"12/2018 IKZ-News: "SiGeQuant: Cooperative endeavour to advance silicon quantum 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.

 Schema

Schematic image of a targeted Si/SiGe quantum well heterostructure with gate-defined quantum dots for qubit applications.

Project leader: This email address is being protected from spambots. You need JavaScript enabled to view it..

 

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"11/2018 IKZ-News: "Single crystals highly enriched in the silicon-28 isotope were grown at IKZ and enabled the redefinition of the kilogram mass unit"

On the 16th of November 2018, the General Conference on Weights and Measures in Paris passed a new definition of the kilogram based on natural constants, which will enter into force on the 20th of May 2019. The Leibniz-Institut für Kristallzüchtung played a part in replacing the almost 140-year-old artificial object of the original kilogram, because perfect crystals of almost isotope-pure silicon-28 (Si-28) were of crucial importance for this project.

 

Si kg

 

In the background of the international Avogadro project, the IKZ has developed the growth of highly perfect and pure Si-28 crystals, including also the deposition of the polycrystalline feedstock.


In high-purity Si-28 crystals, almost all atoms have the same mass and are positioned in a regular three-dimensional lattice, which makes it possible to determine the exact relationship between the mass of the crystal and the number of its atoms. From this relation, the value of the Avogadro constant can be deduced with unprecedented precision and therefore used as a fundamental natural constant to define the kilogram.


Within the "KILOGRAMM" projects led by the Physikalisch-Technische Bundesanstalt in Braunschweig (PTB), the Si-28 crystals from the IKZ were used to prepare several very precise spheres with less than 20nm shape deviations at a diameter of approximately 94mm and with a surface polished free of defects. Under these conditions, the PTB was able to determine the number of Si-28 atoms resulting in a crystal sphere of 1 kg total mass with the required uncertainty of less than 2 x 10-8.


It amounts to:         2,152538397 x 1025 Atoms silicon-28


Crystalline silicon is usually available as a mixture of the stable isotopes of mass numbers 28, 29 and 30. In Russia, the technology of isotope enrichment with ultracentrifuges for civil applications is fortunately available. This technology  was  further developed to achieve record values of enrichment of more than 99.999% Si-28.


To provide the required purity of the crystals grown from this material, further material-intensive melting zone cleaning steps were necessary in addition. The specific challenges were therefore the approx. 1000 times higher material price compared to conventional silicon and the limited amount of the available material.


Silicon is a semiconductor material that has been widely studied over many decades and provides today the basis for microelectronics and almost all daily life communication technologies. IKZ will be dedicated also in future to further improve its material quality to enable highly demanding novel applications like artificial intelligence and quantum technologies.

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"10/2018 IKZ-News:
"Long-term and successful cooperation with Kistler Instrumente AG to be extended again"

For 25 years, the IKZ has been working closely with Kistler Instrumente AG in the field of piezoelectric crystals and development of growth processes. Now the extension of the contract beyond 2019 was discussed at an early stage.

The appointment of a new IKZ director in February 2018 gave Kistler the opportunity to visit the IKZ. Prof. Dr. Thomas Schröder was delighted to welcome the long-standing cooperation partner of the IKZ working group Oxides and Fluorides in order to jointly talk about their future cooperation.

In the future, again a particular focus will be on research into volume crystal growth on high temperature piezoelectric crystals for pressure, force and acceleration sensor products. These are mainly used in industrial process control and automotive R&D. In addition, research will continue in the field of crystal machining as soon as the IKZ has scaled up its capabilities by the planned application science program on crystals for electronics and photonics. Both partners agreed on this.

"For Kistler, this cooperation is very important, since piezoelectric crystals form the heart of the sensors. In the progress of the cooperation, numerous new crystal compounds were grown and evaluated." (Dr. Claudio Cavalloni, Firma Kistler Instrumente AG).

