Fluence and thermal threshold for an effective self-healing in high-energy-neutron-irradiated Al2O3/QFS-graphene/6H-SiC(0001) system

Abstract

This article reveals a unique self-healing ability of the amorphous-aluminum-oxide-passivated p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated nominally on-axis 6H-SiC(0001) system, exposed for 166 h to a destructive flux of 3.3 ×10¹¹ cm⁻² s⁻¹ of mostly fast-neutrons (1–2 MeV), resulting in an accumulated fluence of 2.0 × 10¹⁷ cm⁻². Post-irradiation room-temperature Hall effect characterization proves that the a-Al₂O₃/QFS-graphene/6H-SiC(0001) is n-type, which implies the loss of the quasi-free-standing character of graphene and likely damage to the SiC(0001)-saturating hydrogen layer. Micro-Raman spectroscopy suggests an average defect density in graphene of $n_D$ = 3.1 × 10¹⁰ cm⁻² with an $L_D$ = 32-nm inter-defect distance. Yet, a thermal treatment up to 623 K eliminates defect-related Raman peaks and restores the original p-type conductance. At the same time, 623 K is not enough to recover the initial transport properties in a sample irradiated for 245 h with a total fluence of 2.0 × 10¹⁸ cm⁻². A Density Functional Theory model explains the self-healing phenomenon and restoration of the quasi-free-standing properties through thermally-activated lateral diffusion of the remaining population of hydrogen atoms and re-decoupling of the graphene sheet from the SiC(0001) surface. The thermal regime of 623 K fits perfectly into the operational limits of the a-Al₂O₃/QFS-graphene/6H-SiC(0001) system, defined as 300 K to 770 K. The finding constitutes a milestone for two-dimensional, graphene-based diagnostic and control systems designed for operation in extreme environments.