Growth and fundamentals of oxides for electronic applications – “GraFOx”

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.

Partners

Associate Partners

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.

GraFOx – scientific background

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.

Highlights

Research Highlights

All Highlights in one zip-file. Download >>


  • Structural defects and charge carrier mobility in homoepitaxial layers grown on (100) plane of β-Ga2O3
    pdf-download download PDF >>
  • Suboxide-related kinetics, etching, thermodynamics, and catalysis governing the MBE of Ga2O3, In2O3, SnO2 and (In,Ga)2O3
    pdf-download download PDF >>
  • Ga2O3-based devices
    pdf-download download PDF >>
  • Faceting and step flow growth in the homoepitaxy of Ga2O3
    pdf-download download PDF >>
  • Barium stannate based heterostructures for electronic applications
    pdf-download download PDF >>
  • β-Ga2O3 fundamental properties and their anisotropy
    pdf-download download PDF >>
  • Joint computational and experimental examination of the phase stability of (In xGa1–x)2O3 ternary alloys
    pdf-download download PDF >>
  • Anisotropy of optical and electrical properties of rutile SnO2
    pdf-download download PDF >>
  • Finding the right balance for SnO growth enables the realization of all-oxide SnO/Ga2O3 vertical pn heterojunction diodes
    pdf-download download PDF >>

Further Highlights

Research

GraFOx structure

Four research clusters host the individual research projects:

GraFOX Cluster Graphic

Cluster X: Exploratory materials research

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).

Cluster G: Growth and Surfaces

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:

  1. Bulk growth of highly conducting Ga2O3 substrates with high structural perfection and diameters up to 2 inches.
  2. The development of a predictive and experimentally confirmed model on surface stability and growth for the major surface orientations (100), (010), (100) and (201) of Ga2O3, based on our homoepitaxial growth results of Ga2O3 regarding surface stability as well as thermodynamics and kinetics in the growth by MBE and MOCVD.
  3. Control of stoichiometry and doping of complex oxides by MOCVD (SrTiO3) and MBE (BaSnO3, LaInO3) for fundamental studies on intrinsic properties, atomic defects, and interfaces.

Cluster G is coordinated by Oliver Bierwagen (PDI) and Jutta Schwarzkopf (IKZ).

Cluster P: Physical properties

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:

  1. Optical excitations: Building up on our significant theoretical progress describing electron hole correlations and polarons, we will study excitations in group III-sesquioxides by both theory and experiment. We set a focus on identifying excitons in sesquioxides, and thereby the energetic position of polarons.
  2. Thermal properties: Low thermal conductivity is one of the challenges for power devices. We will study thermal conductivity, thermal expansion and the temperature dependence of the bandgaps by experiment and theory, based on a new theoretical approach that considers the full anharmonicty of lattice vibrations.
  3. Atomic defects: Control of atomic defects and their properties are crucial for device operation. In memristive devices they are even an essential part of the device functionality. We will study atomic defects in sesquioxides and SrTiO3 to understand doping and compensation, defect thermodynamics and diffusion, especially regarding the role of oxygen vacancies.

With respect to (power) devices, we will investigate:

  1. Residual donors, the properties of deep acceptors and defect complexes in sesquioxides.
  2. Heterostructures to obtain 2D electron gases, to electrostatically dope interfaces and to induce electronic phase transitions. We will concentrate on heterostructures of group III sesquioxides, BaSnO3/LaInO3, and KNbO3/NaNbO3.

Cluster P is coordinated by Claudia Draxl (HU Berlin) and Rüdiger Goldhahn (U Magdeburg).

Cluster D: Devices

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:

  1. Lateral and vertical high-voltage power devices (Schottky barrier diodes and field effect transistors). The application of Ga2O3 will be evaluated by focusing on the interactions of processing (deposition, etching, and ion implantation) with the (anisotropic) β-Ga2O3 material and its influence on device properties and reliability. Open questions to be addressed are electronic trap states in the bulk and at the semiconductor/dielectric interface, a gate oxide that does not break down, suitable etching techniques (dry and wet chemical), Schottky and ohmic contact formation mechanisms, as well as issues related to the reduced thermal conductivity of the Ga2O3 material. In addition, the use of deep acceptors or p-type oxides for device insulation will be evaluated.
  2. Solar-blind, deep-UV photo detectors based on different polymorphs of Ga2O3 and its alloys with In2O3 and Al2O3 (enabling bandgaps from about 4 eV to >6 eV), will be processed and benchmarked as metal-semiconductor-metal (MSM) detectors. We will investigate possible causes for the slow temporal response and high gain.
  3. Conductometric gas sensors based on Ga2O3, In2O3, SnO2, and NiO, using well-defined, single-crystalline oxide layers to answer fundamental questions such as bulk- vs. surface conductivity, surface-orientation dependence and the role of local nano-heterojunctions for sensing. Special attention will be given to practical problems, such as cross sensitivity to moisture.

Cluster D is coordinated by Catherine Dubourdieu (HZB) and Joachim Würfl (FBH).

Granted Patents / Patent Applications

Z. Galazka, R. Uecker, R. Fornari,
“Method and Apparatus for Growing Indium Oxide (In2O3) Single Crystals and Indium Oxide (In2O3) Single Crystal”,

  1. No. EP 2841630 (granted on 16.03.2017)
  2. No. JP 6134379 (granted on 28.04.2017)

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).

Dates

The Leibniz ScienceCampus GraFOx will be present at the following events: