Reduction equipment in operation for reducing and conversion of powders into metal bars
High purity (HP)-Ge crystals have an incredible low impurity level in the range down to <1010 cm-3. This means that only one atom out of 1013 is a foreign atom (ppT level !). Such clean HP-Ge material is needed for Gamma-Ray detector applications in X-Ray Physics, Astronomy etc. IKZ is a member in the international LEGEND project  where not natural but even enriched 76Ge detector crystals are needed to make progress on our understanding of the very fundamentals of the Universe.
The CZ-IV working group of the Semiconductors section is specialized in the growth of large diameter Germanium (Ge) single crystals with a low defect density, along with zone-refining Ge bars up to ultra-high purity.
Now additionally, a new reduction furnace is set up at IKZ, which can be operated at high temperatures under H2 atmosphere. This will allow a cost reduction and a better control over the contaminations of the Ge source material for the manufacture of detectors.
Another advantage is to bring-in a full value proposition and complete production chain, starting from Germanium oxide (GeO2) toward the preparation and purification of metal bars till providing the finished high purity crystals (13 N), for e.g. detector-blocks, under one roof. In a single process, the GeO2 powder is first reduced into Ge metallic powder and then melted to prepare the Ge bars for further zone refining.
A reduction process with a very high yield of > 99% was successfully developed by our young research team of the working CZ-IV group, with a capacity of at least 1 kg Ge (and up to 2 kg GeO2) per process and as well with short reduction time (< 48 h), especially suited for materials, which require minimised cosmic ray exposure.
Isotopically-enriched 76Ge material with ultra-high purity is indispensable for the radiation detection applications (e.g. neutrinoless-ßß-decay experiments, a large and grand international collaborative effort by the LEGEND research program). Isotopically enriched Ge is provided as enriched 76GeO2 powder, which then has to be reduced to Ge bars. However, there is no optimized process readily accessible and available for realizing a very high yield reduction of 76Ge, which is an essential requirement, due to the extremely high material cost besides some special care to avoid cosmic-ray exposures.
The process was established and developed by reducing 20 kg of natural GeO2 powder. This process was then seamlessly transferred to produce 23 kg of enriched (88%) 76Ge material from oxide , for our co-operation partner (Technical University Munich). Afterwards, the reduced Ge bars were purified by zone refining up to intrinsic purity to be provided to the other LEGEND partners as per the requirement, for further processing and finally detector fabrication. We were able to achieve a reduction yield of 99.85% (76GeO2 in to 76Ge), which is comparable to (if not, better than) that of commercial producers. Further optimization and scaling-up to mass production could be done for ton-scale requirements of the
 K.-P. Gradwohl, J. Janicskó-Csáthy, O. Moras, S. Schönert, R. R. Sumathi, presented in LEGEND collaboration meeting, 2-6 December 2019, Seattle, USA.
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Figure: Pink lines indicate the newly developed diode-pumped solid-state lasers, black lines indicate commercially available laser diodes
Compact, cost-efficient, and easy-to-handle visible lasers are highly demanded in several applications e.g. in industry, medicine, and imaging applications. Despite strong research activities in this field, as of now there are still no laser diodes available which directly emit in the so-called "green gap" at wavelengths between 540 nm and 600 nm.
The research at the Center for Laser-Materials (ZLM) at the Leibniz-Institut for Crystal Growth (IKZ) now led to the development of an efficient diode-pumped solid-state laser with emission in the green and in the yellow spectral range. The presented laser based on trivalent terbium as the active ion is a simple and compact approach for the direct generation of visible laser light. Pumped by a laser diode emitting at 488 nm in the cyan-blue range, a terbium-doped lithium-yttrium-fluoride crystal emits up to 44 and 14 mW at wavelengths of 542 and 587 nm, respectively. This is the first time, a diode-pumped solid-state laser based on trivalent terbium was realized. It is highlighted that this laser is free of any frequency conversion steps and as such more simple than a green laser pointer. Power scaling will be facilitated by the future progress in cyan-blue diode lasers.
This work was accepted for publication in the prestigious journal Laser & Photonics Reviews  and is featured as a world news report on lasers & sources in Laser Focus World .
 E. Castellano-Hernández, S. Kalusniak, P. W. Metz, and C. Kränkel, "Diode-pumped laser operation of Tb3+-LiLuF4 in the green and yellow spectral range", Laser & Photonics Reviews (accepted for publication) 2019
 Laser Focus World, December 2019, p. 14 ff, available online:
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Growing principle of the new technology to be developed: Local thermal decomposition by laser-assisted Chemical Vapor Deposition (CVD).
In the section "Low-dimensional structures", industrial partners, including BESTEC GmbH, Bundesanstalt für Materialforschung und -prüfung (BAM) and the University of Duisburg/Essen, co-operate to develop a new method for a defined local crystallization on substrates.
