Nanoestruturas de GaN crescidas pelas técnicas de epitaxia por magnetron sputtering e epitaxia por feixe molecular

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Theoretical luminescence spectra in p-type superlattices based on InGaAsN

In this work, we present a theoretical photoluminescence (PL) for p-doped GaAs/InGaAsN nanostructures arrays. We apply a self-consistent method in the framework of the effective mass theory. Solving a full 8 x 8 Kane's Hamiltonian, generalized to treat different materials in conjunction with the Poisson equation, we calculate the optical properties of these systems. The trends in the calculated PL spectra, due to many-body effects within the quasi-two-dimensional hole gas, are analyzed as a function of the acceptor doping concentration and the well width. Effects of temperature in the PL spectra are also investigated. This is the first attempt to show theoretical luminescence spectra for GaAs/InGaAsN nanostructures and can be used as a guide for the design of nanostructured devices such as optoelectronic devices, solar cells, and others.

The influence of different indium-composition profiles on the electronic structure of lens-shaped InxGa1-xAs quantum dots

We present effective-mass calculations of the bound-state energy levels of electrons confined inside lens-shaped InxGa1-xAs quantum dots (QDs) embedded in a GaAs matrix, taking into account the strain as well as the In gradient inside the QDs due to the strong In segregation and In-Ga intermixing present in the InxGa1-xAs/GaAs system. In order to perform the calculations, we used a continuum model for the strain, and the QDs and wetting layer were divided into their constituting monolayers, each one with a different In concentration, to be able to produce a specific composition profile. Our results clearly show that the introduction of such effects is very important if one desires to correctly reproduce or predict the optoelectronic properties of these nanostructures.

Conduction mechanisms in p-type Pb1-xEuxTe alloys in the insulator regime

Electrical resistivity measurements were performed on p-type Pb1-xEuxTe films with Eu content x = 4%, 5%, 6%, 8%, and 9%. The well-known metal-insulator transition that occurs around 5% at room temperature due to the introduction of Eu is observed, and we used the differential activation energy method to study the conduction mechanisms present in these samples. In the insulator regime (x>6%), we found that band conduction is the dominating conduction mechanism for high temperatures with carriers excitation between the valence band and the 4f levels originated from the Eu atoms. We also verified that mix conduction dominates the low temperatures region. Samples with x = 4% and 5% present a temperature dependent metal insulator transition and we found that this dependence can be related to the relation between the thermal energy k(B)T and the activation energy Delta epsilon(a). The physical description obtained through the activation energy analysis gives a new insight about the conduction mechanisms in insulating p-type Pb1-xEuxTe films and also shed some light over the influence of the 4f levels on the transport process in the insulator region. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729813]

Magnetophotoluminescence study of GaAs/AlGaAs coupled double quantum wells with bimodal heterointerface roughness

This work reports on the results of magnetophotoluminescence (MPL) measurements carried out in a sample containing two Al0.35Ga0.65As/GaAs, coupled double quantum wells (CDQWs), with inter-well barriers of different thicknesses, which have the heterointerfaces characterized by a distribution of bimodal roughness. The MPL measurements were performed at 4 K, with magnetic fields applied parallel to the growth direction, and varying from 0 to 12 T. The diamagnetic shift of the photoluminescence (PL) peaks is more sensitive to changes in the confinement potential, due to monolayer variations in the mini-well thickness, rather than to the exciton localization at the local potential fluctuations. As the magnetic field increases, the relative intensities of the two peaks in each PL band inverts, what is attributed to the reduction in the radiative lifetime of the delocalized excitons, which results in the radiative recombination, before the excitonic migration between the higher and lower energy regions in each CDQW occurs. The dependence of the full width at half maximum (FWHM) on magnetic field shows different behaviors for each PL peak, which are attributed to the different levels and correlation lengths of the potential fluctuations present in the regions associated with each recombination channel. (C) 2011 Elsevier B.V. All rights reserved.

Atomically resolved study of the morphology change of InAs/GaAs quantum dot layers induced by rapid thermal annealing

The optoelectronic properties of InAs/GaAs quantum dots can be tuned by rapid thermal annealing. In this study, the morphology change of InAs/GaAs quantum dots layers induced by rapid thermal annealing was investigated at the atomic-scale by cross-sectional scanning tunneling microscopy. Finite elements calculations that model the outward relaxation of the cleaved surface were used to determine the indium composition profile of the wetting layer and the quantum dots prior and post rapid thermal annealing. The results show that the wetting layer is broadened upon annealing. This broadening could be modeled by assuming a random walk of indium atoms. Furthermore, we show that the stronger strain gradient at the location of the quantum dots enhances the intermixing. Photoluminescence measurements show a blueshift and narrowing of the photoluminescence peak. Temperature dependent photoluminescence measurements show a lower activation energy for the annealed sample. These results are in agreement with the observed change in morphology. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770371]

