Thermal conductivity of double polymorph Ga₂O₃ structures

Recently discovered double gamma/beta (γ/β) polymorph Ga₂O₃ structures constitute a class of novel materials providing an option to modulate functional properties across interfaces without changing the chemical compositions of materials, in contrast to that in conventional heterostructures. In this work, for the first time, we investigate thermal transport in such homo-interface structures as an example of their physical properties. In particular, the cross-plane thermal conductivity (k) was measured by femtosecond laser-based time-domain thermoreflectance with MHz modulation rates, effectively obtaining depth profiles of the thermal conductivity across the γ-/β-Ga₂O₃ structures. In this way, the thermal conductivity of γ-Ga₂O₃ ranging from 1.84 to 2.11 W m⁻¹ K⁻¹ was found to be independent of the initial β-substrates orientations, in accordance with the cubic spinel structure of the γ-phase and consistently with the molecular dynamics simulation data. In turn, the thermal conductivity of monoclinic β-Ga₂O₃ showed a distinct anisotropy, with values ranging from 10 W m⁻¹ K⁻¹ for [−201] to 20 Wm⁻¹ K⁻¹ for [010] orientations. Thus, for double γ-/β-Ga₂O₃ polymorph structures formed on [010] β-substrates, there is an order of magnitude difference in thermal conductivity across the γ/β interface, which can potentially be exploited in thermal energy conversion applications.