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News | 02-06-2020

Gallium-based spinel single crystals

There is a continuous search for new transparent semiconducting oxides (TSOs, oxides combining transparency in the UV/visible spectrum and semiconducting behaviour) enabling applications in high power electronics (Schottky barrier diodes, field-effect transistors), optoelectronics (photodetectors, flame detectors), and sensing systems (gas sensors, semiconducting scintillators).

A new class of TSO materials: (from left to right) MgGa₂O₄, ZnGa₂O₄ and CoGa₂O₄.

In addition to the development of binary (b-Ga2O3, SnO2, In2O3), ternary (BaSnO3) and quaternary (InGaZnO4) TSO single crystals, we also focus on Ga-based spinels: MgGa2O4, ZnGa2O4, (Mg,Zn)Ga2O4, and CoGa2O4. They define a new class of TSO materials with a cubic structure for a diversity of applications. (A summary of basic physical properties of Ga-based spinel single crystals is shown in the Table below.)

They all have high melting points and experience a strong decomposition with incongruent evaporation, making the growth of bulk single crystals really challenging. For crystal growth from the melt, we utilized iridium crucibles and innovative tools to control thermodynamics to some extent and stabilize the growth. We applied the Czochralski, Kyropoulos-like, and vertical gradient freeze / Bridgman methods depending on the degree of thermal instability of the compound in quest. The obtained single crystals with several cm3 in volume, as shown in the figure, enable wafer fabrication with a size up to 10x10 mm2.

The availability of Ga-based spinel single crystals may expand fundamental research to explore material’s properties, but also may bring new concepts for device designs and their applications. Cubic ZnGa2O4 crystals exhibit a very high electrical conductivity (one order of magnitude higher than b-Ga2O3 crystals) and high Hall mobility at very high free electron concentrations (twice higher than that of b-Ga2O3).

Therefore, it is a great candidate for vertical power devices, and we are conducting an extensive research in this direction, including homoepitaxy and device fabrication. All the Ga-based spinels have lattice parameters well matched to Fe-based magnetic spinels. Our Ga-based spinel substrates brought the growth of NiFe2O4 films to a higher level by eliminating anti-phase boundaries of the films and improving their magnetic properties. Moreover, the spin-dependent bandgap of CoGa2O4 offers opportunities for spintronic applications.

More Information:
Zbigniew Galazka
Section Oxides & Fluorides