The character of electronic states near the Mott-Anderson metal-insulator transition in the ferromagnetic semiconductor Ga1-xMnxAs is studied by cross sectional scanning tunneling microscopy. 200nm thick samples grown by MBE at UCSB with doping levels from 1.5%, close the metal-insulator transition, to 5%, deep into the metallic regime, were studied at 4.2K. The thickness of the samples ensured that the electronic states were 3D in character. Strong spatial fluctuations in the local density of states, as well as corrections in the density of states at the Fermi energy due to electron-electron interactions in the presence of disorder are observed. The correlation length of the local density of states also grows significantly approaching the Fermi energy. These effects persist even in the highest doped samples, suggesting that disorder plays a profound role in the character of the electronic states and that even metallic samples are still close to the metal-insulator transition. These effects are expected to be of importance in moving beyond simple mean-field models of carrier-mediated ferromagnetism that ignore the spatial fluctuations in the density of states. Moreover, as the Mott-Anderson transition is little understood in the 3D materials where both interactions and strong disorder are equally important, these observations may provide important new insight by direct observation of the states involved. Further work is presented here comparing individual defects present in GaMnAs with their properties in a non-magnetic GaAs environment. Mn and other transition metals substituted into the first atomic layer of GaAs by STM manipulation were also studied to observe chemical trends. It is found that the effects of the symmetry breaking by the surface can account for the observed in-gap states.
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