Teresa Ben

Universidad de Cádiz | UCA · Department of Material Science and Metallurgy Engineering and Inorganic Chemistry

En el contexto de la asignatura de Ingeniería y Tecnología de los Materiales (ITM) de la Escuela Técnica Superior de Ingeniería de Algeciras (ETSIA), se compararon dos metodologías. Una metodología más tradicional basada en i) clases expositivas, ii) clases de problemas y iii) prácticas de laboratorio, y otra metodología docente basada en el aprend...

This paper investigates the effect of GaAsBi strain reduction layers (SRLs) on InAs QDs with different Bi fluxes to achieve nanostructures with improved temperature stability. The SRLs are grown at a lower temperature (370 • C) than the usual capping temperature for InAs QDs (510 • C). The study finds that GaAs capping at low temperatures reduces Q...

En el contexto de la asignatura de Ciencia e Ingeniería de los Materiales (CIM), común para todos los grados de ingeniería de la rama industrial de la Escuela Técnica Superior de Ingeniería de Algeciras (ETSIA), se comparan indicadores de calidad tras aplicar dos aproximaciones de evaluación continua: una más tradicional mediante exámenes escritos...

2D semiconductors (2SEM) can transform many sectors, from information and communication technology to healthcare. To date, top‐down approaches to their fabrication, such as exfoliation of bulk crystals by “scotch‐tape,” are widely used, but have limited prospects for precise engineering of functionalities and scalability. Here, a bottom‐up techniqu...

We present a quantitative analysis of Sb segregation in ultrathin GaAsSb films (1–20 ML) grown with soaking/desorption methods. Sb soaking advances Sb incorporation almost to the beginning of the growth, increasing the compositional gradient for all thicknesses. Application of the desorption step alone increases the compositional gradient at the up...

For optoelectronic devices from the near to the far infrared, the advantages of using ultrathin III-Sb layers as quantum wells or in superlattices are well known. However, these alloys suffer from severe surface segregation problems, so that the actual profiles are very different from the nominal ones. Here, by inserting AlAs markers within the str...

The effect of a Bi supply on the development of InAs/GaAs (001) QDs has been structurally explored in two growth temperature regimes, one for Bi alloying (380 °C) and another for conventional formation of InAs QDs (510 °C). At high growth temperature (HT), there is no trace of Bi incorporation into the film and Bi acts as a surfactant limiting the...

Recently, GaAsSb/GaAsN type II short-period superlattices (SLs) have been proposed as suitable structures to be implemented in the optimal design of monolithic multi-junction solar cells. However, due to strong surface Sb segregation, experimental Sb composition profiles differ greatly from the nominal square-wave design. In this work, the improvem...

The use of thin AlA capping layers (CLs) on InAs quantum dots (QDs) has recently received considerable attention due to improved photovoltaic performance in QD solar cells. However, there is little data on the structural changes that occur during capping and their relation to different growth conditions. In this work, we studied the effect of AlA c...

Recently, thin AlAs capping layers (CLs) on InAs quantum dot solar cells (QDSCs) have been shown to yield better photovoltaic efficiency compared to traditional QDSCs. Although it has been proposed that this improvement is due to the suppression of the capture of photogenerated carriers through the wetting layer (WL) states by a de-wetting process,...

Recently, very thin AlAs capping layers (CLs) have been proposed as a useful tool to increase the performance of InAs/GaAs quantum dot (QDs) devices. However, the structure of QDs after AlAs deposition remains poorly understood and the mechanisms to explain it are often contradictory. In this work, the structural and compositional changes of InAs Q...

In this work, two different strategies to preserve InAs/GaAs QDs against decomposition during the capping process have been compared structurally and optically. They are based on: (i) the control of the growth parameters of the capping layer (CL), such as growth rate, and (ii) the nature of the CL, such as the use of GaAsSb strain-reducing layers (...

We demonstrate type-II GaAsSb/GaAsN superlattices (SL) as a suitable structure to form the lattice-matched 1.0–1.15 eV subcell that would allow the implementation of the optimum monolithic multi-junction solar cell design. The separation of Sb and N atoms during growth leads to an improved composition homogeneity and a lower defect density than in...

The implementation of GaAs0.8Sb0.2 as CL to obtain type-II strain-coupled InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, it has been demonstrated that capping with GaAsSb prevents the formation of In-rich agglomerations that hampered the QD formation as it has been observed in...

The study explores phenomena that occur during the growth of multi-stacked quantum dots (MQD) InAs layers using thin GaAs spacers. An arrangement of plastically relaxed agglomerations that extend along the MQD structure with diameters of about 70–120 nm and separated every 200–400 nm are observed. These agglomerations hinder the regular development...

