β-Ga₂O₃ is a transparent semiconducting oxide that attracted a particular attention in the research community with potential applications especially in high power electronics and UV opto-electronics. This is the result of a wide bandgap of 4.85 eV, good electrical properties enabling a wide doping range, high theoretical breakdown field of 8 MV/cm, and a capability of growing large bulk single crystals and thin films of high structural quality [1].

A yet higher critical breakdown field of β-Ga₂O₃ can be achieved by enlarging its bandgap through heavy doping with Al. For homoepitaxial growth of β-(Al_xGa_1-x)₂O₃ films and subsequent device fabrication, wafers from bulk single crystals of similar composition would be highly beneficial. We have already demonstrated the capability of growing bulk β-(Al_xGa_1-x)₂O₃ single crystals by the Czochralski method with x = 0 – 0.35, and provided basic structural, optical, and electrical properties [2].

The present study provides an overview of the growth of bulk β-(Al_xGa_1-x)₂O₃ single crystals by the Czochralski method, including thermodynamics and limits of Al incorporation in the monoclinic β-Ga₂O₃ crystal lattice, as well as limits of Ga incorporation in the trigonal a-Al₂O₃ crystal lattice. In addition to Al doping, the crystals were co-doped either with Si or Mg. The study is accompanied with extended characterization of physical properties of β-(Al_xGa_1-x)₂O₃ as a function of [Al]. It covers structural (lattice constants), electrical (free electron concentration, electron mobility, BOFM), optical (absorption edge, bandgap, static dielectric constants, refractive index), and thermal (thermal conductivity) properties. A high doping level of bulk β-Ga₂O₃ single crystals with [Al] ≤ 35 mol.%, their high structural quality, and a wide spectrum of physical properties might facilitate homoepitaxial growth of β-(Al_xGa_1-x)₂O₃ films and novel device fabrication.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG) project under Grant Nos. GA 2057/5-1 and PO 2659/3-1. It was partly performed in the framework of GraFOx, a Leibniz-Science Campus, partially funded by the Leibniz Association—Germany.

[1] Eds. M. Higashiwaki and S. Fujita; “Gallium Oxide: Crystal Growth, Materials Properties, and Devices”; Springer Nature Switzerland AG (2020).