With Kistler, the IKZ has a very valuable partner at its side who is a world leader in the field of sensory measurement technology. The personal and efficient cooperation, which has grown over many years, is particularly appreciated.

 

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"10/2018 IKZ-News: "Family-friendly employer: Leibniz-Institut für Kristallzüchtung receives again the certificate for the audit berufundfamilie (work and family audit)"

On September 30, 2018, the Leibniz-Institut für Kristallzüchtung (IKZ) was awarded the berufundfamilie audit certificate for further three years. The certificate is awarded to the institute for its commitment in the area of strategically oriented family and life-phase conscious personnel policy.

Logo audit beruf familie A4

The prerequisite for certification is the successful completion of the auditing process offered by berufundfamilie Service GmbH, which initiates and pursues a systematic process of operational compatibility. In the auditing process, the existing instruments to support the compatibility of work, family and private life were evaluated and further company-specific measures agreed.

 

The renewed certification shows that the IKZ is continuously working on a family-friendly personnel policy. The Institute is dedicated to improve the framework conditions for its employees and to provide them with instruments with which family/private life and career can be better reconciled. This includes, for example, flexible options for the organisation of 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 in child care.

 

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"09/2018 IKZ-News: "IKZ Summer School 2018"

From 24 to 26 September 2018, the IKZ invited to a summer school with the topic "Solid-State Lasers". Half of the more than 50 participants were junior researchers and students from external institutions, some even from abroad - a proof for the topicality of this research topic, which was just established at the IKZ last year.

During the three-day workshop, lecturer Prof. Dr. Günter Huber from the University of Hamburg dealt with fundamental aspects of laser physics and the growth of rare earth-doped laser crystals. In the further course these basics were deepened by many concrete examples and intensively discussed with the very active audience. The program was complemented by laboratory tours at the IKZ and the MBI as well as an evening, organized by IKZ PhD students.

 

In addition to our thanks to the lecturer Prof. Dr. Günter Huber, we would also like to thank all those involved in the organization of the summer school, in particular the colleagues from the MBI for the interesting laboratory tour there.

 

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"10/2018 IKZ-News: "Marthe Vogt Prize 2018 awarded to IKZ scientist Dorothee Braun"

The Forschungsverbund Berlin e.V. (FVB) awards Ms. Dorothee Braun the Marthe Vogt Prize for her dissertation in the field of the development of ferroelectric materials, which was completed with honors. For the first time, the FVB Prize, which has been awarded since 2001, goes to a researcher from the Leibniz-Institut für Kristallzüchtung.

Logo MartheVogt farbe

In her doctoral thesis, Ms. Braun investigated materials with ferro- and piezoelectric properties, such as those used for electronic applications, e. g. for pressure sensors, non-volatile memory devices or ultrasonic devices. The materials currently used pose a potential health risk due to their lead content. Therefore, lead-free materials with comparable or novel properties must be found. For this purpose, she investigated in particular thin crystalline potassium-sodium-niobate layers and examined the relationship between the crystalline structure and the ferroelectric order. Dorothee Braun's doctoral thesis laid the foundation for further, targeted research into the structure-property relationship of oxide layers, especially with perovskite structures. The experimental and theoretical work was carried out at the working group Ferroelectric Oxide Layers at the IKZ, under the supervision of Dr. Jutta Schwarzkopf.

 

"With Dr. Dorothee Braun, the Forschungsverbund Berlin e.V. honors an outstanding scientist who has masterfully worked on a very challenging topic in her doctorate.” (Forschungsverbund Berlin e.V.)

 

The coveted award was officially presented to Ms. Braun on November 8, 2018 at the Haus der Leibniz-Gemeinschaft in Berlin.

