Achim Trampert - Academia.edu (original) (raw)

Papers by Achim Trampert

Physical Review Materials

Fe 3 Si/Ge(Fe,Si)/Fe 3 Si thin film stacks were grown by a combination of molecular beam epitaxy ... more Fe 3 Si/Ge(Fe,Si)/Fe 3 Si thin film stacks were grown by a combination of molecular beam epitaxy and solid phase epitaxy (Ge on Fe 3 Si). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron X-ray diffraction. The Ge(Fe,Si) films crystallize in the well oriented, layered tetragonal structure FeGe 2 with space group P4mm. This kind of structure does not exist as a bulk material and is stabilized by solid phase epitaxy of Ge on Fe 3 Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.

Semiconductor Science and Technology

We investigate the formation of lattice matched single-crystalline Fe3Si/GaAs(001) ferromagnet/se... more We investigate the formation of lattice matched single-crystalline Fe3Si/GaAs(001) ferromagnet/semiconductor hybrid structures by Volmer-Weber island growth, starting from the epitaxial growth of isolated Fe3Si islands up to the formation of continuous films as a result of island coalescence. We find coherent defect-free layers exhibiting compositional disorder near the Fe3Si/GaAsinterface for higher growth rates, whereas they are fully ordered for lower growth rates.

physica status solidi (RRL) – Rapid Research Letters

Several nanometer high steps are observed by (scanning) transmission electron microscopy at the s... more Several nanometer high steps are observed by (scanning) transmission electron microscopy at the surface and interfaces in heteroepitaxially grown III-Sb layers on vicinal Si(001) substrates. Their relations with antiphase boundaries (APBs) and threading dislocations (TDs) are elaborated. An asymmetric number density of TDs on symmetry-equivalent {111} lattice planes is revealed and explained according to the substrate miscut and the lattice misfit in the heteroepitaxial material system. Finally, a step bunching mechanism is proposed based on the interplay of APBs, TDs, and the vicinal surface of the miscut substrate.

Ultramicroscopy, 2017

Melting and crystallization dynamics of the multi-component Ge-Sb-Te alloy have been investigated... more Melting and crystallization dynamics of the multi-component Ge-Sb-Te alloy have been investigated by in-situ transmission electron microscopy (TEM). Starting point of the phase transition study is an ordered hexagonal Ge1Sb2Te4 thin film on Si(111) where the crystal structure and the chemical composition are verified by scanning TEM and electron energy-loss spectroscopy, respectively. The in-situ observation of the liquid phase at 600°C including the liquid-solid and liquid-vacuum interfaces and their movements was made possible due to an encapsulation of the TEM sample. The solid-liquid interface during melting displays a broad and diffuse transition zone characterized by a vacancy induced disordered state. Although the velocities of interface movements are measured to be in the nanometer per second scale, both, for crystallization and solidification, the underlying dynamic processes are considerably different. Melting reveals linear dependence on time, whereas crystallization exhi...

Nano letters, Jan 11, 2017

Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the na... more Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the narrow lattice mismatch range restricting planar heterostructures, owing to misfit strain relaxing at the free surfaces and partitioning throughout the entire nanostructure. In this work, we uncover a novel strain relaxation process in GaAs/InxGa1-xAs core-shell nanowires that is a direct result of the nanofaceted nature of these nanostructures. Above a critical lattice mismatch, plastically relaxed mounds form at the edges of the nanowire sidewall facets. The relaxed mounds and a coherent shell grow simultaneously from the beginning of the deposition with higher lattice mismatches increasingly favoring incoherent mound growth. This is in stark contrast to Stranski-Krastanov growth, where above a critical thickness coherent layer growth no longer occurs. This study highlights how understanding strain relaxation in lattice mismatched nanofaceted heterostructures is essential for designing d...

