Resumo em Inglês:

A brief overview of electron manipulations in nanostructured semiconductors is given. Such manipulations go average transfer of less than a single electron per controlled step. The applications to nanodevices the scaled down silicon microelectronics is also discussed.

Resumo em Inglês:

Semiconductor superlattices are artificial crystals with a periodicity much larger than lattice constants of the constituent materials. Applying an electric field parallel to the superlattice axis, i.e., perpendicular to the layers, results in unique electronic and transport properties such as miniband transport and Wannier-Stark localization, which are the basis for the observation of Bloch oscillations. At larger field strengths, resonant tunneling between different mini- or subbands dominates the electronic transport properties. Miniband transport and resonant tunneling are highly nonlinear process resulting in interesting phenomena of self-organization of the field distribution.

Resumo em Inglês:

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple Kane band-structure scheme and the interaction with the laser field is incorporate through the renormalization of the semiconductor energy gap and conduction/valence effective masses. Far from resonances, such one-body approach allows the study of the effects of laser fields on a variety of optoelectronic phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description. We calculate the effects originated by the laser-dressing on the donor and exciton peack energies in quantum-well heterostructures, and show that they may be quite considerable and observable.

Resumo em Inglês:

We present a theoretical study of the spin dynamics of site-localized excitons in semiconductor quantum wells driven by the electron-hole interaction. Using a simple model for the distribution of sites dephasing leading to a strong spin polarization decay.

Resumo em Inglês:

GaAs/AlGaAs multiple quantum well structures containing an nipi delta-doping superlattice, where the n-type doping is inserted in the quantum wells and the p-type in the barriers, have been studied in detail to evaluate their potential for use in the fabrication of amplitude modulators. It is shown that C is an adequate p-type dopant for such structures, however, little exibility is found in the growth conditions, in particular for the V to III uxes ratio, for obtaining such layers. It is also observed that the required balance between n and p type doping levels is not trivial to be achieved due to the presence of interface hole traps whose population depends on the quantum well doping concentration. In addition, the observed photoluminescence near-edge emission at room temperature occurs at essentially the same energy as that of an equivalent undoped structure. Finally, no deep level emissions are observed which could deteriorate the device performance.

Resumo em Inglês:

The phenomenon of electromagnetically induced transparency (EIT) was discovered by Adriano Gozzini and coworkers in 1976 in Pisa. Novel schemes to investigate and exploit EIT in the optical domain have attracted much interest both in atomic and solid state systems. We discuss some of our recent theoretical results, in particular: i) a well developed EIT regime based on free exciton levels in undoped bulk crystalline Cu2O; ii) light dragging effects in moving media under EIT; iii) the coherent control of Cherenkov radiation in the EIT regime.

Resumo em Inglês:

The electron{acoustic-phonon scattering process in spherical II-VI quantum dots is discussed. The quantized acoustic modes are described in terms of the Lamb's classical theory. We considered two mechanism for the interaction between electrons and acoustic modes: microscopic deformation potential, and macroscopic acoustic deformation, also called the ripple mechanism. We also discuss the in uence of the glass matrix on the electron-phonon coupling. Our calculations show that the ripple mechanism scattering rates become dominant by more than an order of magnitude, for small dot radius. In general, the total scattering rate depends on the acoustical properties of the glass matrix.

Resumo em Inglês:

The characteristics of multi-layered InAs/GaAs self assembled quantum dots (SAQDs) annealed after the growth were here studied using a combination of capacitance-voltage (C-V) measurements,Raman scattering and photoluminescence (PL) spectroscopy. The combination of the results obtained with the three techniques, gave evidences that the annealing at 500º C causes the sharpness of the SAQDs interfaces, while the annealing at 600º C eliminated the SAQDs. However, the comparison with the case of single layered SAQDs, revealed a thermal stability of the last system even at an annealing temperature of 700º C, thus confirming the role of the interlayer strain in the low temperature diffusion process.

Resumo em Inglês:

The vertical coupling of InAs quantum dots and their capping with In0. 1Ga0. 9As layers were investigated in order to shift the optical emission of the structures toward longer wavelengths. We observed that both ways can not be used simultaneously by just replacing part of the usual GaAs spacer layer by In0. 1Ga0. 9As because the intermixing of Ga and In atoms is enhanced in stacked layers and causes a blueshift of the emission instead of the expected redshift.