 

Read the complete press release of the Forschungsverbund Berlin e.V.:
Spannende Kristalle: Dorothee Braun erhält Marthe-Vogt-Preis

 

Further information on the Marthe Vogt Prize: 
http://www.fv-berlin.de/nachwuchs/nachwuchswissenschaftlerinnen-preis-1

 

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"09/2018 IKZ-News: "IKZ Summer School 2018"

From 24 to 26 September 2018, the IKZ invited to a summer school with the topic "Solid-State Lasers". Half of the more than 50 participants were junior researchers and students from external institutions, some even from abroad - a proof for the topicality of this research topic, which was just established at the IKZ last year.

During the three-day workshop, lecturer Prof. Dr. Günter Huber from the University of Hamburg dealt with fundamental aspects of laser physics and the growth of rare earth-doped laser crystals. In the further course these basics were deepened by many concrete examples and intensively discussed with the very active audience. The program was complemented by laboratory tours at the IKZ and the MBI as well as an evening, organized by IKZ PhD students.

 

In addition to our thanks to the lecturer Prof. Dr. Günter Huber, we would also like to thank all those involved in the organization of the summer school, in particular the colleagues from the MBI for the interesting laboratory tour there.

 

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"09/2018 IKZ-News: "IKZ successfully no less than 3 times"

Last Thursday, September 6th, 3 teams competed for the Leibniz-Institut für Kristallzüchtung at the 6th Adlershof Company Relay in good weather and a great mood.

 

On a total length of 8.7 km, teams of 3 competed against other companies located in Adlershof. Each runner had to overcome a distance of 2.9 km until he could hand over the baton to his/her team mate or had reached the finish line. Altogether 209 teams fought for a good race time at the running event organized by Gesundheitssport Berlin-Brandenburg e.V., whereby the IKZ could successfully reach places 25, 29 and 98.

 

We congratulate the participating teams on these great results!

 

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"09/2018 IKZ-News: IKZ undertakes the initiative to develop deep-UV light emitting diodes"

The new joint project funded by the BMBF consortium “Advanced UV for Life” started on 1st of September under the coordination of IKZ. Over the next three years, IKZ group “Aluminium Nitride” together with the partners will build up and develop the value-added chain to facilitate commercial production of UV light emitting diodes (LEDs) with ultra-short wavelength of 230 nm on AlN substrates. Such components enable applications in gas and biochemical sensors, medical and environmental technology, water, surface, and air disinfection, but they are not yet available on the market.

 

The development UV LED devices with the wavelength in the range of 265-310 nm comprised the center of activities in the first stage of the “Advanced UV for Life” project. Thanks to the successfully implemented technologies of AlGaN layers epitaxy on sapphire substrates, such components are about to become available on the market. In contrast, the devices with the wavelength below 245 nm require AlGaN layers with Al content above 80%. For such layers, sapphire substrates are no longer suitable, and native AlN substrates with low defect density should be applied.

 

Being the first link in the value-added chain, the “Aluminium Nitride” team of the IKZ develops the core technology for the production of AlN substrates using physical vapor transport (PVT) technique. The current project requires AlN substrates with a specified, low density of defects and industrially relevant substrate diameter of at least 25 mm. Besides, the substrates have to be optically transparent at the application wavelength to provide efficient extraction of photons generated in the active layers. For the projected industrial production, it is also crucial to increase reproducibility and yield of AlN crystal growth.

 

Apart from the production of low-defect AlN crystals, the joint work plan of IKZ and its partners – the Ferdinand-Braun-Institut (FBH), the group “Nanophotonics” of Prof. Kneissl at the Technical University of Berlin, and the companies Freiberger Compound Materials and CrysTec – includes the preparation of epi-ready wafers, the epitaxy of pseudomorphically strained layer systems, and the development of specific device technology (see the figure).

AlN230 Eng

Added-value chain in the joint project „AlN-230nm“. (Photo: IKZ; C. Kuhn (TU Berlin))

 

For further information, please contact This email address is being protected from spambots. You need JavaScript enabled to view it..

