Electrical properties of ScN(111) layers grown on semi-insulating GaN(0001) by plasma-assisted molecular beam epitaxy After more than half a century of research, understanding electron scattering mechanisms in scandium nitride (ScN) has remained a significant challenge, primarily due to high impurity concentrations in scandium sources. In a groundbreaking study published in Phys. Rev. Appl. 22, 014067 (2024), researchers at PDI overcame this long-standing limitation by growing high-purity ScN(111) layers on nearly lattice-matched semi-insulating GaN(0001) using plasma-assisted molecular beam epitaxy.
Advances in Terahertz Quantum-Cascade Lasers for Real-World Applications An invited paper authored by PDI scientists in collaboration with DLR, INP, and Lytid SAS, has recently been selected as a Featured Paper in the Special Issue of IEEE Transactions on Terahertz Science and Technology “Selected Emerging Trends in Terahertz Science and Technology”. The paper details advancements in designing and fabricating terahertz quantum-cascade lasers and explores their applications in high-resolution spectroscopy, demonstrating their potential for atmospheric studies and industrial diagnostics.
Dynamical reorientation of spin multipoles Color centers in solids, particularly optically addressable spin centers, are promising for quantum technologies due to their ability to interact with light. Silicon carbide (SiC) is gaining attention for its compatibility with CMOS technology and its coherent spin centers, such as the negatively charged silicon vacancy (VSi) with spin S=3/2. A recent study by researchers from the Paul-Drude-Institut, Helmholtz-Zentrum Dresden-Rossendorf, and ICFO shows that weak magnetic fields applied perpendicular to the VSi symmetry axis invert the optically detected magnetic resonance (ODMR) contrast. This inversion is attributed to the reorientation of spin polarization, offering insights into spin dynamics and potential applications in quantum sensing and spin-photon interfaces.
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.
“Acceleration beats” shine bright light on novel universal modulation regime PDI researchers have discovered a novel modulation regime in semiconductor-based lasers, characterized by "acceleration beats". The universal effect, observed experimentally for the first time, suggests potential for new high-frequency spectral features and quantum control protocols, with implications extending to cosmic and high-energy particle phenomena.
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.
PDI confirms existence of Bi_Ga hetero-antisites in landmark study An invited paper by PDI Senior Scientist Esperanza Luna Garcia de la Infanta was recently selected as Featured article (Journal Editors’ choice) in a special issue of the Journal of Applied Physics. The research conducted at PDI marks the second-ever published experimental confirmation of Bi_Ga hetero-antisites and provides a crucial step towards enhancing material quality and facilitating applications.
Advanced imaging techniques on a semiconductor material reveal ‘surprising’ hidden activity New research, led by a partnership between the Paul Drude Institute (PDI) and Penn State University, reveals surprising activity within an often overlooked material in computer chip design. This discovery has the potential to revolutionize electronics, leading to faster and more energy-efficient devices.
Thick does the trick: Ferroelectricity in two-dimensional (GeTe)m(Sb2Te3)n lamellae A recent paper published in Advanced Science and featuring contributions from PDI researchers demonstrated that molecular beam epitaxy allows fabricating lamellar GeTe-rich (GeTe)m(Sb2Te3)n alloys which show ferroelectric properties. The material was developed at PDI within the European project BeforeHand (GA 824957, Horizon2020) – a collaboration between PDI and six other European partners.
Above-room-temperature ferromagnetism in large-area van der Waals heterostructures Two-dimensional (2D) magnetic materials are promising building blocks for the realization of novel, ultra-compact devices for spintronics. Combining them with other 2D crystals such as graphene is a very attractive route to realize hybrid material systems exhibiting integrated magnetic and electronic functionalities.
Researchers demonstrate solution for long-term challenge that has benefits for spintronics and data storage In a paper published recently in Advanced Science, researchers from the Paul Drude Institute in Berlin, Germany, and the Xiamen University, Xiamen, China, demonstrated that ferrimagnetic NiCo2O4 (NCO) constitutes a solution for the long-term challenge of finding materials with a robust out-of-plane magnetization.
PDI shares insights in prominent international collaboration 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'.
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.
New research on self-locking light sources presents opportunities for quantum technologies In a paper published today in Nature Communications, researchers from the Paul-Drude-Institut in Berlin, Germany, and the Instituto Balseiro in Bariloche, Argentina, demonstrated that the mixing of confined quantum fluids of light and GHz sound leads to the emergence of an elusive phonoriton quasi-particle – in part a quantum of light (photon), a quantum of sound (phonon) and a semiconductor exciton.
Terahertz quantum-cascade lasers for high-resolution absorption spectroscopy of atoms and ions in plasmas 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.
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
Carrier diffusion length in GaN revisited 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.
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
Correlation between emission frequency and location on the wafer for terahertz quantum-cascade lasers 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.
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.
Controlling free electrons in quantum circuits 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.
Nanoearthquakes control spin centers in SiC 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.
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
Large non-reciprocal propagation of surface acoustic waves in epitaxial Fe3Si/GaAs structures Non-reciprocal propagation of sound 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.
Can we determine the carrier diffusion length in GaN from cathodoluminescence maps around threading dislocations? 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.
High-performance GaAs/AlAs terahertz quantum-cascade lasers for spectroscopic applications 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.
Acoustic modulation of light emission centres in hexagonal boron nitride 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.
Quantum confinement of exciton-polaritons in a structured (Al,Ga)As microcavity 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.