Eingefrorene Unordnung und Magnetismus in TiFe_1tn2-Laves-Phase-Dünnschichten

In der vorliegenden Arbeit wird die binäre intermetallische Verbindung TixFe1-x im C14 Laves-Phase Stabilitätsbereich anhand von dünnen Schichten untersucht. TiFe2 weist zwei energetisch nahezu entartete magnetische Grundzustände auf. Dies führt zu einer starken Korrelation von strukturellen und magnetischen Eigenschaften, die im Rahmen dieser Arbeit untersucht wurden. Es wurden daher epitaktische Schichten mit variabler Zusammensetzung im C14 Stabilitätsbereich auf Al2O3 (001)-orientierten Substraten mittels Molekularstrahlepitaxie präpariert und strukturell charakterisiert. Die temperatur- und magnetfeldabhängigen magnetischen Eigenschaften dieser Proben wurden mittels DC-SQUID Magnetisierungsmessungen bestimmt. Es zeigte sich eine magnetische Phasenseparation von Antiferromagnetismus und Ferromagnetismus in Abhängigkeit von der Zusammensetzung. Aus den charakteristischen Ordnungstemperaturen konnte ein magnetisches Phasendiagramm für dünne Schichten und niedrige Aligning-Felder erstellt werden. Ein Phasendiagramm für Volumenproben bei hohem Magnetfeld unterscheidet sich von diesem im Wesentlichen durch den Einfluß von Fe-Segregation in den Volumenproben, welche bei der epitaktischen Präparation nicht auftritt. Anhand von Monte-Carlo Verfahren, denen ein „quenched random disorder“ Modell zugrunde lag, wurde das Verhalten der Dünnschichtproben simuliert und daraus ein magnetisches Phasendiagramm abgeleitet. Das simulierte und experimentelle Phasendiagramm stimmt in den wesentlichen Punkten überein. Die Unterschiede sind durch die speziellen Wachstumseigenschaften von TiFe2 erklärbar. Als Ergebnis kann die magnetische Phasenseparation in diesem System als Auswirkung einer Symmetriebrechung durch Substitution in der Einheitszelle beschrieben werden.

Magnetic properties of selected 3d- and 4f-systems on alpha-Al 2 O 3 substrates

In this work the growth and the magnetic properties of the transition metals molybdenum, niobium, and iron and of the highly-magnetostrictive C15 Laves phases of the RFe2 compounds (R: Rare earth metals: here Tb, Dy, and Tb{0.3}Dy{0.7} deposited on alpha-Al2O3 (sapphire) substrates are analyzed. Next to (11-20) (a-plane) oriented sapphire substrates mainly (10-10) (m-plane) oriented substrates were used. These show a pronounced facetting after high temperature annealing in air. Atomic force microscopy (AFM) measurements reveal a dependence of the height, width, and angle of the facets with the annealing temperature. The observed deviations of the facet angles with respect to the theoretical values of the sapphire (10-1-2) and (10-11) surfaces are explained by cross section high resolution transmission electron microscopy (HR-TEM) measurements. These show the plain formation of the (10-11) surface while the second, energy reduced (10-1-2) facet has a curved shape given by atomic steps of (10-1-2) layers and is formed completely solely at the facet ridges and valleys. Thin films of Mo and Nb, respectively, deposited by means of molecular beam epitaxy (MBE) reveal a non-twinned, (211)-oriented epitaxial growth as well on non-faceted as on faceted sapphire m-plane, as was shown by X-Ray and TEM evaluations. In the case of faceted sapphire the two bcc crystals overgrow the facets homogeneously. Here, the bcc (111) surface is nearly parallel to the sapphire (10-11) facet and the Mo/Nb (100) surface is nearly parallel to the sapphire (10-1-2) surface. (211)-oriented Nb templates on sapphire m-plane can be used for the non-twinned, (211)-oriented growth of RFe2 films by means of MBE. Again, the quality of the RFe2 films grown on faceted sapphire is almost equal to films on the non-faceted substrate. For comparison thin RFe2 films of the established (110) and (111) orientation were prepared. Magnetic and magnetoelastic measurements performed in a self designed setup reveal a high quality of the samples. No difference between samples with undulated and flat morphology can be observed. In addition to the preparation of covering, undulating thin films on faceted sapphire m-plane nanoscopic structures of Nb and Fe were prepared by shallow incidence MBE. The formation of the nanostructures can be explained by a shadowing of the atomic beam due to the facets in addition to de-wetting effects of the metals on the heated sapphire surface. Accordingly, the nanostructures form at the facet ridges and overgrow them. The morphology of the structures can be varied by deposition conditions as was shown for Fe. The shape of the structures vary from pearl-necklet strung spherical nanodots with a diameter of a few 10 nm to oval nanodots of a few 100 nm length to continuous nanowires. Magnetization measurements reveal uniaxial magnetic anisotropy with the easy axis of magnetization parallel to the facet ridges. The shape of the hysteresis is depending on the morphology of the structures. The magnetization reversal processes of the spherical and oval nanodots were simulated by micromagnetic modelling and can be explained by the formation of magnetic vortices.