Bernd Jenichen

This work presents the in situ transmission electron microscopy (TEM) investigation of the solid-phase epitaxy of Ge on cubic Fe ₃ Si. Amorphous Ge layers were deposited at room temperature in an ultra-high vacuum growth chamber. Subsequent annealing experiments in the microscope between 200 °C and 500 °C are performed both with slow heating rate and under rapid thermal annealing conditions to observe the amorphous/crystalline phase transition with high spatial resolution and to clarify the crystal lattice alignment during the solid phase epitaxial growth. Time-dependent high-resolution TEM measurements are used to analyse the crystallization kinetics following a square root characteristic. By means of energy-dispersive x-ray spectroscopy in the TEM, Fe diffusion into the crystallized Ge film is detected at higher temperature and the influence on the crystal phase formation of FeGe ₂ will be discussed with respect to epitaxial strain and thermal treatments.

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/GaAs—interface for higher growth rates, whereas they are fully ordered for lower growth rates.

We investigate the formation of lattice matched single-crystalline Fe$_3$Si/GaAs(001) ferromagnet/semiconductor hybrid structures by Volmer-Weber island growth, starting from the epitaxial growth of isolated Fe$_3$Si 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 Fe$_3$Si/GaAs-interface for higher growth rates, whereas they are fully ordered for lower growth rates.

We analyze fluctuation of the layer thicknesses and its influence on the strain state of (In,Ga)As/(Al,Ga)As micro-tubes containing quantum well structures. In those structures a curved high-mobility two-dimensional electron gas (HM2DEG) is established. The layer thickness fluctuation studied by atomic force microscopy, x-ray scattering, and spatially resolved cathodoluminescence spectroscopy occurs on two different lateral length scales. On the shorter length scale of about 0.01~$\mu$m, we found from atomic force micrographs and the broadening of the satellite maxima in x-ray diffraction curves a very small value of the mean square roughness of 0.1~nm. However, on a longer length scale of about 1.0~$\mu$m, step bunching during epitaxial growth resulted in layer thickness inhomogeneities of up to 2~nm. The resulting fluctuation of the strain in the micro-tubes leads to a local variation of the chemical potential, which results in the fluctuation of the carrier density as well. This leads to a phase cancelation of the Shubnikov-de-Haas oscillations in the curved HM2DEG and a reduction of the single-electron scattering time, while the electron mobility in the structures remains high. The estimated fluctuation of the carrier density agrees well with the energy fluctuation measured in the cathodoluminescence spectra of the free-electron transition of the quantum well.

Electrically induced resistive switching resulting from ionic transport and electrochemical redox reactions is promising for future generation non-volatile memory devices and artificial neural computing. The key ingredient for the highly efficient neural computing in this context is a memristor, which is a special type of a resistive two-terminal element whose electrical properties depend on not only the state of the element but also how the state has been achieved in its history. Memristor characteristics are demonstrated in a bilayer junction of Al and a Bi–Cu–S alloy utilizing electrically reversible generation of an insulating interface. The high resistance due to the interface layer drops abruptly by orders of magnitude when the barrier is annihilated electrochemically under a bias. The barrier is made to be robust by applying a reverse bias, giving rise to a controllable memory effect on the switching phenomenon. The switching mechanism based on the manipulation of a barrier, which is complementary to conventional bridging conductive filaments, will open the way for new functionalities as device elements.

The direct growth of semiconductors (SC) over metals by molecular beam epitaxy is a difficult task due to the large differences in crystallization energy between these types of materials. This aspect is problematic in the context of spintronics, where coherent spin injection must proceed via ballistic transport through sharp interfacial Schottky barriers. We report the realization of single-crystalline ferromagnet/SC/ferromagnet hybrid trilayers using solid-phase epitaxy, with combinations of Fe ₃ Si, Co ₂ FeSi, and Ge. The slow annealing of amorphous Ge over Fe ₃ Si results in a crystalline film identified as FeGe ₂ . When the annealing is performed over Co ₂ FeSi, reflected high-energy electron diffraction and x-ray diffraction indicate the creation of a different crystalline Ge(Co,Fe,Si) compound, which also preserves growth orientation. It was possible to observe independent magnetization switching of the ferromagnetic layers in a Fe ₃ Si/FeGe ₂ /Co ₂ FeSi sample, thanks to the different coercive fields of the two metals and to the quality of the interfaces. This result is a step towards the implementation of vertical spin-selective transistor-like devices.

The direct growth of semiconductors over metals by molecular beam epitaxy is a difficult task due to the large differences in crystallization energy between these types of materials. This aspect is problematic in the context of spintronics, where coherent spin-injection must proceed via ballistic transport through sharp interfacial Schottky barriers. We report the realization of single-crystalline ferromagnet/semiconductor/ferromagnet hybrid trilayers using solid-phase epitaxy, with combinations of Fe3Si, Co2FeSi, and Ge. The slow annealing of amorphous Ge over Fe3Si results in a crystalline filmlm identified as FeGe2. When the annealing is performed over Co2FeSi, reflected high-energy electron diffraction and X-ray diffraction indicate the creation of a different crystalline Ge(Co,Fe,Si) compound, which also preserves growth orientation. It was possible to observe independent magnetization switching of the ferromagnetic layers in a Fe3Si/FeGe2/Co2FeSi sample, thanks to the different coercive fields of the two metals and to the quality of the interfaces. This result is a step towards the implementation of vertical spin-selective transistor-like devices.

Ferromagnetic Heusler alloys can be used in combination with semiconductors to create spintronic devices. The materials have cubic crystal structures, making it possible to grow lattice-matched heterojunctions by molecular beam epitaxy. However, the development of devices is limited by the difficulty of growing epitaxial semiconductors over metallic surfaces while preventing chemical reactions, a requirement to obtain abrupt interfaces and achieve efficient spin-injection by tunneling. We used a solid-phase epitaxy approach to grow crystalline thin film stacks on GaAs(001) substrates, while preventing interfacial reactions. The crystallized Ge layer forms superlattice regions, which are caused by the migration of Fe and Si atoms into the film. X-ray diffraction and transmission electron microscopy indicate that the trilayers are fully crystalline, lattice-matched, and have ideal interface quality over extended areas.