“Quantitative analysis of the interfacial intermixing in InAs/GaSb short-period-superlattices using transmission electron microscopy”


Seminario: “Quantitative analysis of the interfacial intermixing in InAs/GaSb short-period-superlattices using transmission electron microscopy”, por la Dra. Esperanza Luna (Paul-Drude Institut für Festkörperelektronik, Berlin, Germany)

Noviembre – 13. 2013  Sala de Seminarios del IMM, 12h

Resumen (en inglés):

InAs/GaSb short-period-superlattices (SPS) show unique potential device applications, which are strongly dependent on the quality of the interfaces. While most of the studies focus on the interfacial roughness (i.e., steps and islands), little is known about the interfacial intermixing (i.e., composition profile).

We propose an innovative method for a reliable and systematic characterization of these noncommon-atom interfaces. It is based on transmission electron microscopy (TEM) and, although a dedicated data analysis is required, all the steps are straightforward to implement. In particular, the compositional sharpness is obtained from the comparison of the experimental contrast in chemical sensitive g002 two-beam dark-field (DF) TEM micrographs with simulated intensity profiles. The later are calculated assuming that the element distribution profiles are described by sigmoidal functions, with the interface width as the main fitting parameter. The procedure (i.e. search of the composition profile that after being input into the calculation best fits the experiment) would resemble the analysis of x-ray diffraction data. The method is of general applicability and not only restricted to the InAs/GaSb case.

To illustrate the potential of the method, we analyse InAs/GaSb SPS grown by molecular beam epitaxy with different interface intermixing and correlate the results with the specific growth conditions. As will be shown, the method allows the detection of few-monolayer-thin layers, as well as the identification of unintentional interfacial layers and segregation effects. Simulations of the SPS band-structure using the obtained distribution profiles predict values for the emission wavelengths in agreement with the measured values.

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Categorias: Q&CLight, Seminarios

Seminario: Nanomechanics and strain engineering in atomically thin MoS2


Nanomechanics and strain engineering in atomically thin MoS2

Andres Castellanos-Gomez*

Delft University of Technology, 2628 CJ Delft, The Netherlands

*E-mail: a.castellanosgomez@tudelft.nl

17 de octubre a las 12.00 horas ISOM-UPM, C-206.1

http://www.isom.upm.es/sem.php

 

Single-layer MoS2 is an attractive two-dimensional material that combines the mechanical flexibility of graphene with a large direct bandgap. While graphene is very interesting as a transparent electrode, its lack of a bandgap limits its usefulness in semiconducting and optoelectronic devices. Atomically thin MoS2, on the other hand, has a large intrinsic bandgap. This attractive feature has been employed to fabricate many devices not possible in graphene, such as field-effect transistors with high mobility and current on/off ratio, logic gates and efficient photo-transistors. Our recent work on MoS2 has been focused on developing new methods to fabricate single layer MoS2 [1] and to characterize the intrinsic mechanical [2], optical and electrical properties [3-4] of this atomically thin material.

Here, I will show an overview of our last results on MoS2 paying special attention to our studies on nanomechanical resonators based on MoS2 layers [5] and on the effect of localized strain on the optoelectronic properties of MoS2 [6].

[1] Castellanos-Gomez, A., et al., Laser-Thinning of MoS2: On Demand Generation of a Single-Layer Semiconductor. Nano Letters, 2012. 12(6): p. 3187-3192.

[2] Castellanos-Gomez, A., et al., Elastic Properties of Freely Suspended MoS2 Nanosheets. Advanced Materials, 2012. 24(6): p. 772-775.

[3] Castellanos-Gomez, A., et al., Electric-Field Screening in Atomically Thin Layers of MoS2: the Role of Interlayer Coupling. Advanced Materials, 2013. 25(6): p. 899-903.

[4] Buscema, M., et al., Large and Tunable Photothermoelectric Effect in Single-Layer MoS2. Nano Letters, 2013.13 (2), p. 358–363

[5] Castellanos-Gomez, A., et al., Single-layer MoS2 mechanical resonators. Advanced Materials, 2013 (in press).

