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Group Activities

Most Recent Highlights


Forming-Free Grain Boundary Engineered Hafnium Oxide Resistive Random Access Memory Devices

S. Petzold, A. Zintler, R. Eilhardt, E. Piros, N. Kaiser, S. U. Sharath, T. Vogel, M. Major, K. P. McKenna, L. Molina‐Luna, L. Alff

Advanced Electronic Materials, early access (2019)
doi: 10.1002/aelm.201900484

Within the project of WAKeMeUP and in close collaboration with the Advanced Electron Microscopy division of Dr. Molina-Luna, a new method of directed defect engineering has been developed, namely grain boundary engineering.

With this new method it is possible to create low energy grain boundaries of high symmetry which thread the whole dielectric layer, thus, providing a defined path for facilitated filament formation. This method renders forming free devices with a narrow distribution of forming voltages which makes it a powerful tool to overcome one of the main challenges for resistive switching devices: device-to-device variation. An exemplary transmission electron microscopy image and further analysis of the engineered grain boundaries is depicted in the Figure.

Invited talk at ISAF 2019 in Lausanne

ATFT doctoral student Juliette Cardoletti gave an invited talk at the International Symposium on Applications of Ferroelectrics (ISAF), sponsored by the IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society (UFFC-S). Juliette’s 30 minutes invited talk was about deposition of ferroelectric thin films on stainless steel for actuator applications. After a research stay at Penn State University in the group of Susan Trolier-McKinstry (supported by IDS-FunMat-INNO), Juliette worked within an interdisciplinary DFG funded project (AL 560/19), in collaboration with electrical engineers, on direct deposition of functional ferroelectrics on common stainless steel, including modelling of the device performance of a bending tongue (AIP Adv. 9, 025017 (2019), doi: 10.1063/1.5082392). Juliette has shown that international and interdisciplinary collaboration can be the base for outstanding success already at an early stage of career.

Interviews with Lambert Alff and Leo Molina-Luna on the occasion of the E-MRS 2019

Interviews with Lambert Alff and Leo Molina from E-MRS-TV are now online.

Analysis and simulation of the multiple resistive switching modes occurring in HfOₓ-based resistive random access memories using memdiodes

S. Petzold, E. Miranda, S. U. Sharath,, J. Muñoz-Gorriz, T. Vogel E. Piros, N. Kaiser, R. Eilhardt, A. Zintler, L. Molina-Luna, J. Suñé, and L. Alff

J. Appl. Phys. 125, 234503 (2019)
doi: 10.1063/1.5094864

Within the project of WAKeMeUP and in cooperation with Prof. Miranda, a simulation of all observed switching modes reported for hafnium oxide based resistive switching devices has been achieved.
Comparing stoichiometric vs. highly deficient hafnia devices by compact simulation, we found that a single or two memdiodes connected in anti-series can be utilized to simulate all reported switching modes. We found that all switching modes of the stoichiometric device could be simulated utilizing only one memdiode, while for the deficient device a second memdiode in opposite polarity was necessary to fit its switching modes. By correlating the different interfaces to the two memdiodes taken for compact simulation, we were able to gain additional information of the switching process, as e.g. the voltage drop at the individual interfaces.

Tunable capacitors based on all-oxide technology supported by the Pioneer Fund

The „ALOVA“ Team in the Lab, next to the PLD. Photo: Katrin Binner
The „ALOVA“ Team in the Lab, next to the PLD. Photo: Katrin Binner

We received supporting by the Pioneer Fund program for our project ALOVA. Our two PhD students Patrick Salg and Lukas Zeinar are working within an interdisciplinary project together with Prof. Jakoby’s group of Mikrowellentechnik on tunable capacitors (varactors) based on SrMoO3 bottom electrodes. This approach allows an epitaxial growth of all-oxide heterostructures. This leads to lower defect densities and thus allowing lower tuning voltage extending battery lifetime. This technology could replace expensive platinum based applications in mobile 5G communication.
See Latest News (german) on the TU-Darmstadt homepage.

15th Meeting of the working group materials for nonvolatile memories of the GMM – VDE/VDI at TU Darmstadt

The working group materials for non-volatile memories headed by Thomas Mikolajick deals with current research topics in the field of highly integrated semiconductor memories and future memory cell concepts. One important aspect is the investigation of emerging memories such as phase change, resistive switching and ferroelectric memories. The group covers developments from materials, to devices and systems. From TU Darmstadt ATFT and AEM are jointly involved in the research of HfO2 based memory cells from structure to function.

