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Scientific Highlights

Epitaxy of highly dissimilar transition metal nitride-semiconductor heterostructures with low defect density – the example ScN/GaN(1-100)

We demonstrate twin-free epitaxial growth of rocksalt ScN on wurtzite GaN by using the two-fold symmetric M-plane surface of GaN, avoiding twinning issues. This enables high-quality integration of transition metal nitrides with III-nitrides, enhancing device performance and enabling new applications with additional functionalities like superconductivity and photonic resonances.

Breakthrough Research Uncovers Hidden Phenomena in Ultra-Clean Quantum Materials

An international research team led by Roman Engel-Herbert has reported previously unobserved phenomena in an ultra-clean sample of the correlated metal SrVO3. The study challenges existing theoretical models by providing new insights, suggesting a need to re-evaluate current theories on electron interactions. The breakthrough was achieved using an innovative growth technique, leading to the synthesis of SrVO3 with unprecedented purity, enabling detailed exploration of its true properties.

On-chip GHz time crystals with semiconductor photonic devices pave way to new physics and optoelectronic applications

Researchers have observed a time crystal on a microscale semiconductor chip oscillating at a rate of several billion times per second, unveiling exceptionally high non-linear dynamics in the GHz range. The experiment results, published today in Science, establish a firm connection between formerly uncorrelated areas of non-linear exciton-polariton dynamics and coherent optomechanics at GHz frequencies.

Carrier Recombination in Highly Uniform and Phase-Pure GaAs/(Al,Ga)As Core/Shell Nanowire Arrays on Si(111): Implications for Light-Emitting Devices

GaAs-based nanowires are among the most promising candidates for realizing a monolithic integration of III-V optoelectronics on the Si platform. To realize their full potential for applications as light absorbers and emitters, it is crucial to understand their interaction with light governing the absorption and extraction efficiency, as well as the carrier recombination dynamics determining the radiative efficiency.

Flying electron spin control gates

Electron spins are attractive qubits for the implementation of quantum functionalities in semiconductor devices. In this context, the spin transistor proposed by Datta and Das [Datta and Das, Appl. Phys. Lett. 56, 665, (1990)] has been a guiding concept towards the implementation of spin-based functionalities in III-V semiconductor structures. The acoustically driven flying spin gates enable a high degree of dynamic spin control as well as on-chip spin transfer over several tens of micrometers by simply changing the amplitude of the carrier acoustic wave. The approach is compatible with planar technology and also offers a convenient interface for the interconversion between electron spins and polarized phonons for long-distance quantum information transfer. ... read more

The azimuthal cell arrangement in molecular beam epitaxy drastically affects the luminescence efficiency of nanowire shells

The chamber geometry of a system for molecular beam epitaxy (MBE) is known to affect technical aspects such as the macroscopic deposition homogeneity across a wafer. However, for the microscopic mechanisms governing the growth of thin films on a planar substrate, the chamber geometry does not play any role. In marked contrast, we show here that the luminescence efficiency of (In,Ga)As/GaAs shell quantum wells grown around GaAs nanowires changes by more than two orders of magnitude depending on the relative position of the As cell compared to the group-III cells. ... read more

Large-area van der Waals epitaxy of ferromagnetic Fe3GeTe2 films on graphene

Magnetic two-dimensional (2D) materials are promising building blocks for realizing ultra-compact spintronic devices with enhanced performance. Their study is also expected to open new perspectives on a more versatile modulation of magnetic properties, beyond what can be achieved in traditional three-dimensional (3D) magnetic thin films and superlattices. In particular, combining magnetic 2D materials with other 2D crystals such as graphene and transition metal dichalcogenides (TMDCs) to create van der Waals (vdW) heterostructures offers great potential to tailor magnetism via proximity-induced phenomena. ... read more

Selective-area epitaxy of 2D heterostructures via defect engineering using a focused He ion beam

