David Robledo - Academia.edu (original) (raw)

Papers by David Robledo

Applied Physics Letters, 2014

ABSTRACT The application of a GaAsSb/GaAsN short-period superlattice capping layer (CL) on InAs/G... more ABSTRACT The application of a GaAsSb/GaAsN short-period superlattice capping layer (CL) on InAs/GaAs quantum dots (QDs) is shown to be an option for providing improved luminescence properties to this system. Separating both GaAsSb and GaAsN ternaries during the growth in 2 monolayer-thick phases solves the GaAsSbN immiscibility-related problems. Strong fluctuations in the CL composition and strain field as well as in the QD size distribution are significantly reduced, and a more regular CL interface is also obtained. Room-temperature (RT) photoluminescence (PL) is obtained for overall N contents as high as 3%, yielding PL peak wavelengths beyond 1.4 μm in samples with a type-II band alignment. High external quantum efficiency electroluminescence and photocurrent from the QD ground state are also demonstrated at RT in a single QD-layer p-i-n device. Thus, it becomes possible to combine and transfer the complementary benefits of Sb- and N-containing GaAs alloys to InAs QD-based optoelectronics.

Springer Proceedings in Physics

ABSTRACT This paper shows the first experimental evidence of anticorrelated InAs/GaAs quantum dot... more ABSTRACT This paper shows the first experimental evidence of anticorrelated InAs/GaAs quantum dot structures grown by molecular beam epitaxy. As previous authors have predicted theoretically, a transition occurs between correlated and anticorrelated vertical arrangements depending on the ratio between the layer separation and the average spacing between quantum dots in a single plane. These vertically anticorrelated quantum dot systems are observed to be an efficient way to keep the size and density of the islands constant, which is of crucial importance for the optoelectronic applications of these heterostructures.

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany

ABSTRACT GaN and InGaN with a high content of Ga became the most important semiconductors for lig... more ABSTRACT GaN and InGaN with a high content of Ga became the most important semiconductors for light emission and detection in the visible and the near UV spectral regions. On the other hand, the fabrication of InGaN epilayers with medium and high concentration of In is still not well developed. However, the control of the growth of alloys within this In-rich range would give rise to expand the optical activity of InGaN till the near IR. Recently, the compositional dependencies of photoconductivity and electron transport properties for metal-semiconductor-metal photodetectors based on Inrich InGaN ultra-thin films were studied [1]. There, the electron density profiles and low-field mobilities for different compositions of InGaN were calculated. It was demonstrated that in contrast to bulk InN exhibiting a dominating surface electron accumulation, the free electrons in ultra-thin InxGa1−xN/GaN (0.5

physica status solidi (c), 2008

ABSTRACT The occurrence of cubic indium nitride thin layers grown by molecular beam epitaxy on to... more ABSTRACT The occurrence of cubic indium nitride thin layers grown by molecular beam epitaxy on top of c-plane sapphire substrates modified by an intermediate layer of cubic indium oxide is reported. An orientation relationship between the (0001) plane of Al2O3 and both (001) surfaces of body-centered cubic In2O3 and zinc-blende InN is demonstrated by means of electron and X-ray diffraction and by transmission electron microscopy. We propose that the demonstrated approach is able to stabilize the non equilibrium phase of InN (i. e., the cubic polytype) due to a low lattice mismatch together with a four fold surface atomic arrangement of the indium oxide-indium nitride interface. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

physica status solidi c, 2007

A study of the impact of different pseudosubstrates as AlN, GaN or AlN/GaN on the crystalline qua... more A study of the impact of different pseudosubstrates as AlN, GaN or AlN/GaN on the crystalline quality of InN layers by transmission electron microscopy is reported. The temperature of the substrate was found to play an important role: for higher temperatures, low quality InN layers are obtained, poorly supported on the buffers and consequently almost free of dislocations; whereas for lower temperatures, higher qualities, with smooth surfaces and continuous interfaces are observed. In these cases, high resolution electron microscopy showed that the high lattice mismatch existing between the pseudosubstrate and the InN epilayer is mainly accommodated by a network of geometrical misfit dislocations at the heterointerface. Additionally, low densities of threading dislocations, also decaying exponentially with the increasing thickness were observed. The introduction of a GaN buffer layer on top of AlN promotes the reduction in one order of magnitude the density of threading dislocations in the epilayer, partially due to the reduction of dislocations propagating from the pseudosubstrate. However, other relevant factors as the domains coalescence or the generation of secondary misfit dislocation must be taken into account.