E-MRS Spring meeting in Nice (now interview online)

The European Materials Research Society (E-MRS) has held its annual spring meeting in the congress centre Acropolis (see foto) in Nice at the Côte d’Azur. One of the chair persons of the conference was Anke Weidenkaff from TU Darmstadt and Fraunhofer IWKS. ATFT has presented the latest results in Symposium O about nanoscale multi functional oxides. We highlighted SrMoO3 as highly-conducting bottom electrode material replacing expensive platinum in applications for mobile communication (5G).

Editor’s pick in APL Materials: Our latest article in oxide electronics

Our article on an all-oxide device based on our special material SrMoO3 which has a higher conductivity than platinum has been chosen editor’s pick by the renowned journal APL Materials. In the paper with the title “Atomically interface engineered micrometer-thick SrMoO3 oxide electrodes for thin-film BaxSr1-xTiO3 ferroelectric varactors tunable at low voltages”, a joint work between ATFT, AEM and IMP we explain the growth (see individual atoms in the figure) of this extraordinary material in devices for application in smart antenna in mobile communication. We congratulate Patrick Salg, Dominik Walk and Lukas Zeinar to their successful work!

P. Salg, D. Walk, L. Zeinar, A. Radetinac, L. Molina-Luna, A. Zintler, R. Jakoby, H. Maune, P. Komissinskiy, and L. Alff
APL Materials 7, 051107 (2019)
doi: 10.1063/1.5094855

Prize for outstanding dissertation 2018 for Dr. Sharath

Dr. Sharath (2nd one from the left in the back row) and the awardees 2019 with the chairmen of Freunde der TU-Darmstadt Matthias W. Send. Photo: Felipe Fernandes
Dr. Sharath (2nd one from the left in the back row) and the awardees 2019 with the chairmen of Freunde der TU-Darmstadt Matthias W. Send. Photo: Felipe Fernandes

We are proud to announce that Dr. Sharath Ulhas has received a prize for outstanding scientific achievements within the best dissertation of the Materials Science department in 2018. This prize is part of a new funding concept of the association “Freunde der TU Darmstadt” that took effect for the first time in 2014.

Dr. Sharath graduated about novel non-volatile memory for energy-efficient data storage (see Adv. Funct. Mater. 27, 1700432 (2017), doi: 10.1002/adfm.201700432, Video Abstract).
He is now working at Micron Technology, a world-leading computer memory company. Dr. Sharath said: “My thesis got me my job.” Congratulations!

Gradual Reset and Set Characteristics in Yttrium Oxide based Resistive Random Access Memory

S. Petzold, E. Piros, S. U. Sharath, A. Zintler, E. Hildebrandt, L. Molina-Luna, C. Wenger, and L. Alff

Semicond. Sci. Technol., 34, 075008 (2019)
doi: 10.1088/1361-6641/ab220f

Within the project of WAKeMeUP (https://www.ecsel.eu/projects/wakemeup), funded by BMBF and ECSEL JU, we investigated the resistive switching behavior of yttrium oxide based devices.

In this study, we report for the first time the coexistence of bipolar resistive switching and unipolar resistive switching within one stack combination based on yttrium oxide. The BRS is characterized by low variability, good data retention, high DC endurance, and average on-off-ratio above 100.

The obtained concurrent gradual transitions during BRS set and reset which have been employed in pulse depression and potentiation operations (see figure) with shown plasticity over about two orders of magnitude make RRAM based on yttrium oxide an interesting alternative candidate for neuromorphic synapses. Combined with its characteristic of being forming-free and having low operation voltages, RRAM based on yttrium oxide is highlighted as a suitable material candidate for new high density nonvolatile memory technologies.

Heavy Ion Radiation Effects on Hafnium Oxide based Resistive Random Access Memory

S. Petzold, S.U. Sharath, J. Lemke, E. Hildebrandt, C. Trautmann, and L. Alff

IEEE Trans. Nucl. Sci., Early Access (2019)
doi: 10.1109/TNS.2019.2908637

Within the project of WAKeMeUP, funded by BMBF and ECSEL JU, we investigated the effect of ionizing radiation on resistive switching devices based on hafnium oxide.

Devices were exposed to high energetic heavy ion irradiation with Energies of 5.6 MeV per nucleus resulting in an overall Ion energy of 1.1 GeV. The effect of heavy ion irradiation on different electrical statistics of RRAM devices was investigated showing extraordinary data retention towards ionizing radiation. The written information was found to be stable up to very high total ionization doses of up to 880 Mrad. As a result, RRAM devices based on hafnium oxide are a very interesting candidate for application in high-dose environments, as for example in satellites or other space applications.

