Fe-based semimagnetic semiconductors (invited)

Recently, the family of semimagnetic semiconductors (SMSC) or diluted magnetic semiconductors (DMS) typically involving manganese ions, i.e., Cd/sub 1-//sub x/Mn/sub x/Te, Cd/sub 1-//sub x/Mn/sub x/Se, Zn/sub 1-//sub x/Mn/sub x/Te, Hg/sub 1-//sub x/Mn/sub x/Te, was extended to include iron ions in selenium and tellurium II-VI compounds. Contrary to Mn-based DMS where d states of Mn are energetically superimposed on the valence-band continuum, for Cd/sub 1-//sub x/Fe/sub x/Se and Zn/sub 1-//sub x/Fe/sub x/Se the Fe2+(3d6) is a donor whose energy level lies deeply in the energy gap. On the other hand, for Hg/sub 1-//sub v//sub -//sub x/Cd/sub v/Fe/sub x/Se with x≤0.15 and vapprox. <0.40, the Fe2+ level is a resonant donor located in the conduction band. Also in the case of this material, the mobility of free electrons is surprisingly high, while the Dingle temperature is low. Because of the Coulomb interaction between the ionized donors Fe3+(3d5) at low T, there will appear a correlation of the positions of charges in this system. This leads to a kind of ''condensation'' of the charges in the system of donors and to their ''crystallization'' at even lower T (i.e., formation of a ''charge superlattice'' or a localized Wigner crystal of ionized donors). The space ordering of ionized donors dramatically influences the free-carrier scattering and consequently leads to high mobilities and low Dingle temperatures. Another implication of intersite repulsion is the existence of the Coulomb gap in the one-electron d-band density of states. We discuss some of the effects related to its formation