Intrinsic-Vacancy Mediated, Room-Temperature Ferromagnetism in a Silicon-Compatible Dilute Magnetic Semiconductor: Cr-doped Ga$_2$Se$_3$

Room temperature ferromagnetism (RTFM) and silicon compatibility are key criteria for useful spintronic materials. We present RTFM in a new class of dilute magnetic semiconducor (DMS) involving intrinsic vacancies: epitaxial Cr-doped Ga$_2$Se$_3$ on Si(001). With Ga$_2$Se$_3$'s ordered defects responsible for band-edge states, Cr:Ga$_2$Se$_3$ lies at the intersection between traditional DMS materials, where FM is mediated by dopant-induced carriers, and ones in which defects play a predominate role. Semiconducting Cr:Ga$_2$Se$_3$ films have a RT magnetic moment of 4 $\mu_B$/Cr and Cr-induced states overlapping the vacancy-derived states. Cr substitution for Ga, combined with a rotation of bonds around a single Se, can explain the observed octahedral local environment without long-range lattice disruption and lead to FM mediated by states associated with intrinsic vacancies. Cr incorporates into laminar Ga$_2$Se$_3$ films up to about 8 \% Cr, disaggregating into Cr-rich islands at higher concentration; islanded films remain FM, but with altered magnetic moment per Cr.