Microscopy and Microanalysis, 2007

Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Fl... more Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Microscopy and Microanalysis, 2014

A study by electron microscopy techniques of the structural and compositional properties of AlxGa... more A study by electron microscopy techniques of the structural and compositional properties of AlxGa1-xN/GaN nanowire (NW) heterostructures on Si(111) is presented. AlxGa1-xN depositions grown without catalyst by plasma-assisted molecular beam epitaxy were designed to form NWs in the range of 0.20

Microscopy and Microanalysis, 2006

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois,... more Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Microelectronics Journal, 1999

A series of InGaAs/GaAs Single Strained Quantum Wells (SSQWs) with indium content ranging from 25... more A series of InGaAs/GaAs Single Strained Quantum Wells (SSQWs) with indium content ranging from 25% to 35% and 100 Å well thickness were grown on two different (111)B GaAs off-axis substrates under optimized growth conditions for simultaneous growth. Optoelectronic properties were studied in terms of low temperature photoluminescence (PL). Results indicate a PL emission dependence with the substrate used, this dependence being stronger for highly strained systems. In order to determine the source of this dependence, samples were studied by Planar View Transmission Electron Microscopy (PVTEM). Relaxation mechanisms seem to act in a different way regarding the misoriented substrate used. Although previous theoretical results have already reported this dependence, this is the first direct evidence of this phenomenon for SSQWs. The results of these two different techniques will be compared and discussed.

Journal of Crystal Growth, 2007

ABSTRACT In this work, coalescence aspects of InN epitaxy are addressed. The coalescence phenomen... more ABSTRACT In this work, coalescence aspects of InN epitaxy are addressed. The coalescence phenomena have been studied in thin InN epilayers by means of electron microscopy and X-ray diffraction. Coalescence time and the corresponding diffusion coefficients at elevated temperatures were estimated for InN deposition. The substrate temperature was found to impact drastically the coalescence of the epilayer, and consequently, the electrical and transport properties of hexagonal InN material. Additionally, a simple growth model was suggested to explain the formation of domain boundaries and (0001) stacking faults formed during the coalescence. In particular, it is shown that two adjacent and tilted, hexagonal-shaped InN domains may form a non-coherent boundary along a {11¯00} plane. We also suggest that the interaction between tilted domains induces formation of basal dislocations. This interaction has two consequences: a localized lateral growth of the most epitaxially oriented domain (forming a basal (0001) stacking fault) followed by the formation of a surface step, and consequently the termination of a threading dislocation by its dissociation and propagation under the formed (0001) stacking fault.

Journal of Applied Physics, 2006

The strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers du... more The strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H-InN. Edge-type threading dislocations and dislocations of mixed character have been found to be dominating defects in the wurtzite InN layers. It has been shown that the threading dislocation density decreases exponentially during the film growth due to recombination and, hence, annihilation of dislocations, reaching ϳ10 9 cm −2 for ϳ2200 nm thick InN films.

Applied Physics Letters, 2005

InN quantum dots (QDs) on GaN (0001) grown by metalorganic vapor phase epitaxy onto a sapphire su... more InN quantum dots (QDs) on GaN (0001) grown by metalorganic vapor phase epitaxy onto a sapphire substrate were studied by transmission electron microscopy (TEM). We found that the nucleation of InN QDs on GaN is directly related to the presence of threading dislocations (TDs) in the center of the QDs. The TEM analysis revealed that the TDs finish at the

Applied Physics Letters, 2012

Applied Physics Letters, 2011

The effect of low N-alloying on the structure of capped InAs/GaAs quantum dots is analyzed by tra... more The effect of low N-alloying on the structure of capped InAs/GaAs quantum dots is analyzed by transmission electron microscopy related techniques. A statistical study of interplanar distances in InAsN quantum dots shows an increase in the lattice parameter compared to the InAs case. We suggest that the addition of nitrogen blocks the Ga/In exchange processes during the quantum dot capping

Applied Physics Letters, 1998

Using previously published relaxation models [D. J. Dunstan, P. Kidd, L. K. Howard and R. H. Dixo... more Using previously published relaxation models [D. J. Dunstan, P. Kidd, L. K. Howard and R. H. Dixon, Appl. Phys. Lett. 59, 3390 (1991) and D. González, D. Araújo, G. Aragón, and R. García, Appl. Phys. Lett. 71, 2475 (1997)] that predict the strain relaxation in the InGaAs/GaAs system, before and during the stage of relaxation saturation, the critical thickness where

Acta Materialia, 2010

A group of InAlN films was fabricated by molecular beam epitaxy and investigated by X-ray diffrac... more A group of InAlN films was fabricated by molecular beam epitaxy and investigated by X-ray diffraction, transmission electron microscopy and element nano-analyses. All top InxAl1−xN layers have compositions around lateral lattice-matching to GaN (x≈0.18) and are pseudomorphic. For a growth rate of 350nmh−1, each InAlN film separated into two sublayers with different In/Al-ratios. Micrographs reveal sharp transitions both at the