. The study in this paper aims at reducing this freedom. We adopt a
finite element approach to systematically study the elastic strain induced by different
configurations of lithographically-created SiGe nanostructures on a Ge substrate, by focusing on
their composition and geometries. We numerically investigate the role played by the Ge substrate
by comparing the strain induced on a bulk or on a suspended membrane. These results and their
interpretation can provide the community starting guidelines to choose the appropriate subset of parameters to achieve the desired strain. A case of a very large optically active area of a Ge membrane is reported.
Ge heteroepitaxy on Si (1 1 10) substrates induces the formation of prism-shaped in-plane nanowir... more Ge heteroepitaxy on Si (1 1 10) substrates induces the formation of prism-shaped in-plane nanowires bounded
with {1 0 5} facets. In this work, in-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si
̄
(1 1 10) templates oriented in the [551] direction. Atomic force microscopy (AFM) reveals that a self-elongation of
the nanowires occurs, resembling the phenomena observed on rib-patterned Si (0 0 1) templates, which indicates
that this is a universal effect for nanowires grown on rib patterns. Finite-element simulations, performed with input
from the latest ab initio calculations, reveal that the mechanism behind these phenomena is the minimization of
the total energy density of the epilayer under rib-dominated geometry. Ge surface diffusion leads to a broadening
of the Ge nanowires at the rib shoulder sites, which is proved to be an effective route to reduce the total energy
density. Our results provide a straightforward solution for the realization of a single or a few Ge nanowires for
potential device applications.
The evolution of the composition of tungsten carbide and silicon surfaces initiated by the bombar... more The evolution of the composition of tungsten carbide and silicon surfaces initiated by the bombardment
with Zr and Cr ions has been investigated as a function of the substrate bias voltage. Surface composition
profiles were measured by Rutherford backscattering and have been compared with the results obtained
by the TRIDYN simulation program. It is found that the general dependence of film thickness on substrate
bias is satisfactorily reproduced by this model. Deviations between experiment and simulation are attrib-
uted to possible partial oxidation of the surface or uncertainties in the charge state distribution of metal
ions. The results confirm that TRIDYN facilitates the predictability of the nucleation of metallic vapor at
substrate surfaces.
The stability and growth of three-dimensional (3D) nanostructures in the Ge on Si system is contr... more The stability and growth of three-dimensional (3D) nanostructures in the Ge on Si system is controlled in
part by the strain- and overlayer-thickness-dependent surface energies of the crystal facets involved. Here, we
use density functional theory (DFT) with local-density approximation calculations to calculate the strain- and
thickness-dependent energy of various Ge(113) and Si(113) surface reconstructions. Results of DFT calculations
are compared to Tersoff potential calculations to assess the relative importance of stress-strain effects compared
to electronic effects not captured by empirical atomistic potentials. We find that the self-interstitial-based
3 × 2 adatom-dimer-interstitial and 3 × 2 adatom-interstitial surface reconstructions are stable for Ge overlayer
thicknesses from 0 to 4 monolayers and at applied biaxial strains from ∼−4% to 0%. We leverage calculated
surface energies to determine net effective surface energies of various experimentally observed 3D Ge on Si
nanostructures.
A Fortran90 program for the determination of the Wulff construction, starting solely from the dir... more A Fortran90 program for the determination of the Wulff construction, starting solely from the directions of the bounding facets (defined by the user), is presented. SOWOS stands for solid of Wulff open source, and the program is distributed freely with no charge to the user, being readily available to the community for immediate use. Its simple algorithm (which will be explained) allows the determination of complex solids with hundreds of facets in just seconds on any machine, requiring only a small amount of memory. It is able to determine even the smallest facets and shortest edges and to distinguish almost adjacent vertices. The output files give a complete range of information about the structure: the coordinates of the vertices and the facets common to them, the extension of the facets and bounding vertices, and the length of the edges and extreme vertices. These details enable the reconstruction of the shape in any other (commercial) software for further processing. Visualization is straightfor-ward via the free program gnuplot. A feature for the creation of cubic crystal atomistic models of the resultant solids is included. The program may be a useful tool for crystallography, nanostructures and any other field where crystal facets are involved.
Ge/Si(001) is a prototypical system for investigating three-dimensional island self-assembly
owed... more Ge/Si(001) is a prototypical system for investigating three-dimensional island self-assembly
owed to the Stranski–Krastanow growth mode. More than twenty years of research have
produced an impressive amount of results, together with various theoretical interpretations.
