Ferdinand-Braun-Institute, Leibniz-Institut für Höchstfrequenztechnik

Amano H., Collazo R., Santi C.D., Einfeldt S., Funato M., Glaab J., Hagedorn S., Hirano A., Hirayama H., Ishii R., Kashima Y., Kawakami Y., Kirste R., Kneissl M., Martin R., et. al.

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

Frye K., Abend S., Bartosch W., Bawamia A., Becker D., Blume H., Braxmaier C., Chiow S., Efremov M.A., Ertmer W., Fierlinger P., Franz T., Gaaloul N., Grosse J., Grzeschik C., et. al.

Microgravity eases several constraints limiting experiments with ultracold and condensed atoms on ground. It enables extended times of flight without suspension and eliminates the gravitational sag for trapped atoms. These advantages motivated numerous initiatives to adapt and operate experimental setups on microgravity platforms. We describe the design of the payload, motivations for design choices, and capabilities of the Bose-Einstein Condensate and Cold Atom Laboratory (BECCAL), a NASA-DLR collaboration. BECCAL builds on the heritage of previous devices operated in microgravity, features rubidium and potassium, multiple options for magnetic and optical trapping, different methods for coherent manipulation, and will offer new perspectives for experiments on quantum optics, atom optics, and atom interferometry in the unique microgravity environment on board the International Space Station.

Assmann R.W., Weikum M.K., Akhter T., Alesini D., Alexandrova A.S., Anania M.P., Andreev N.E., Andriyash I., Artioli M., Aschikhin A., Audet T., Bacci A., Barna I.F., Bartocci S., Bayramian A., et. al.

This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years.

Laucht A., Hohls F., Ubbelohde N., Fernando Gonzalez-Zalba M., Reilly D.J., Stobbe S., Schröder T., Scarlino P., Koski J.V., Dzurak A., Yang C., Yoneda J., Kuemmeth F., Bluhm H., Pla J., et. al.

Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon.

Lachmann M.D., Ahlers H., Becker D., Dinkelaker A.N., Grosse J., Hellmig O., Müntinga H., Schkolnik V., Seidel S.T., Wendrich T., Wenzlawski A., Carrick B., Gaaloul N., Lüdtke D., Braxmaier C., et. al.

Bose-Einstein condensates (BECs) in free fall constitute a promising source for space-borne interferometry. Indeed, BECs enjoy a slowly expanding wave function, display a large spatial coherence and can be engineered and probed by optical techniques. Here we explore matter-wave fringes of multiple spinor components of a BEC released in free fall employing light-pulses to drive Bragg processes and induce phase imprinting on a sounding rocket. The prevailing microgravity played a crucial role in the observation of these interferences which not only reveal the spatial coherence of the condensates but also allow us to measure differential forces. Our work marks the beginning of matter-wave interferometry in space with future applications in fundamental physics, navigation and earth observation. Conducting atom-optical experiments in space is interesting for fundamental physics and challenging due to different environment compared to ground. Here the authors report matter-wave interferometry in space using atomic BECs in a sounding rocket.

Lobo-Ploch N., Mehnke F., Sulmoni L., Cho H.K., Guttmann M., Glaab J., Hilbrich K., Wernicke T., Einfeldt S., Kneissl M.

Deep UV-LEDs (DUV-LEDs) emitting at 233 nm with an emission power of (1.9 ± 0.3) mW and an external quantum efficiency of (0.36 ± 0.07) % at 100 mA are presented. The entire DUV-LED process chain was optimized including the reduction of the dislocation density using epitaxially laterally overgrown AlN/sapphire substrates, development of vanadium-based low resistance n-metal contacts, and employment of high thermally conductive AlN packages. Estimated device lifetimes above 1500 h are achieved after a burn-in of 100 h. With the integration of a UV-transparent lens, a strong narrowing of the far-field pattern was achieved with a radiant intensity of 3 mW/sr measured at 20 mA.

Glaab J., Lobo-Ploch N., Cho H.K., Filler T., Gundlach H., Guttmann M., Hagedorn S., Lohan S.B., Mehnke F., Schleusener J., Sicher C., Sulmoni L., Wernicke T., Wittenbecher L., Woggon U., et. al.

Multiresistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) cause serious postoperative infections. A skin tolerant far-UVC (< 240 nm) irradiation system for their inactivation is presented here. It uses UVC LEDs in combination with a spectral filter and provides a peak wavelength of 233 nm, with a full width at half maximum of 12 nm, and an irradiance of 44 µW/cm2. MRSA bacteria in different concentrations on blood agar plates were inactivated with irradiation doses in the range of 15–40 mJ/cm2. Porcine skin irradiated with a dose of 40 mJ/cm2 at 233 nm showed only 3.7% CPD and 2.3% 6-4PP DNA damage. Corresponding irradiation at 254 nm caused 11–14 times higher damage. Thus, the skin damage caused by the disinfectant doses is so small that it can be expected to be compensated by the skin's natural repair mechanisms. LED-based far-UVC lamps could therefore soon be used in everyday clinical practice to eradicate multiresistant pathogens directly on humans.