The objective is the realization of localized structure formation on amorphous surfaces without the application of complex and expensive mask or lithography processes. For this purpose, focused laser radiation of suitable wavelength and intensity is to be used, which is guided over the surface by a scanner in a grid-like manner, so that a pattern-shaped energy input can be made possible. The resulting thermal inhomogeneity is intended to cause the selective formation of liquid and ultimately crystalline structures on the surface.
Such islands can serve as precursors for the locally defined growth of compound semiconductors. The ministry of economics is funding the project with 1.5 million euros, of which around 700T euros will be allocated to the IKZ.
Figure: Gallium oxide chip with lateral transistor and measurement structures, manufactured at FBH by projection lithography. "ForMikro-GoNext" aims at a vertical device architecture. | ©FBH/schurian.com
The recently launched joint project "ForMikro-GoNext" of the Leibniz-Institut für Kristallzüchtung (IKZ), the Ferdinand Braun Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH), the University of Bremen and industrial partners ABB Power Grids Switzerland Ltd. and AIXTRON studies beta-gallium oxide (β-Ga2O3). The project partners are investigating this semiconductor material utilizing a new vertical device architectures in order to exploit its outstanding properties for transistors more effectively. The joint project is funded by the German Federal Ministry of Education and Research (BMBF) with approximately €2 million over 4 years.
Modern society relies on a wide range of electrical and electronic systems, from communication to industrial production and e-mobility. About 80% of them require the conversion of primary electricity into another form of electricity. Therefore, the conversion of electrical energy must be carried out as efficient as possible. New semiconductor materials with a high band gap such as silicon carbide (SiC) and gallium nitride (GaN) offer a higher electrical breakdown field strength than silicon, allowing the fabrication of components in much smaller and compact dimensions. The semiconductor β-Ga2O3 features a breakdown field strength of twice the value of SiC and GaN and thus offers the potential to further increase power converter efficiency. High voltages can be switched with significantly smaller semiconductor drift regions - the basis for more compact systems. In addition, transistors based on β-Ga2O3 are characterized by a low on-resistance at a given breakdown voltage and faster switching transients, which leads to lower power losses. Due to these properties β-Ga2O3 has the best prerequisites to become the high-performance material for next generation power applications.
So far, lateral Ga2O3 devices have been investigated. In this configuration, the voltage is switched across the device surface, making large chip size areas combined with complex isolation techniques necessary for high voltages. ForMikro-GoNext aims at vertical device structures in order to more efficiently utilize the high electrical breakdown field strength of the material β-Ga2O3. The better exploitation of the chip size also opens the potential of upscaling device geometries towards technically relevant very high current level switching. The development of these transistors requires a synchronized process chain from crystal growth, epitaxy and device processing to characterization, which is completely covered within the project.
By focusing the expertise of the Leibniz Institutes IKZ (gallium oxide crystal growth, epitaxy and material characterization) and FBH (device design, manufacturing and testing) it is expected to efficiently transfer the achieved results from basic research into application- and industry-oriented research. The Institute for Electrical Drives, Power Electronics and Devices (IALB) at the University of Bremen provides qualified assessment of the application potential of the new devices with its power electronic characterization capabilities. Reliability tests will provide information about the stability of the Ga2O3 transistors. The project will be supported by industrial partners ABB Power Grids Switzerland Ltd. and AIXTRON - AIXTRON in the field of epitaxy, ABB in the design and testing of the devices.
Read the full press release
Section Fundamental Discription at IKZ - Achieved accelerated
process development using artificial intelligence (AI)
Crystal growth is essential for the development of new technological functional materials. A special challenge is the reduction of costs and time in the production of industrially important materials. However, a general approach based on trial-and-error experiments and CFD simulations (Computational Fluid Dynamic) is too slow to provide fast answers. For example, increasing the diameter of silicon wafers from 1 inch to 12 inches took 40 years using this method.
However, artificial intelligence (AI) can significantly shorten the development time of crystal growth processes. In order to come closer to this goal, the section "Fundamental Description" of the IKZ has extended its research topics to the investigation of various applications of AI in the growth of volume crystals.
The IKZ used static ANNs (artificial neural networks) for pattern recognition and parameter optimization in the magnetic driven growth of crystalline materials [1,2]. A current research topic is the application of dynamic neural networks for real-time prediction in the transient VGF-GaAs crystal growth process. For example, temperature distributions in the melting furnace as well as the position of the crystallization front during the growth process can be predicted . Such dynamic ANNs enable process automation and control as a decisive step in the development of smart factories in the context of Industry 4.0. The Application of the AI technologies to other crystal growth topics is currently in progress.
 N. Dropka, M. Holena, Optimization of magnetically driven directional solidification of silicon using artificial neural networks and Gaussian process models, Journal of Crystal Growth 471 (2017) 53-61.
 N. Dropka, M. Holena and Ch. Frank-Rotsch, TMF optimization in VGF crystal growth of GaAs by artificial neural networks and Gaussian process models, Proceedings of XVIII International UIE-Congress on Electrotechnologies for Material Processing, Eds. E. Baake, B. Nacke, Hannover, June 6 - 9, 2017, p.203-208.