Resumo em Inglês:

We theoretically investigate the effect of the lead dimensionality on the non-equilibrium electron transport through quantum dots. More precisely, we study nonlinear transport in a quantum dot coupled to leads of diverse dimensionality. We show that the presence of the latter strongly determines the resulting transport properties. Differently from higher dimensional leads (wide and smooth band limit), van Hove singularities in the density of states of low-dimensional reservoirs determine sharp resonances in the differential conductance at -nite applied voltages as well as in the dot spectral density. It is also shown that, due to the finiteness of the terminal bandwidth, the differential conductance change its sign at higher biases. These results clearly indicate that the environment doesplay an important rôle in determining transport properties in mesoscopic systems.

Resumo em Inglês:

We present calculations of the conductance in semiconductor quasi-one-dimensional systems using the Landauer formalism. We consider the effects on the transport properties inside semiconductor quantum waveguides of different shapes when a defect is located either in the wire region or in the quasi-two-dimensional region. We observe changes on the plateau's threshold when the defect is placed inside the wire and lowering of the conductance plateaus themselves below the conductance quantum G0 =2e² /h when the defect is outside the wire.

Resumo em Inglês:

We discuss the main theoretical approaches for the phonon-assisted tunneling in double barrier resonant tunneling devices and introduce a quantum coherent treatment based on the mapping of the many-body problem into a higher dimensional one-body system. Conditions for a maximized phonon-emission are established.

Resumo em Inglês:

We have investigated the electronic structure of Cd1_xMn xTe parabolic quantum wells and half-parabolic quantum wells heterostructures in the envelope function approximation using the k · p method. We have considered an external magnetic field applied in two different configurations: in the plane of the layer and in the growth direction. The confined states have been calculated taking into account the effective mass dependence of the Mn concentration and the strain effects. In this work we analyze the giant Zeeman splitting due to applied magnetic fields and we havecompared the energies of the excitonic transitions hh (lh) ->el for the magnetic field in the two considered configurations. Comparison of our results for the half-parabolic quantum wells with available experimental data is made.

Resumo em Inglês:

Self-consistent numerical calculation and photoluminescence (PL) measurements have been used to investigate the temperature dependence of the optical Stark effect in n-doped GaAs/AlGaAs single asymmetric quantum wells (SAQWs), grown by molecular beam epitaxy. In the low-temperature regime (5 to 40 K) a remarkable blue shift (9.8 meV) is observed in the PL peak energy, as the optical excitation intensity increases from 0.03 to 90 W/cm2. The blue shift is well explained by the reduction of the two-dimensional electron gas (2DEG) density, due to a charge-transfer mechanism. At about 80 K, however, an anomalous behavior of the PL peak energy was found, i.e. a red shift has been observed as the optical excitation intensity increases. This anomalous behavior has been explained by combining the effects of band gap renormalization, band bending, temperature dependence of the band gap, temperature dependence of the 2DEG density, and temperature dependence of the fundamental energy position.

Resumo em Inglês:

The behavior of excitons in spatially modulated magnetic fields is described taking into account theb exciton spin contribution. The results show that the exciton trapping in periodic magnetic fields is possible and dependent on the modulation profile.

Resumo em Inglês:

Photoluminescence (PL) and excitation PL measurements have been performed at different temperatures in a number of lattice-matched GaAs/In0.49Ga0.51P quantum wells, where the uctuations of the potential energy are comparable with the thermal energy of the photocreated carriers. Two samples with different well widths allow to observe a series of anomalous e ects in their optical response. The observed effects are related to the disorder in the interface, characterizing uctuations in the confinement potential energy. It is proposed that the carrier relaxation processes occur either at the local minima or at the absolute minimum of the confinement potential, depending on the ratio of the thermal energy and the magnitude of the potential fluctuations.

Resumo em Inglês:

Optical experiments on electric field tunable AlAs/GaAs coupled quantum wells clearly demonstrate that the optical nature of these structures can be directly controlled by an applied electric field. In this work we are concerned with calculation and analysis of the first electron and holOe states in these structures. We take into account the effects of the heterostrucuture geometry and of the external uniform electric field applied perpendicularly to the layers, which may lead to an anticrossing of low-lying electron levels in the system. Calculations are performed within the tight-binding supercell formalism with interactions between atomic orbitals up to second nearest-neighbors. Our results show that for GaAs layers less than <img src="http:/img/fbpe/bjp/v32n2a/a18img01.gif" align="absMiddle"> 30 Å wide, application of electric fields takes the structures from typeII to type I heteroestructure behavior. Our estimated value of the electric field intensity needed to cause this transition is in good agreement with the experimental results for comparable heteroestructures geometries .

