Exploring oxides for novel electronics
The Berlin-centered research network GraFOx merges activities in crystal growth, epitaxy, theory, and
fundamental physical investigations towards one goal: to create and explore oxide systems for new
generations of electronic devices.
With startup funding as a Leibniz ScienceCampus researchers from 8 institutions team up in their enthusiasm and expertise for oxides. GraFOx combines many unique research facilities, equally in experiment and theory, in more than 33 coordinated projects, involving 40 PIs and 25 PhD students.
Paul-Drude-Institut für Festkörperelektronik
Leibniz-Institut für Kristallzüchtung
Humboldt-Universität zu Berlin
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Helmholtz-Zentrum Berlin für Materialien und Energie
Prof. Norbert Esser of the Leibniz-Institut für Analytische Wissenschaften (ISAS) closely collaborates with Prof. Goldhahn by providing unique expertise in synchrotron-based ellipsometry in Berlin.
The workgroup “Chemistry of Inorganic Materials” at Ruhr-Universität Bochum (RUB) lead by Prof. Anjana Devi develops and provides the precursors for MOCVD of complex oxides at IKZ.
At the Technische Universität Berlin (TUB) Dr. Susi Lindner and the group of Prof. Markus R. Wagner support GraFOx with their strong expertise in scanning tunneling microscopy/spectroscopy and optical spectroscopy, respectively.
Based on their large activity on the gas sensing applications of semiconducting oxides, the group of Prof. Udo Weimar and Dr. Nicolae Barsan at Eberhard-Karls-Universität Tübingen (UT) collaborates towards the practical application of the oxides investigated in GraFOx.
Oxides are among the materials with the widest tunability of physical properties. Spanning insulators, semiconductors, metallic conductors and superconductors, magnetic materials, ferro-/antiferro- and other dielectrics, oxides are a materials class with high potential for a new generation of electronic devices. Particularly in energy applications, they are expected to exhibit outstanding performance.
Yet, control of oxides is in its infancy. Compared to more conventional semiconductors, the strong ionicity of bonds in oxides poses big challenges, such as a variety of crystal phases, non-stoichiometry and defects. Harnessing oxides for electronic devices, like the quest to control GaN, therefore requires both, the growth of well-defined material and a broad understanding of device-relevant physical properties.
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Exploratory materials research focuses on early stage development of oxide materials, on their fundamentals, and on their screening for novel applications. This cluster intentionally overlaps with the clusters on Growth and on Physical properties, highlighting their fundamental research aspects.
Following our work in the first phase, we target alloys that extend the compositional range of (In,Ga)2O3 and (Al,Ga)2O3 in order to provide Ga2O3-based heterostructures with large band-offsets. Moreover, we will investigate novel aspects, such as different deep acceptors and semiconducting p-type oxides (based on our results on p-type SnO and NiO), for insulation layers or heterojunctions in Ga2O3-based power devices.
Another focus lies on novel ultra-wide band gap semiconducting oxides, such as ZnGa2O4.
GraFOx partners bring in a wide and comprehensive range of growth techniques. For rapid synthesis of solid solutions, compositional spread PLD is used as a high throughput method. Once promising ranges of materials have been developed by rapid methods, materials with the highest material quality will be grown in bulk form or as thin films by MBE and by MOCVD.
ALD-grown Ga2O3 will be explored with the goal of realizing ultra-low power synaptic nanoscale devices.
GraFOx builds on powerful, and in several cases unique, in-situ analytical methods. We use in-situ heating in TEM, in-situ Raman spectroscopy, and XRD performed in an MBE at the BESSY II synchrotron to study phase diagrams and phase transitions.
Cluster X is coordinated by Martin Albrecht (IKZ) and Holger von Wenckstern (U Leipzig).
Crystal growth intimately relates to phenomena at the surface. Cluster G therefore addresses surface processes, both theoretically and experimentally, to control stoichiometry, formation of surface facets, interfaces, and atomic defects.
For bulk growth, critical issues are growth stability, defect formation, and the impact of growth conditions on crystal diameter and properties.
MBE and MOCVD often suffer from facet formation and have their individual challenges: The low growth pressures during MBE tend to form volatile suboxides that limit the growth temperature. MOCVD allows for conditions closer to thermodynamic equilibrium and higher oxygen pressures, decreasing intrinsic defect concentrations. However, the more complex chemistry during growth, poor volatility and stability of most metalorganics requires precursor development in the case of complex oxides, which explains the very limited work in complex oxide MOCVD.
Cluster G performs growth, ab-initio theory, and characterization, addressing three major goals:
Cluster G is coordinated by Oliver Bierwagen (PDI) and Jutta Schwarzkopf (IKZ).
Cluster P exploits the unique materials perfection gained from work in cluster G to study the inherent fundamental properties of oxides, particularly as needed in device applications.
Oxides differ from classical semiconductors by the high ionicity in bonding and the variability in cation valencies. This impacts phase formation, the physics of defects, the coupling of lattice vibrations to optical excitations as well as heat and charge transport. Examples for the peculiar properties of oxides are the formation of small polarons and the extraordinary shift of the bandgap with temperature.
Cluster P will primarily address:
With respect to (power) devices, we will investigate:
Cluster P is coordinated by Claudia Draxl (HU Berlin) and Rüdiger Goldhahn (U Magdeburg).
This cluster was newly introduced to highlight the potential of the oxides studied in GraFOx for device-applications. Devices will be processed on layers grown in cluster G and their performance will be investigated.
Device applications addressed are:
Cluster D is coordinated by Catherine Dubourdieu (HZB) and Joachim Würfl (FBH).
Z. Galazka, R. Uecker, R. Fornari,
“Method and Apparatus for Growing Indium Oxide (In2O3) Single Crystals and Indium Oxide (In2O3) Single Crystal”,
Z. Galazka, R. Uecker, D. Klimm, M. Bickermann,
“'Method for growing beta phase of gallium oxide (β-Ga2O3) single crystals from the melt contained within a metal crucible”,
International Patent Applications No. PCT/EP2015/079938, publication No. WO 2016/110385A1 (14.07.2016)
Accepted also in Europe (EP3242965 (26 June 2019) and Korea (KR101979130 (15 May 2019).
The Leibniz ScienceCampus GraFOx will be present at the following events: