Research Topics

Graphene and other two-dimensional (2D) materials are promising building blocks for the fabrication of novel, atomically thin devices for future applications. In addition, combining them to form the so-called van der Waals (vdW) heterostructures enables novel, promising phenomena which are not observed in the single materials. For instance, in heterostacks composed of graphene and hexagonal BN (h-BN) layers the band structure of graphene can in principle be engineered via the design of stacking characteristics such as number of layers as well as crystallographic alignment. ... read more

In the nanowire geometry, the chemical composition can be changed in both the axial and the radial direction, leading to corresponding heterostructures and/or doping profiles. Such design concepts still strongly resemble established planar structures. Beyond those strategies, we investigate more advanced structures with the aim to tailor functionality in entirely new directions. ... read more

Our research relates to applications based on either light emission or light absorption, e.g. optical data transmission or energy harvesting. Since the nanowire geometry facilitates the integration of dissimilar materials, substrates with attractive properties can be employed that were excluded for planar growth. Furthermore, the spectral region of optoelectronic devices can be widened. Also, radial nanowire heterostructures drastically increase the size of the active region, which is associated with stronger light emission and absorption. In addition, light-matter interaction can be tailored by design of nanowire dimensions and their arrangement in arrays. ... read more

We use cryogenic scanning tunneling microscopy (STM) to create individual nanostructures on semiconductor surfaces and explore their quantum properties by scanning tunneling spectroscopy. The structures are assembled from native indium adatoms on an indium arsenide surface. The adatoms are positively charged and can be repositioned by the STM tip, making it possible to engineer the electrostatic surface potential landscape and confine surface-state electrons on the atomic length scale and with atomic precision. In this way, quantum dots with a perfectly defined level structure can be constructed, as well as quantum-dot molecules whose coupling has no intrinsic variation but can nonetheless be tuned over a wide range. ... read more

Collective excitations of magnetic order such as spin waves (their corresponding quanta are called magnons) are promising alternatives to charge carriers for efficient on-chip transport and information processing. The frequencies and wave vectors of spin waves are similar to those of SAWs, allowing the coupling between phonons and magnons to be exploited for the excitation, detection, and manipulation of spin waves. The work could lead to the realization of sound-driven magnonic memories and logic gates, as well as magnetically controlled SAW isolators and circulators for the efficient acoustic interfacing between localized qubits in quantum networks. ... read more

The fabrication of ferromagnet semiconductor hybrid structures in high crystal perfection is a prerequisite for spintronic device concepts. Our focus is on the growth of half-metallic Heusler alloys on GaAs-related structures. As an amendment, we investigate MgO interlayers at the Heusler alloy/GaAs interface as diffusion and spin-filtering barriers. The growth of these materials is the long-standing basis for spin transport and spin manipulation studies in the core research areas Nanoanalytics and Control of Elementary Excitations by Acoustic Fields. ... read more

Phonons with frequencies in the range from a few GHz to a few tens of GHz (so-called hypersound) are particularly interesting for quantum technologies because they are resonant to many types of qubits, can be operated in the phonon ground state, and can be piezoelectrically excited. We develop technology and tools to efficiently excite and detect SAWs exceeding few GHz and 5-20 GHz BAWs. Using high-resolution scanning-probe techniques, we investigate fundamental limits of focusing and confinement of GHz SAWs and BAWs. The ability to generate and guide GHz strain will be important for on-chip transport and storage of quantum information as well as for the development of new types of compact and fast sensors. ... read more

These alloys are well-known semiconductors which can be used to produce heterostructures of highest structural and optical quality. They are still the first choice for many device applications and basic research experiments. ... read more

These alloys enable the realization of efficient light emitters used for solid-state lighting, display technologies, and diode lasers. Consequently, this material class is now second only to Si as the commercially most important semiconductor. Related to these applications, we recently concluded the project SPRInG [Short Period Superlattices for Rational (In,Ga)N] that was funded by the European Union. In particular, we investigated superlattices made of alternating binary InN and GaN layers as an ordered alternative to (In,Ga)N layers with high In content. ... read more

Interactions between photons, excitons and phonons in small volumes open a way to realize novel hybrid architectures. We develop hybrid (Al,Ga)As microcavities for the coupling of GHz vibrations, quantum well excitons and optical photons. On the one hand, the strong light-exciton coupling, leading to exciton-polaritons, enhances the coupling to phonons. On the other hand, it allows us to exploit the long temporal coherence of polariton BEC condensates, which exceeds the phonon oscillation period. With this novel platform, we aim to explore fundamental physics of resonant polaromechanical metamaterials based on confined, strongly nonlinear-driven dissipative systems. The research could lead to the development of new optical devices (e.g., polarization modulators, synchronized inversionless lasers) operating at tens of GHz. ... read more

A central goal in preparing modern artificial heterostructures and nanomaterials is the control of interfaces and the understanding of their building principles. Interfaces have a crucial effect on both, physical properties and device performance, in particular as structure sizes become smaller. The detailed analysis and understanding of morphology and chemical intermixing of epitaxial interfaces allows a better control during growth and opens the possibility to tune the interface property with the appropriate functionality in nano-structured materials. ... read more

In this research topic, we have realized a resistance switching device operating based on a phenomenon that occurs at the interface between a chalcogenide alloy based on (Bi,Sb)-(Cu,Ag,Ni)-(S,Se,Te) and a metal such as Al and Ag. Our goal is to demonstrate machine learning using an artificial neural network, constructed by interfacial-resistance-switching devices. ... read more