Resumo em Inglês:

The quantum transport of spin-polarized electrons across nonmagnetic III-V semiconductor multiple barriers is considered theoretically. We have calculated the spin dependent transmission coeficient, for conducting electrons transversing lattice-matched In0. 53Ga0. 47As/GaAs0. 5Sb0: /In0. 53Ga0. 47As/ InP/In0. 53Ga0. 47As nanostructures with different numbers of asymmetric double barriers, as a function of electron energy and angle of incidence. Spin-orbit split resonances, due to the Rashba term, are observed. The envelope function approximation and the Kane k ·p model for the bulk are used. For an unpolarized incident beam of electrons, we also obtain the spin polarization of the transmitted beam. The formation of spin dependent minibands of energy with nonzero transmission is observed.

Resumo em Inglês:

Shot noise is a time-dependent current uctuation due to the discrete character of the electron charge. Here we address shot noise in a spin-resolved tunneling system under the influence of spin-flip scattering within a master equation approach. We find that the average current <FONT FACE=Symbol>á</font>I<FONT FACE=Symbol>ñ</FONT> and Fano factor gamma("normalized noise") present contrasting behavior for differing spin-flip time ratios: <FONT FACE=Symbol>á</FONT>I<FONT FACE=Symbol>ñ</FONT> decreases while gamma increases for <img src="http:/img/fbpe/bjp/v32n2a/a20img03.gif" align="absMiddle"> > <img src="http:/img/fbpe/bjp/v32n2a/a20img01.gif" align="absMiddle"> as compared to the <img src="http:/img/fbpe/bjp/v32n2a/a20img03.gif" align="absMiddle"> = <img src="http:/img/fbpe/bjp/v32n2a/a20img01.gif" align="absMiddle"><FONT FACE=Symbol> </FONT>case and vice versa for <img src="http:/img/fbpe/bjp/v32n2a/a20img03.gif" align="absMiddle"><FONT FACE=Symbol> </FONT>< <img src="http:/img/fbpe/bjp/v32n2a/a20img01.gif" align="absMiddle">.

Resumo em Inglês:

Here we investigate the spin-dependent subband structure of newly-developed Mn-based modulation-doped quantum wells. In the presence of an external magnetic field, the s-d exchange coupling between carriers and localized d electrons of the Mn impurities gives rise to large spin splittings resulting in a magnetic-field dependent subband structure. Within the framework of the effective-mass approximation, we self-consistently calculate the subband structure at zero temperature using Density Functional Theory (DFT) with a Local Spin Density Approximation (LSDA). We present results for the magnetic-field dependence of the subband structure of shallow ZnSe/ZnCdMnSe modulation doped quantum wells. Our results show a significant contribution to the self-consistent potential due to the exchange-correlation term. These calculations are the first step in the study of a variety ofinteresting spin-dependent phenomena, e.g., spin-resolved transport and many-body effects in polarized two-dimensional electron gases.

Resumo em Inglês:

Transport properties of a GaAs/AlAs superlattice-like double barrier diode are studied in this work as a function of the sample temperature. An activation energy of about 60meV obtained from the Arrhenius plot is in good agreement with the confined level in the central well. Numerical simulations also confirm the importance of bound levels in the gammaand X bands for the resonant tunneling process. The enhancement of photoluminescence as the temperature is increased is also studied. This behavior is associated to the transport properties of holes in the collector contact, which control the supply of minority carriers, which tunnel into the well. The description of the observed results requires the modi-cation of simple known models to take into account the two contributions to the pair generation rate in the well, responsible of the photoluminescence at zero and finite bias.

Resumo em Inglês:

The interfaces in InP=In0.53 Ga0.47As superlattices modulation doped with Si were investigated using magneto-transport, capacitance-voltage, and high resolution X-ray diffraction measurements. Results indicate that a thick interfacial layer is formed when InP is grown on top of In xGa1-xAs, and that Si atoms that fall in the interfacial layer have a high probability of not forming a shallow donor center. Using a simple theoretical model the width of the interfacial layer which was estimated to be 7±1 monolayers.

Resumo em Inglês:

Using the balance of forces in the linear regime, it is shown that the ratio between the induced voltage in an open circuit Coulomb drag experiment, and the induced current in closed circuit, equals the internal resistence of the circuit in which the induction occurs.

Resumo em Inglês:

The existence of quantized levels, as a consequence of spatial confinement, does not necessarily imply in formation of superlattices minibands due to the coupling between these quantum wells. The present work discuss a framework to analyze the analoge to minibands in periodic superlattices made of disordered quantum wells. The inter quantum well coupling leads to diffusive-like minibands.

Resumo em Inglês:

We present a theoretical study on the electron mobility and scattering mechanism in a remotely doped AlGaAs wide parabolic quantum well. Electron mobilities in different subbands are calculated from the self-consistent results of the subband energy and wavefunction in the system. The scattering due to ionized impurities and alloy disorder is considered. We show theinterplay of the different scattering mechanisms.

Resumo em Inglês:

Shubnikov-de Haas oscillations are measured in a wide parabolic quantum well with 6 subbands in a tilted magnetic fi eld. We find two types of oscillations. The oscillations at low magnetic field are shifted towards higher fields with tilted angles, and can be attributed to the two-dimensional Landau state at the bottom subband. The position of the second type oscillations do not shift with tilted angles indicating a three-dimensional character of the Landau state formed by the highest subbands. The bottom level in the quantum well is not overlapped with the highest subbands due to the enhanced quantum scattering time of the lowest subbands.

Resumo em Inglês:

The effective magnetoconductivity tensor of selectively doped heterostructures with non-screened potential is evaluated for high-frequency regime. The lateral inhomogeneity of the donor distribution in the structure results in a large-scale variation of the energy levels. The contribution due to such inhomogeneity takes place in addition to the standard Drude mechanism of absorption and this mechanism is dominant for the high-frequency regime.

Resumo em Inglês:

We have investigated the transport of carriers in GaAs using time resolved optical spectroscopy with picosecond resolution. Carriers are optically created to the sample surface by an ultra-fast laser pulse. They diffuse and drift throught a thick GaAs layer, until they are captured by an InGaAs quantum well, where they recombine with holes from a p-type doped layer at an inner InGaP barrier. Our study was performed with a set of samples with different GaAs layer thickness. As the GaAs thickness increases, the emission from the quantum well is delayed and its decay slows down significantly. We have investigated the effect of an applied DC field between the surface and the InGaAs quantum well. The transient of the quantum well emission is mostly independent of the applied DC voltage up to field of the order of 20 KV/cm, including both polarities. This is a clear indication that the carrier transport is dominated by ambipolar diffusion due to the Coulomb interaction that strongly couples photoinjected electrons and holes. On the other hand, the decay of the GaAs emission varies signi-cantly when a DC gate voltage is applied such as a current appears at the structure.

Resumo em Inglês:

A study of the dynamical behavior of the charge in an asymmetric double quantum well is presented. In particular, the time behavior of intrasub-band dipole moment which is induced by intersub-band optical excitation is analyzed in two aspects: first, design of the quantum wells in order to spatially separate the charge and store it and second study the dynamic tunneling effect between the two wells. Since both conduction band levels are optically populated by transitions from a single band level, they are coherently coupled. The intrasub-band density matrix completely characterizes the dynamics of the conduction band electrons. These dipole moment oscillations are entirely equivalent to the tunneling oscillations between the two wells. The time dependent dipole moment produced by these real-space charge oscillations is obtained as a source of dipole electromagnetic radiation in THz region, which has been detected experimentally. The calculations show the geometry dependence of the emitted radiation. Based on the reported decoherence time for this process a coherent control mechanism of the THz radiation is introduced by varying the phase of a second laser.

Resumo em Inglês:

The surface segregation of indium atoms during the growth of InGaAs/GaAs heterostructures has been investigated in situ by reflection high-energy electron diffraction (RHEED). We pointed out that strong damping of the RHEED oscillations during the deposition of InGaAs on GaAs was related to the segregation strength of indium atoms in the InGaAs layer. A simple model shows that the decay constant of the RHEED oscillations may be used to determine accurately the segregation coefficient R, as confirmed by photoluminescence (PL) measurements.

