An "s-Electron" Donor Band Driven Metallic Ferromagnetism in Co-Doped ZnO Films

Achieving room-temperature ferromagnetism (RTFM) in diluted magnetic semiconductors (DMSs) has been a long-standing challenge, with doping transition metals (TM) into oxide semiconductors being one of the most common approaches. However, the underlying physical mechanisms remain poorly understood, particularly for Co-doped ZnO (Co:ZnO) films, which exhibit high Curie temperatures (Tc) above 300 K. A promising mechanism proposed for high-Tc ferromagnetism is the donor impurity band exchange model, in which donor electrons mediate the coupling between TM spins. Despite its theoretical significance, the nature of the donor band electrons has yet to be experimentally identified. In this work, we use polarization-dependent, bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) to investigate Co-doped ZnO epitaxial films. Our results reveal the presence of a weak electron donor band, crossing the Fermi level, and from a polarization dependence analysis, it is unambiguously identify it as having "s-character." This finding offers new insight into the ferromagnetic mechanism in Co-doped ZnO, where Zn¹⁺4s¹ states mediate the ferromagnetism, contributing to metallic-like transport and Co²⁺ spin ordering. These results not only elucidate the complementary role of dopant-host electronic states but also open avenues for designing novel room-temperature magnetic semiconductors, particularly in the context of 2D DMSs.