In order to go beyond the state of the art, we develop novel materials and nanostructures with enhanced properties. For example, novel piezoelectric materials are needed for more efficient and compact acoustic devices operating well into GHz frequency range. Investigation of coupling between phonons and quantum systems motivates us to research growth of high-quality semiconductor quantum wells and quantum dots, as well as deterministically fabricated color centers. We also push the limits of the micro- and nanostructure fabrication by using advanced patterning and growth methods. These advances are a backbone of our research. ... read more

The physical properties of ferromagnetic alloys and semiconducting oxide layers depend on their structural order and phase stability. The underlying physical principles of phase stabilities and phase transformation processes are explored with high spatial and temporal resolution in dependence on internal parameters like chemical composition and strain state as well as external parameters given by temperature and growth kinetics. The degree of order/disorder and their spatial distribution within thin films are determined quantitatively by in-situ and ex-situ X-ray diffraction and electron microscopy methods. ... read more

In our studies of nanowire properties, we pay special attention to phenomena that are a direct consequence of the peculiar wire-like shape and nanometric size and are independent of the material the nanowires consist of. For example, the high surface-to-volume ratio of nanowires has several important consequences. In particular, lattice-mismatched axial heterostructures relax their strain elastically at the free sidewalls, and the formation of dislocations is avoided. For radial heterostructures, a new type of structure only possible due to the nanowire geometry, the strain is shared between core and shell. In both cases, the strain distribution is very complex. Since many semiconductor properties depend sensitively on strain, the dedicated investigation of strain in nanowires is crucial for an understanding of their optoelectronic properties. ... read more

The everlasting trend to miniaturize classical electronic devices has guaranteed progress over half a century. This evolution is now reaching its fundamental limits due to the granularity of matter and the quantum nature of the electron dynamics, which modifies the circuit properties on extremely short length and time scales. Therefore, future progress of electronics will be increasingly based on the coherent quantum dynamics of individual states. This development is often denoted as the second quantum revolution. ... read more

The functionality of many metastable or nanoscale materials (such as graphene or 2D layered materials) depends essentially on their structural perfection and compositional homogeneity on the atomic to mesoscopic scale. In-situ/ex-situ X-ray diffraction and various microscopy techniques are used to comprehensively explore these materials with respect to their structural, electronic and mechanical properties on all relevant length scales. The exact knowledge of these properties serves as a basis for the targeted adjustment and optimization of the material functionality. ... read more

Oxides are among the materials with the widest tunability of physical properties. Spanning insulators, semiconductors, metallic conductors and superconductors, magnetic materials, ferro-/antiferro- and other dielectrics, oxides are a materials class with high potential for a new generation of electronic devices for energy and sensing applications. ... read more

Spintronics based on hybrid devices comprising magnetic and semiconducting (nonmagnetic) materials can perform the processing, communication, and storage of information within one chip. The investigations on spin transport phenomena in ferromagnet/nonmagnet hybrid structures aim at the understanding of the underlying physical mechanisms as well as the evaluation of their relevance for spintronic device concepts. ... read more

On-chip transfer of information in quantum networks requires both channels and means for non-destructive transport of qubits. We develop nano-channels, which exploit traveling SAWs to transport electronic excitations along these channels. We seek to understand the mechanisms and the limits of charge and spin transport in these nanostructures. The work could be a building block for quantum links connecting on-chip qubits separated by tens or hundreds of micrometers. ... read more

Wide bandgap semiconductors contain electrons trapped in localized defects ("color centers"). Because of their atomic-like optical emission, characterized by sharp lines and non-classical statistics, they can be used as single-photons sources. In addition, their long spin coherence times and spin-dependent optical cycles make them very attractive for spintronic applications. Our goal is to exploit their high sensitivity to local lattice distortions to dynamically manipulate their orbital and spin states. By combining acoustically and optically induced transitions, we aim at on-demand excitation of spin quantum states with long coherence times and ultrafast optical readout of the quantum information stored in single color centers. ... read more

The best-known bottom-up method resulting in uniaxial growth of nanowires utilizes the vapor-liquid-solid mechanism in which the formation of a solid nanowire from the vapor phase is mediated by a tiny liquid metal droplet. We employ this approach (using Ga as the metal) for the synthesis of group-III-arsenide nanowires on Si(111) substrates. Selective area vapor-liquid-solid growth is readily obtained by patterning these substrates by electron beam lithography and reactive ion etching, leading to ordered nanowire arrays. ... read more

The excellent state of semiconductor science and technology allows for the development and application of sophisticated devices such as quantum-cascade lasers (QCLs). These devices rely on the comprehensive understanding and the high-quality growth of complex planar heterostructures. For the terahertz spectral region, GaAs/(Al,Ga)As-based heterostructures are the material of choice. Our activities in the field of THz QCLs cover the design, growth, fabrication, and determination of the operating parameters of these lasers as well as their provision for spectroscopic applications. ... read more

Topological insulators possess conductive surface states while the bulk states are insulating. Due to the strong spin-orbit coupling in topological insulators, the surface states are helical, i.e., the spin and momentum of a conduction electron are coupled to each other. Therefore, topological insulators are an interesting system for investigating spin transport. ... read more