Scientific Highlights

Roman Engel-Herbert, Director of PDI, and Joao Marcelo J. Lopes, a Senior Scientist at PDI, were honored recently with an invitation to review the field of two-dimensional layered materials (2DLM) in a paper for ACS titled 'Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications'. ... read more

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

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

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

A popular method to experimentally determine the diffusion length of minority carriers or excitons in semiconductors relies on the perception that threading dislocations are line defects that act as nonradiative sinks for carriers. The zone of reduced luminescence intensity around the dislocation is thus directly related to the carrier or exciton diffusion length. Here, we show that this understanding of a diffusion-controlled intensity contrast around threading dislocations in GaN{0001} is a misconception. ... read more

A figure of merit for the potential performance of any bipolar semiconductor device is the carrier diffusion length. For the technologically secondmost relevant semiconductor GaN, popular ways to determine this quantity using cathodoluminescence (CL) have been found to be based on incomplete physical models. This background motivates us to take a fresh, in-depth look on how to determine the carrier diffusion length in GaN using CL spectroscopy. ... read more

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

The progress of electronic devices is increasingly linked to the utilization of quantum effects. A future scenario are integrated quantum circuits containing coupled nanostructures. Interconnects, coupling distant on-chip components, could then be realized by the exchange of ballistic electrons. Our work aims at optimizing the coherent exchange of ballistic electrons between quantum point contacts, fundamental building blocks of quantum circuits. ... read more

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. Here, we study the spontaneous emission of zincblende GaAs/(Al,Ga)As core/shell nanowire arrays by µ-photoluminescence spectroscopy. These ordered arrays are synthesized on patterned Si(111) substrates using molecular beam epitaxy, and exhibit an exceptionally low degree of polytypism for interwire separations exceeding a critical value. ... read more

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

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

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

Light emission centres in solids are promising candidates for applications in quantum information processing due to their capability of acting as single photon sources. One of the challenges related to these kinds of quantum light sources is to find mechanisms for the efficient control of their optoelectronic properties. Here, we show that surface acoustic waves can modulate the optical properties of light emission centres in hexagonal boron nitride. These results have been published as a Featured Article in Applied Physics Letters. ... read more

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. ... read more

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. ... read more

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

Non-reciprocal propagation of sound, that is, the different transmission of acoustic waves traveling along opposite directions, is an essential requirement for the realization of devices such as acoustic isolators and circulators. Here, we demonstrate the efficient non-reciprocal transmission of surface acoustic waves propagating in a GaAs substrate coated with an epitaxial Fe3Si film. ... read more

Researchers from the Paul-Drude-Institut in Berlin, the Helmholtz-Zentrum in Dresden and the Ioffe Institute in St. Petersburg have demonstrated the use of elastic vibrations to manipulate the spin states of optically active color centers in SiC at room temperature. They show a non-trivial dependence of the acoustically induced spin transitions on the spin quantization direction, which can lead to chiral spin-acoustic resonances. These findings are important for applications in future quantum-electronic devices and have recently been published in Physical Review Letters. ... read more

Interactions of light and material excitations (e.g., excitons) are at the heart of the modern and future optoelectronics. Efficient control and spatial localization (confinement) of the optical fields and particles is a crucial challenge. The latter can be addressed by the strong coupling of the quantum well excitons and photons in semiconductor microcavities, which results in microcavity exciton-polaritons (simply, polaritons - MP). Polaritons display a wide range of attractive properties arising from their dual light-matter nature, such as bright directional emission, lasing, macroscopic spatial coherence at Kelvin-temperatures, Bose-Einstein-like condensation and strong nonlinearities. Therefore, polaritons offer a pathway to bridge conventional microelectronics and quantum technologies in an all-semiconductor platform. ... read more

The correlation between the emission frequency and the location on the wafer due to an inhomogeneous growth rate across the wafer was investigated for GaAs/AlAs terahertz (THz) quantum-cascade lasers (QCLs) using experiments and simulations. For several wafers, we observe a blue shift of the emission frequency from the center to the edge of the wafer. This blue shift is attributed to a decrease of the thickness of the QCL, which can be determined with spectroscopic techniques. The correlation of the calculated frequencies with the actual emission frequencies of QCLs fabricated from different locations on the wafer allows to establish an effective method for the fabrication of THz QCLs emitting at a particular target frequency. ... read more

Terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures have been developed for application-defined emission frequencies between 3.4 and 5.0 THz. These THz QCLs can be used as local oscillators in airborne or satellite-based astronomical instruments or as radiation sources for high-resolution absorption spectroscopy. The GaAs/AlAs THz QCLs can be operated in mechanical cryocoolers and even in miniature mechanical cryocoolers due to their comparatively high wall-plug efficiency of around 0.2% and typical current densities below 500 A/cm2. These lasers exhibit output powers of more than 1 mW at operating temperatures up to about 70 K, which is sufficient for most of the abovementioned applications. ... read more

In this CReA we have developed terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures, which exhibit single-mode emission at 3.360, 3.921, and 4.745 THz. These frequencies are in close correspondence to fine-structure transitions of Al atoms, N+ ions, and O atoms, respectively. Due to the low electrical pump power of these THz QCLs, they can be operated in a mechanical cryocooler in continuous-wave mode, while a sufficient intrinsic tuning range of more than 5 GHz is maintained. The single-mode operation and the intrinsic tuning range of these THz QCLs allow for the application of these lasers as radiation sources for high-resolution absorption spectroscopy to determine the absolute densities of Al atoms, N+ ions, and O atoms in plasmas. ... read more