Damage accumulation induced metal-insulator transition through ion implantation of ScN thin films

Ion implantation is a powerful approach for tuning the electrical properties of materials through controlled doping and defect engineering, with applications in thermoelectrics and microelectronics. Scandium nitride (ScN) is particularly sensitive to irradiation-induced disorder, with transport properties spanning several orders of magnitude and multiple conduction mechanisms involved. In this study, we investigate the evolution of electrical transport in epitaxial ScN thin films undergoing accumulated irradiation damage at an initial defect state. A phenomenological damage-accumulation model was successfully combined with temperature dependent resistivity and Hall effect measurements to elucidate the impact of defect buildup on electrical transport and to provide physically grounded, quantitative insight into the nature and accumulation of irradiation-induced defects. It reveals two distinct defects-generation regimes of electrically active defects. At low doses, direct-impact damage produces stable and isolated acceptor-type complex defects, (VSc-X) with VSc a scandium vacancy and X denoting residual impurities, leading to a gradual increase in resistivity. At higher doses, defect accumulation dominates through a multi-hit process, giving rise to point-defect buildup and carrier localization, resulting in hopping-dominated transport. This localized regime is thermally unstable and recovers upon low-temperature annealing. We further demonstrate that the residual defect landscape strongly influences both the critical dose for the metal-insulator transition and the localization strength: films grown on Al2O3 exhibit an earlier transition and weaker localization than those grown on MgO. These results highlight ion implantation as an effective route for engineering disorder-induced localization in ScN, with the initial film quality playing a decisive role.

💡 Research Summary

In this work the authors systematically investigate how accumulated ion‑implantation damage controls the electrical transport and metal‑insulator transition (MIT) in epitaxial scandium nitride (ScN) thin films. Two sets of 150 nm‑thick ScN layers were grown by DC magnetron sputtering at 850 °C on (001) MgO and c‑cut Al₂O₃ substrates, providing distinct initial defect landscapes: the MgO‑based film exhibits lower residual resistivity (0.048 mΩ cm), higher mobility (≈187 cm² V⁻¹ s⁻¹) and a larger Debye temperature (≈926 K) than the Al₂O₃‑based film (ρ_R ≈ 0.122 mΩ cm, μ_R ≈ 68 cm² V⁻¹ s⁻¹, θ_D ≈ 886 K). Both as‑grown films are degenerate n‑type conductors with carrier concentrations around 9 × 10²⁰ cm⁻³, showing metallic temperature dependence (dρ/dT > 0) that can be described by a Bloch‑Grüneisen model.

The films were then subjected to cumulative oxygen ion (O²⁺) implantation at 180 keV, performed at room temperature in steps ranging from 0.005 dpa up to 2.25 dpa (MgO) or 2.15 dpa (Al₂O₃). SRIM simulations ensured that the implanted oxygen concentration remained below 10⁻³ at.% throughout the film, so that the dominant effect was lattice damage rather than chemical doping. After each implantation step, in‑plane resistivity ρ(T), Hall carrier concentration n_H(T) and mobility μ(T) were measured between 80 K and 300 K using a Van‑der‑Pauw/Hall setup with a 0.58 T magnetic field.

Analysis of the transport data reveals two distinct defect‑generation regimes. At low doses (≤ 0.2 dpa) the dominant defects are isolated acceptor‑type complexes V_Sc‑X (a scandium vacancy bound to residual impurities). These complexes act as shallow acceptors, modestly reducing the carrier concentration and increasing resistivity in a linear fashion, while the overall metallic conduction mechanism remains unchanged. The resistivity increase can be expressed as ρ = ρ_R + α₁·dpa with α₁≈1.2 × 10⁻³ Ω·cm·dpa⁻¹.

At higher doses (> 0.2 dpa) a multi‑hit process leads to the accumulation of point defects that strongly scatter carriers and induce electronic localization. The temperature dependence of resistivity switches to Mott variable‑range hopping (VRH), ρ ∝ exp