It is commonly believed that lattice-mismatch strain relief is the major driving force
leading to the formation of these islands. However, a set of recent results on Si(001)
and vicinals point out that, under suitable conditions, this is not the case. Indeed, we
here review experimental and theoretical results dealing with nanostructures mainly
determined by surface-energy minimization. Results are intriguing, as they reveal the
existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.
We report the observation of large scale self-assembly of long horizontal nanowires into orthogon... more We report the observation of large scale self-assembly of long horizontal nanowires into orthogonally
oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are
interpreted in terms of a collective wave-propagation mechanism, previously suggested for
interpreting ripple faceting on Ge/Si(1 1 10) surfaces. Quantitative agreement between experiments
and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total
density can be controlled by carefully tuning the growth parameters.
Self-assembled Ge wires with a height of only 3 unit cells and a length of up to 2 micrometers we... more Self-assembled Ge wires with a height of only 3 unit cells and a length of up to 2 micrometers were
grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow
horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly
uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a
preferential base width for longitudinal expansion, in quantitative agreement with the experimental
findings. Despite the absence of intentional doping, the first transistor-type devices made from single
wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. In view
of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization
of hole systems with exotic properties and provide a new development route for silicon-based
nanoelectronics.
SiGe heteroepitaxy on vicinal Si (1 1 10) is studied as a model system for one-dimensional (1D) t... more SiGe heteroepitaxy on vicinal Si (1 1 10) is studied as a model system for one-dimensional (1D) to
three-dimensional growth mode transitions. By in situ scanning tunneling microscopy it is shown that the
1D-3D transition proceeds smoothly from perfectly facetted 1D nanoripples to coarsened superripples,
tadpoles, asymmetric domes, and barns without involving coalescence or agglomeration. By extension of
the studies to a wide range of SiGe compositions, a 1D-3D growth phase diagram is obtained. Total energy
calculations reveal that the observed critical transition volumes are fully consistent with thermodynamic
driven strain relaxation.
Ge growth on high-indexed Si (1110) is shown to result in the spontaneous formation of a perfectl... more Ge growth on high-indexed Si (1110) is shown to result in the spontaneous formation of a perfectly
f105g faceted one-dimensional nanoripple structure. This evolution differs from the usual Stranski-
Krastanow growth mode because from initial ripple seeds a faceted Ge layer is formed that extends
down to the heterointerface. Ab initio calculations reveal that ripple formation is mainly driven by
lowering of surface energy rather than by elastic strain relief and the onset is governed by the edge energy
of the ripple facets. Wavelike ripple replication is identified as an effective kinetic pathway for the
transformation process.
Using both density functional theory–local density approximation and Tersoff potentials, we calcu... more Using both density functional theory–local density approximation and Tersoff potentials, we calculate
(1 1 10), (105), and (001) surface energies relevant to the self-assembly of Ge {105} ripples on Si (1 1 10) substrates.
Surface energies are calculated as a function of Ge overlayer thickness and applied strain. Comparison of density
functional theory (DFT) and Tersoff potential results reveals qualitative differences in the predicted dependence
of surface energies on Ge overlayer thickness and the stability of the Ge on Si (1 1 10) surface relative to the Ge
(1110)
on Si (001) surface. DFT calculations show that γGe/Si is strongly influenced by the presence of tilted dimers, and
provide an explanation for the differing stability predictions. Finally, a multiscale model including strain- and
thickness-dependent γ Ge/Si is used to show that surface energy is a driving force for formation of Ge {105} ripples
on Si (1 1 10), supporting recent experimental results of Ge-deposition-induced {105} faceting on Si (1 1 10).
The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays... more The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays very
uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower
‘‘dome’’ and steeper ‘‘barn’’ morphologies. Correspondingly, the average Ge content in the alloyed
islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing
size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the
islands to keep growing in size while avoiding plastic relaxation.
