Control of electrical and structural properties in Ga2O3 epitaxy

The ultrawide bandgap semiconductor material gallium oxide (Ga2O3) features promising properties for next generation electronic devices. In particular, the bandgap of ≈ 5 eV, the high predicted breakdown field, the possibility to engineer its n-type conductivity through extrinsic doping, and the high chemical stability, are key factors to foresee its extensive application in the field of (power) electronics and optoelectronics devices, especially in the case of the thermodynamically stable β phase. In fact, this material system presents polymorphism: different metastable structures of Ga2O3 (α, β, γ, δ and κ) can be stabilized though various epitaxial growth techniques. Among the metastable polymorphs, κ-Ga2O3 has been theoretically predicted to be characterized by a large spontaneous polarization, driving significant interest into this material system. Nonetheless, for κ-Ga2O3 a throughout understanding over the structural and point defects towards the determination of its functional properties is still at the early stage. Therefore, the present work focuses primarily on these aspects related to the κ-Ga2O3 material system. The heteroepitaxy of κ-Ga2O3 results on various substrates in the formation of an intrinsically structurally defective structure (e.g., vertically oriented rotational domains). This work investigates the defective nature of κ-Ga2O3 to gain a comprehensive understanding of its impact on the functional properties of this material, presenting the first evidence of defect-mediated in-plane transport. The study then explores potential strategies to mitigate the presence of structural defects in two different epitaxial growth techniques, MOVPE and MBE. Following the analysis of structural defects, the focus shifts to the role of point defects in κ-Ga2O3, offering a practical approach to engineer them through mild annealing treatments well-below its temperature stability threshold. In the final section, the diffusion of Li into different Ga2O3 structures is discussed, along with insights into the diffusion mechanism driven by the defective nature of the studied layer.