Resumo em Inglês:

Carbon doped AlInAs layers grown by low pressure metalorganic vapor phase epitaxy are investigated as a function of growth temperature. Photoluminescence spectra show a significant drop in peak intensity as the growth temperature is reduced, due to the incorporation of non-radiative defects. It is shown that the C doping is not related to the deterioration of the optical properties of the layers. On the other hand, both the net hole concentration and the conductivity increase as the growth temperature is reduced due to a more efficient C incorporation and a lower incorporation of the passivating H atoms.

Resumo em Inglês:

In this work we study the structural properties of amorphous oxynitride films (a-SiOxNy), grown by plasma enhanced chemical vapour deposition (PECVD)at 320° C. The films were deposited at different ow ratio of N2O and SiH4. The atomic composition of the samples was determined by means of Rutherford backscattering spectrometry (RBS). The local order structure was studied by X-ray absorption spectroscopy (XAS) and the chemical bondings were investigated by Fourier transform infrared (FTIR) spectroscopy. The results evidence a tetrahedric arrangement of the oxynitride network. The tetrahedrons are similiar to SiO3N for x values between 1.43 and 1.64. For x values higher than 1.64 the tetrahedrons are similar to SiO4.

Resumo em Inglês:

We have studied numerically the stability and defect nucleation in epitaxial layers on a substrate with lattice mismatch. Stress relaxation and energy barriers for misfit dislocation nucleation are estimated using modern methods for saddle point search based on a combination of activation with local repulsive potential and the Nudged Elastic Band method. Stress relaxation processes correspond to different transition paths from coherent to incoherent states of the epitaxial layer.Using a two-dimensional atomistic model with Lennard-Jones interacting potential, we and different equilibrium critical thickness and activation energy behavior for dislocation nucleation of epitaxial films under tensile and compressive strain. For tensile strain, the energy barrier decreases with thickness while it reaches a constant value for compressive strain.

Resumo em Inglês:

This work describes a systematic study of the preparation of nano-crystalline thin h-BN films by Plasma-Enhanced Chemical Vapor Deposition (PECVD) technique. The samples were prepared at low temperatures using B2H6 and N2 as gas precursors. It is shown that the flow ratio among these gases has an important influence on the size of the crystallites (deduced by Raman spectroscopy). The 2H6/N2 flow ratio was varied from 1,6 x 10-2 to 6.7 x 10-4 leading to films presenting a crystallite size, which varied from 90 nm to amorphous, respectively. The XRD spectra show two peaks at 2 values around 42° and 44°, which are associated to <img src="http:/img/fbpe/bjp/v32n2a/menor.gif"> 110 <img src="http:/img/fbpe/bjp/v32n2a/maior.gif"> and <img src="http:/img/fbpe/bjp/v32n2a/menor.gif"> 100 <img src="http:/img/fbpe/bjp/v32n2a/maior.gif"> h-BN crystalline directions, indicating, a <img src="http:/img/fbpe/bjp/v32n2a/menor.gif"> 002 <img src="http:/img/fbpe/bjp/v32n2a/maior.gif"> preferential orientation for the h-BN crystallites. The thermo-mechanical properties, as stress, hardness, and Young modulus were also studied and correlated with the structural properties. The composition of the films was obtained by RBS and EDS indicating Boron to Nitrogen ratio, close to stoichiometry for all the studied deposition conditions.

Resumo em Inglês:

We have investigated the crystallization of amorphous SiGe films deposited on crystalline GaAs (001) substrates using ns laser pulses. Analysis of the film structure using Raman spectroscopy indicates the formation of heteroepitaxial Si xGe1/GaAs structures for Si compositions up to x = 25%. Higher compositions lead to polycrystalline films. This is attributed to the increased lattice mismatch between Si xGe1 and GaAs as the Si fraction in the alloy increases

Resumo em Inglês:

This paper addresses the doping mechanism of amorphous semiconductors through the investigation of boron doped rf co-sputtered amorphous hydrogenated silicon. The activation energy and room temperature conductivity varied from 0.9 to 0.3 eV and from 10-12 to 10-4 Ohm-1 .cm-1, respectively, by ranging the boron concentration from 0 to 3 at.%. These ranges of electronic properties are of the same order of those reported for samples prepared by plasma enhanced chemical vapor deposition (PECVD). In spite of these similarities, there are some relevant differences. The doping effciency, at low boron concentrations, is much lower than that of PECVD samples. In addition, the creation of deep defects (dangling bonds) does not follow the square root power law dependence on the boron concentration as proposed by Street and observed in PECVD samples. These differences are associated with the density of defects, which is much higher in films prepared by sputtering. The increase in the deep defect density is more likely related to topological disorder, introduced by the presence of a high concentration of inactive impurities.