Susilo N., Ziffer E., Hagedorn S., Cancellara L., Netzel C., Ploch N.L., Wu S., Rass J., Walde S., Sulmoni L., Guttmann M., Wernicke T., Albrecht M., Weyers M., Kneissl M.

We report on the performance of AlGaN-based deep ultraviolet light-emitting diodes (UV-LEDs) emitting at 265 nm grown on stripe-patterned high-temperature annealed (HTA) epitaxially laterally overgrown (ELO) aluminium nitride (AlN)/sapphire templates. For this purpose, the structural and electro-optical properties of ultraviolet-c light-emitting diodes (UVC-LEDs) on as-grown and on HTA planar AlN/sapphire as well as ELO AlN/sapphire with and without HTA are investigated and compared. Cathodoluminescence measurements reveal dark spot densities of 3.5×109  cm−2, 1.1×109  cm−2, 1.4×109  cm−2, and 0.9×109  cm−2 in multiple quantum well samples on as-grown planar AlN/sapphire, HTA planar AlN/sapphire, ELO AlN/sapphire, and HTA ELO AlN/sapphire, respectively, and are consistent with the threading dislocation densities determined by transmission electron microscopy (TEM) and high-resolution X-ray diffraction rocking curve. The UVC-LED performance improves with the reduction of the threading dislocation densities (TDDs). The output powers (measured on-wafer in cw operation at 20 mA) of the UV-LEDs emitting at 265 nm were 0.03 mW (planar AlN/sapphire), 0.8 mW (planar HTA AlN/sapphire), 0.9 mW (ELO AlN/sapphire), and 1.1 mW (HTA ELO AlN/sapphire), respectively. Furthermore, Monte Carlo ray-tracing simulations showed a 15% increase in light-extraction efficiency due to the voids formed in the ELO process. These results demonstrate that HTA ELO AlN/sapphire templates provide a viable approach to increase the efficiency of UV-LEDs, improving both the internal quantum efficiency and the light-extraction efficiency.

Utke I., Swiderek P., Höflich K., Madajska K., Jurczyk J., Martinović P., Szymańska I.B.

• Group 10 & 11 molecules for gas-assisted FEBID nanoprinting and thin film ALD and CVD. • Surface reactions: adsorption, dissociation, desorption - electron induced vs thermal. • Detailed survey of electron irradiation studies on condensed films and gas phases. • Comparative compilation of FEBID, thermal and plasma CVD, and ALD processes. • Emerging methods: Electron enhanced ALD and CVD, pulsed FEBID with laser heating. Nanostructured materials made from group 10 (Ni, Pd, Pt) and group 11 (Cu, Ag, Au) elements have outstanding technological relevance in microelectronics, nano-optics, catalysis, and energy conversion. Processes that allow for the easy and reliable fabrication of such nanostructures are heavily sought after. Focused electron beam induced deposition (FEBID) is the only direct-write technique that can fabricate nanostructures with arbitrary shape and dimensions down to the sub-10 nm regime. However, the complex chemistry of FEBID involving electron-induced dissociation processes of metalorganic precursors molecules, surface kinetics, and thermal effects is poorly understood and far from being optimized. Here, we review in a comparative manner the performance and the underlying chemical reactions of surface deposition processes, namely, chemical vapour deposition (CVD), atomic layer deposition (ALD), and FEBID itself. The knowledge gained in CVD and ALD as related surface deposition techniques will help us to understand the spatially selective chemistry occurring in FEBID. Fundamental surface and gas phase studies provide insight to electron-induced chemistry and desorption of precursor fragments. Specific emphasis is put on the type of the ligands and their different behaviour under thermal, surface-related, and electron-induced processes. The comprehensive overview of the current state of FEBID for group 10 and 11 metals includes reactive environments and purification approaches as these may provide valuable information on the design of novel precursors. The evaluation of the precursor and process performance is extended to include W, Co, Fe, Ru, Rh, and Ir to represent a general guide towards future developments in FEBID. These may not only rely on the design of novel compounds but also on optimized deposition strategies inspired by ALD and CVD.

Sulmoni L., Mehnke F., Mogilatenko A., Guttmann M., Wernicke T., Kneissl M.

The electrical and structural properties of V/Al-based n-contacts on n‐AlxGa1−xN with an Al mole fraction x ranging from x=0.75 to x=0.95 are investigated. Ohmic n-contacts are obtained up to x=0.75 with a contact resistivity of 5.7×10−4  Ω·cm2 whereas for higher Al mole fraction the IV characteristics are rectifying. Transmission electron microscopy reveals a thin crystalline AlN layer formed at the metal/semiconductor interface upon thermal annealing. Compositional analysis confirmed an Al enrichment at the interface. The interfacial nitride-based layer in n-contacts on n‐Al0.9Ga0.1N is partly amorphous and heavily contaminated by oxygen. The role and resulting limitations of Al in the metal stack for n-contacts on n-AlGaN with very high Al mole fraction are discussed. Finally, ultraviolet C (UVC) LEDs grown on n‐Al0.87Ga0.13N and emitting at 232 nm are fabricated with an operating voltage of 7.3 V and an emission power of 120 μW at 20 mA in cw operation.