Spatially indirect excitons in type-II quantum dots

The authors have calculated the electronic structure for type-II InP/GaAs quantum dot systems considering a three-dimensional geometry including the wetting layer and the electron-hole interaction, which is the only responsible for the hole localization. Their results for the InP/GaAs structure show the electron confined inside the dot and the hole in the GaAs layer, partially above and below the dot. The authors propose structures with InGaAs or InGaP layers, where the hole wave function forms a ring around the dot walls. The electron-hole overlap, and therefore, the carrier lifetimes are very sensitive to the structural geometry, which is an important tool for device engineering. (c) 2007 American Institute of Physics.902

Light scattering and atomic force microscopy study of InAs island formation on InP

Some aspects of the morphology of InAs island formation on InP have been studied by atomic force microscopy, photoluminescence, photoluminescence excitation spectroscopy, and Raman scattering. The InAs layer is grown by chemical beam epitaxy on top of InP surfaces with sawtooth-like channels. The deposition of a thin InAs layer results in quantum dots strongly aligned along the InP channels. Subsequent annealing in an arsenic atmosphere produces growth and loss of coherency of the islands. Atomic force microscopy shows the changes in size and alignment of the islands. Optical measurements serve to give quantitative estimates of the strain distribution among the top of the InP buffer layer, the wetting layer and the islands for the differently treated samples. (C) 2000 American Institute of Physics. [S0021-8979(00)02003-X].8731165117

On the optical properties of InAs/InP systems: The role of two-dimensional structures and three-dimensional islands

We investigate the effects of the interface morphology on the electronic properties of InAs/In systems using in-air atomic force microscopy and low temperature photoluminescence. Atomic force microscopy results show that the distribution of InAs strained film into three-dimensional islands and the two-dimensional wetting layer-typical of the Stranski-Krastanov growth mode-is strongly affected by the characteristics of the substrate and by the morphology of the InP buffer layer. The differences in the optical data are correlated to the different interface characteristics observed by atomic force microscopy. We discuss the origin of emission peaks taking into account the diffusion process of adsorbed atoms on the different types of surface. (C) 1998 American Institute of Physics.7291015101

Strain redistribution at the phase transition of MnAs/GaAs(001) films

We investigated the thermal evolution of the lattice parameters of a MnAs film epitaxially grown on GaAs(001) around its magnetostructural first-order phase transition using x-ray diffraction. Despite the substrate constraint, large variation of one of the in-plane lattice parameters is preserved, typical of bulk MnAs phase transition, during a large temperature range where two phases coexist. We demonstrated that the condition of the constant film length along this direction, in accord to the substrate length, is always fulfilled during the process. The effect is attributed to the gliding of misfit dislocations present on the film.881

Light controlled spin polarization in asymmetric n-type resonant tunneling diode

The authors have observed a strong dependence of the circular polarization degree from the quantum well emission in an asymmetric n-type GaAs/AlAs/AlGaAs resonant tunneling diode on both the laser excitation intensity and the applied bias voltage. The sign of the circular polarization can be reversed by increasing the light excitation intensity when the structure is biased with voltages slightly larger than the first electron resonance. The variation of polarization is associated with a large density of photogenerated holes accumulated in the quantum well, which is enhanced due to the asymmetry of the structure. (C) 2007 American Institute of Physics.91

Optical properties of multi-layer type II InP/GaAs quantum dots studied by surface photovoltage spectroscopy

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details. (C) 2011 American Institute of Physics. [doi:10.1063/1.3638705]1106Bulgarian National Science Fund [D01-463/12.7.06]Alexander von Humboldt FoundationCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Bulgarian National Science Fund [D01-463/12.7.06

An optical study of self-assembled InxGa1-xAs/GaAs quantum dots embedded in a two-dimensional electron gas

We studied the low temperature (77 K) photomodulated reflection and transmission as well as the photoluminescence at 2.2 K of a self-assembled InxGa1-xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of the substrate in order to achieve a gradual variation of the In concentration along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a characteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra leads to an estimate of the average value of the Fermi energy in the conduction band of the wetting layer (E(F)similar or equal to 13.4 meV). The variation of this Fermi energy along the composition gradient can be obtained from the spectra, and an estimate of the gradient of the density of quantum dots along this direction can be made. A careful comparison of the variation of the critical energy of the different lines suggests that the average quantum dot size depends on the In molar fraction of the alloy, which is seen to vary more or less linearly across the wafer diameter. (C) 2000 American Institute of Physics. [S0021-8979(00)06510-5].87117994799

Enhanced magneto-optical oscillations from two-dimensional hole-gases in the presence of Mn ions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We investigated the effects of nearby Mn ions on the optical properties of two-dimensional hole-gases confined in InGaAs/GaAs quantum wells. We observed energy oscillations on both the averaged emission and the spin-splitting energies, whereas the first one presents maxima at all integer filling factors, and the second one, solely at odd-filling factors. The strength of the oscillations clearly increases with the Mn concentration. Furthermore, considering the relatively low-mobility of our structures, the oscillations are surprisingly strong and robust, persisting up to relatively high temperatures and excitation intensities. (C) 2011 American Institute of Physics. [doi:10.1063/1.3601477]9825Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Ministry of Education and Science of Russian Federation [RNP 2.2.2.2/4737, 2.2.2.2/11107]Civilian Research and Development Foundation [RUXO-001-NN-06/BP1M01]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Ministry of Education and Science of Russian Federation [RNP 2.2.2.2/4737, 2.2.2.2/11107]Civilian Research and Development Foundation [RUXO-001-NN-06/BP1M01

Polarization resolved luminescence in asymmetric n-type GaAs/AlGaAs resonant tunneling diodes

We have investigated the polarized emission from a n-type GaAs/AlGaAs resonant tunneling diode under magnetic field. The GaAs contact layer emission shows a large constant negative circular polarization. A similar result is observed for the quantum well, but only when electrons are injected from the substrate, while for inverted biases, the polarization tends to become positive for small voltages and large laser excitation intensities. We believe that the quantum well polarization may be associated to the partial thermalization of minority carriers on the well subbands and is thus critically dependent on the bias-controlled density of carriers accumulated in the well.921

Optical and structural investigation of In1-xGaxP free-standing microrods

We present a structural and optical characterization of scepterlike micrometer-sized free-standing structures, composed of a long InGaP rod with a metallic sphere on its top, grown on polycrystalline InP substrates. In contrast to the conventional vapor-liquid-solid growth method, no catalyst was deposited on the substrate. Instead, metallic In liberated from the InP substrate by phosphor evaporation works as the catalyst metal. We performed Raman scattering, photoluminescence spectroscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy measurements on individual structures. The alloy composition measured by microscopic techniques is in agreement with the values obtained by the optical measurements considering that the rod is strain free. The InGaP rods present essentially constant Ga composition within a fluctuation of similar to 10% and efficient optical emission. We also observed a marked increase in the Raman-scattering signal at rod positions near the metallic sphere (the "neck"), which was attributed to a surface-enhanced Raman-scattering effect. Our results demonstrate the possibility of using InGaP rods for optical device applications. (c) 2005 American Institute of Physics.98