Structural stability and formation of MnAs inclusions in (Ga,Mn)As
|Pawel Jakubas , Piotr Boguslawski|
Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
(Ga,Mn)As is one of the most intensively investigated magnetic semiconductors due to its high Curie temperature and compatibility with the GaAs technology. While annealing at about 200 C results in a substantial improvement of its properties , annealing at higher tempera-tures leads to the formation of inclusions. Depending on the annealing conditions, inclusions consist in smaller zinc-blende Mn-rich (Ga,Mn)As nanoclusters, or larger clusters of MnAs in the NiAs phase .
To understand structural stability of (Ga,Mn)As we performed first principles calculations using the local spin density approximation, and we generalized the definition of the mixing enthalpy to the case of alloys with a varying crystalline structure. We find that the stability of the zinc blende phase persists up to about x=0.75, while for higher Mn contents the equilib-rium structure is the NiAs phase. Moreover, due to the lattice mismatch, cubic GaMnAs is intrinsically unstable with respect to the segregation into pure end compounds. This suggests that formation of MnAs inclusions occurs in two steps. First, intrinsic instability results in fluctuations of the alloy composition, i.e., formation of Mn-rich clusters. In the second step, the clusters undergo a structural transformation from the metastable zinc blende to the stable NiAs structure.
Finally, we extend the analysis to (Cd,Mn)Te. The phase diagrams of both (Ga,Mn)As and (Cd,Mn)Te are similar, exhibiting a transition from the cubic to the NiAs phase with the in-creasing Mn content. For both systems we discuss magnetic ordering (which is ferromagnetic for (Ga,Mn)As, and antiferromagnetic for (Cd,Mn)Te), and its contribution to the structural stability.
The work has been supported by grant PBZ-KBN-044/P03/2001.
 K. Edmonds et al., Phys. Rev. Lett. 92, 37201 (2004).
 M. Moreno et al., Phys. Rev. B 67, 235206 (2003); J. Appl. Phys. 92, 4672 (2002).
Presentation: oral at E-MRS Fall Meeting 2005, Symposium D, by Piotr Boguslawski
See On-line Journal of E-MRS Fall Meeting 2005
Submitted: 2005-06-02 11:04 Revised: 2009-06-07 00:44