Resumo em Inglês:

Potentiostatic electrodeposition methods have been used to grow polycrystalline ZnSe and ZnSe-Fe granular thin films on stainless steel substrates from aqueous solutions. These films were characterized by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy and Raman spectroscopy.

Resumo em Inglês:

A photon Green's function theory is used to incorporate Bethe- Salpeter-like many body corrections in the the computations of nonlinear optical spectra of semiconductor quantum well laser materials. The numerical results presented reproduce several features found in the experimental characterization of the actual devices.

Resumo em Inglês:

We performed comparative studies of radiation-induced interface trap generation at n-6H-SiC/SiO2 and Si/SiO2 interfaces for high doses of low energy x-rays in the range 0 to 200 Mrad (Si). We found that radiation-induced interface trap generation at the n-6H-SiC/SiO2 interface is much lower than at the Si/SiO2 interface. The experiments provide evidence of interface trap generation near the SiC conduction band associated with oxide traps.

Resumo em Inglês:

Using a first-principles pseudopotential method we have compared the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(21) and Ge(001){(21) surfaces. We find that the dissociated state is energetically more favourable than the molecular state by 1.70(0.81) eV, whereas the latter is 0.58(0.25) eV more stable than the system composed of the free silicon(germanium) surface and PH3(g). The chemisorbed system is characterised by elongated Si{Si(Ge{Ge) dimers that are symmetric in the dissociative case and asymmetric in the molecular case and by the fact that the Si(Ge){PH2 as well as the PH3(ads) groups retain the pyramidal geometry of the phosphine molecule. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.

Resumo em Inglês:

In this work, using Density Functional, Hartree-Fock and Extended Hückel Theories together with the supercell and cluster model approaches, we present our preliminary results for the simulation of the adsorption of Si and C atoms over the (111) SiC surfaces as well as the torsion of SiC molecules at that surfaces. Our results shown that, before and after the adsorption, the cubic structure are the most stable one, when compared with the hexagonal ones. However, the torsional energy barrier between the cubic and the hexagonal structures is small enough that, at the conditions of SiC growth, it facilitates the SiC polytype formation.

Resumo em Inglês:

Atomic Force microscopy was used to investigate the morphology of GaAs(001) layers deposited by molecular beam epitaxy using a non conventional growth method that involves the supply of a few monolayers of gallium (the arsenic cell is shuttered) followed by the annealing of the surface under an arsenic ux (the gallium cell is shuttered). When the silicon shutter is opened and closed together with the gallium one, this particular growth mode allows the silicon atoms to be incorporated into the arsenic sites and p-type GaAs(001) layers can be obtained. Several sets of samples were grown and analyzed in order to better understand the microscopic growth mechanisms of this kind of layers and minimize the number and size of the structural defects that are characteristic of this peculiar technique.

Resumo em Inglês:

The steady state hole transport properties of GaSb were investigated by the Ensemble Monte Carlo method (EMC). In our model we taken in to account heavy and light hole valence bands, and the following scattering mechanisms: inelastic acoustic phonon; polar optical phonon; nonpolar optical; ionized impurity. The theoretical calculations are compared with available experimental results for the GaSb hole mobility shown good agreement with temperatures from 90K up to 300K.

Resumo em Inglês:

First-principles calculations of electronic band structures of the ordered cubic alloys Al xGa1-xN and Cd xZn1-xTe are carried out. The band structures are used to provide e ective masses and Luttinger parameters which are useful in the parametrization of theories based on e ective hamiltonians.