[6] Castellanos-Gomez, A., et al., Local strain engineering in atomically thin MoS2. 2013 (under review)

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Categorias: Q&CLight, Seminarios

Seminario: Direct growth of InGaN on Si(111) – ISOM-UPM


Direct growth of InGaN on Si(111)

Praveen Kumar and Richard Nötzel

ISOM, UPM Madrid

Abstract:

I will start my presentation with a brief introduction about III-nitrides, especially  the novel application potential of InGaN with high In composition and then discuss the main challenges in context with our research: The growth of high-In-composition InGaN directly on Si(111) without any buffer layers – towards novel device designs and the direct integration of III-nitride with Si technology.

The growth of InGaN on Si(111) is commonly performed by employing high band gap buffer layers such as AlN or GaN which create an energy barrier to electrically isolate the InGaN layer from the Si substrate. This makes novel device designs complex and costly, if not impossible. Thus, the direct growth of InGaN on Si(111) without any buffer layers is highly desirable. Towards this goal, a series of experiments were performed at various substrate temperatures and Ga, In, and N fluxes on a MECA 2000 MBE system. In a narrow window of growth conditions, laterally extended, micrometer-sized planar areas are formed together with trenches and holes. Detailed structural and optical analyses reveal that the planar areas comprise the InGaN layer with high and uniform In composition, while the trenches and holes are associated with pure GaN and low-In-composition InGaN. Photoluminescence at low temperature is observed from the high-In-composition InGaN layer, which forms an ohmic contact with a p-Si substrate and is large enough to serve as template for planar heterostructure based devices.

 

Associated group members for this work:

Paul E. D. Soto Rodriguez, Victor J. Gomez, Naveed H. Alvi and Pavel Aseev

Fuente: http://www.isom.upm.es/eng/sem.php

Date: Friday, 26 of April of 2013
Time: 12:00
Place: Sala 206-1 Edificio López Araújo. ETSI de Telecomunicación.

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Seminario: “Mesoscopic entanglement in solid-state quantum optics” – 17 Abril 2013


Seminario: “Mesoscopic entanglement in solid-state quantum optics” por el Dr. Diego Porras Torre (Departamento de Física Teórica I, Facultad de Ciencias Físicas, Univ. Complutense de Madrid)

http://www.imm.cnm.csic.es/es/eventos/detalle/10

Abril – 17. 2013 Sala de Seminarios del IMM, 12h

Resumen: (en inglés)

In this talk I will review the theory of collective effects in quantum optics, with a focus on superradiant spontaneous decay. I will explain how collective effects allow us to generate entangled states of quantum emitters, as well as multi-mode entangled states of the radiation field [1]. Those ideas can be applied to variety of setups, including trapped atoms, as wel as superconducting qubits [2] and quantum dots. For example, by coupling quantum dots to one-dimensional waveguides, the superradiant spontaneous decay may induce entangled steady-states, even in the presence of disorder induced by fabrication [3].

[1] D. Porras and J. I. Cirac, Phys. Rev. A 78, 053816 (2008)
[2] D. Porras and J.J. García-Ripoll, Phys. Rev. Lett. 108, 043602 (2012)
[3] A. González-Tudela and D. Porras, Phys. Rev. Lett. 110, 080502 (2013)

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Categorias: Q&CLight, Seminarios

Seminario Q&Clight: Critical role of two-dimensional island-mediated growth on the formation of semiconductor heterointerfaces


Seminario en IMM-CSIC, día 9 de abril 2012, hora 12:30

Esperanza Luna, Paul-Drude Institute, Berlín, Alemania

Critical role of two-dimensional island-mediated growth on the formation of semiconductor heterointerfaces

 

Abstract

We experimentally demonstrate a sigmoidal variation of the composition profile across semiconductor

heterointerfaces. The wide range of material systems (III-arsenides, III-antimonides, III-V quaternary

compounds, III-nitrides) exhibiting such a profile suggests a universal behavior. We show that sigmoidal profiles emerge from a simple model of cooperative growth mediated by two-dimensional island formation, wherein cooperative effects are described by a specific functional dependence of the sticking coefficient on the surface coverage. Experimental results confirm that, except in the very early stages, island growth prevails over nucleation as the mechanism governing the interface development and ultimately determines the sigmoidal shape of the chemical profile in these two-dimensional grown layers. In agreement with our experimental findings, the model also predicts a minimum value of the interfacial width, with the minimum attainable value depending on the chemical identity of the species.