 N. Dropka, M. Holena, S. Ecklebe, Ch. Frank-Rotsch, J. Winkler, Fast forecasting of VGF crystal growth process by dynamic neural networks, Journal of Crystal Growth 521(2019) 9-14.
Ba2ScNbO6 single crystal containing multicrystalline regions at the rim / Christo Guguschev © IKZ
Functional thin films are of strong contemporary interest in materials physics. Examples include La:BaSnO3, a semiconducting oxide exhibiting a very high electron mobility, LaInO3 as ionic gate oxide, BiScO3 as important constituent of a new class of high-temperature piezoelectrics and PbZrO3 as antiferroelectric material relevant to energy storage. These materials all feature a perovskite crystal structure with (pseudo-)cubic lattice parameters in slight excess of 4.1 Å, for which no substrate crystal was available. As a result, current studies are based on highly lattice-mismatched substrates leading to films with low structural quality and in case of electronic demonstration devices to insufficient performances.
This situation will change now, as IKZ and the Cornell University invented a novel bulk crystal growth technique tailored to the double-perovskite Ba2ScNbO6 . The growth conditions at about 2150°C are suitable to yield relatively large crystals (see Fig.) and (100)-oriented single-crystalline substrates with surface areas as large as 10 x 10 mm2. While the composition was chosen on a lattice parameter survey, the ability to grow the crystal from the melt was assessed by a thermo-chemical study. The achievement was published and a joint patent has been applied for. First La-doped BaSnO3 films grown on the new Ba2ScNbO6 substrates at the Cornell University already demonstrated the strong impact and benefit of using lattice-matched high-quality substrates .
 C. Guguschev et al., https://arxiv.org/ftp/arxiv/papers/1907/1907.02719.pdf
 H. Paik et al., “High mobility La-doped BaSnO3 thin film growth on lattice-matched Ba(Sc0.5Nb0.5)O3 (001) substrate by molecular-beam epitaxy”, poster at the 19th International Conference on Crystal Growth and Epitaxy (ICCGE-19/OMVPE-19), July 28 – August 2, 2019, Keystone, Colorado, USA.
Kaspars Dadzis with the demo setup for a crystal growth process
For the first time in the history of the Leibniz-Institut für Kristallzüchtung (IKZ) the prestigious grant from the European Research Council (ERC) is awarded to an IKZ researcher. Kaspars Dadzis receives a total of 1.5 million euros over a period of 5 years for his project “Next Generation Multiphysical Models for Crystal Growth Processes (NEMOCRYS)”. As one of a total of four scientists in Germany, Kaspars Dadzis asserted himself in the "Products and Processes Engineering" panel.
Crystal growth processes are highly complex physical phenomena. In that context numerical simulation is often used for process optimization. However, the lack of possibilities for direct measurements inside of crystal growth environments limits the achievable accuracy of the underlying theoretical models. Consequently, an experimental trial-and-error approach still dominates the practice of crystal growth development. This could change in the future by the work of the junior research group "Model Experiments" under the leadership of Kaspars Dadzis.
The awarded project "Next Generation Multiphysical Models for Crystal Growth Processes (NEMOCRYS)" is dedicated to the development of a new experimental platform (the "MultiValidator") which includes a unique crystal growth setup for model materials. The dedicated design of this setup, reduced working temperatures, and relaxed vacuum-sealing requirements will enable a convenient experimental access for various in-situ measurement techniques. The simultaneous observation of thermal fields, fluid flows, stress distributions and other physical phenomena will allow for the first time to thoroughly validate a series of fundamental assumptions in multiphysical macroscopic models for crystal growth. The NEMOCRYS project has the goal to reach a new level of physical understanding and to change the paradigm how we observe, describe and develop crystal growth processes and similar complex multiphysical systems. The practical results in form of new physical models and optimized measurement techniques will be applied to support various development projects at the IKZ.
Four hundred and eight early-career researchers have been awarded European Research Council grants in this year’s first completed ERC competition. The highly-coveted funding will help individual scientists and scholars to build their own teams and conduct pioneering research across all disciplines. The grants, worth in total €621 million, are part of the EU’ Research and Innovation programme, Horizon 2020.
After the completion of his PhD thesis, Kaspars Dadzis worked in the industrial research at SolarWorld in Freiberg focusing on silicon crystal growth for solar cells. In 2016 he moved to the Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin-Adlershof. He received the LIMTECH Young Scientist Award for his work in the field of model experiments and numerical simulation in crystal growth in 2017.
Read the full press release.
Inductively heated reactors for sublimation growth of Aluminium nitride single crystals
In 2019, the Aluminium Nitride group launched a new crystal growth laboratory with three identical reactors for the sublimation growth of aluminium nitride monocrystals. The equipments were designed and built in close cooperation with the Design Group at the IKZ.
This measure not only increases the growth capacity significantly, but also helps to guarantee reproducible growth conditions from test to test or plant to plant, even from an industrial point of view, i.e. within a very narrow parameter range. In particular, the focus is on the reliable optical measurement of temperatures > 2000°C at the top and bottom of the crucible.
The aim of the work in the new laboratory is to develop a process for the production of low-defect (EPD < 105 cm-2) AlN substrates with a diameter of 10 mm to 1" (25.4 mm) and defined properties (doping, orientation, geometry, surface quality).
With these prototype series of substrates with reproducible properties, the potential of AlN for various applications in the field of semiconductor electronics can be evaluated together with partners. The substrate requirements vary depending on the potential applications.
In the current BMBF project "UV LEDs for ultra-short wavelengths around 230 nm based on AlN substrates (AlN-230nm)", for example, UV-transparent AlN substrates which are grown at IKZ and processed by the substrate manufacturers FCM and Crystec are used by epitaxy and device developers FBH and TUB for the production of UVC LEDs. Within the framework of the Advanced UV for Life Consortium, these can be used, for example, to develop modules for disinfection in medical technology.
For other non-optical applications, however, no UV transparency is required. In the case of high-temperature sensors, for example, the focus is on the influence of doping on the electromechanical properties, while for acoustic surface wave filters (SAW filters) the sample geometry and surface quality are the most important properties.
Prof. Dr. Darrell Schlom, Cornell University, USA, and Dr. Reinhard Uecker, Leibniz-Institut für Kristallzüchtung (IKZ), Germany, received the Frank Prize of the International Organization for Crystal Growth (IOCG) for their pioneering contributions to the development of new perovskite substrates enabling strain engineering of functional oxides.
The prize was jointly awarded to both scientists at the ICCGE-19 in Keystone, Colorado, USA, from 28 July to 2 August 2019. The IOCG Frank Prize will be awarded for significant fundamental contributions to crystal growing and its global impact on science and technology.
Images: Laureate Prof. Dr. Darrell Schlom, Cornell University, USA (left) and Dr. Reinhard Uecker, Leibniz-Institut für Kristallzüchtung (IKZ), Germany (right)
Ferro-/piezoelectric thin films can potentially be used for memory, sensor or microwave applications. The use of surface acoustic waves (SAW) in thin film form is a technologically highly interesting, but also demanding application. They offer increased sensitivity compared to conventional volume-based SAW sensors. Potassium-sodium-niobate (KxNa1-xNbO3) is a material system that has not only high piezoelectric and electromechanical properties, which are essential for thin film devices. But it is also a lead-free material, which has so far been little investigated as thin film because it contains volatile components (sodium, potassium).
Figure 1: Comparison of the lattice parameters of the substrates used (lower part). The widths of the unit cells of the KxNa1-xNbO3 layers also result from the ratio of NaNbO3 and KNbO3 in the layers. These lattice parameters are shown above the black line.
Figure 2: Phase transition temperature as a function of overall lattice strain.
For several years now, the IKZ group "Ferroelectric Oxide Layers" has been the only group worldwide that can epitaxially grow this material system through the so-called metal-organic vapour phase epitaxy (MOVPE). In close cooperation with Roger Wördenweber's research group at Forschungszentrum Jülich (FZJ), we were able to successfully detect for the first time the propagation of acoustic surface waves in 30 nm thin potassium-sodium-niobate layers (K0.7Na0.3NbO3) on terbium- and gadolinium-scandate substrates [1,2]. First attempts to use these SAW structures as sensors for biomolecules are currently being carried out at the FZJ.
Piezo- and ferroelectric materials are characterized by phase transitions in which both the symmetry of the material and its functional properties change with temperature. In potassium-sodium-niobate volume crystals, the symmetry changes with decreasing temperature from a paraelectric, cubic to a sequence of ferroelectric phases with tetragonal, orthorhombic and rhombohedral symmetry. In the temperature range in the vicinity of these phase transitions the electromechanical properties are often significantly increased. One way of exploiting these enhanced properties for applications is to shift the phase transition temperature into the working range of the device. One approach is given by the so-called strain engineering, in which the phase transitions can be shifted intentionally on the temperature scale by adjusting lattice strains.
In addition to the generation of surface acoustic waves in potassium-sodium-niobate thin films, the phase transition temperature of strained potassium-sodium-niobate layers has recently been systematically adjusted in a large temperature range between -15°C and 400°C . The layers were deposited epitaxially on different rare-earth scandates. These rare-earth scandates are grown in the group "Oxide/Fluorides" in the IKZ and have - depending on the rare-earth metal - different lattice parameters. Together with the lattice parameters of NaNbO3 and KNbO3, this is shown in Figure 1. Figure 2 represents the relationship between the temperature of the phase transition and the incorporated (overall) lattice strain in the films. These investigations were carried out in close cooperation with the group “Physical Characterization” at IKZ and took place within the framework of a DFG project and a doctoral thesis.
