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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