Nano letters, Jan 11, 2017

We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The ... more We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The approach is based on the fact that these nanostructures do not form in plasma-assisted molecular beam epitaxy on structurally and chemically uniform cation-polar substrates. By in situ depositing and nitridating Si on a Ga-polar GaN film, we locally reverse the polarity to induce the selective area epitaxy of N-polar GaN nanowires. We show that the nanowire number density can be controlled over several orders of magnitude by varying the amount of predeposited Si. Using this growth approach, we demonstrate the synthesis of single-crystalline and uncoalesced nanowires with diameters as small as 20 nm. The achievement of nanowire number densities low enough to prevent the shadowing of the nanowire sidewalls from the impinging fluxes paves the way for the realization of homogeneous core-shell heterostructures without the need of using ex situ prepatterned substrates.

Nano letters, Aug 27, 2017

The fabrication of nanowires with axial multiquantum wells or disks presenting a homogeneous size... more The fabrication of nanowires with axial multiquantum wells or disks presenting a homogeneous size and shape distribution along the whole stack is still an unresolved challenge, despite being essential for narrowing their light emission bandwidth. In this work we demonstrate that the commonly observed change in the shape of the disks along the stacking direction proceeds in a systematic, predictable way. High- resolution transmission electron microscopy of stacked (In,Ga)N quantum discs embedded in GaN nanowires with diameters of ∼40 nm and lengths of ∼700 nm and finite element method calculations show that, contrary to what is normally assumed, this change is not related to the radial growth of the nanowires, which is shown to be negligible, but to the strain relaxation of the whole active region. A simple model is proposed to account for the experimental observations. The model assumes that each disk reaches an equilibrium shape that minimizes the overall energy of the system, give...

Nano letters, Sep 6, 2017

We demonstrate an all-epitaxial and scalable growth approach to fabricate single-crystalline GaN ... more We demonstrate an all-epitaxial and scalable growth approach to fabricate single-crystalline GaN nanowires on graphene by plasma-assisted molecular beam epitaxy. As substrate, we explore several types of epitaxial graphene layer structures synthesized on SiC. The different structures differ mainly in their total number of graphene layers. Because graphene is found to be etched under active N exposure, the direct growth of GaN nanowires on graphene is only achieved on multilayer graphene structures. The analysis of the nanowire ensembles prepared on multilayer graphene by Raman spectroscopy and transmission electron microscopy reveals the presence of graphene underneath as well as in between nanowires, as desired for the use of this material as contact layer in nanowire-based devices. The nanowires nucleate preferentially at step edges, are vertical, well aligned, epitaxial, and of comparable structural quality as similar structures fabricated on conventional substrates.

Advanced Materials, 2016

The metal-semiconductor phase-transition in the ternary transitional metal dichalcogenides (TMDs)... more The metal-semiconductor phase-transition in the ternary transitional metal dichalcogenides (TMDs) monolayer was achieved by alloying Te into WSe2 (WSe2(1-x) Te2x , where x = 0%-100%). The optical band gaps of the WSe2(1-x) Te2x monolayer can be tuned from 1.67 to 1.44 eV (2H semiconductor) and drop to 0 eV (1Td metal), which opens up an exciting opportunity in functional electronic/optoelectronic devices.

Journal of Applied Physics, 2016

Applied Physics Letters, 2016

Applied Physics Letters, Apr 23, 1995

ABSTRACT

Physica E Low Dimentional Systems and Nanostructures, 2006

ABSTRACT

Physica Status Solidi B Basic Research, Jun 13, 2007

ABSTRACT We give a bird's-eye view of the interface formation during MBE growth of variou... more ABSTRACT We give a bird's-eye view of the interface formation during MBE growth of various quasi lattice-matched as well as highly-mismatched semiconductor heterostructures. We show that the parameters controlling the growth mode and the interface formation are numerous and that the optimum interface configuration depends on the materials system and on the target application. Our results obtained from various semiconductor materials systems demonstrate that this growth and interface-related problem is much more complex than it has often been considered for many years. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal of Crystal Growth, Apr 1, 2007

