Repositório RCAAP

This paper focuses on recent results on the optical properties of self-assembled quantum dots involving type-I InGaAs/GaAs and type-II InP/GaAs interfaces. In the first part, we focus on the InGaAs/GaAs quantum dots, that were used to study the influence of a two-dimensional electron gas on the optical emission of single quantum dots. In the second part, we present the results on type-II InP/GaAs quantum dots. In this system, we observed an experimental evidence indicating that the model used to interpret the blue shift of the type-II quantum well optical emission cannot be applied to type-II quantum dots.

A short overview is given of recent advances in the field of carrier-controlled ferromagnetism in Mn-based III-V and II-VI diluted magnetic semiconductors and their nanostructures. The tailoring of domain structures and magnetic anisotropy by strain engineering and confinement is described. Experiments demonstrating the tunability of T C by light and electric field are presented.

The adsorption of maleic anhydride on the Si(001) surface was investigated using the first-principles pseudopotential formalism. Our results indicate that C2H2-C2O3 adsorbs through a [2+2] cycloaddition of the C-C bond to the dangling bonds of Si-Si dimers, as observed for other small hydrocarbons. According to our calculations, the adsorption of a second maleic anhydride molecule will preferentially occur on first-neighbours Si-Si dimer sites in either the dimer chain our dimer row, i.e. considering a (4×2) reconstruction, C2H2-C2O3 can either form a linear chain or a zig-zag chain, depending on the growth conditions. Although both structures are found to be stable from the energetic point of view, only our STM theoretical images for the linear chain are comparable to available experimental data.

Using the Keldysh nonequilibrium technique, we study current and the tunnelling magnetoresistance (TMR) in a quantum dot coupled to two ferromagnetic leads (FM-dot-FM). The current is calculated for both parallel and antiparallel lead alignments. Coulomb interaction and spin-flip scattering are taken into account within the quantum dot. Interestingly, we find that these interactions play a contrasting role in the TMR: there is a parameter range where spin flip suppresses the TMR, while Coulomb correlations enhance it, due to Coulomb blockade.

We report ab-initio all electrons density- functional calculations for the electronic structure of the compounds MnAs and MnN, in the zinc-blende phase. They are potential materials for use in fabrication of new functional semiconductors taking advantage of the spin degree of freedom. The aim is the establishing of the semiconductor spin electronics (spintronics) as a practical technology [H. Ohno, Semiconductor Science and Technology 17, 4 (2002).]. We compare results obtained using the theoretical approaches LDA (Local Density Approximation) and GGA (Generalized Gradient Approximation). The calculations are spin-polarized and we follow the evolution of the band structures as a function of lattice parameter. We compare also the evolution of the density of states of the majority-spins and of the minority-spins. We conclude that, depending on the lattice parameter, both materials may be half-metallic, therefore showing conduction by charge carriers of one spin direction exclusively: the majority- spin band is continuous, while the minority-spin has a gap. Both materials reach a total magnetization of the order of 4 mB. MnN changes from paramagnetic to ferromagnetic with the increase of the lattice parameter.

In this work several structural and chemical properties of self-assembled InAs islands grown on GaAs(001) are studied using surface x-ray scattering with synchrotron radiation. The technique of x-ray diffraction under grazing incidence condition was employed to differentiate coherent and incoherent islands. We used a model of a strained pyramidal island to interpret the x-ray results and correlate size and strain-state of these islands. The degree of GaAs interdiffusion in the islands was inferred from the variation of volume of the unit cell. The Poisson's ratio of the two materials involved establishes a limit of tetragonal distortion for this material. Any variation in this distortion is associated with the presence of Ga inside the islands.

4H- and 6H-SiC Schottky diodes responding down to 5 ppm of NO and NO2 gases at temperatures up to 450ºC were fabricated. Upon exposure to gas, the forward current of the devices changes due to variations in the Schottky barrier height. For NO gas, the response follows a simple Langmuir adsorption model. Device parameters were evaluated from linear conductance/current versus conductance plots, with improved accuracy. The linearity of these plots is an indication that the devices are not being affected by interfacial oxide layers or pinning of the Fermi level. 4H-SiC devices showed slightly superior stability and sensitivity for this application.

Ferromagnet-semiconductor heterostructures are promising materials for the integration of magnetic or spin related functions into semiconductor materials and devices. Fe-on-GaAs and MnAs-on-GaAs are canditates for potential room temperature applications. We review our recent results on growth, structural and magnetic properties of these materials, and their use as spin injectors. The most critical issue of Fe/GaAs heterostructures is the delicate interface formation during epitaxy and its possible modification during subsequent processing. The properties of the MnAs/GaAs heterostructures are intimately related to the phase transition between ferromagnetic and paramagnetic phases.

The possibility of coupling two Y-junctions of carbon nanotubes, forming a type of nanotube ring is addressed here. Recent experimental evidences of growing carbon nanotube structures forming basically Y-kind junctions give support to this theoretical proposition. A single-band tight-binding Hamiltonian is considered and local electronic properties are investigated via the Green function formalism following renormalization techniques. Modulation on the transport properties of those nanostructured systems may be induced by considering different kind of junctions and different symmetries. Quantum interference phenomena are proven to reduce partial or completely the conductance of such nanoring structures.

Site-selective luminescence spectroscopy (SSL) was employed to resolve the vibracional modes in self-assembled poly(p-phenylene vinylene), PPV. Using Franck-Condon analysis, the PPV spectra can be well described by three effective vibrational modes at 330, 1160 and 1550 cm-1. As the temperature increases from 4 to 180 K the electron-phonon coupling (S factor) remains constant for the 1550 cm-1 mode while for other lower energy modes the coupling increases monotonically. This trend is consistent with the decrease of the conjugation length due to the increase of thermal disorder. We find that the temperature dependence of vibrational progression in PPV films is determined mainly by low frequency torsional modes.

