Built-in electric field assisted spin injection in Cr and Mnδ-layer doped AlN/GaN(0001) heterostructures from first principles

Highly spin-polarized diluted ferromagnetic semiconductors are expected to be widely used as ideal spin injectors. Here, extensive first-principles density-functional theory calculations have been performed to investigate the feasibility of using Cr- and Mn-doped wurtzite polar AlN/GaN(0001) heterostructures, with the aim to realize the appealing half-metallic character and, hence, efficient electrical spin injection. To overcome the formation of detrimental embedded clusters, we propose digital δ-layer doping perpendicular to the growth direction so as to realize enhanced performance at room temperature. The formation energy, electronic and magnetic properties, and the degree of spin polarization for both neutral and charged valence states for various concentrations are studied. Under both metal-rich (Al- or Ga-rich) and N-rich conditions, Cr and Mn dopants prefer to segregate into the GaN region and reside close to the interface, where dopant incorporation occurs more readily under N-rich conditions. The doped Cr and Mn atoms introduce $3d$ states in the band gap of the host semiconductor heterostructure. The spin injection channels are constructed via the hybridization between dopant $3d$ and surrounding host atoms, up to a few monolayers around the interface, where the spin-polarized t₂ electrons are injected into AlN without the conductivity mismatch problem. Significantly, for the energetically favorable configurations, the built-in electric field in the AlN/GaN(0001) heterointerface serves as a driving force for efficient spin injection through the interface and spin transport in the AlN region. Also importantly, the electronic properties of the heterostructures (half metallic, semiconducting, or metallic) are found to depend sensitively upon the doping concentration and valence charge states. In general, charged valence states can destroy the ferromagnetic half metallicity for both systems, particularly in n-type materials. These results will be useful with regard to the practical fabrication of desirable heterostructures for optoelectronic and semiconductor spintronic devices.