Nanoscale Potential Fluctuations in (GaMn)AsGaAs Heterostructures: From Individual Ions to Charge Clusters and Electrostatic Quantum Dots

During growth of the dilute p-type ferromagnetic semiconductor GaMnAs, interstitial manganese is formed when the Mn concentration exceeds 2{\%}. This interstitial Mn acts as a double donor which compensates 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 interstitial Mn in annealed p-i-n (GaMn)As-GaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual interstitial Mn ions, and show that the ions are doubly charged.