Abstract
Substantial ab initio calculations are carried out in this study to further support the conceptual
idea proposed by us recently, which is increasing the percentage of substitutional doping of magnetic
ions (Mn) in group-IV-semiconductor-based diluted magnetic semiconductors (DMS) by co-doping
with another conventional electronic dopant. Kinetic as well as energetic characteristics of the microscopic
co-doped system are explored in detail. The n-p pair formed by an n-type electronic dopant
and a p-type substitutional Mn atom is found to be a stable con guration in both Ge and Si. The
Mn atoms are also found to be kinetically easier to move from interstitial sites to substitutional sites
in the presence of a neighboring n-type electronic dopants. Magnetic coupling between two Mn ions
in Ge is found to be oscillatory between positive (ferromagnetic) and negative (antiferromagnetic)
values with increasing Mn-Mn distance, whereas in Mn/As co-doped Ge all coupling parameters are
positive except for the the nearest-neighbor one, and this qualitative di erence does not change with
doping level. On the other hand, in Mn doped Ge, when the magnetic coupling is plotted along
di erent directions, the oscillatory behavior is gone, indicating the oscillation is from anisotropy
rather than a RKKY-form interaction. For Mn doped Si, all coupling values except for the nearest
neighbor one are positive and do not change much upon the co-doping. An unconventional magnetic
anisotropy, which is the dependence of magnetic coupling on the relative positions of magnetic ions
and their neighboring assistant dopants, is also studied. Then the calculated magnetic coupling is
mapped to a classical Heisenberg model and Monte Carlo simulation is employed to get the Curie
temperature Tc. Tc of Mn/As co-doped Ge is found to be 264K at 5% Mn doping, whereas no
ferromagnetic order is present in Mn doped Ge with the Mn concentration ranging from 3.13% to
6%. The homogeneously doped Ge by Mn is thus a spin glass, and Monte Carlo simulation yield a
spin-glass phase transition happens at 5K at 5% doping.