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Papers by Daniele Scopece
of their elastic (volumetric), surface, and edge energy contribution, all of them being shape dependent. Due to the
size and multifaceted morphology of these islands, the estimation of the first term is typically a time-consuming
or complicated task. A general rule to predict it from the sole morphologies would guarantee a precious advantage
in this field. Here we present an interpolating function to fulfill this purpose for the prototypical systems of Ge/Si
and InAs/GaAs. The trend is first extracted from a systematic analysis of realistic shapes observed on (001)
substrates. It is then tested and corroborated for selected vicinal (tilted) substrates. Finally, the deviations due
to intermixing and the underlying wetting layer are quantified. Of fundamental importance in this process is the
identification of a morphological descriptor more accurate than the widely adopted aspect ratio, the limitations
of which are discussed.
order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically pre-
dicted to transform germanium from an indirect to a direct gap semiconductor. These values of
strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures
acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers
strain of 2% (Vogl et al 1993 Phys. Scr. T49B 476) or uniaxial tensile strain of 4% (Aldaghri
et al 2012 J. Appl. Phys. 111 053106). This makes it appealing for the integration of
optoelectronics into current CMOS technology. It is known that the induced strain is highly
dependent on the geometry and composition of the whole system (stressors and substrate),
leaving a large number of variables to the experimenters willing to realize this transition and just
a trial-and-error procedure. The study in this paper aims at reducing this freedom. We adopt a
finite element approach to systematically study the elastic strain induced by different
configurations of lithographically-created SiGe nanostructures on a Ge substrate, by focusing on
their composition and geometries. We numerically investigate the role played by the Ge substrate
by comparing the strain induced on a bulk or on a suspended membrane. These results and their
interpretation can provide the community starting guidelines to choose the appropriate subset of parameters to achieve the desired strain. A case of a very large optically active area of a Ge membrane is reported.
with {1 0 5} facets. In this work, in-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si
̄
(1 1 10) templates oriented in the [551] direction. Atomic force microscopy (AFM) reveals that a self-elongation of
the nanowires occurs, resembling the phenomena observed on rib-patterned Si (0 0 1) templates, which indicates
that this is a universal effect for nanowires grown on rib patterns. Finite-element simulations, performed with input
from the latest ab initio calculations, reveal that the mechanism behind these phenomena is the minimization of
the total energy density of the epilayer under rib-dominated geometry. Ge surface diffusion leads to a broadening
of the Ge nanowires at the rib shoulder sites, which is proved to be an effective route to reduce the total energy
density. Our results provide a straightforward solution for the realization of a single or a few Ge nanowires for
potential device applications.
with Zr and Cr ions has been investigated as a function of the substrate bias voltage. Surface composition
profiles were measured by Rutherford backscattering and have been compared with the results obtained
by the TRIDYN simulation program. It is found that the general dependence of film thickness on substrate
bias is satisfactorily reproduced by this model. Deviations between experiment and simulation are attrib-
uted to possible partial oxidation of the surface or uncertainties in the charge state distribution of metal
ions. The results confirm that TRIDYN facilitates the predictability of the nucleation of metallic vapor at
substrate surfaces.
part by the strain- and overlayer-thickness-dependent surface energies of the crystal facets involved. Here, we
use density functional theory (DFT) with local-density approximation calculations to calculate the strain- and
thickness-dependent energy of various Ge(113) and Si(113) surface reconstructions. Results of DFT calculations
are compared to Tersoff potential calculations to assess the relative importance of stress-strain effects compared
to electronic effects not captured by empirical atomistic potentials. We find that the self-interstitial-based
3 × 2 adatom-dimer-interstitial and 3 × 2 adatom-interstitial surface reconstructions are stable for Ge overlayer
thicknesses from 0 to 4 monolayers and at applied biaxial strains from ∼−4% to 0%. We leverage calculated
surface energies to determine net effective surface energies of various experimentally observed 3D Ge on Si
nanostructures.
owed to the Stranski–Krastanow growth mode. More than twenty years of research have
produced an impressive amount of results, together with various theoretical interpretations.
It is commonly believed that lattice-mismatch strain relief is the major driving force
leading to the formation of these islands. However, a set of recent results on Si(001)
and vicinals point out that, under suitable conditions, this is not the case. Indeed, we
here review experimental and theoretical results dealing with nanostructures mainly
determined by surface-energy minimization. Results are intriguing, as they reveal the
existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.
oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are
interpreted in terms of a collective wave-propagation mechanism, previously suggested for
interpreting ripple faceting on Ge/Si(1 1 10) surfaces. Quantitative agreement between experiments
and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total
density can be controlled by carefully tuning the growth parameters.
grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow
horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly
uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a
preferential base width for longitudinal expansion, in quantitative agreement with the experimental
findings. Despite the absence of intentional doping, the first transistor-type devices made from single
wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. In view
of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization
of hole systems with exotic properties and provide a new development route for silicon-based
nanoelectronics.