 

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"09/2018 IKZ-News: Symposium "Epitaxial oxide films for electronic applications" on the EMRS Fall Meeting 2018 in Warsaw "

From Sep 16-20, the EMRS symposium hosts an exciting program of 34 invited, 29 contributed and 16 poster presentations about state-of-the-art functional oxide electronics research. The symposium is organized by IKZ department head Matthias Bickermann (IKZ) together with Pavlo Zubko (London), Debdeep Jena (Cornell, USA) and Ulrike Diebold (Vienna).

 

The symposium topics range from growth, characterization and devices of gallium oxide semiconductor, conducting oxide interfaces and thin films, perovskite and transition metal oxide heterostructures, ferroelectrics, to defects and resistive switching. The main challenge addressed is to transfer elements of the control and perfection customary in semiconductor epitaxy to the production of functional oxide layers.

 

This is the first time a meeting is dedicated to such a combination of topics. The international attendance and renowned speakers illustrate the topicality of this kind of research. IKZ contributes with nine presentations from the teams "Ferroelectric Oxide Layers", "Semiconducting Oxide Layers", "Physical Characterization", "Electron Microscopy" and "Oxides & Fluorides". Also, our partners from the Leibniz ScienceCampus "GraFOx" are heavily involved in the symposium.

 

Full details can be found at the Symposium homepage:
https://www.european-mrs.com/epitaxial-oxide-films-electronic-applications-emrs.
GraFOx: http://grafox-pdi-berlin.de

 

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"07/2018 IKZ-News: Unveiling the mechanism of strain relaxation in III-group nitride films"

Researchers from the Electron Microscopy group resolved the fundamental ambiguity behind the formation of misfit dislocations in strained c-plane wurtzite layers. The new findings, published in the Journal of Applied Physics, will help to optimise, in particular, the growth of AlGaN/GaN heterostructures for the deep-UV emitters and achieve either films with low defect density or, on the contrary, to intentionally promote plastic relaxation in such structures for strain-engineering purposes.

 

The scientists from the Leibniz Institute for Crystal Growth considered the crucial step of misfit dislocation nucleation and investigated the influence of different growth modes on the strain relaxation process. Their work provides for the first time a reliable quantitative model for the plastic relaxation process of strained c-plane wurtzite films and allows to accurately predict the critical thickness as dependent on the actual surface morphology.

 

Whether to relax the strain between two layers caused by the lattice mismatch or to grow pseudomorphic defect-free heterojunctions, it is critical to understand under which conditions and how misfit dislocations form in the material. Previous quantitative studies of plastic relaxation in wurtzite films adopted the classical models deduced from cubic materials (InGaAs on GaAs, SiGe on Si). In these works, the authors only considered the energy balance aspect, i.e. the moment when plastic relaxation becomes energetically favourable, while completely neglecting the nucleation process of misfit dislocations itself. However, for wurtzite materials, it is particularly important to consider this process, due to the limitation of how dislocations can move within the wurtzite crystal lattice. As an outcome, a significant discrepancy between theoretical predictions and experimental observations of plastic relaxation in III-nitride thin films has existed for many years.

 

The current study combines thorough experimental analysis of dislocation formation in AlGaN/GaN heterostructures at each growth step by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) technics with theoretical calculations of the stress distribution at the dislocation nucleation sites obtained from the finite element method.

 

 “We have found, that for c-plane wurtzite films only the presence of three-dimensional surface structures – islands’ edges and corners, crack fronts, surface macro-steps or V-pits – and the high concentration of the shear stress at these sites enables the nucleation and spread of misfit dislocations in the interfacial plane. This provides an efficient pathway for the plastic relaxation.” – says Dr. Toni Markurt, the author of the work and a researcher from the Leibniz Institute for Crystal Growth.


More important, according to the authors, the critical thickness of the films strongly depends on the surface morphology and the actual geometry of surface structures: for example, two-dimensional layers with cracks can be grown thicker than films with three-dimensional islands, before undergoing plastic relaxation.

 

The work was done in a close collaboration with OSRAM Opto Semiconductors, who provided dedicated samples for the experimental study.