Superlattice structures (SLs) with type-II (GaAsSb/GaAsN) and-I (GaAsSbN/GaAs) band alignments have received a great deal of attention for multijunction solar cell (MJSC) applications, as they present a strongly intensified luminescence and a significant external quantum efficiency (EQE), with respect to the GaAsSbN bulk layers. Despite the difficu...

Samples of InAs(1-x)Bix/InAs multi-quantum wells with different concentrations of bismuth have been analyzed in order to determinate how bismuth segregation occurs. This element presents a tendency to segregate from the layers to the barriers. Also, in all samples the first and the tenth layer are more inhomogeneity than the others, in the first on...

The effect of Sb and N incorporation in the CLs of (un)coupled QDs were analysed by TEM. InAs coupled QDs with GaAs and GaAsSb CLs produced agglomerations with high densities of TDs due to a collapse of the structure degrading their optical properties. On the contrary, simultaneous incorporation of Sb and N lead to well aligned QDs. However, an inc...

The practical realization of epitaxial quantum dot (QD) nanocrystals led before long to impressive experimental advances in optoelectronic devices, as well as to the emergence of new technological fields. However, the necessary capping process is well-known to hinder a precise control of the QD morphology and therefore of the possible electronic st...

GaAsSbN dilute nitrides are potential candidates for integration in high-performance multi–junction solar cells due to the bandgap tunability in the 1.0-1.15 eV range and the possibility to match the lattice constant to the GaAs substrates. Recently, the use of GaAsSb/GaAsN superlattices (SLs) has been shown as an effective way to enhance photovolt...

As promising candidates for solar cell and photodetection applications in the range 1.0–1.16 eV, the growth of dilute nitride GaAsSbN alloys lattice matched to GaAs is studied. With this aim, we have taken advantage of the temperature gradient in the molecular beam epitaxy reactor to analyse the impact of temperature on the incorporation of Sb and...

A procedure to quantitatively analyse the relationship between the wetting layer (WL) and the quantum dots (QDs) as a whole in a statistical way is proposed. As we will show in the manuscript, it allows determining, not only the proportion of deposited InAs held in the WL, but also the average In content inside the QDs. First, the amount of InAs de...

This work reports on the benefits from using thin GaAsSb capping layers (CLs) on InAs/GaAs quantum dot (QD) solar cells. The application of such CLs allows the tunability of the QD ground state, switching the QD-CL band alignment from type I to type II for high Sb contents and extending the photoresponse beyond 1.5 µm. Two different structures with...

This chapter reviews the impact of using thin alloyed capping layers (CLs) for InAs/GaAs quantum dot (QD) solar cells. This novel approach can be considered as a particular way to develop hybrid QD-quantum well (QW) solar cells in which a thin QW is additionally used to modify the properties of QDs. The tunability of the absorption edge, defined by...

Dilute nitride III–V alloys have attracted a lot of attention in the last decade due to its wide tunability of both band gap and lattice constant that makes them a potential candidate in multi-junction solar cell technology. For certain, these alloys can be used to improve the conventional lattice matched three-junction solar cell by the replacemen...

Nowadays, stacked InAs/GaAs quantum dots (QDs) capped with layers different than GaAs are being applied in photovoltaic and photodetector technologies due to their potential for tailoring the optical properties and enhancing the device efficiency.1,2 The particular use of GaAsSbN capping layers (CLs) allows controlling the QD size as well as the el...

We propose type-II GaAsSb/GaAsN superlattices (SLs) lattice-matched to GaAs as a novel material for the 1 eV sub-cells present in highly efficient GaAs/Ge-based multi-junction solar cells. We demonstrate that, among other benefits, the spatial separation of Sb and N allows a better control over composition and lattice matching, avoiding the growth...

The built-in piezoelectric fields in group III-nitrides can act as road blocks on the way to maximizing the efficiency of opto-electronic devices. In order to overcome this limitation, a proper characterization of these fields is necessary. In this work nano-beam electron diffraction in scanning transmission electron microscopy mode has been used t...

Stranski-Krastanov In(Ga)As/GaAs quantum dots (QDs) are the subject of great scientific and technological interest, primarily aimed to broaden the emission wavelength towards 1.3-1.55 μm, required for optical fiber telecommunications [1]. More recently, this QD system has drawn a lot of attention owing to their unique properties, which could be als...

