156 research outputs found
Strain relaxation of GaAs/Ge crystals on patterned Si substrates
Taboada, A. G. et al.We report on the mask-less integration of GaAs crystals several microns in size on patterned Si substrates by metal organic vapor phase epitaxy. The lattice parameter mismatch is bridged by first growing 2-μm-tall intermediate Ge mesas on 8-μm-tall Si pillars by low-energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs crystals towards full pyramids exhibiting energetically stable {111} facets with decreasing Si pillar size. The release of the strain induced by the mismatch of thermal expansion coefficients in the GaAs crystals has been studied by X-ray diffraction and photoluminescence measurements. The strain release mechanism is discussed within the framework of linear elasticity theory by Finite Element Method simulations, based on realistic geometries extracted from scanning electron microscopy images. © 2014 AIP Publishing LLC.Financial support by the Swiss Federal Program Nano-Tera through projects NEXRAY and COSMICMOS and Spanish MINECO and CAM through projects EPIC-NANOTICS and Q&C Light are gratefully acknowledged.Peer Reviewe
X-Ray Nano-Diffraction on Epitaxial Crystals
The concept of growing epitaxial Ge and SiGe crystals onto tall Si pillars may provide a means for solving the problems associated with lattice parameter and thermal expansion coefficient mismatch, i.e., dislocations, wafer bowing and cracks. For carefully tuned epitaxial growth conditions the lateral expansion of crystals stops once nearest neighbors get sufficiently close. We have carried out scanning nano-diffraction experiments at the ID01 beam-line of the European Synchrotron Radiation Facility (ESRF) in Grenoble on the resulting space-filling arrays of micron-sized crystals to assess their structural properties and crystal quality. Elastic relaxation of the thermal strain causes lattice bending close to the Si interface, while the dislocation network is responsible for minute tilts of the crystals as a whole. To exclude any interference from nearest neighbors, individual Ge crystals were isolated first by chemical etching followed by micro-manipulation inside a scanning electron microscope. This permitted us to scan an X-ray beam, focused to a spot a few hundreds of nm in size, along the height of a single crystal and to record three-dimensional reciprocal space maps at chosen heights. The resolution limited width of the scattered X-ray beams reveals that the epitaxial structures evolve into perfect single crystals sufficiently far away from the heavily dislocated interface
Perfect crystals grown from imperfect interfaces
The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.ISSN:2045-232
Strain driven migration of In during the growth of InAs/GaAs quantum posts
Using the mechano-optical stress sensor technique, we observe a counter-intuitive reduction of the compressive stress when InAs is deposited on GaAs (001) during the growth of quantum posts. Through modelling of the strain fields, we find that such anomalous behaviour can be related to the strain-driven detachment of In atoms from the crystal and their surface diffusion towards the self-assembled nanostructures.We acknowledge the financial support by Spanish MINECO through Grant Nos. ENE2012-37804-C02-02 and TEC2011-29120-C05-04, and by Spanish CAM through Grant Nos. S2009/ESP-1503 and S2009/ENE-1477.Peer Reviewe
Emission polarization control in semiconductor quantum dots coupled to a photonic crystal microcavity
8 páginas, 5 figuras.-- OCIS codes: (160.4760) Optical properties; (230.5298) Photonic crystals; (230.5590) Quantumwell,
-wire and –dot devices.We study the optical emission of single semiconductor quantum dots weakly coupled to a photonic-crystal micro-cavity. The linearly polarized emission of a selected quantum dot changes continuously its polarization angle, from nearly perpendicular to the cavity mode polarization at large detuning, to parallel at zero detuning, and reversing sign for negative detuning. The linear polarization rotation is qualitatively interpreted in terms of the detuning dependent mixing of the quantum dot and cavity states. The present result is relevant to achieve continuous control of the linear polarization in single photon emitters.This
work has been supported by research contracts of the Spanish Ministry of Education Grants
MAT2008-01555/NAN, Consolider CSD 2006-19 and Naninpho-QD TEC2008-06756-C03-
01, and the Community of Madrid Grant Grant CAM (S2009/ESP-1503).Peer reviewe
Theoretical modelling of quaternary GaInAsSb/GaAs self-assembled quantum dots
Trabajo presentado al "Quantum Dot", celebrado en Nottingham (UK) del 26 al 30 de Abril de 2010.InAs/GaAs quantum dots exposed to Sb after growth exhibit spectral changes. We study
in the present paper an idealized nanostructure consisting of a homogeneous distribution of the
quaternary GaInAsSb surrounded by a barrier of GaAs. We nd that the valence band o set is
a critical parameter in modelling its electronic structure. Depending on this value, we predict a
transition from type-I to type-II band alignment at a di erent Sb concentration. The addition
of Sb to reduce the transition energy while keeping a type-I alignment is only of bene t at low
Sb concentrationThis work was supported by the Spanish MICINN (projects TEC2008-06756-C03-01/02/TEC,
CONSOLIDER INGENIO 2010 CSD2006-0019 and CSD2009-00013), the Junta de Andalucía
(PAI research groups TEP-120 and TIC-145; project P08-TEP-03516) and Comunidad Autónoma de Madrid S2009ESP-1503.Peer reviewe
Stress compensation by GaP monolayers for stacked InAs/GaAs quantum dots solar cells
Trabajo presentado en el 33rd IEEE Photovoltaic Specialists Conference, celebrado en San Diego, CA (Estados Unidos), del 11 al 16 de mayo de 2008In this work we report the stacking of 10 and 50
InAs quantum dots layers using 2 monolayers of GaP for
stress compensation and a stack period of 18 nm on GaAs
(001) substrates. Very good structural and optical quality is
found in both samples. Vertical alignment of the dots is
observed by transmission electron microscopy suggesting
the existence of residual stress around them. Photocurrent
measurements show light absorption up to 1.2 µm in the
nanostructures together with a reduction in the blue
response of the device. As a result of the phosphorus
incorporation in the barriers, a very high thermal activation
energy (431 meV) has also been obtained for the quantum
dot emission.The authors gratefully acknowledge financial
support by the Spanish MEC and CAM through projects
200560M089, S-05050/ENE-0310, TEC-2005-05781-C03-
01 and -02, Consolider-Ingenio 2010 CSD2006-0004, the
Junta de Andalucia (project TEP383, group TEP120) and
by the European Commission through the SANDIE
Network of Excellence (NMP4-CT-2004-500101).Peer reviewe
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