Repositório RCAAP

In this work we present the structural properties and stoichiometry analysis of thin films of lead iodide (PbI2). This material is a very promising semiconductor material for the development of X-ray detectors in digital medical imaging. An alternative deposition method called Spray Pyrolysis was used. We discuss the main advantages and limitations of the deposition process comparing three different starting material powders. Extra iodine atmosphere during deposition and the effect of post-deposition thermal treatment is also discussed. The structural properties were studied by X-ray diffraction, Atomic Force Microscopy (AFM), Scanning Electronic Microscopy (SEM) and stoichiometry analysis were performed using Energy Dispersive Spectroscopy (EDS).

A series of SnTe layers with thicknesses varying from 0.42 to 9.1 µm were grown by molecular beam epitaxy on (111) BaF2 substrates. The SnTe lattice parameter was found to be 6.331 Å as determined from x-ray diffraction spectra measured in the triple-axis configuration. The FWHM of the (222) SnTe x-ray rocking curves indicated a good crystalline quality and an unusual dependence on layer thickness. Atomic force microscopy (AFM) of the SnTe surface revealed spirals with monolayer steps formed around threading dislocations, similar to the PbTe on BaF2 epitaxy. The dislocation density was estimated from the AFM picture to be 9x10(8) cm-2. Small black pits corresponding to holes that were left during growth were also observed on the AFM images. Sn diffusion can be a possible reason for these pits and the relatively high dislocation density. Electrical measurements showed that the SnTe epilayers present a typical p-type carrier concentration around 10(20) cm- 3 almost temperature independent and a Hall mobility which decreases from 10(4) to 10³ cm²/V.s as the temperature increases from 10 to 350K.

We study numerically the equilibrium shape, shape transition and dislocation nucleation in strained epitaxial islands with a two-dimensional atomistic model, using interatomic potentials of Lennard-Jones type. The phase diagram for the equilibrium island shapes as a function of island size and lattice misfit with the substrate is obtained by an energy minimization procedure which does not require predefined faceted shapes. We determine the energy barrier and transition path for transition between different shapes of the islands and for dislocation nucleation in initially coherent islands using a method introduced recently, based on a systematic search of the transition paths for activated events.

A new dc hollow cathode plasma source has been assembled whith a conventional planar magnetron cathode used together with another plane cathode plate to form a hollow cathode cavity. The system comprises two cathode plates of aluminium separated by a distance d, one of them acting as target of the magnetron cathode, the other being an ordinary plate. The discharge anode is a metallic flange of the vacuum chamber. This leads to enhanced ionization in the cathode cavity region and enables the discharge to operate at significantly lower pressures than for a typical planar magnetron configuration. As a consequence, sputtered atoms can reach a substrate with minimum energy loss due to collisions with filling gas atoms. The discharge gas was a mixture of argon and nitrogen. AlN thin films were grown on silicon substrates, at ambient temperature, and characterized with respect to the structure and morphology by XRD and AFM analyses respectively. The structure and roughness of the AlN films were studied as a function of the deposition parameters.

We compare the transport properties for triangular, parabolic and cubic quantum wells. We calculate the transport mobility for electrons belonging to the different subbands. We obtain the energy spacing between first and second subbands from the electron sheet density and compare results for different potential profiles. We find that experimental results are in quantitative agreements with our calculations.

It is reported a theoretical and numerical study of non-markovian memory effects in backscattering of ballistic electrons constrained to move in a corrugated surface topography. Two approaches to model the electron trajectories are used, better approximation is obtained with the Hamilton-Dirac method for constrained system.

The aim of this work is to study the importance of minority carrier transport in double barrier diodes (DBD). We propose a theoretical model capable to describe the photoluminescence properties observed in GaAs-Al0.35Ga0.65As double barrier diodes with the increase of temperature. In this model, we considered that the minority carries (holes) photocreated at the contact of the structure diffuse, drift and then tunnel to the well. To study the influence of previous transport mechanisms, we solved the continuity equation under the influence of an applied bias and an excitation laser light. The theoretical photocurrent and photoluminescence calculations agree quite well with the experimental observations. Within the approaches of our model, we are able to simulate the minority holes mobility from the photocurrent measurements suggesting the use of the experimental technique as a probe of the carrier mobility.

In the present work we develop a theoretical study to analyze how the image charges effects can modify the electronic properties in Si and SrTiO3-based quantum wells. We have used the method based on the calculation of the image charge potential by solving Poisson equation in cylindrical coordinates. The numerical results show that the electron-heavy hole recombination energy can be shifted by more than 200 meV due to the combination of charge image and SiO2 (SrTiO3) interface thickness effects.

Using the effective-mass approximation and the variational method, we have calculated the effects of hydrostatic pressure on the donor- and acceptor-related optical absorption spectra in symmetrical GaAs double quantum well structures. A central finite potential barrier and two infinite external barriers constitute the profile of the potential barrier considered for the wells. Our results are presented as a function of the well and barrier widths and hydrostatic pressure. For the pressure dependence we consider the gamma-X mixing in the central barrier layer. For symmetrical and infinite-external-barrier quantum wells, and depending on the sizes of the structure and the hydrostatic pressure, the donor-related spectra show three special structures, whereas for the acceptor one only two structures appear.

In this work we present a calculation of the intradonor 1s-2p+ transition energies for an impurity donor present in a GaAs-AlGaAs Double Quantum Well structure as a function of an applied external magnetic field. In our calculation the impurity energy levels were obtained from a variational method by choosing a Gaussian trial wave function, and the energy corrections due to the polaronic effect were included by second order perturbation theory IWBPT as modified by Cohn, Larsen and Lax. We have considered only the GaAs bulk LO phonon in the electron-phonon coupling. A very good agreement between the theoretical and experimental results for a DQW consisting of two 100 Å well widths separated by a 100 Å potential barrier width was obtained.