The effect of the capping process on the morphology of InAs/GaAs quantum dots (QDs) by using different GaAs-based capping layers (CLs), ranging from strain reduction layers to strain compensating layers, has been studied by transmission microscopic techniques. For this, we have measured simultaneously the height and diameter in buried and uncapped...

The built-in piezoelectric fields in group III-nitrides can act as road blocks on the way to maximizing the efficiency of opto-electronic devices. In order to overcome this limitation, a proper characterization of these fields is necessary. In this work nano-beam electron diffraction in scanning transmission electron microscopy mode has been used t...

Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N(+) implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and...

We present the mapping of the plasmonic properties of gold nanoparticles that are embedded in a TiO2 thin film deposited over two different substrates, glass and silicon. An improved electron energy-loss spectroscopy (EELS) imaging technique was used to extract plasmon maps with nanometre resolution. Several representative cases of randomly dispers...

We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-S...

1 ZnO microrods were grown by laser assisted flow deposition technique in order to study their luminescence behaviour in the near band edge spectral region. Transmission electron microscopy analysis put in evidence the high crystallinity degree and microrod's compositional homogeneity. Photoluminescence revealed a dominant 3.31 eV emission. The cor...

In this manuscript we carry out a comparative analysis of p-i-n junction solar cells based on 10 stacks of InAs/GaAs quantum dots (QDs) capped with GaAs(Sb)(N) capping layers (CLs). The application of such CLs allows to significantly extend the photoresponse beyond 1.3 μm. Moreover, a strong photocurrent from the CLs is observed so that the devices...

Changing the growth rate during the heteroepitaxial capping of InAs/GaAs quantum dots (QDs) with a 5 nm-thick GaAsSbN capping layer (CL) strongly modifies the QD structural and optical properties. A size and shape transition from taller pyramids to flatter lens-shaped QDs is observed when the CL growth rate is decreased from 1.5 to 0.5 ML/s. This i...

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 fluc...

A study by electron microscopy techniques of the structural and compositional properties of Al x Ga1-x N/GaN nanowire (NW) heterostructures on Si(111) is presented. Al x Ga1-x N depositions grown without catalyst by plasma-assisted molecular beam epitaxy were designed to form NWs in the range of 0.20<x<0.40 with different lengths and growth tempera...

Laser assisted flow deposition (LAFD) is a very high yield method based on a vapor-solid mechanism, allowing the production of ZnO crystals in a very short time. The LAFD was used in the growth of different morphologies (nanoparticles, tetrapods and microrods) of ZnO micro/nanocrystals and their microstructural characterization confirms the excelle...

With the aim to improve the design and potential use of 1D-nanostructures as functional materials, structural and compositional changes of these nanostructures must be correlated with the growth conditions. The process temperature, deposition time, diffusivity of participant specimens, the buffer or intermediate layer characteristics grown previous...

In this chapter, we review results obtained by conventional and advanced electron microscopy related techniques of complex 1-D nanostructures such as spontaneous core–shell AlxGa1−x N/GaN (0001) nanowires, ZnO/Si(111) nanorods, ZnO/CdTe (111) nanowires, and InAsxP1−x/InP(001) quantum wires.

As an alternative to Core-Loss Electron Energy Loss Spectroscopy, Low-Loss EELS is suitable for compositional analysis of complex heterostructures, such as the InAs-GaAs- GaSb system, since in this energy range the edges corresponding to these elements are better defined than in Core-Loss. Furthermore, the analysis of the bulk plasmon peak, which i...

Ion-induced intermixing and damage is evaluated in GaN/AlN superlattices of quantum dots (QDs) and quantum wells (QWs) using 100 keV Ar(+) implantation at low temperature (15 K). Despite the similar damage build up at low fluences, a significant increase of the damage accumulation takes place for QDs at high fluences. Elemental depth profiles were...

The effects of low-energy (<=2 kV) Ar+ irradiation on the optical and structural properties of zinc oxide (ZnO) nanowires (NWs) grown by a simple and cost-effective low-temperature technique were investigated. Both photoluminescence spectra from ZnO NW-coated films and cathodoluminescence analysis of individual ZnO NWs demonstrated obvious evidence...

EDX-SEM analysis of ZnO nanowires before the irradiation process. This file displays a SEM image at low magnification showing the initial sample just after growing the nanowires. On the right of the SEM image, an EDX spectrum is presented with a table containing the quantitative analysis and confirming that the composition was very close to the sto...

Compositional analysis carried out by EDX spectroscopy of the superficial particles. This file presents an EDX spectrum carried out in the superficial particles. The quantitative analysis shown in the table confirms that the superficial particles are made up of ZnO.

