Effective dilution of Mn impurities constitutes the main issue in the fabrication of diluted magnetic semiconductors. However, also the study of the extrinsic phases, that Mn impurities form with host atoms, deserves interest for future application in nanomagnetics. Samples for our analysis has been prepared through high temperature (240-300°C) implantation of Mn⁺ ions onto Ge (100) substrates at 100 keV and subsequent 400°C annealing. TEM and EELS measurements reveal the coexistence (in a 130 nm thick implanted layer) of Mn dilution into the crystalline Ge matrix and of MnGe precipitates embedded in it. The precipitates with smaller size (3-10 nm) are principally amorphous, whereas the greater ones (8-15 nm) are mainly in the Mn₅Ge₃ crystalline phase. The samples have been progressively sputtered with Ar ions, in order to explore with surface probes different depths of the implanted layer. STM measurements show the presence of nanometer sized mounds that protrude few nm out a flat surface. These clusters are clearly identified as the MnGe precipitates (harder to etch than the surrounding Mn-diluted Ge matrix). Current Image Tunneling Spectroscopy (CITS) maps evidence significantly higher current values in correspondence with the protruding MnGe clusters, in particular for those of greater dimensions, well distinguished from the much less conductive Mn-diluted Ge matrix. Averaged I-V measurements show that the smaller MnGe precipitates (identifiable as the amorphous ones) are semiconductive (band gap of 0.45±0.05 eV), whereas the bigger ones (identifiable as the Mn₅Ge₃ nanocrystals) are metallic This is the first direct investigation of the electronic structure of the two types of clusters. The Mn-diluted Ge matrix has a band gap of 0.60±0.05 eV (close to intrinsic Ge). The experimental findings on the electronic structures are also compared with ab-initio calculations.

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