Recent results on magnetoresistance in a two dimensional electron gas (2DEG) under crossed magnetic and microwave fields show a new class of oscillations, suggesting a new kind of zero-resistance states. We consider the problem from the point of view of the electronic structure dressed by photons due to an in-plane linearly polarized ac field. In the strong modulation limit predictions on dressed Hofstadter spectra are discussed, which could be of interest since the bare spectra have been observed in the past few years.

A series of PbTe/PbEuTe double barrier samples with different barrier widths were successfully grown on BaF2 substrates by molecular beam epitaxy. The electron concentration of PbTe spacer and well layers was controlled by the deviation from stoichiometry, while the buffer and cap layers were intentionally doped with bismuth to obtain low-resistivity layers to be used as top and bottom contacts. Assuming 50% of band offset, a barrier height of 150 meV was determined by infrared transmission measurements, corresponding to a PbEuTe barrier with 5% of europium content. The structural parameters of the samples were accurately determined by combining the measurement in a high-resolution x-ray diffractometer in the triple-axis configuration with a simulation within the framework of dynamical theory of diffraction.

The low-temperature electron mobility is investigated here for electrons confined in modulation-doped In0.53Ga0.47As/InP single symmetric quantum wells. The subband structure calculation is developed via variational method, both Schrödinger and Poisson equations being solved simultaneously with adequate heterointerface matching conditions. With this in hands, the main electron scattering rates are computed, namely alloy disorder, remote ionized impurity, and interface roughness. As a result, interesting interchanges in these scattering rates were found by varying the well width and the spacer width, which show that some scattering mechanisms can surpass the alloy disorder scattering rate and come to limit the electron mobility, a behavior not reported in the literature.

The influence of dimensionality on the conductance of semiconductor cross junctions is investigated in the effective mass approximation and quantum ballistic regime. Our calculations exhibit some similar features for both two- and three-dimensional models, namely the conductance peaks before the fundamental eigenenergy of the quantum wire that represents the sidearms and the zero conductance dip. Despite this, we show that, in general, the conductance and the electronic wave function are strongly affected when the third spatial dimension is taken into account. In other words, we prove that two-dimensional models are not suitable for representing this kind of system.

Theoretical calculations of electron-phonon scattering rates in GaAs/Al xGa1 - xAs spherical quantum dots have been performed by means of effective mass approximation in the frame of finite element method. The influence of a roughness interface and external magnetic fields are analysed for different scattering rate transition. Our results open interesting channels for electron dephasing times manipulation.

The electronic band structure of various Ge quantum wires of different sizes, with hydrogenated surfaces, is studied using a nearest-neighbor empirical tight-binding Hamiltonian by means of a sp³s* atomic orbitals basis set. We suppose that the nanostructures have the same lattice structure and the same interatomic distance as in bulk Ge and that all the dangling bonds are saturated with hydrogen atoms. These atoms are used to simulate the bonds at the surface of the wire and sweep surface states out of the fundamental gaps. One of the most important features is a clear broadening of the band gap due the quantum confinement. Comparing to experimental data, we conclude that, similar to the case of Si, the size dependent PL in the near infrared may involve a trap in the gap of the nanocrystals.

Motivated by recent experiments and theoretical works that contradict the original explanation for fractal conductance fluctuations (FCF) in electron billiards, based on the "mixed" structure of the classical phase space, we propose an alternative approach to investigate FCF using Random Matrix Theory (RMT). By means of a semiclassical estimate for value of the magnetic correlation field B C we conclude that most of the experiments on FCF were performed for magnetic fields around or greater than B C. This strongly suggests that the appropriate explanation for the observed FCF should rely on the absence of long-range correlations and not on the structure of the classical phase space. This idea is supported by a numerical study of parametric variations within the framework of RMT, which validates our surmise that the observed FCF actually reflect a diffusive scenario for electronic transport.

We have studied the collective magneto-optical response of layers of semiconductor quantum dots and nano-rings. Expressions for both the polarizability of the individual nano-objects have been used to determine the magneto-optical response on square two-dimensional lattices on those nano-objects as a function on frequency, magnetic field and angle of incidence. The magnetic field dependence of the response for layers of rings gives rise to the Aharonov-Bohm type oscillatory behavior in the reflectance and absorbance that can be measurable. Layers of dots do not display any remarkable magnetic field dependence.

Using a variational procedure for a hydrogenic donor-impurity we have investigated the influence of an axial magnetic field and hydrostatic pressure in the binding energy and the impurity-related photoionization cross-section in 1D and 0D GaAs low dimensional systems. Our results are given as a function of the radius, the impurity position, the polarization of the photon, the applied magnetic field, the normalized photon energy, and the hydrostatic pressure. In order to describe the gamma-X mixing in the Ga1-xAl xAs layer, we use a phenomenological procedure to describe the variation of the potential barrier that confines the carriers in the GaAs layer. Our results agree with previous theoretical investigations in the limit of atmospheric pressure. We found that the binding energy and the photoionization cross-section depend on the size of the structures, the potential well height, the hydrostatic pressure, and the magnetic field.

In this work we have studied the electron LO phonon interaction on the pair-correlation function g(x) and its dependence on the electronic density for a GaAs-AlGaAs rectangular quantum wire within the random-phase approximation (RPA). We assumed two different values of the wire width. As negative non-physical results are found for lower electronic densities and small interparticle separations in the RPA approach, we have delimited the regions where the RPA approach cannot be used for the calculation of the Q1D electron gas collective excitation.