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Seminario: “Aharonov-Bohm Interference in Neutral Excitons: Effects of Built-In Electric Fields”


Dr. Marcio Daldin Teodoro. Universidad Federal de San Carlos, Brasil

Seminario: “Aharonov-Bohm Interference in Neutral Excitons: Effects of Built-In Electric Fields”

Abstract:

I present a comprehensive discussion of quantum interference effects due to the finite structure of neutral excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. The signatures of built-in electric fields and temperature on quantum interference are demonstrated by theoretical models that describe the modulation of the interference pattern and confirmed by complementary experimental procedures.

MIÉRCOLES, 20 de Febrero de 2013- 12h.
IMM-CSIC (http://www.imm.cnm.csic.es/)

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Seminario: “ZnO-based resonators – from growth to Bose-Einstein condensation”


Seminario: “ZnO-based resonators – from growth to Bose-Einstein condensation”

Lugar: 23/11/2012 en la sala de seminarios, C4 Facultad de Ciencias, a las 15:00, Universidad Autónoma de Madrid.

Rüdiger Schmidt-Grund

Semiconductor Physics Group
University of Leipzig, Germany
Schmidt-Grund@physik.uni-leipzig.de
http://polariton.exphysik.uni-leipzig.de/index.html
http://www.uni-leipzig.de/~hlp/

In the first part of the talk, I will discuss electronic properties observed in the dielectric function of semiconductors (ZnO), ferroelectrics (BaTiO3), and dielectric ferromagnets ((Zn,Co)Fe2O4), polarization interaction in ZnO/BaTiO3 heterostructures, and plasmonic effects in thin metal structures deposited on artificial regular surface structures.
The second part addresses ZnO-based resonators: I will present some growth aspects and optical properties in the weak and strong coupling regime of different resonator types which are: 1.) Two-dimensionally confined whispering-gallery and Fabry-Perot mode resonators based on ZnO micro- respective nanowires which have a perfect hexagonal cross-section and are coated in the Fabry-Perot case conformally with concentric all-oxide Bragg reflectors. 2.) One-dimensionally confined planar Fabry-Perot resonators with a ZnO bulk cavity layer embedded between two all-oxide Bragg reflectors. The different growth methods for the different types of resonators, the dimension of the confinement as well as the nature of the photonic modes leads to different physical processes and effects which I will discuss in detail concerning their influence on the properties of the Bose-Einstein condensate (BEC) which we have observed up to room temperature.
In the planar resonators, excitons bound to impurities play an important role. They provide a high background potential causing a blueshift of the condensate up to 30 meV (~1/3 of the Rabi splitting). Further, disorder effects (electronic and photonic) provide insight into non-linear non-equilibrium processes like density-dependent condensate depletion which is possibly connected to phase de-synchronization.
In contrast to the planar case, the self-organized grown microwires exhibit a crystal quality comparable to bulk single crystals. Especially, structural disorder and bound excitons are almost absent. Thus the background potential is low and therewith the observed blueshift is typically below 1 meV. The whispering-gallery modes are formed due to total internal reflection at the side facets of the hexagonal wire. The large diameter of the microwires (in the μm range) leads to a high spectral mode density yielding multiple polariton states. We have observed interbranch energy-relaxation processes of the condensate at low temperature which can be related to parametric processes. During the relaxation, momentum and energy conservation along with the specific dispersion of the corresponding branches causes step-by-step conversion of potential into kinetic energy of the condensate. At room temperature, this process is supressed due to the presence of phonons and the relaxation is assisted by scattering with optical phonons.