 

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"06/2018 IKZ-News: One summer night in a crystal world"

On the 9th of June, the IKZ again opened the doors for the visitors of the Long Night of Sciences – an annual event that aims to increase the awareness about science and technology.

Despite the tropical heat in Berlin, 1452 visitors came on Saturday evening to the institute to learn about crystal growth, modern crystals and their application in technology. 

 

By combining the laser and crystal growth demonstrations with the tours through the growth halls and scientific lectures, we managed to keep the balance between the “wow”-effect and more serious food for thought for the inquisitive guests.

 

 

 

 

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"06/2018 IKZ-News: The world's most efficient direct emitting yellow solid-state laser"

Researchers of the Center for Laser Materials (ZLM) at the IKZ obtained a direct yellow emitting solid-state laser with record efficiency based on a fluoride crystal doped with terbium ions (Tb3+). Since the emission wavelength of this Tb3+-laser is very close to the sodium (Na) absorption D-lines at 589 nm, it may find applications in fields that require sodium detection or excitation such as astronomical optics (e.g., laser guide stars) or microscopy.

 

Direct laser emission in the visible spectral range is often not easy to achieve. Green laser pointers, for example, rely on a complicated nonlinear mechanism converting infrared (1064 nm) to green light (532 nm), which limits the total device efficiency and the battery lifetime. Other visible lasers are based on organic dyes, which are highly impractical gain media due to their liquid and often toxic nature.

 

Therefore one of the ZLM research topics is the investigation of rare-earth doped solid-state crystals for lasers directly emitting within the visible range of the electromagnetic spectrum.

 

In the past, there has been tremendous progress in red, orange and green emitting solid-state lasers with trivalent praseodymium (Pr3+) as an active ion. However, this ion is not suitable for generating emission in the yellow spectral range. Therefore, recently Tb3+-doped fluoride crystals were suggested as laser active materials.


For a long time, terbium was not considered as a good candidate for laser operation due to its low capability to absorb and emit light and a high risk of detrimental parasitic absorption, intrinsic to this ion. In their recent work, ZLM researchers successfully circumvented these limitations by using fluorides as host crystals and significantly increasing the concentration of Tb3+-ions in these hosts.


This approach led to the world’s most efficient direct yellow emitting solid-state laser, pumped by a blue semiconductor laser. A 28% Tb3+-doped lithium lutetium fluoride crystal (Tb:LLF) enabled an output power of 0.5 W at a laser wavelength of 588 nm with an efficiency as high as 25%. The power of the available blue pump light currently limits the output power of the laser, but the prospects for future improvement are good since LLF is a well-established host crystal known to withstand high powers when doped with other laser ions.

 

For further information, please contact This email address is being protected from spambots. You need JavaScript enabled to view it.

 

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"05/2018 IKZ-News: CHEETAH finishes the race: towards the future of photovoltaics through European collaboration"

In March 2018, the department "Layers and Nanostructures" of the Leibniz-Institute for Crystal Growth (IKZ) successfully completed the EU project CHEETAH. For more than four years this joint scientific project, created within the 7th European Framework Program, supported the development of new photovoltaic technology at every stage of the value chain. The goal was to develop a new process, which will reduce the costs by saving material and increase energy conversion efficiency, in comparison with the existing multi- and polycrystalline silicon-based technology. The project brought together the expertise of 33 member institutions of the European Energy Research Alliance (EERA), including the IKZ.

Firstly, the IKZ department has shown the feasibility of crystalline Si layers grown on thin films of reorganized porous silicon and glass in application to solar cells (fig. 1), which have potential advantages over the conventional Si wafers. In a second work package, they have developed a method to grow insular Cu(InxGa1-x)Se2 (CIGSe) micro-crystals (about 50 µm in size) at defined locations on a glass substrate. These structures are the basis for the development of cost-effective CIGSe micro-concentrator solar cells.