 L. von Helden, M. Schmidbauer, S. Liang, M. Hanke, R. Wördenweber, J. Schwarzkopf; Ferroelectric monoclinic phases in strained K0.70Na0.30NbO3 thin films promoting selective surface acoustic wave propagation; Nanotechnology 29, 415704 (2019), https://doi.org/10.1088/1361-6528/aad485
 S. Liang, Y. Dai, L. von Helden, J. Schwarzkopf, R. Wördenweber; Surface acoustic waves in strain-engineered K0.7Na0.3NbO3 epitaxial films on (110) TbScO3, Appl. Phys. Lett 113, 052901 (2018), https://doi.org/10.1063/1.5035464
 L. von Helden, L. Bogula, P.-E. Janolin, M. Hanke, T. Breuer, M. Schmidbauer, S. Ganschow, J. Schwarzkopf; Huge impact of compressive strain on phase transition temperatures in epitaxial ferroelectric KxNa1 xNbO3 thin films; Appl. Phys. Lett. 114, 232905 (2019), https://doi.org/10.1063/1.5094405
In its funding recommendation, the senate of the Leibniz Association attests the Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin a positive development and recommends the Joint Science Conference (GWK) to continue funding the institute as a Leibniz institution.
The Leibniz senate concurs with the positive evaluation report of the international expert commission that visited the institute in December 2018. The reviewers emphasize that the Institute maintained its leading international position in the fields of science & technology and service & transfer for crystalline materials during the past evaluation period. In order to further expand this leading position, the international panel of experts explicitly supports the planned strategy - expansion of the IKZ. The Institute plans to close a central innovation gap in the field of innovative crystalline materials for future applications in electronics and photonics by intensifying prototype research and development.
The IKZ is looking forward to working on these future scientific-technological challenges and thanks the members of the Evaluation Commission for their committed, prudent and outstanding work within the framework of the IKZ Leibniz Evaluation 2018.
The complete statement of the senate on the evaluation of the IKZ is available for download.
Background to the evaluation:
Each institution in the Leibniz Association is evaluated externally every 7 years usually. An international review panel evaluates the research strategy as well as the scientific achievements of the institute on the basis of written documents. During an evaluation visit on site, the committee is then convinced of the quality of the work performed in a direct discussion and records the results in an evaluation report. On this basis, the senate of the Leibniz Association decides whether the Federal Government and the Federal States recommend further Leibniz funding for the institution.
STEM HAADF images of the superlattice structure under investigation with thick barrier (left) and one of the quantum wells from the stack (right) taken at the transmission electron microscope in IKZ
Today, indium gallium nitride (InGaN)-based quantum structures have been widely established in solid-state lightning, reaching efficiencies of up to 90 %. However, the physical reason for these high efficiencies, despite a considerable amount of structural defects, is still under debate. It is widely recognized in the scientific community that the localization of holes within the InGaN quantum well plays a central role, which, however, has been mostly discussed in theoretical works. One of the reasons is related to the fact that in a conventional quantum well, typically both charge carriers – electrons and holes - are confined. In addition, their degree of localization is influenced by various factors such as InGaN alloy fluctuations, quantum well thickness variations and polarization fields, which impedes an experimental access to the recombination processes.
For this reason, a research consortium consisting of the Leibniz-Institut für Kristallzüchtung (IKZ, Berlin), Paul-Drude Institut (PDI, Berlin), Max-Born Institute for Nonlinear Optics (MBI, Berlin) and Max Planck Institute für Eisenforschung (MPIE, Düsseldorf) has investigated quantum systems where the recombination almost exclusively depends on the localization of holes within the quantum well. To achieve this less complicated scenario, the InGaN quantum wells, examined in the recent work of Anikeeva et al., exhibit a thickness of only a single atomic layer with an average indium content of 25%. In this paper, the authors present extensive studies of optical (MBI, IKZ) and structural properties (IKZ) combined with theoretical calculations (MPIE).
Anikeeva et al. demonstrate that despite the reduced complexity with regard to the charge carrier localization, many optical properties resemble those of conventional quantum wells. This highlights already the overall important role of hole localization for the recombination process of charge carriers in InGaN quantum wells. By stacking the monolayer thick quantum wells in superlattice and by carefully tuning its periodicity, the authors were able to reduce the degree of hole localization. As a result, they find a strong impact on emission, most notably in a decreased radiative recombination efficiency and changes in the optical properties, i.e. temporal and temperature behavior. In summary, the authors show by means of experiments and theoretical calculations that hole localization is a decisive factor for optical phenomena in InGaN quantum structures. This enables deeper insights into the basic recombination mechanism, and allows a critical review of models represented in literature.
On 25 June 2019 the IKZ was awarded the certificate for the audit berufundfamilie for further 3 years. With the certificate the IKZ is honoured for its commitment in the area of the strategically aligned family and life-phase conscious personnel policy.
The prerequisite for the certification was the successful completion of the auditing process offered by berufundfamilie Service GmbH. The IKZ has set itself the goal of creating family-friendly working conditions. Over a period of three years, the Institute will actively pursue the family and life-phase conscious measures agreed in the target agreement.
The renewed certification shows that the IKZ is continuously working on a family-friendly personnel policy. The Institute aims to improve the framework conditions for its employees and to provide them with instruments by which family/private life and career can be better reconciled. This includes, for example, flexible options for organising working time, whether it be daily working time or (temporary) part-time employment. A parent-child room is available to employees to bridge short-term bottlenecks.