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Categorias: Q&CLight, Seminarios

SEMINARIO: GENERACIÓN DE LUZ BLANCA EFICIENTE CON NANOESTRUCTURAS SEMICONDUCTORAS


SEMINARIOS INTERNACIONALES DE FRONTERAS DE LA CIENCIA DE MATERIALES

Los Seminarios Internacionales de Fronteras de la Ciencia de Materiales son organizados por el Departamento de Ciencia de Materiales de la Universidad Politécnica de Madrid, y tienen periodicidad semanal. Su objetivo es servir de punto de encuentro, interacción y difusión de problemáticas actuales y destacadas dentro del área de la Ciencia e Ingeniería de Materiales; con una visión amplia que va desde los materiales biológicos a los materiales funcionales, pasando por aplicaciones puramente tecnológicas. En ellos se cuenta con la participación desinteresada de relevantes investigadores y tecnólogos de Universidades, Empresas y Centros de Investigación del ámbito nacional e internacional.

GENERACIÓN DE LUZ BLANCA EFICIENTE CON NANOESTRUCTURAS SEMICONDUCTORAS

LUNES, 19 DE NOVIEMBRE DE 2012 A LAS 9:30 H DE LA MAÑANA

ENRIQUE CALLEJA

INSTITUTO DE SISTEMAS OPTOELECTRÓNICOS Y MICROTECNOLOGÍA (ISOM), UPM, ESPAÑA

RESUMEN

Se analiza en crecimiento epitaxial selectivo de nanoestructuras de InGaN mediante la técnica MBE (Molecular Beam Epitaxy) sobre substratos con máscaras de nanoagujeros de titanio. El éxito en este tipo de crecimiento depende de un balance crítico entre la difusividad de los átomos metálicos y su desorción, la rugosidad de la superficie, y la razón III/V. Se discuten diferentes aproximaciones a la generación de luz blanca de estado sólido sin utilización de fósforo conversor, como es el caso de zonas activas de InGaN en forma de : i) pozo cuántico; ii) region extensa con composición fija; iii) región extensa con composición gradual; y iv) estructura RGB (rojo-verdeazul)

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Seminario UAM: “Carbononics: carbon based electronics, photonics and spintronics”


Title: “Carbononics: carbon based electronics, photonics and spintronics”

Speaker: P. Hawrylak (Emerging Technologies Division, National Research Council of Canada, Ottawa, Canada)

Date: Monday 22th October, 15:00-16:00.

Place: Depto. Física Teórica de la Materia Condensada (Facultad de Ciencias), módulo 5, seminar room (5th floor).

More info about our seminars in: www.uam.es

We briefly review electronic and optical properties of graphite and graphene, atomically thin layer of carbon atoms, and discuss its strength and shortcomings. We next discuss how one can engineer electronic, optical and magnetic properties of graphene by control of lateral size, character of the edge and number of layers. We describe gate controlled triangular graphene quantum dots
with zigzag edges which exhibit a shell of degenerate states at the Fermi level, a prerequisite for strongly correlated electron system.

We describe transport, optics and magnetism controlled by the gate and by the number of layers. Potential applications of graphene in spintronics, photovoltaics and quantum information are discussed.

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Seminario UPM: Nanostructures and their Applications


Fecha: miércoles, 3 de octubre de 2012
Hora: 12.00 hora
Lugar: Sala 206-1 Edificio López Araújo. ETSI de Telecomunicación

Título: Nanostructures and their Applications
Ponente: Prof. Magnus Willander
Organismo: Department of Science and Technology, Linkoping University, Sweden

Resumen:
He will give a short background of low temperature chemical growth of several different nanostructures in several different materials and on different substrates. Their optical, electrical, mechanical and chemical properties will be discussed. Finally applications of the nanostructures for lasers, LEDs, optical detectors, chemical sensors and piezoelectrical generators will be reviewed.

http://www.isom.upm.es/sem.php

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Categorias: Q&CLight, Seminarios