Distinct local electronic structure and magnetism for Mn in amorphous Si and Ge

Transition metals such as Mn generally have large local moments in covalent semiconductors due to their partially filled $d$ shells. However, Mn magnetization in group-IV semiconductors is more complicated than often recognized. Here we report a striking crossover from a quenched Mn moment $(<0.1{\ensuremath{\mu}}_{B})$ in amorphous Si $(a\text{-Si})$ to a large distinct local Mn moment $(\ensuremath{\ge}3{\ensuremath{\mu}}_{B})$ in amorphous Ge $(a\text{-Ge})$ over a wide range of Mn concentrations (0.005--0.20). Corresponding differences are observed in $d$-shell electronic structure and the sign of the Hall effect. Density-functional-theory calculations show distinct local structures, consistent with different atomic density measured for $a\text{-Si}$ and $a\text{-Ge}$, respectively, and the Mn coordination number ${N}_{c}$ is found to be the key factor. Despite the amorphous structure, Mn in $a\text{-Si}$ is in a relatively well-defined high coordination interstitial type site with broadened $d$ bands, low moment, and electron ($n$-type) carriers, while Mn in $a\text{-Ge}$ is in a low coordination substitutional type site with large local moment and holes ($p$-type) carriers. Moreover, the correlation between ${N}_{c}$ and the magnitude of the local moment is essentially independent of the matrix; the local Mn moments approach zero when ${N}_{c}>7$ for both $a\text{-Si}$ and $a\text{-Ge}$.