ABSTRACT

Review of Scientific Instruments, 2005

Physics and Simulation of Optoelectronic Devices VI, 1998

Physical Review Materials

Fe 3 Si/Ge(Fe,Si)/Fe 3 Si thin film stacks were grown by a combination of molecular beam epitaxy ... more Fe 3 Si/Ge(Fe,Si)/Fe 3 Si thin film stacks were grown by a combination of molecular beam epitaxy and solid phase epitaxy (Ge on Fe 3 Si). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron X-ray diffraction. The Ge(Fe,Si) films crystallize in the well oriented, layered tetragonal structure FeGe 2 with space group P4mm. This kind of structure does not exist as a bulk material and is stabilized by solid phase epitaxy of Ge on Fe 3 Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.

Semiconductor Science and Technology

We investigate the formation of lattice matched single-crystalline Fe3Si/GaAs(001) ferromagnet/se... more We investigate the formation of lattice matched single-crystalline Fe3Si/GaAs(001) ferromagnet/semiconductor hybrid structures by Volmer-Weber island growth, starting from the epitaxial growth of isolated Fe3Si islands up to the formation of continuous films as a result of island coalescence. We find coherent defect-free layers exhibiting compositional disorder near the Fe3Si/GaAsinterface for higher growth rates, whereas they are fully ordered for lower growth rates.

physica status solidi (RRL) – Rapid Research Letters

Several nanometer high steps are observed by (scanning) transmission electron microscopy at the s... more Several nanometer high steps are observed by (scanning) transmission electron microscopy at the surface and interfaces in heteroepitaxially grown III-Sb layers on vicinal Si(001) substrates. Their relations with antiphase boundaries (APBs) and threading dislocations (TDs) are elaborated. An asymmetric number density of TDs on symmetry-equivalent {111} lattice planes is revealed and explained according to the substrate miscut and the lattice misfit in the heteroepitaxial material system. Finally, a step bunching mechanism is proposed based on the interplay of APBs, TDs, and the vicinal surface of the miscut substrate.

Ultramicroscopy, 2017

Melting and crystallization dynamics of the multi-component Ge-Sb-Te alloy have been investigated... more Melting and crystallization dynamics of the multi-component Ge-Sb-Te alloy have been investigated by in-situ transmission electron microscopy (TEM). Starting point of the phase transition study is an ordered hexagonal Ge1Sb2Te4 thin film on Si(111) where the crystal structure and the chemical composition are verified by scanning TEM and electron energy-loss spectroscopy, respectively. The in-situ observation of the liquid phase at 600°C including the liquid-solid and liquid-vacuum interfaces and their movements was made possible due to an encapsulation of the TEM sample. The solid-liquid interface during melting displays a broad and diffuse transition zone characterized by a vacancy induced disordered state. Although the velocities of interface movements are measured to be in the nanometer per second scale, both, for crystallization and solidification, the underlying dynamic processes are considerably different. Melting reveals linear dependence on time, whereas crystallization exhi...

Nano letters, Jan 11, 2017

Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the na... more Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the narrow lattice mismatch range restricting planar heterostructures, owing to misfit strain relaxing at the free surfaces and partitioning throughout the entire nanostructure. In this work, we uncover a novel strain relaxation process in GaAs/InxGa1-xAs core-shell nanowires that is a direct result of the nanofaceted nature of these nanostructures. Above a critical lattice mismatch, plastically relaxed mounds form at the edges of the nanowire sidewall facets. The relaxed mounds and a coherent shell grow simultaneously from the beginning of the deposition with higher lattice mismatches increasingly favoring incoherent mound growth. This is in stark contrast to Stranski-Krastanov growth, where above a critical thickness coherent layer growth no longer occurs. This study highlights how understanding strain relaxation in lattice mismatched nanofaceted heterostructures is essential for designing d...

