Different magnetic moment in Mn-doped amorphous group-IV semiconductors: a comparison study between Si and Ge matrices.

Mn-doped amorphous Si ($a-$Si) and Ge ($a-$Ge) are prepared by $e$-beam co-evaporation for a wide range of concentrations (0.5-18 at.{\%}) to explore the Mn local moment in group-IV semiconductors. We find that Mn behaves quite differently in these two matrices: in $a$-Si, the Mn local moment is quenched, even for the lowest doping (0.5 at.{\%}), while in $a-$Ge, a large Mn moment is observed, with a spin-glass ground state. X-ray absorption spectra (XAS) of $a-$Mn$_{x}$Si$_{1-x}$ have very broad $L$-edge absorption peaks which correlate with the quenched magnetic state. The quenched Mn moment in $a$-Si is unexpected and can be understood by the formation of Anderson-localized itinerant states even on the insulating side of the metal-insulator transition. By contrast, XAS of $a-$Mn$_{x}$Ge$_{1-x}$ show atomic multiplets. $a$-Mn$_{x}$Si$_{1-x}$ has positive magnetoresistance (MR) like typical non-magnetic disordered electronic systems, while $a-$Mn$_{x}$Ge$_{1-x}$ has negative MR, consistent with magnetization data.