The electronic, structural, and thermodynamic properties of cubic (zinc blende) group-III nitride ternary In xGa1-xN and quaternary Al xIn yGa1-x-yN alloys are investigated by combining first-principles total energy calculations and cluster expansion methods. External biaxial strain on the alloys are taken into account in the calculations. While alloy fluctuations and strain effects play a minor role in the physical properties of AlGaN, due to the small lattice-mismatch, in the InGaN alloys we found a remarkable influence of strain on the phase separation. The effect of biaxial strain on the formation of ordered phases has been also investigated, and the results are used to clarify the origin of the light emission process in In xGa1-xN based optoelectronic devices. The dependence of the lattice constant and energy bandgap in quaternary Al xIn yGa1-x-yN alloys was obtained as function of the compositions x and y. PACS: 61.66.Dk, 64.75.+g, 71.20.Nr

We review our investigations on nanostructured ferromagnets fabricated on semiconductor heterostructures. We show that ballistic electrons in the semiconductor are in particular sensitive to local stray fields. The reversal of the ferromagnets give rise to characteristic magnetoresistance traces in the ferromagnet/semiconductor hybrid structures. We use these stray-field effects for detailed studies on the magnetic properties of rectangular micromagnets, nanodisks and nanorings. The nanostructures might be favorable candidates for ferromagnetic contacts in future spin-injection experiments in lateral feromagnet/semiconductor hybrid structures.

We carried a theoretical investigation on the electronic and structural properties of titanium impurities in silicon, diamond, and silicon carbide. The calculations were performed using the spin-polarized full-potential linearized augmented plane wave method in the supercell approach. The atomic configurations and transition and formation energies of isolated Ti impurities were computed.

Miniband charge transport was investigated by Monte Carlo simulations of electronic motion in short period superlattices of type-II energy band alignment (InAs/GaSb composition). The strong decrease of the miniband energy width when the electronic in-plane energy increases, characteristic of type-II superlattices, leads to a conductivity that is very sensitive to a magnetic field applied parallel to the axis of the superlattice. For structures with a miniband energy width greater than the optic phonon energy, the differential conductance can be changed from positive to negative by the magnetic field, due to the suppression of optic phonon emission.

We present, a systematic theoretical study of the adsorption of Ga, In and N over GaAs (110) surfaces based on parameter-free, self-consistent total energy and force calculations using the density functional theory. We analyzed the changes in the bond-lengths and in the bond-angles before and after deposition, as well as the total energy behaviour with the adsorbate chemical potential variation. Our results show that the GaN growth over GaAs (110) is energetically more favorable than the InN growth. However, the resulting InN film can be cubic only if we apply small strains on the substrate, while the GaN one is hexagonal.

We performed the first-principles band-structure calculations for a pure and a Cd-doped AgInO2 delafossite compound. The results are carefully analyzed and compared with the results obtained for a pure and a Cd-doped CuAlO2 compound, calculated previously by our group. The electronic structures of both systems are found to be similar in many details, being characterized by the same hybridization scheme that occurs at both Cu and Ag positions. Introduction of Cd impurity into the Cu site produces a shallow band within the CuAlO2 gap, while the Cd presence at the Ag site creates a wide band pinned at the Fermi level in the AgInO2 spectrum. In both cases, however, the n-type conductivities are predicted.

In this paper we theoretically investigate the magnetic-field and temperature dependence of the Shubnikov-de Haas oscillations in group II-VI modulation-doped Digital Magnetic Heterostructures. We self-consistently solve the effective-mass Schrödinger equation within the Hartree approximation and calculate the electronic structure and the magneto-transport properties. Our results show i) a shift of the Shubnikov-de Haas minima to lower magnetic fields with increasing temperature, and ii) an anomalous oscillation which develops when two opposite Landau levels cross near the Fermi energy. Both of these are consistent with recent magneto-transport measurements in such heterostructures.

In this work, we have calculated ab initio the phonon dynamics of the Ga-rich GaAs (100) (1 × 1), GaAs (100) (2 × 1), GaN (100) (1 × 1), GaAs (110) and GaN (110) surfaces. Our results for the (100) surfaces show that, while the (2 × 1) reconstruction is characterized by dimer vibrations, the (1 × 1) reconstruction presents interesting features which is closely related to the reduced mass of the compound. All the studied cases show the presence of a Rayleigh mode. The calculated properties for the (110) surfaces agree very well with the available experimental data from HEELS and inelastic He-atom scattering as well as with other theoretical calculations.

In this work we investigated the optical and structural properties of GaAs/GaInP quantum wells (QW) grown by Chemical Beam Epitaxy (CBE). The samples were characterized by photoluminescence (PL), photoluminescence excitation (PLE) and transmission electron microscopy (TEM). Simulations of the quantum well potential profiles, using the Van De Walle-Martin model, supplemented by our experimental results, allowed us to associate the interface properties with the growth procedures. We concluded that a thin GaP layer grown at the interface improves its quality and also that the observed broad emission band in the PL spectrum is related to quaternary Ga1-xIn xAs1-yPy.

We perform a theoretical investigation of the absorption and emission properties of quantum confined SiC/SiO2 spherical quantum dots, focusing on the the size-dependent emission and absorption spectra of an independent single dot. We observed strong oscillations in the absorption lines as a consequence of the interband transitions rules, and a blue-shift of the emission spectra lines with the decrease of the quantum dot diameter.