 

In addition to research activities, the promotion of young European researchers and the establishment of a network for long-term cooperation in the field played a significant role at CHEETAH. The project financed two postdoctoral positions at the IKZ and provided five young scientists the opportunity to make short research stays and share their results at workshops and conferences in the partner institutions, as well as in the USA, Singapore, and Japan.

The direct research partners of the IKZ for the development of thin-film silicon solar cells were IMEC (Belgium), INES (France), SINTEF (Norway), ECN (the Netherlands) and ISE (Germany). In the research of CIGSe micro-concentrator solar cells, the institute cooperated with ENEA (Italy), University of Estonia, INL (Portugal) as well as with Helmholtz-Zentrum and Federal Institute for Materials Research and Testing (BAM) in Berlin, Germany.

For further information, please contact This email address is being protected from spambots. You need JavaScript enabled to view it.

 

fig2 solarcell klein

Fig 1.: Growth principle of silicon epitaxial layer on reorganized porous Si



 

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"04/18 IKZ-News: Advanced carrier lifetime measurements in silicon and germanium crystals"

Lateral Photovoltage Scanning (LPS) and Scanning Photoluminescence (SPL) are among the key methods used worldwide and in IKZ to visualise electrically active crystalline defects in silicon (Si) or germanium (Ge) samples. The recent upgrade of these systems allows to simultaneously analyse the distribution of both resistivity gradients and structural defects in the crystals.

 

The latter is well known to reduce charge carrier lifetime in semiconductors – the decisive characteristic of wafers used later on for photovoltaic solar cells and other micro- and nano-electronic devices.

 

LPS and SPL methods were created and established at IKZ several years ago, and since then our researchers further develop and routinely use these methods to image various defects in Si and Ge crystals, such as grain boundaries, dislocations or growth-induced doping inhomogeneities (striations). The new detection and evaluation mode extends the functionality of these measurement techniques and allows to map the charge carrier lifetimes while scanning across the sample.

 

The possibility to visualise defects, resistivity inhomogeneities, and carrier lifetime distribution at the same time on the same sample area is the main advantage of the upgraded combined LPS & SPL system. It allows avoiding uncertainties in data interpretation, which might happen, e.g., if the measurements are carried out separately on different stations. Moreover, the new system is equipped with two solid-state lasers with a variable laser power from 1 µW to 100 mW, and the laser beam can be focused down to 5 µm, providing the flexibility of the experiment and precise localisation of recombination centres on the image.

 

Structural and deep-level impurity defects are among the most important causes of charge carrier recombination events in crystals. When an excited electron-hole pair is trapped by such a defect, it may eliminate – i.e., recombine – and release energy, either thermal or in the form of photons, instead of contributing to the desired (opto)-electronic performance of the device. The longer the charge carriers can stay around before recombining – the longer is their “lifetime” and the better the resulting device performance. Generally, the recombination centres have a detrimental effect on the quality of the wafers used in photovoltaics or electronics. Thus the possibility to visualize the recombination centres and to understand their origin is a significant part of the crystal growth research and development.

 

The LPS / SPL research work is carried out at IKZ in close collaboration with the company LPCon: https://www.lpcon.com/

 


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"03/18 IKZ-News: A new way for growing perovskite crystals"

In the case of the perovskite prototype CaTiO3, crystal growth using a melt solution is the method of choice. At the IKZ, a mixture of calcium fluoride (CaF2) and titanium (IV) oxide (TiO2) was identified as an advantageous solvent for these crystals.

 

Calcium titanate (CaTiO3) occurs in the nature as a mineral. However, the direct growth from the melt is impossible since phase transition at 1625 K leads to strong formation of twin defects and thus to damage of the crystals. This problem can be avoided by adding so-called melt solution. This is the substance with a relatively low melting point, which is dissolved in the crystallising substance in liquid phase. During cooling down, the melt solvent is excreting the crystallising component out again.