Modelling of the vertical deflection of ferroelectric bending tongues loaded at their free end

Juliette Cardoletti, Aldin Radetinac, Daniel Thiem, Julian Walker, Philipp Komissinskiy, Bai-Xiang Xu, Helmut Schlaak, Susan Trolier-McKinstry, and Lambert Alff

AIP Advances 9, 025017 (2019)
doi: 10.1063/1.5082392

Ferroelectric bending tongues loaded at their free end have been modelled to calculate their vertical deflection. The model relies on a ferroelectric switching criterion and on the Euler-Bernoulli beam theory. It takes into account the geometry of the bending tongue, the mechanical and piezoelectric material properties, the applied electrical field, the crystallographic state of the ferroelectric thin film and its built-in strain. The developed model differentiate the different contributions to the bending tongue vertical deflection. The results are characteristic of a ferroelectric with the modelled butterfly loop incorporating strain saturation at high electrical field. It allows accurate estimates of the vertical deflection for ferroelectric bending tongue based applications and show the importance of the ferroelectric thin film crystallographic orientation.

Electrochemical Performance of All-Solid-State Sodium-Ion Model Cells with Crystalline NaₓCoO₂ Thin-Film Cathodes

All-solid-state sodium batteries are attractive due to the abundance of sodium and advantageous for safe battery operation by avoiding flammable organics and liquids and suppressed dendrite formation. Herein, we present a NaxCoO2/Nasicon/Na thin-film model sodium solid-state battery using a Sc-substituted Nasicon solid electrolyte. Excellent chemical stability of this solid electrolyte at high voltages of up to 4.2 V increases the accessible sodium (de)intercalation range and battery capacity. Direct extraction of the interface resistances between the electrode materials of the thin-film model cell using electrochemical impedance spectroscopy gives a unique opportunity of correlation the electrochemical performance with properties of electrode materials and their interfaces.

doi: 10.1016/j.jpowsour.2018.10.089
doi: 10.1149/2.0491903jes

Magnetic attraction

The mixed German-Indian team of ATFT @ JEMS.
The mixed German-Indian team of ATFT @ JEMS.

The 9th Joint European Magnetic Symposia was organized by our RMU-colleagues from University of Mainz. Several members of the ATFT group (Berthold Rimmler, Jasna Palakkal, Shalini Sharma, Thorsten Schneider, Supratik Dasgupta) presented their recent work on magnetism related topics and informed themselves about the research in the field. ATFT always supports young researchers from the Master stage to the PhD and postdoc phase to attend international scientific conferences to boost their career possibilities and come into contact with world-wide leading researchers in the field.

Oxide electronics on top


The 25th International Workshop on Oxide Electronics took place in the mountains of Switzerland. During the interdisciplinary workshop the state-of-the-art of mostly epitaxial functional oxide materials and their potential for electronic application are discussed. The ATFT group was represented by Patrick Salg, Lukas Zeinar and Lambert Alff. Former group member Vikas Shabadi now working for his own technology company also joined the foto. Our contributions to the conference were focusing on the perovskite SrMoO3 as outstanding electrode material for high-frequency devices.

Doctoral graduation in the ATFT group

We congratulate our colleague Sharath S. Ulhas to his outstanding doctoral defense (January 31,2018). The title of his PhD thesis is Defect engineering in HfO2/TiN-based resistive random access memory (RRAM) devices by reactive molecular beam epitaxy. As external examiners we had Thomas Schroeder (IKZ Berlin) and Klaus Hofmann (Electrical Engineering department of TU Darmstadt) as our guests. The defense was followed by a social event with food catering in the presence of Dr. Sharath’s parents and wife which traveled all the way from India and US. We are extremely thankful to Sharath for his remarkable advances in science and for having been a much valued member of our research group. We look forward to continuing scientific exchange and wish him all the best for his future career!

Doctoral graduation in the ATFT group

We congratulate our colleague Dominik Gölden to his very successful defense (January 24, 2018). The title of his PhD thesis is Magnetocrystalline anisotropy of iron thin films with interstitial nitrogen and boron. The defense was followed in the evening by a social event in the department with food and liquid relaxation medicine after all the excitement. We thank Dominik very much for his advances in science and for having been an important member of our group in many respects. It is good news that Dominik immediately obtained an attractive job in science related industry showing the value of the doctorate at ATFT and TU Darmstadt.