Color change detected in ZnO irradiated areas. This file shows samples irradiated with different energies. As can be seen, a clear color change is observed in the irradiated area by the naked eye when illuminating under UV light. The irradiated areas appear black.

In this work, we have analyzed the effect of intentional carbon doping on molecular beam epitaxy grown In-polar InN epilayers using carbon bromide (CBr4) as dopant source. Hall effect measurements, high resolution X-ray diffraction, atomic force microscopy, transmission electron microscopy, secondary ion mass spectrometry, spectroscopic ellipsometr...

Using Transmission Electron Microscopy-related techniques, we study the effect of the high temperature in the Vapour Phase Transport (VPT) process on the morphology and chemistry of VPT ZnO nanorod arrays deposited on a two-step Chemical Bath Deposition (CBD) buffer layers on silicon substrates. Though well-aligned and c-axis oriented arrays of ZnO...

This paper reports the formation and characterization of spherical GaAs quantum dots obtained by nanosecond pulsed laser ablation in a liquid (ethanol or methanol). The produced bare GaAs nanoparticles demonstrate rather narrow size distribution which depends on the applied laser power density (from 4.25 to 13.9 J/cm2 in our experiments) and is as...

In the last decade several prototypes of intermediate band solar cells (IBSCs) have been manufactured. So far, most of these prototypes have been based on InAs/GaAs quantum dots (QDs) in order to implement the IB material. The key operation principles of the IB theory are two photon sub-bandgap (SBG) photocurrent, and output voltage preservation, a...

Growth of catalyst-free AlxGa1-xN (0.15<x<0.50)/GaN nanowires by plasma-assisted molecular beam epitaxy is thoroughly structurally and chemically analyzed by using transmission electron microscopy techniques. We found that well-defined and defect-free core--shell structures are spontaneously formed during the AlxGa1-xN section growth. An Al-rich sh...

Arsenic alloying is observed for epitaxial layers nominally intended to be In0.75Ga0.25N. Voids form beneath their interfaces with GaAs substrates, acting as sources of Ga + As out-diffusion into the growing epilayers. As a result, heteroepitaxial single-phase quaternary InxGa1−xAsyN1−y films are formed with x ∼ 0.55 and 0.05 < y < 0.10. While an u...

This chapter briefly describes the fundamentals of high-resolution electron microscopy techniques. In particular, the Peak Pairs approach for strain mapping with atomic column resolution, and a quantitative procedure to extract atomic column compositional information from Z-contrast high-resolution images are presented. It also reviews the structur...

We report that Sb irradiation combined with the presence of a GaAs intermediate layer previous to the deposition of a GaSb layer over InAs quantum dots grown by molecular beam epitaxy improves the crystalline quality of these nanostructures. Moreover, this approach to develop III–V-Sb nanostructures causes the formation of quantum dots buried by a...

c-axis aligned ZnO nanorods were deposited by vapor phase transport on textured chemical bath deposited buffer layers. In this work, we examine the role of the buffer layer and how it influences the vapor phase transport deposition process using both scanning and transmission electron microscopes and related techniques. Vapor phase transport deposi...

Photoluminescence (PL) studies of GaN/AlxGa1−xN quantum dots (QDs) in nanowires demonstrate an efficient carrier confinement, resulting in thermally stable decay times up to 300 K. The evolution of the PL transition energy as a function of both the QD height and the Al mole fraction in the barriers, as well as the evolution of the decay time as a f...

Growth of catalyst-free AlxGa1-xN (0.15<x<0.50)/GaN nanowires by plasma assisted molecular beam epitaxy is thoroughly structural and chemical analyzed by using transmission electron microscopy related techniques. We found that well-defined and defect-free core-shell structures are spontaneously formed during the wire growth. An Al-rich shell with s...

GaSb incorporation to InAs/GaAs quantum dots is considered for improving the opto-electronic properties of the systems. In order to optimize these properties, the introduction of an intermediate GaAs layer is considered a good approach. In this work, we study the effect of the introduction of a GaAs intermediate layer between InAs quantum dots and...

〈1 1 2¯ 0〉 wurtzite ZnO nanowires (NWs) have been obtained by oxidizing in air at 500°C thermally evaporated Zn metal films deposited onto CdTe substrates. The presence of Cd atoms from the substrate on the ZnO seeding layer and NWs seems to affect the growth of the NWs. The effects of the oxidation time on the structural and optical properties of...

Type-II self-assembled GaSb/GaAs nanostructures have been grown by molecular-beam epitaxy and studied by atomic-force microscopy, transmission electron microscopy, and power-dependent magnetophotoluminescence. Nanostructures on the sample surface are found to be entirely dotlike, while capped nanostructures are predominantly ringlike. Moreover, an...

Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation o...

The effects of a high ion dose irradiation on TiO2 thin films under different conditions of temperature and ion nature are discussed. We have shown that anatase TiO2 thin films irradiated with N+ ions at room temperature develop a typical microstructure with mounds and voids open to the surface whereas irradiations at 700 K generate a surface patte...

Within the context of quantum dot Intermediate Band Solar Cells (QD-IBSC), it is of interest to investigate the maximum value that can be achieved for the smaller of the transitions (ELEL), since values larger than 0.3 eV are required for improved performance. This work provides both theoretical and experimental arguments to verify the shift of the...

Prototypes based on InAs/GaAs QDs have been manufactured in order to realize the theoretically predicted high efficiency intermediate band solar cells (IBSCs). Unfortunately, until now, these prototypes have not yet demonstrated the expected increase in efficiency when compared with reference samples without IB material. One of the main arguments e...

By using a marker technique based on nanowire (NW) heterostructure, we have identified the Ga-limited and N-limited GaN NW growth regimes, which are shifted in comparison to those in two-dimensional GaN layers. The results show that the Ga atoms diffusing along NW sidewalls have a significant contribution to the NW vertical growth. By reducing the...

The addition of antimony to III-V nanostructures is expected to give greater freedom in bandgap engineering for device applications. One of the main challenges to overcome is the effect of indium and antimony surface segregation. Using several very high resolution analysis techniques we clearly demonstrate blocking of indium incorporation by antimo...

The addition of antimony to III–V nanostructures is expected to give greater freedom in bandgap engineering for device applications. One of the main challenges to overcome is the effect of indium and antimony surface segregation. Using several very high resolution analysis techniques we clearly demonstrate blocking of indium incorporation by antimo...

We show in this work that GaAs capping of 2 ML of GaSb grown by molecular beam epitaxy results in the formation of very small �with heights of about 1 nm� GaAsxSb1−x nanostructures surrounded by a GaAs rich layer. This conclusion is obtained by analyzing the morphology of the GaAsxSb1−x nanostructures by high resolution scanning transmission electr...

In this work we report on the ability to form low density Ga(As)Sb quantum ring-shaped nanostructures (Q-rings) on GaAs(001) substrates by the droplet epitaxy technique. The Q-rings are formed by crystallization of Ga droplets under antimony flux. After being capped by a GaAs layer, these nanostructures show surface mounding features that are corre...

A study was conducted to demonstrate the synthesis of an aqueous soluble near-infrared (NIR) fluorescent composite, based on a PbS quantum dot (QD), entrapped in the hollow core of a horse spleen apoferritin (AFt) protein case. The synthesis was performed by two different approaches,such as reassembly and nanoreactor processes. It was demonstrated...

We determine the compositional distribution with atomic column resolution in a horizontal nanowire from the analysis of aberration-corrected high resolution Z-contrast images. The strain field in a layer capping the analysed nanowire is determined by anisotropic elastic theory from the resulting compositional map. The reported method allows prefere...

ABSTRACT Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

In this work we report on the ability to form low density Ga(As)Sb quantum ring-shaped nanostructures (Q-rings) on GaAs(001) substrates by the droplet epitaxy technique. The Q-rings are formed by crystallization of Ga droplets under antimony flux. After being capped by a GaAs layer, these nanostructures show surface mounding features that are corre...

One of the main requirements for the development of quantum computing is the ability of manufacturing single photon emitters. Self assembled semiconductor nanostructures are ones of the most reliable candidates for this application, once control of size and spatial location is achieved. The droplet epitaxy technique allows growth of low density dis...

It is well known that there is intense interest in expanding the usable wavelength for electronic devices. Research efforts are dedicated to develop GaAs technology in order to achieve emission at 1.3 and 1.55 µm, so GaAs could be used for telecommunication applications [1]. Ga(As)Sb on InAs/GaAs quantum dots (QDs) is a promising nanostructure to b...

The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capped InAs/GaAs quantum dots emitting at 1.3 mu m. The existence of the four constituent elements is demonstrated by using spatially resolved low-loss electron energy loss spectroscopy and aberration-corrected high angle annular dark field scanning transmi...

Strain mapping is defined as a numerical image-processing technique that measures the local shifts of image details around a crystal defect with respect to the ideal, defect-free, positions in the bulk. Algorithms to map elastic strains from high-resolution transmission electron microscopy (HRTEM) images may be classified into two categories: those...