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Identification of point defects and their influence on
(photo-)electrical transport ex-situ and in-situ (scanning)
transmission electron microscopy

 

 

Overview of the research project:

Growth and physical properties of transparent semiconducting oxide crystals and of ferroelectric perovskites have been studied at IKZ for some time. In particular, the PhD student A. Fiedler (IKZ budget financed since March 2015) is working on “Electrical and optical characterization of β-Ga2O3 layers and bulk crystals”. This subject is treated in the context of the proposed project for another two years.

The focus of this work is the investigation of how efficient n-type doping of β-Ga2O3 layers, homoepitaxially grown by metal-organic chemical vapor deposition (MOCVD) at IKZ, can be accomplished by using the doping elements silicon and tin. For this purpose we have to find out how the doping elements are incorporated into the crystal and which defects and impurities are responsible for electrical compensation and reduced electron mobility. The same issues are investigated for the β-Ga2O3 bulk crystals (grown by the Czochralski method at our institute) from which the substrate wafers for the MOCVD layer growth are prepared.

The identification of harmful defects and impurities is prerequisite to propose measures for growth improvements. The principal methods we apply are temperature dependent conductivity and Hall effect measurements (TDH), deep level transient spectroscopy (DLTS), electron paramagnetic resonance spectroscopy (EPR), and optical absorption spectroscopy from ultraviolet to far-infrared wavelengths (UV-Vis-NIR, FTIR). The electrical measurements by TDH and DLTS yield the most important electrical parameters of the investigated samples such as electron concentration and mobility, donor and compensating acceptor concentrations, ionization energies of shallow and deep defects or impurities as well as concentrations and capture cross sections of deep traps. Including optical excitation capabilities to these methods allow us to determine optical ionization energies and photoionization cross sections. With all these parameters one can model the (photo‑)electrical behavior of the crystals and of simple device structures (here Schottky contacts are prepared for DLTS, C-V, I-V measurements) phenomenologically. However, a direct identification of the defect nature is not possible. One may speculate about the underlying defects by correlating electrical measurement results with variations of the growth conditions (e.g. chemical potentials, impurities). Although challenging, the more reliable way we want to go in addition is to complement the electrical with spectroscopic methods. In particular, EPR and local vibrational mode (LVM) spectra may contain information that allows a direct identification of, or gives at least strong hints on, the defect or impurity nature. Using identical samples and correlating the signals from electrical measurements with those from spectroscopy, for instance by the photoionization cross section as “fingerprint”, is the preferred way of unique defect identification. It is to be mentioned that we are measuring LVM spectra not only by using FTIR, but also by Raman spectroscopy in close collaboration with PDI (M. Ramsteiner).

Structural investigations, in particular performed by transmission electron microscopy (TEM) in the group of M. Albrecht, are presently used to correlate structural defects with electrical properties. In future, also correlations of electrical and optical features with point defects, which can be detected in aberration-corrected high resolution TEM, will be strived.

Medium and long term planning of investigations:

  • Surface and interface conductivity, in particular electron accumulation at the In2O3 surface (oxygen vacancy vs. hydrogen)
  • Detection of small polarons and their influence on electrical transport
  • Magnetic doping to investigate possible ferromagnetic coupling mechanisms (Collaboration with PDI)
  • Heterostructures of perovskites


Major Accomplishments expected:

  • Identification of defects/impurities which are responsible for electrical compensation and reduced mobility in β-Ga2O3 bulk crystals and layers.
  • Measures to improve crystal growth (bulk and epitaxy) with respect to efficient intentional doping and reduced formation or incorporation of point defects/impurities.


Collaboration with partners in the project:

  • C3—Growth projects: Günter Wagner (IKZ), Zbigniew Galazka (IKZ), Jutta Schwarzkopf (IKZ), Oliver Bierwagen (PDI)—Feedback of the point defect and doping analysis for crystal growth improvements; growth of tailored samples (e.g. magnetic doping, doping levels)
  • C1—Project on optical and band structure properties: Manfred Ramsteiner (PDI), Rüdiger Goldhahn (UM)
  • C4—Project on electronic transport and application to devices: Ted Masselink (HUB)
  • C4—Project on kinetics and thermodynamics of atomic defects studied by HRTEM: Martin Albrecht (IKZ)
  • C4—Project on theoretical investigations of optoelectronic excitations: Matthias Scheffler (FHI),


The Research Team

 

Andreas Fiedler

Andreas Fiedler
PhD student

Andreas Fiedler is from Berlin and did his Bachelor and Master in Physics at Humboldt-Universität zu Berlin. His PhD thesis is on Electrical and optical characterization of the transparent semiconducting oxide, Ga2O3 and the identification of point defects and their influence on (photo-)electrical transport.

 


Project lead

If you have queries about the project, please contact the PI:
Klaus Irmscher, Institut für Kristallzüchtung

 

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coordination:
Paul-Drude-Institut für
Festkörperelektronik
Leibniz-Insitut im Forschungsverbund Berlin e.V.
Hausvogteiplatz 5-7
10117 Berlin, Germany 

The Leibniz ScienceCampus GraFOx is a network of two Leibniz institutes, two universities and one institute of the Max Planck Society. The Network is based in Berlin, Germany.

 

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