The joint press release of berufundfamilie Service GmbH and the Federal Ministry for Family Affairs, Senior Citizens, Women and Youth (BMFSFJ) on the awarding of certificates can be found here (German only).
In addition to Ampere, Kelvin, Mol and Co., the kilogram also is now defined by a natural constant. In concrete terms, this means that the original kilogram, which has been the measure of all things for 130 years, has now served its purpose in Paris. This is made possible by the single crystals grown from the highly enriched isotopic silicon-28 at the Leibniz-Institut für Kristallzüchtung (IKZ).
Prototype of a silicon-28 single crystal after growth in a floating zone plant in the context of the KILOGRAM project. (Source: IKZ)
The new International System of Units (SI) was adopted at the 26th General Conference on Weights and Measures in Paris on 16 November 2018. Now the system officially came into force on 20 May 2019, the World Metrology Day. From now on, 7 natural constants form the foundation of all measures.
Hereafter, a new definition for the kilogram is valid using the Planck constant and thus this unit is no longer determined through the mass of the „original kilogram“. The scientific and high-technology communities mostly benefit from this. The IKZ played a decisive role in replacing the almost 130-year-old artificial object of the original kilogram, because the structurally perfect crystals of isotopically-pure silicon-28 (28Si, enrichment up to 99.9995 %) grown at the IKZ were of decisive importance for this project.
In these crystals, almost all the atoms have the same mass and are arranged in a regular three-dimensional lattice, which makes a very exact assignment possible between the mass of the crystal and the number of its atoms. From this relation, the value of the Avogadro constant could be derived with unprecedented precision and thus used as a fundamental natural constant for the definition of the kilogram, since the Plank constant could be determined more precisely with the help of the Avogadro constant. In the new SI system, the value of the Avogadro constant is determined and one mole therefore contains exactly 6.02214076×1023 individual particles.
But that is not everything. Now all 7 basic units are defined by natural constants. This has been the case for many years for the second (with the hyperfine structure transition of the ground state in the Cs atom), the metre (via the speed of light) and the candela (via the photometric radiation equivalent of a special radiation). Now the other units also follow, whereby here the elementary charge (for the ampere), the Boltzmann constant (for the Kelvin), the Avogadro constant (for the mole) and the Planck constant (for the kilogram) play the decisive roles.
IWithin the framework of the "KILOGRAM" projects led by the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, several very precise spheres with shape deviations of less than 20 nm at a diameter of about 94 mm and with a defect-free polished surface were prepared from the 28Si crystals grown at IKZ using the float-zone method (FZ). Under these preconditions, PTB succeeded in determining the number of 28Si atoms in a crystal sphere of 1 kilogram total mass, with the required uncertainty of less than 2 x 10-8
It amounts to: 2,152538397 x 1025 atoms of silicon-28
In order to guarantee the necessary purity of the crystals grown from this material, various material-intensive molten-zone cleaning steps are necessary. The special challenges were therefore the approx. 1000 times higher material price compared to conventional silicon as well as the limited amount of material availability.
Silicon is regarded as a very comprehensively investigated semiconductor material that dominates microelectronics and thus communication technologies worldwide. The IKZ will continue to work on the extreme requirements for the further improvement of material properties in order to enable future applications such as artificial intelligence and quantum technologies. „IKZ´s expertise on isotope pure Si crystals, developed during this metrology project, will allow us to play in the next round a key role as materials science institute for the development of innovative quantum technologies“, states Prof. Dr. Thomas Schröder, Scientific Director at IKZ.
More information can be found at https://www.ptb.de/cms/en/research-development/research-on-the-new-si.html
Erbium-doped lutetia crystal (Er3+:Lu2O3) grown by the OFZ method | Photo: Anastasia Uvarova © IKZ
The world’s first diode-pumped terbium-laser
| Photo: Christian Ehlers © IKZ
The laboratories and research facilities at the Center for Laser Materials - Crystals (ZLM-K) were set up since August 2016 under the guidance of Dr. Christian Kränkel. Initially, the aims of the BMBF-funded Project "EQuiLa" on the research and qualification of innovative laser materials and crystals were the main focus of the research at the ZLM-K.
In materials processing, medicine or in metrology - lasers are ubiquitous in daily life. Nevertheless, even nearly 60 years after the invention of the laser, optimized lasers are not yet available for every area. In many cases this results from the lack of a suitable laser material for the laser light generation. To find a remedy in such cases is the aim of the future research at the new Center for Laser Materials at the Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin Adlershof.
For the project acquired in cooperation with the Ferdinand-Braun-Institut (FBH) state-of-the art furnaces for the growth of highly melting crystals by the optical floating zone technique (OFZ) were set up at the IKZ. The successful growth of erbium-doped lutetia is a first evidence for the suitability of this approach. The crystal grower, Anastasia Uvarova, PhD-student at the ZLM-K, is happy about this result: "This otherwise difficult to grow crystal is important for mid-infrared lasers, which are needed for example in laser scalpels in medicine."
Visible lasers were another topic in EQuiLa, which is investigated at the ZLM-K. Due to their emission in the visible, terbium-doped fluoride crystals as active media are suitable for the direct and efficient generation of green and yellow laser radiation. Within the project, the first diode-pumped terbium-doped solid-state laser based on terbium-doped lithium-lutetium-fluoride (Tb:LiLuF4) was set up. "The first application of such a simple pumping source is an important step toward the commercial use of terbium-lasers", says laser specialist Elena Castellano, who is also a PhD-student at the ZLM-K.
Dr. Christian Kränkel, leader of the Center for Laser Materials, elucidates the concept: "The research at the Center for Laser Materials at IKZ and FBH covers the whole value chain of diode-pumped solid-state lasers starting from the growth of laser crystals and their polishing over the growth of semiconductor-films for laser diodes and their packaging and ending with setting up laser demonstrators".
A diode-pumped chromium-laser was set up as a proof-of-concept at the end of the project “EQuiLa”: The gain material, a chromium-doped Lithium-calcium-aluminium-fluoride crystal (Cr3+LiCaAlF6), was grown and polished at IKZ, while the pump source, an indium-gallium-phosphide-based (InxGa1-xP) red-emitting laser diode, was built at the FBH.
These results pave the way for further successful research and development works in the area of innovative diode-pumped solid-state lasers based on laser crystals grown at IKZ.
Molecular Beam Epitaxy (MBE):
Plant used for the deposition of Silicon and Silicon/Germanium structures
| Photo: IKZ
Today, quantum technologies gain momentum in research & development for game-changing solutions in the area of computing, communication, sensing, cryptography etc.
The successful realization of quantum technologies depends decisively on atomically engineered crystalline materials with ultra-high precision and here, the expertise of the Leibniz Institut für Kristallzüchtung (IKZ) is worldwide recognized. Specifically, the use of isotope pure Silicon and Germanium (Si/Ge) materials systems is considered a key enabling approach for quantum devices. IKZ substantially benefits in this area of preparing very pure and isotopically enriched Si and Ge crystals from its world leading experience e.g. by metrology projects to define the new kg calibration standard.
On 22nd March 2019, IKZ organized a Si/Ge Quantum Materials workshop to establish the complete value chain from isotopically enriched Si/Ge material purification, single crystal growth as well as heteroepitaxy preparation by CVD and MBE up to manufacturing of Qubit structures, including materials and device characterization on a state-of-the-art level.
For this purpose, IKZ welcomed a group of world leading researchers from Russia (Prof. Petr G. Sennikov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Nizhny Novgorod; Prof. Andrey D. Bulanov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Nizhny Novgorod; Prof. Alexander A. Ezhevskii, Department of Physics of Semiconductors and Optoelectronics, Nizhny Novgorod Lobachevsky University), from Australia (Prof. Sven Rogge, Department of Condensed Matter Physics, University of New South Wales, Sydney), from Canada (Prof. Oussama Moutanabbir, Department of Engineering Physics, Polytechnique Montréal), from Italy (Prof. Giovanni Capellini, Department of Science, Roma Tre University) and from Germany (Dr. Lars Schreiber, Institute for Quantum Information, RWTH University, Aachen; Dr. Wolfgang Klesse, Leibniz-Institut für innovative Mikroelektronik (IHP), Frankfurt/Oder) to strengthen international links and define solid R & D concepts towards SiGe quantum devices.
IKZ will use its international guest scientist program to strengthen links with these leading research groups by mutual research visits and by initiating common, third party funded research projects.
For his research in the context of his doctoral thesis on the influence of growth conditions on the optical properties of strontium titanate (SrTiO3), Dr. Dirk Johannes Kok receives the Young Scientists Award of the Deutsche Gesellschaft für Kristallzüchtung und Kristallwachstum (DGKK).
Strontium titanate crystals are an important base material for superconducting layers (e.g. extremely sensitive magnetic field sensors) as well as for novel electronic applications which exploit the interesting properties of oxide layer structures. These include switchable resistors (memristors), interface conduction and non-volatile memories by controlled formation of ferroelectric domains. At the Leibniz-Institut für Kristallzüchtung (IKZ), a crystal growth technique was developed to produce strontium titanate crystals from the melt with very high structural quality. Dirk Kok supported this project with his doctoral thesis, in which he investigated the influence of growth conditions on optical properties. The results and models found and published by him show application-relevant correlations between the growth atmosphere, the stoichiometric deviation in the crystal and the associated change of the lattice parameter. His observations on the course of temperature-dependent thermal conductivity and band gap are applicable to many other complex oxide materials.
During his chemistry studies at Radboud University in Nijmegen, the Netherlands, Mr. Kok already became familiar with crystallization topics. The doctoral thesis was written from 2013 to 2017 at the Leibniz-Institut für Kristallzüchtung (IKZ) in the group Oxides and Fluorides under the direct supervision of Dr. Christo Guguschev, PD Dr. Detlef Klimm, Dr. Klaus Irmscher and Prof. Dr. Matthias Bickermann. After a successful defense at the Humboldt-Universität zu Berlin, Mr. Kok worked at the Helmholtz-Zentrum Berlin on the synthesis of tungsten bronzes (department of Priv.-Doz. Dr. Klaus Habicht). From May 2019 he will return to Radboud University Nijmegen for a postdoctoral position (group of Prof. Elias Vlieg).
With the DGKK Young Scientists Award, the association honors outstanding scientific achievements of young scientists in the field of crystal growth. The award is worth 2.500 € and the ceremony will take place on 20 March 2019 at the DGKK Crystal Grower Conference in Poznan (Poland). Dirk Kok shares the prize for 2019 with Dr. Pascal Puphal from PSI Villingen (Switzerland).
Gallium arsenide single crystal 4", grown
with the VGF method | Photo: IKZ
New platform for very high performance X-ray detectors:
G-ray Nanotech and IKZ have entered into a research and development collaboration covering the doping of Gallium Arsenide structures and the manufacturing of high purity crystals in wafer form factor for detector applications.
We are delighted to work with a world-leading institute in the field of material sciences, says Philippe Le Corre, CEO of G-ray Nanotech. The competencies of IKZ will allow us to accelerate significantly the expansion of our latenium™ detector architecture into medium-large energy X-rays applications as well as in the infra-red spectrum.
G-ray Industries SA, a Neuchâtel start-up, is currently developing ultra-high performance detectors dedicated to industrial non-destructive testing solutions. These ultra-high-performance detectors are developed in partnership with CSEM, based on G-ray’s revolutionary patented latenium™ technology.
The latenium™ Evaluation kits are available for evaluation purposes as of Q1-2019. In addition, the G-ray technologies - in particular the covalent bonding of a silicon wafer to a GaAs, Ge or Si wafer at low temperatures and the very fast epitaxial growth of Germanium structures - are being positioned in the fields of high-energy physics research for new particle detectors and in vision systems for the automotive industry.
We are pleased to start a long-term collaboration with G-ray Industries, says Prof. Thomas Schroeder, IKZ´s Scientific Director: IKZ is committed to push high performance crystalline materials to market applications and the state-of-the-art X-ray imaging detector development at G-Ray is a nice opportunity for us. We consider 3D heterointegration via bonding approaches as a fruitful strategy for us to innovate technologies by high quality, precisely tailored crystalline materials.
“With our expertise in materials science and technology we have supported the G-ray team right from the start. This is an outstanding opportunity to bring a ground-breaking X-ray detector technology to the market”, says Gian-Luca Bona, CEO of Empa, the Swiss Federal Laboratories for Material Science and Technology.
End of November brought exciting news from the Leibniz Association headquarters: the project SiGeQuant, led by the IKZ department “Layers & Nanostructures”, was granted in the frame of the Leibniz Competition and will receive the funding of 998 T€ over the next three years. Within this project, two Leibniz and two RWTH Aachen university institutions unite their expertise to investigate high-purity isotope-enriched Si and Si/Ge structures to fabricate devices for quantum electronics.
In recent years, quantum computing evolved from fundamental research to emerging technology and set to revolutionise fields like cryptography, weather forecasting, modelling of new materials or logistics. However, the upscaling of quantum devices, essential to perform practical large-scale parallel computations, is still challenging and impedes many developing technologies. A thin film of isotope-enriched 28Si buried between two SiGe layers provides a promising approach to design scalable spin qubits, as it improves the coherence properties of qubits and is compatible with the CMOS-technology. In this approach, the qubit hosting material properties play an essential role: crystal defects, impurities, the inclusion of 29Si isotopes, and low-quality interfaces jeopardies the qubit control. However, the project focusing on this fundamental part of quantum device production was missing until now.
Within the extern collaboration, the IKZ will provide its intern expertise for the growth of high-purity 28Si, for the development of an epitaxial growth process of high-purity, elastically strained, dislocation-free stacks of SiGe/28Si/SiGe by molecular beam epitaxy technique (MBE), and for investigation of electrical, optical, and structural properties of the grown films. The partners from the Leibniz-Institut für innovative Mikroelektronik (IHP) will develop complementary growth and characterisation techniques, while the Institut für Halbleitertechnik (IHT) and the Institut für Quanteninformation (IQI) of RWTH Aachen University will, in turn, fabricate and characterise quantum dot devices on the wafer scale, based on these structures.
“[…] Silicon quantum electronics, is one of the hottest scientific contests at the moment, but no other project exists with the scope and depth of the present one, in the specific (and challenging) aspect of material fabrication and characterisation. […] Starting up a project like this without the expertise and infrastructure already existing at IHP, IHT, IKZ and IQI would be prohibitively expensive or outright impossible. “ – describes the project one of the reviewers.
The SiGeQuant addresses both fundamental scientific and application-relevant questions, which will foster the involved German institutes to build new bridges with the international quantum information community.
Schematic image of a targeted Si/SiGe quantum well heterostructure with gate-defined quantum dots for qubit applications.
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