Resumo em Inglês:

Electron spin resonance (ESR) and dc-magnetization experiments have been performed in Gd-doped amorphous (a-)silicon films. The films were deposited by the cosputtering technique following different conditions rendering samples with varying Gd and hydrogen concentrations. In addition to films with different contents of impurities and, in order to probe the inuence of the atomic structure on the magnetic properties of the Gd species, the films were also submitted to laser-induced crystallization processing. Both ESR and dc-magnetization results show that Gd is incorporated as a trivalent ion (Gd3+) in the Si host. ESR data indicate a strong dependence between the Gd concentration and the density of Si dangling bonds. Moreover, the Gd3+ local environment is nearly insensitive to the Gd and H content, as well as to the atomic structure of the Si host. Along with gadolinium, other rare-earth species have been investigated and the main results are discussed.

Resumo em Inglês:

We have carried out our experiments on a semi-insulating GaAs sample grown by low temperature Molecular Beam Epitaxy. Three parameters were used to fine tune the experiments, namely, temperature, illumination, and applied bias. We have used powerful tools of time series analysis in order to assess the embedding dimension through near false nearest neighbors. We have also measured the maximum Lyapunov exponent and the correlation dimension [1]. The main contribution was in presenting new experimental data and its analysis which presents self-generated chaos.

Resumo em Inglês:

We present a normal-mode analysis of coupled photocarrier-phonon dynamics in Te. We consider a dielectric function which accounts for LO phonons and the electron-hole gas within the Debye- Hüuckel model and RPA. Our main finding is the existence of a bare LO phonon mode in the system even at high carrier density. This oscillation is an unscreened L - (q) mode arising from ineffective screening at large wave vectors. This mode is consistent with the bare LO-phonon peak in recent THz-emission spectra of Te.

Resumo em Inglês:

Iron-acceptor pairs (Fe-A, A = B, Al, Ga, and In) in silicon were investigated using an ionic-based model, which incorporates the valence electron cloud polarization and the lattice relaxation.Our results are generaly in good agreement with the experimental trends among the Fe-A pairs, describing the increase in the pair donor energy level with increasing A principal quantum number and decreasing pair separation distance, and the pair configurational symmetries.

Resumo em Inglês:

We report the development and characterization of high quality polyaniline-silicon heterojunction diodes appropriated for use as gas and/or ionizing radiation sensors. Polyaniline thin films 40 nm thick are an active part of the junction structure, that presents excellent electrical characteristics, with rectifying ratio of 50,000 at ± 1.0 Volt bias. The devices are very sensitive to gamma-radiation up to 6,000 Gy and to gas moistures such ammonia, nitric acid and trichloroethylene. The sensitivity of the diodes is observed through shifts of the current-voltage (I-V) curves which can be easily monitored to provide a calibration curve of the sensor either as a radiation dosimeter or as a gas sensor for use in applications in environments demanding gas monitoring or radiation dosimetry.

Resumo em Inglês:

We present a theoretical study on local electronic properties of quantum-dot nanotubes formed by connecting pure semiconducting and metallic nanotubes via appropriate junctions. The junctions are formed by introducing pair-defects composed by heptagon and pentagon along the axial direction of pure nanotubes. We investigate the dependence of the confined electronic states with the characteristic sizes of the quantum dot taking it account different based-nanotube heterostructures. Quantum-well like and interface states are characterized by investigating the spatial dependence of the local density of states of the discrete levels. We follow the Green function formalism and adopt real-space renormalization techniques in the derivation of local density of states.

Resumo em Inglês:

Carbon nanotubes have recently attracted interest for their possible applications as nanoactuators and nanoswitches, as well as possible building blocks for nanoelectronics. We present ab initio calculations for uniformly charged graphene and (11,0),(9,0) and (5,5) carbon nanotubes. We also consider the effects of polaron formation in these systems. The strain-charge coeÆcient is calculated and compared for both graphene and the nanotubes under study.

Resumo em Inglês:

A self-consistent calculation of the electronic properties of GaAlAs:Mn magnetic semiconductor quantum well structures is performed including the Hartree term and the sp-d exchange interaction with the Mn magnetic moments. The spin polarization density is obtained for several structure configurations.