Nano letters, Jan 11, 2017

We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The ... more We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The approach is based on the fact that these nanostructures do not form in plasma-assisted molecular beam epitaxy on structurally and chemically uniform cation-polar substrates. By in situ depositing and nitridating Si on a Ga-polar GaN film, we locally reverse the polarity to induce the selective area epitaxy of N-polar GaN nanowires. We show that the nanowire number density can be controlled over several orders of magnitude by varying the amount of predeposited Si. Using this growth approach, we demonstrate the synthesis of single-crystalline and uncoalesced nanowires with diameters as small as 20 nm. The achievement of nanowire number densities low enough to prevent the shadowing of the nanowire sidewalls from the impinging fluxes paves the way for the realization of homogeneous core-shell heterostructures without the need of using ex situ prepatterned substrates.

Nano letters, Aug 27, 2017

The fabrication of nanowires with axial multiquantum wells or disks presenting a homogeneous size... more The fabrication of nanowires with axial multiquantum wells or disks presenting a homogeneous size and shape distribution along the whole stack is still an unresolved challenge, despite being essential for narrowing their light emission bandwidth. In this work we demonstrate that the commonly observed change in the shape of the disks along the stacking direction proceeds in a systematic, predictable way. High- resolution transmission electron microscopy of stacked (In,Ga)N quantum discs embedded in GaN nanowires with diameters of ∼40 nm and lengths of ∼700 nm and finite element method calculations show that, contrary to what is normally assumed, this change is not related to the radial growth of the nanowires, which is shown to be negligible, but to the strain relaxation of the whole active region. A simple model is proposed to account for the experimental observations. The model assumes that each disk reaches an equilibrium shape that minimizes the overall energy of the system, give...

Nano letters, Sep 6, 2017

We demonstrate an all-epitaxial and scalable growth approach to fabricate single-crystalline GaN ... more We demonstrate an all-epitaxial and scalable growth approach to fabricate single-crystalline GaN nanowires on graphene by plasma-assisted molecular beam epitaxy. As substrate, we explore several types of epitaxial graphene layer structures synthesized on SiC. The different structures differ mainly in their total number of graphene layers. Because graphene is found to be etched under active N exposure, the direct growth of GaN nanowires on graphene is only achieved on multilayer graphene structures. The analysis of the nanowire ensembles prepared on multilayer graphene by Raman spectroscopy and transmission electron microscopy reveals the presence of graphene underneath as well as in between nanowires, as desired for the use of this material as contact layer in nanowire-based devices. The nanowires nucleate preferentially at step edges, are vertical, well aligned, epitaxial, and of comparable structural quality as similar structures fabricated on conventional substrates.

Advanced Materials, 2016

The metal-semiconductor phase-transition in the ternary transitional metal dichalcogenides (TMDs)... more The metal-semiconductor phase-transition in the ternary transitional metal dichalcogenides (TMDs) monolayer was achieved by alloying Te into WSe2 (WSe2(1-x) Te2x , where x = 0%-100%). The optical band gaps of the WSe2(1-x) Te2x monolayer can be tuned from 1.67 to 1.44 eV (2H semiconductor) and drop to 0 eV (1Td metal), which opens up an exciting opportunity in functional electronic/optoelectronic devices.

Journal of Applied Physics, 2016

Applied Physics Letters, 2016

Applied Physics Letters, Apr 23, 1995

ABSTRACT

Physica E Low Dimentional Systems and Nanostructures, 2006

ABSTRACT

Physica Status Solidi B Basic Research, Jun 13, 2007

ABSTRACT We give a bird's-eye view of the interface formation during MBE growth of variou... more ABSTRACT We give a bird's-eye view of the interface formation during MBE growth of various quasi lattice-matched as well as highly-mismatched semiconductor heterostructures. We show that the parameters controlling the growth mode and the interface formation are numerous and that the optimum interface configuration depends on the materials system and on the target application. Our results obtained from various semiconductor materials systems demonstrate that this growth and interface-related problem is much more complex than it has often been considered for many years. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal of Crystal Growth, Apr 1, 2007

ABSTRACT

Review of Scientific Instruments, 2005

Physics and Simulation of Optoelectronic Devices VI, 1998