three-dimensional growth mode transitions. By in situ scanning tunneling microscopy it is shown that the
1D-3D transition proceeds smoothly from perfectly facetted 1D nanoripples to coarsened superripples,
tadpoles, asymmetric domes, and barns without involving coalescence or agglomeration. By extension of
the studies to a wide range of SiGe compositions, a 1D-3D growth phase diagram is obtained. Total energy
calculations reveal that the observed critical transition volumes are fully consistent with thermodynamic
driven strain relaxation.
f105g faceted one-dimensional nanoripple structure. This evolution differs from the usual Stranski-
Krastanow growth mode because from initial ripple seeds a faceted Ge layer is formed that extends
down to the heterointerface. Ab initio calculations reveal that ripple formation is mainly driven by
lowering of surface energy rather than by elastic strain relief and the onset is governed by the edge energy
of the ripple facets. Wavelike ripple replication is identified as an effective kinetic pathway for the
transformation process.
(1 1 10), (105), and (001) surface energies relevant to the self-assembly of Ge {105} ripples on Si (1 1 10) substrates.
Surface energies are calculated as a function of Ge overlayer thickness and applied strain. Comparison of density
functional theory (DFT) and Tersoff potential results reveals qualitative differences in the predicted dependence
of surface energies on Ge overlayer thickness and the stability of the Ge on Si (1 1 10) surface relative to the Ge
(1110)
on Si (001) surface. DFT calculations show that γGe/Si is strongly influenced by the presence of tilted dimers, and
provide an explanation for the differing stability predictions. Finally, a multiscale model including strain- and
thickness-dependent γ Ge/Si is used to show that surface energy is a driving force for formation of Ge {105} ripples
on Si (1 1 10), supporting recent experimental results of Ge-deposition-induced {105} faceting on Si (1 1 10).
uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower
‘‘dome’’ and steeper ‘‘barn’’ morphologies. Correspondingly, the average Ge content in the alloyed
islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing
size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the
islands to keep growing in size while avoiding plastic relaxation.
of their elastic (volumetric), surface, and edge energy contribution, all of them being shape dependent. Due to the
size and multifaceted morphology of these islands, the estimation of the first term is typically a time-consuming
or complicated task. A general rule to predict it from the sole morphologies would guarantee a precious advantage
in this field. Here we present an interpolating function to fulfill this purpose for the prototypical systems of Ge/Si
and InAs/GaAs. The trend is first extracted from a systematic analysis of realistic shapes observed on (001)
substrates. It is then tested and corroborated for selected vicinal (tilted) substrates. Finally, the deviations due
to intermixing and the underlying wetting layer are quantified. Of fundamental importance in this process is the
identification of a morphological descriptor more accurate than the widely adopted aspect ratio, the limitations
of which are discussed.
order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically pre-
dicted to transform germanium from an indirect to a direct gap semiconductor. These values of
strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures
acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers
strain of 2% (Vogl et al 1993 Phys. Scr. T49B 476) or uniaxial tensile strain of 4% (Aldaghri
et al 2012 J. Appl. Phys. 111 053106). This makes it appealing for the integration of
optoelectronics into current CMOS technology. It is known that the induced strain is highly
dependent on the geometry and composition of the whole system (stressors and substrate),
leaving a large number of variables to the experimenters willing to realize this transition and just
a trial-and-error procedure. The study in this paper aims at reducing this freedom. We adopt a
finite element approach to systematically study the elastic strain induced by different
configurations of lithographically-created SiGe nanostructures on a Ge substrate, by focusing on
their composition and geometries. We numerically investigate the role played by the Ge substrate
by comparing the strain induced on a bulk or on a suspended membrane. These results and their
interpretation can provide the community starting guidelines to choose the appropriate subset of parameters to achieve the desired strain. A case of a very large optically active area of a Ge membrane is reported.
with {1 0 5} facets. In this work, in-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si
̄
(1 1 10) templates oriented in the [551] direction. Atomic force microscopy (AFM) reveals that a self-elongation of
the nanowires occurs, resembling the phenomena observed on rib-patterned Si (0 0 1) templates, which indicates
that this is a universal effect for nanowires grown on rib patterns. Finite-element simulations, performed with input
from the latest ab initio calculations, reveal that the mechanism behind these phenomena is the minimization of
the total energy density of the epilayer under rib-dominated geometry. Ge surface diffusion leads to a broadening
of the Ge nanowires at the rib shoulder sites, which is proved to be an effective route to reduce the total energy
density. Our results provide a straightforward solution for the realization of a single or a few Ge nanowires for
potential device applications.