Resumo em Inglês:

Mn in GaAs substitutes Ga as a p dopant and introduces a localized magnetic moment of 5<img src="http:/img/fbpe/bjp/v32n2a/a54img01.gif" align="absMiddle">/2. Ga1–xMn xAs (for x <FONT FACE=Symbol>»</font> 0.04) has been proved to be an extremely interesting material, for its intrincated transport and magnetic properties. In the first part of this work a structure is considered in which a series of Ga1–xMn xAs layers is grown inside a GaAs quantum well. The electron-electron interaction through a Hartree potential, together with the magnetic interaction with the DMS layers (assumed in the ferromagnetic phase) produces an e effctive potential corresponding to a sequence of small barriers which depend (height and width) on the hole spin orientation. For a large number of such layers it is shown that the model structure approaches a magnetic superlattice. A self-consistent calculation is performed for the electronic structure, resulting into spin-polarized energy levels. The polarized miniband Bloch transport properties are calculated for different structural arrangements. In the second part we calculate the transversal spin polarized current induced by an applied voltage through an AlAs/Ga1–xMn xAs/AlAs double barrier heterostructure (DBH). The magnetic layer is assumed in its metallic and ferromagnetic phase. As a consequence of the magnetic interaction a strong polarization of the hole subbands inside the quantum well. In that case the resonant tunneling occurs at different voltages for different spin orientation (parallel or anti-parallel to the average magnetization of Ga1–xMn xAs) throughout. Based on a many-body field quantization formalism that makes use of the tight binding method we obtain, in an essentially exact way, an analytic expression for the spin polarized tunneling current and the escape rate from the right barrier of the DBH. The characteristic spin polarized I(V) curves, as well as the relaxation times, are obtained for different hole concentrations and magnetic layer widths, providing important informations for the possibility of building spin valves made those materials.

Resumo em Inglês:

By combining ab initio methods and interatomic potentials, we investigated the electronic and the structural properties of amorphous silicon nitride. Our results show the trends on the electronic band structure of SiNx (0.5 < x < 1.8) as the nitrogen content changes, and are in good agreement with the experimental data.Hydrogenation of the systems was found to considerably reduce the number of energy levels in the gap. The energy levels which appeared in the materials band gap were compared to the electrically active centers, which have been identified by electron-paramagnetic resonance experiments.

Resumo em Inglês:

We contribute here a theoretical study of the electron mobility in n-doped GaN, InN, and AlN at moderate to high electric fields. We solve the set of coupled nonlinear integro-differential equations of evolution to obtain the steady-state values of the basic intensive nonequilibrium thermodynamic variables for the three materials. The regions with ohmic and non-ohmic behavior in the electron drift velocity dependence on the electric field strength are characterized in the three nitrides. The electron mobility is calculated, and it is shown that the larger corresponds to InN, and the smaller to AlN.

Resumo em Inglês:

The complex mobility of electrons in 3C - and 6H-SiC subjected to intense high frequency electric fields is calculated taking into account effects of band nonparabolicity. The electric field, given by a dc component plus an ac component in the frequency range 0.1-100 THz, is applied along the [0001] ([111]) direction in the hexagonal (cubic) polytype. The real electron mobility presents a characteristic maximum peaking around 6-8 THz, while the imaginary electron mobility is structured, with characteristic minimum and maximum around 2-3 THz and 20{30 THz, respectively. These mobilities are seen to smooth down for higher temperatures in both polytypes.

Resumo em Inglês:

Calculation of the Raman spectra of zinc{blende InN/AlN superlattices were carried out assuming the existence of an interface region with thickness deltavarying from zero to three monolayers. Frequency shifts up to 80 cm were observed for some of the optical frequencies when delta = 3. Many peaks appearing at the low frequency side, shift toward the center position of the spectrum. As a consequence, pairs of the Raman modes become quasi-degenerate giving rise to highly prominent structures in the spectrum, for delta = 2 and 3. These effects are tracked to localization of atomic displacements mainly at the interface regions.

Resumo em Inglês:

Using the density-functional theory within the Full Potential Linear Augmented Plane-Wave (FP-LAPW) method, we have calculated, in this work, the equation of state, the effective charges and the phonon dispersion along [100] and [111] directions for the cubic III-Nitrides(c-BN, c-AlN, c-GaN and c-InN), including hydrostatic strains dependence. A good agreement with the micro-Raman scattering experiment is obtained for the phonon modes at <FONT FACE=Symbol>G.</FONT> Our results show that, for c-InN case, the apparent discrepancy between the known experimental results is a consequence of hydrostatic effects on the sample due to differences of the used Raman methods.