The stacking of different two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) allows to realize novel 2D heterostructures with tailored properties for application in atomically thin optoelectronic, electronic, and sensing devices. To achieve a high-density, bottom-up integration of such heterostructures in future technologies, the synthesis of 2D layers on top of each other via van der Waals epitaxy (vdWE) is a promising alternative to flake exfoliation followed by mechanical transfer, which is problematic in terms of scaling and reproducibility. However, due to the weak bonding between 2D crystals, vdWE is sensitive to various surface defects, usually leading to uncontrolled nucleation and thus non-uniform growth of polycrystalline material. Hence, the control over nucleation location is one of the key challenges to achieve scalable and high-quality fabrication of 2D heterostructures. ... read more

Understanding exciton recombination in GaN nanowires

A strong asset of bottom-up nanowires and related nanowire heterostructures is the enhanced surface strain relaxation that delays the onset of plastic relaxation. As a matter of fact, GaN nanowires essentially free of dislocations can be directly grown on technologically relevant substrates such as silicon or metals. Yet, in spite of their high structural perfection – on par with bulk GaN – the electron-hole recombination in these nanoscopic structures remains predominantly nonradiative, even at cryogenic temperature. Here, we provide first evidence that the efficient nonradiative channel does not take place at point defects located at the nanowire surface or in the bulk, but stems instead from field-ionization of the exciton in the surface electric field. ... read more

Electrically driven GHz sound meets opto-electronic resonators

Electrically driven, coherent phonons (sound waves in solids) with frequencies in the super-high-frequency (SHF) 3-30 GHz range are an important tool for the coherent manipulation of solid-state excitations. These phonons play a crucial role in optomechanics by enabling processes such as laser cooling of resonators to the mechanical ground state as well as in the quantum-coherent coupling between microwaves and near-infrared photons.

Evolution of low-frequency vibrational modes in ultrathin GeSbTe films

Phase change materials (PCM) are compounds employed in non-volatile random-access memory and rewriteable optical storage media thanks to the rapid and reversible transformation between the amorphous and crystalline states. GeSbTe alloys are the most studied PCMs and consist of lamellae separated by van der Waals gaps in the ordered crystalline phase. Here we report a methodology for the study of the two-dimensional (2D) character of epitaxial GeSbTe by looking at the weak inter-lamellae interactions. Our unique approach is the use of molecular beam epitaxy for the controlled synthesis of ultrathin films combined with Raman spectroscopy for the investigation of the PCM properties. The shift of the vibrational modes in the low-frequency range results in a direct probe of the film thickness. ... read more

A phonon laser – coherent vibrations from a self-breathing resonator

Lasing – the emission of a collimated light beam of light with a well-defined wavelength (color) and phase - results from a self-organization process, in which a collection of emission centers synchronizes itself to produce identical light particles (photons). A similar self-organized synchronization phenomenon can also lead to the generation of coherent vibrations – a phonon laser, where phonon denotes, in analogy to photons, the quantum particles of sound.

Strategies for Analyzing Non-Common-Atom Heterovalent Interfaces: The Case of CdTe-on-InSb

Semiconductor heterostructures are intrinsic to a wide range of modern-day electronic devices, such as computers, light-emitting devices and photodetectors. Knowledge of chemical interfacial profiles in these complex structures is critical to the task of optimizing the device performance. Here, we report on an innovative methodology that enables reliable interface structure analysis of non-common-atom heterovalent interfaces on all relevant length scales from hundred-nm to atomic resolution. ... read more

GaAs-based nanowire heterostructure for light generation in the telecommunication O band on Si

The monolithic integration of light emitters on Si remains an important technological challenge for the development of intra-chip optical connections, as well as inter-chip connections. In this context, GaAs nanowires grown on Si substrates offer great potential, but their effective use for Si photonics technologies requires operation in the Si transparent window, with special relevance of the telecommunication bands for potential data transfer applications. Here, we present a novel coaxial nanowire heterostructure to extend the emission range of GaAs-based nanowire devices into the telecommunication O band. ... read more