with Zr and Cr ions has been investigated as a function of the substrate bias voltage. Surface composition
profiles were measured by Rutherford backscattering and have been compared with the results obtained
by the TRIDYN simulation program. It is found that the general dependence of film thickness on substrate
bias is satisfactorily reproduced by this model. Deviations between experiment and simulation are attrib-
uted to possible partial oxidation of the surface or uncertainties in the charge state distribution of metal
ions. The results confirm that TRIDYN facilitates the predictability of the nucleation of metallic vapor at
substrate surfaces.
part by the strain- and overlayer-thickness-dependent surface energies of the crystal facets involved. Here, we
use density functional theory (DFT) with local-density approximation calculations to calculate the strain- and
thickness-dependent energy of various Ge(113) and Si(113) surface reconstructions. Results of DFT calculations
are compared to Tersoff potential calculations to assess the relative importance of stress-strain effects compared
to electronic effects not captured by empirical atomistic potentials. We find that the self-interstitial-based
3 × 2 adatom-dimer-interstitial and 3 × 2 adatom-interstitial surface reconstructions are stable for Ge overlayer
thicknesses from 0 to 4 monolayers and at applied biaxial strains from ∼−4% to 0%. We leverage calculated
surface energies to determine net effective surface energies of various experimentally observed 3D Ge on Si
nanostructures.
owed to the Stranski–Krastanow growth mode. More than twenty years of research have
produced an impressive amount of results, together with various theoretical interpretations.
It is commonly believed that lattice-mismatch strain relief is the major driving force
leading to the formation of these islands. However, a set of recent results on Si(001)
and vicinals point out that, under suitable conditions, this is not the case. Indeed, we
here review experimental and theoretical results dealing with nanostructures mainly
determined by surface-energy minimization. Results are intriguing, as they reveal the
existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.
oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are
interpreted in terms of a collective wave-propagation mechanism, previously suggested for
interpreting ripple faceting on Ge/Si(1 1 10) surfaces. Quantitative agreement between experiments
and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total
density can be controlled by carefully tuning the growth parameters.
grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow
horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly
uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a
preferential base width for longitudinal expansion, in quantitative agreement with the experimental
findings. Despite the absence of intentional doping, the first transistor-type devices made from single
wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. In view
of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization
of hole systems with exotic properties and provide a new development route for silicon-based
nanoelectronics.
three-dimensional growth mode transitions. By in situ scanning tunneling microscopy it is shown that the
1D-3D transition proceeds smoothly from perfectly facetted 1D nanoripples to coarsened superripples,
tadpoles, asymmetric domes, and barns without involving coalescence or agglomeration. By extension of
the studies to a wide range of SiGe compositions, a 1D-3D growth phase diagram is obtained. Total energy
calculations reveal that the observed critical transition volumes are fully consistent with thermodynamic
driven strain relaxation.
f105g faceted one-dimensional nanoripple structure. This evolution differs from the usual Stranski-
Krastanow growth mode because from initial ripple seeds a faceted Ge layer is formed that extends
down to the heterointerface. Ab initio calculations reveal that ripple formation is mainly driven by
lowering of surface energy rather than by elastic strain relief and the onset is governed by the edge energy
of the ripple facets. Wavelike ripple replication is identified as an effective kinetic pathway for the
transformation process.
(1 1 10), (105), and (001) surface energies relevant to the self-assembly of Ge {105} ripples on Si (1 1 10) substrates.
Surface energies are calculated as a function of Ge overlayer thickness and applied strain. Comparison of density
functional theory (DFT) and Tersoff potential results reveals qualitative differences in the predicted dependence
of surface energies on Ge overlayer thickness and the stability of the Ge on Si (1 1 10) surface relative to the Ge
(1110)
on Si (001) surface. DFT calculations show that γGe/Si is strongly influenced by the presence of tilted dimers, and
provide an explanation for the differing stability predictions. Finally, a multiscale model including strain- and
thickness-dependent γ Ge/Si is used to show that surface energy is a driving force for formation of Ge {105} ripples
on Si (1 1 10), supporting recent experimental results of Ge-deposition-induced {105} faceting on Si (1 1 10).
uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower
‘‘dome’’ and steeper ‘‘barn’’ morphologies. Correspondingly, the average Ge content in the alloyed
islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing
size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the
islands to keep growing in size while avoiding plastic relaxation.