Enhancement of Curie temperature in Ga_1-xMn_xAs epilayers grown on cross-hatched In_yGa_1-yAs buffer layers

Relaxed In_yGa_1-yAs epilayers grown on (001) GaAs are known to exhibit a cross-hatched surface with ridges running along the [110] and [11̄0] directions. We find that Ga_1-xMn_xAs epilayers grown on such buffer layers can have as-grown Curie temperatures (T _C) that are higher than the as-grown 110K value typical of Ga _1-xMn_xAs/GaAs heterostructures. Further, low-temperature annealing leads to only modest additional increases in T_C, contrasting with the behavior in Ga_1-xMn_xAs/GaAs where T_C typically increases significantly upon annealing. Our observations suggest that the initial concentration of Mn interstitials in as-grown Ga _1-xMn_xAs /In_yGa_1-yAs heterostructures is smaller than that in as-grown Ga_1-xMn _xAs/GaAs heterostructures. We propose that strain-dependent diffusion may drive Mn interstitials from the bulk of the growing crystal to more benign locations on the ridged surface, providing a possible route towards defect-engineering in these materials.