Detection of stacking faults breaking the [110]/[11¯0] symmetry in ferromagnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P)

We report on high-resolution x-ray diffraction measurements of (Ga,Mn)As and (Ga,Mn)(As,P) epilayers. We observe a structural anisotropy in the form of stacking faults that are present in the (111) and (111¯) planes and absent in the (1¯11) and (11¯1) planes. They occupy 10^-2%-10^-1% of the ferromagnetic epilayer volume while no stacking faults are detected in the controlled, undoped GaAs epilayer. Full-potential density functional calculations provide additional evidence that the formation of the stacking faults is promoted by Mn attracted to these structural defects. The enhanced Mn density along the common [11¯0] direction of the stacking fault planes produces a symmetry-breaking mechanism of a strength and sense that can account for the uniaxial [110]/[11¯0] magnetocrystalline anisotropy of these ferromagnetic semiconductors.