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Register a new userThe electronic structure around As antisite near (110) surface of GaAs
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The electronic structure around a single As antisite in GaAs is investigated in bulk and near the surface both in the stable and the metastable atomic configurations. The most characteristic electronic structures of As antisite is the existence of the localized p-orbitals extending from the As antisite. The major component of the highest occupied state on As antisite in the stable configuration is s-orbital connecting with neighboring As atoms with nodes whereas that in the metastable configuration is p-orbital connecting without nodes. Localized p-orbitals on the surrounding As atoms around the As antisite exist in every configuration of As antisite. Such features are retained except the case of the As antisite located just in the surface layer in which the midgap level is smeared into the conduction band and no localized states exist near the top of the valence band. Scanning tunneling microscopic images of defects observed in low-temperature grown GaAs, possibly assigned as As antisite, the origin of the metastability, and the peculiarity of the defects in the surface layer are discussed.
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Combining density-functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the $[rm Mn^{2+}]^0$ plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity $d$-electrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic $[rm Fe^{3+}]^0$ and the ionized $[rm Fe^{2+}]^-$ states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of $d$-orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state $[{rm Fe}^{2+}, h]^0$, the magnetic-anisotropy dependence on the impurity sublayer resembles the case of $[{rm Mn}^{2+}, h]^0$. In contrast, for $[{rm Fe}^{3+}]^{0}$ electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e.g. by an external magnetic field, with detectable consequences in the local density of states probed by STM.
We report on the experimental observation by scanning tunneling microscopy at low temperature of ring-like features that appear around Co metal clusters deposited on a clean (110) oriented surface of cleaved p-type InAs crystals. These features are visible in spectroscopic images within a certain range of negative tunneling bias voltages due to the presence of a negative differential conductance in the current-voltage dependence. A theoretical model is introduced, which takes into account non-equilibrium effects in the small tunneling junction area. In the framework of this model the appearance of the ring-like features is explained in terms of interference effects between electrons tunneling directly and indirectly (via a Co island) between the tip and the InAs surface.
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We investigate theoretically the effect of nearby As (arsenic) vacancies on the magnetic properties of substitutional Mn (manganese) impurities on the GaAs (110) surface, using a microscopic tight-binding model which captures the salient features of the electronic structure of both types of defects in GaAs. The calculations show that the binding energy of the Mn-acceptor is essentially unaffected by the presence of a neutral As vacancy, even at the shortest possible ${rm V}_{rm As}$--Mn separation. On the other hand, in contrast to a simple tip-induced-band-bending theory and in agreement with experiment, for a positively charged As vacancy the Mn-acceptor binding energy is significantly reduced as the As vacancy is brought closer to the Mn impurity. For two Mn impurities aligned ferromagnetically, we find that nearby charged As vacancies enhance the energy level splitting of the associated coupled acceptor levels, leading to an increase of the effective exchange interaction. Neutral vacancies leave the exchange splitting unchanged. Since it is experimentally possible to switch reversibly between the two charge states of the vacancy, such a local electric manipulation of the magnetic dopants could result in an efficient real-time control of their exchange interaction.
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