 

However, the search for suitable melt solvents is often an almost alchemical procedure. For CaTiO3, potassium fluoride and lead (II) fluoride have been described in the literature. However, both have the disadvantage that only very small portions of CaTiO3 (about 1:12) can be dissolved in them. Furthermore, undesired chemical reactions between the solvent and CaTiO3 lead to contamination of the solvent.

As a part of a master's thesis at the IKZ, a mixture of the lead-free substances calcium fluoride (CaF2) and titanium (IV)-oxide (TiO2) was identified to be a more advantageous solvent for CaTiO3. From a mixture of these substances with the molar ratio 3:1:1 (green dot on the figure) CaTiO3 crystallises below the critical phase transformation, resulting in still small (approx. 2.5 mm edge lengths), but high quality crystals. The results were obtained on the basis of extensive thermoanalytical measurements and a thermodynamic model of the ternary phase diagram based on them. The proportion of dissolved CaTiO3 has been improved to 1:4, no undesirable chemical reactions have been detected and thus no contamination is present.

Although pure CaTiO3 has little technical relevance, the knowledge and understanding of its properties is of fundamental importance. A number of important ferroelectric materials (e.g. barium titanate, (potassium, sodium) niobate etc.) and other functional materials, such as substrates for oxide electronics (e. g. strontium titanate, rare earth scandates etc.) crystallize in the perovskite or related crystal structures. Therefore, the supply of CaTiO3 high quality single crystals as suitable model systems to answer fundamental questions is essential for basic investigations.


The article is published in the Journal of Crystal Growth.
https://doi.org/10.1016/j.jcrysgro.2018.01.025


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"01/18 IKZ-News: Fundamental limitation in the key material for solid-state lighting revealed"

International researchers have revealed the mechanism that limits the indium (In) content in indium gallium nitride ((In, Ga)N) thin films - the key material for blue light emitting diodes (LED).

 

Increasing the In content in InGaN quantum wells is the common approach to shift the emission of III-Nitride based LEDs towards the green and, in particular, red part of the optical spectrum, necessary for the modern RGB devices. The new findings answer the long-standing research question: why does this classical approach fail, when we try to obtain efficient InGaN-based green and red LEDs?

Despite the progress in the field of green LEDs and lasers, the researchers could not overcome the limit of 30% of indium content in the films. The reason for that was unclear up to now: is it a problem of finding the right growth conditions or rather a fundamental effect that cannot be overcome? Now, an international team from Germany, Poland and China has shed new light on this question and revealed the mechanism responsible for that limitation.

In their work the scientists tried to push the indium content to the limit by growing single atomic layers of InN on GaN. However, independent on growth conditions, indium concentrations have never exceeded 25% - 30% – a clear sign of a fundamentally limiting mechanism. The researchers used advanced characterization methods, such as atomic resolution transmission electron microscope (TEM) and in-situ reflection high-energy electron diffraction (RHEED), and discovered that, as soon as the indium content reaches around 25 %, the atoms within the (In, Ga)N monolayer arrange in a regular pattern – single atomic column of In alternates with two atomic columns of Ga atoms. Comprehensive theoretical calculations revealed that the atomic ordering is induced by a particular surface reconstruction: indium atoms are bonded with four neighboring atoms, instead of expected three. This creates stronger bonds between indium and nitrogen atoms, which, on one hand, allows to use higher temperatures during the growth and provides material with better quality. On the other hand, the ordering sets the limit of the In content of 25%, which cannot be overcome under realistic growth conditions.

The work is a result of a collaboration between Leibniz-Institut für Kristallzüchtung (Berlin, Germany), Max-Planck-Institut für Eisenforschung (Düsseldorf, Germany), Paul-Drude Institut für Festkörperelektronik (Berlin, Germany), Institute of High-Pressure Physics (Warsaw, Poland), and State Key Laboratory of Artificial Microstructure and Mesoscopic Physics (Beijing, China).


Read the full press release.

The article is published in:
https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.2.011601


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