During growth of the dilute p-type ferromagnetic semiconductor Ga1âxMnxAs, interstitial manganese, Mni2, is formed when xexceeds 2%. The double donor Mni2犉ꥺ笚뗫 the free holes that mediate ferromagnetism. Annealing causes out-diffusion of these interstitials, thereby increasing the Curie temperature. Here, we use cross sectional scanning tunneling microscopy and spectroscopy to visualize the potential landscape which arises due to the clustering of Mni2 annealed pâiân (GaMn)AsâGaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual Mni2誉, and show that the ions are doubly charged. [ABSTRACT FROM AUTHOR]