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Large-scale XSTM topographic image acquired with a constant tunneling current of 0.5 nA and a sample bias of −2.0 V. The bright regions toward the top (bottom) of the image correspond to the InAs:Mn (InAs:Be) QDs. The gray-scale range displayed is 0.7 nm.
Plot of normalized conductance versus sample bias, collected from the (a) core, (b) edge, and (c) GaAs matrix surrounding InAs, InAs:Be, and InAs:Mn QDs. The sample voltage corresponds to the energy relative to the Fermi level. The effective valence and conduction band edges are indicated by vertical dashed lines at negative and positive sample voltages, respectively. The spectra from the QD core are similar for all cases. In the spectra from the edge of the InAs:Mn QDs and from the GaAs near the edges of InAs:Mn QDs, mid-gap features indicated by vertical arrows presumably correspond to Mn-induced electronic states.
The LDOS calculated at the QD core (a)–(c), QD edge (d)–(f), and GaAs matrix (g)–(i), using order (N) tight binding. LDOS from in and near the InAs:Mn (InAs) QDs are plotted as dashed (solid) lines toward the top (bottom). The LDOS was calculated assuming three different cases: Mn located at the QD core [(a), (d), and (g)], Mn located at the QD edge [(b), (e), and (h)], and Mn located in the surrounding GaAs matrix [(c), (f), and (i)]. The trends observed in the “Mn at QD edge” case are consistent with the trends observed in the measured STS spectra.
Normalized Gibbs free energies for single Mn impurities, plotted as a function of InAs QD diameter. The solid and dashed curves represent the free energy for Mn impurities at the QD core and surface. Above (below) a critical QD diameter of 26 nm, core (surface) doping is expected to dominate.
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