Enhanced Valley Splitting of Transition-Metal Dichalcogenide by Vacancies in Robust Ferromagnetic Insulating Chromium Trihalides

Recently, single-layer CrI₃, a member of the chromium trihalides CrX₃ (where X = Cl, Br, or I), has been exfoliated and experimentally demonstrated as an atomically thin material suitable for two-dimensional spintronics. Valley splitting due to the magnetic proximity effect has been demonstrated in a WSe₂/CrI₃ van der Waals heterojunction. However, the understanding of the mechanisms behind the favorable performance of CrI₃ is still limited. Here, we systematically study the carrier mobility and the intrinsic point defects in CrX₃ and assess their influence on valley splitting in WSe₂/CrI₃ by first-principles calculations. The flat-band nature induces extremely large carrier mass and ultralow carrier mobility. In addition, intrinsic point defects - localized states in the middle of the band gap - show deep transition energy levels and act as carrier recombination centers, further lowering the carrier mobility. Moreover, vacancies in CrI₃ can enhance ferromagnetism and valley splitting in a WSe₂/CrI₃ heterojunction, proving that chromium trihalides are excellent ferromagnetic insulators for spintronic and valleytronic applications.