Understanding the Selective Area Nucleation and Growth of GaN nanocolumns by MBE using Ti nanomasks

•Self- assembled Ga(In)N Nanorods and Nanostructures •Ordered growth of GaN Nanorods: masks issues •Ordered growth of GaN Nanorods: mechanisms •White NanoLEDs

MBE growth and characterization of InGaN-based films and nanocolumns on Silicon substrates and GaN templates

InN layers: MBE growth issues Growth of InN-based thin films: InN/InGaN QWS on GaN Growth of InN-based nanorods ● Self Self-assembled assembled InN InN nanorods nanorods onon different different substrates substrates ● Self-assembled InGaN nanorods ● Broad- Broad-emission emission nanostructures ● Self Self--assembled assembled InGaN InGaN--based based Qdisks Qdisks ● Selective area growth (SAG) of InGaN Qdisks

Influence of substrate type and orientation on the morphology and optical properties of selective area growth of GaN and InGaN nanocolumns

A relevant issue concerning optoelectronic devices based on III-nitrides is the presence of strong polarization fields that may reduce efficiency.

Mechanical Characterization of Lead-Free Sn-Ag-Cu Solder Joints by High-Temperature Nanoindentation

The reliability of Pb-free solder joints is controlled by their microstructural constituents. Therefore, knowledge of the solder microconstituents’ mechanical properties as a function of temperature is required. Sn-Ag-Cu lead-free solder alloy contains three phases: a Sn-rich phase, and the intermetallic compounds (IMCs) Cu6Sn5 and Ag3Sn. Typically, the Sn-rich phase is surrounded by a eutectic mixture of β-Sn, Cu6Sn5, and Ag3Sn. In this paper, we report on the Young’s modulus and hardness of the Cu6Sn5 and Cu3Sn IMCs, the β-Sn phase, and the eutectic compound, as measured by nanoindentation at elevated temperatures. For both the β-Sn phase and the eutectic compound, the hardness and Young’s modulus exhibited strong temperature dependence. In the case of the intermetallics, this temperature dependence is observed for Cu6Sn5, but the mechanical properties of Cu3Sn are more stable up to 200°C.

Polyaniline-coated carbon papers for supercapacitor electrodes

Graphene and carbon nanotubes are promising materials for supercapacitor electrodes because of their high specific surface area and excellent electrical, thermal, and mechanical properties. However, these materials suffer from a high manufacturing cost and some aggregation of graphene layers or the presence of toxic residual metallic impurities of carbon nanotubes.

Ti/Pd/Ag Contacts to n-Type GaAs for High Current Density Devices

The metallization stack Ti/Pd/Ag on n-type Si has been readily used in solar cells due to its low metal/semiconductor specific contact resistance, very high sheet conductance, bondability, long-term durability, and cost-effectiveness. In this study, the use of Ti/Pd/Ag metallization on n-type GaAs is examined, targeting electronic devices that need to handle high current densities and with grid-like contacts with limited surface coverage (i.e., solar cells, lasers, or light emitting diodes). Ti/Pd/Ag (50 nm/50 nm/1000 nm) metal layers were deposited on n-type GaAs by electron beam evaporation and the contact quality was assessed for different doping levels (from 1.3 × 1018 cm−3 to 1.6 × 1019 cm−3) and annealing temperatures (from 300°C to 750°C). The metal/semiconductor specific contact resistance, metal resistivity, and the morphology of the contacts were studied. The results show that samples doped in the range of 1018 cm−3 had Schottky-like I–V characteristics and only samples doped 1.6 × 1019 cm−3 exhibited ohmic behavior even before annealing. For the ohmic contacts, increasing annealing temperature causes a decrease in the specific contact resistance (ρ c,Ti/Pd/Ag ~ 5 × 10−4 Ω cm2). In regard to the metal resistivity, Ti/Pd/Ag metallization presents a very good metal conductivity for samples treated below 500°C (ρ M,Ti/Pd/Ag ~ 2.3 × 10−6 Ω cm); however, for samples treated at 750°C, metal resistivity is strongly degraded due to morphological degradation and contamination in the silver overlayer. As compared to the classic AuGe/Ni/Au metal system, the Ti/Pd/Ag system shows higher metal/semiconductor specific contact resistance and one order of magnitude lower metal resistivity.

First principles study of Bi dopen CdTe thin film solar cells: electronic and optical properties

Nowadays, efficiency improvement of solar cells is one of the most important issues in photovoltaic systems and CdTe is one of the most promising thin film photovoltaic materials we can found. CdTe reported efficiencies in solar energy conversion have been as good as that found in polycrystalline Si thin film cell [1], besides CdTe can be easily produced at industrial scale.

Electronic damage in quartz (c-SiO2) by MeV ion irradiations: Potentiality for optical waveguiding applications

The damage induced on quartz (c-SiO2) by heavy ions (F, O, Br) at MeV energies, where electronic stopping is dominant, has been investigated by RBS/C and optical methods. The two techniques indicate the formation of amorphous layers with an isotropic refractive index (n = 1.475) at fluences around 1014 cm−2 that are associated to electronic mechanisms. The kinetics of the process can be described as the superposition of linear (possibly initial Poisson curve) and sigmoidal (Avrami-type) contributions. The coexistence of the two kinetic regimes may be associated to the differential roles of the amorphous track cores and preamorphous halos. By using ions and energies whose maximum stopping power lies inside the crystal (O at 13 MeV, F at 15 MeV and F at 30 MeV) buried amorphous layer are formed and optical waveguides at the sample surface have been generated.

Electronic property analysis of O-doped Cu3SbS3

The ternary Cu-Sb-S semiconductors are considered to be sustainable and potential alternative absorber materials in thin film photovoltaic applications. In these compounds, several phases may coexist, albeit in different proportions depending on experimental growth conditions. Additionally, the photovoltaic efficiency could be increased with isoelectronic doping. In this work we analyze the electronic properties of O-doped Cu3SbS3 in two structures: the wittichenite and the skinnerite. We use first-principles within the density functional formalism with two different exchange-correlation potentials. In addition, we estimate the potential of these compounds for photovoltaic applications.

Crack mechanical failure in ceramic materials under ion irradiation: case of lithium niobate crystal

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion- generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

IBPOWER Project, Intermediate band materials and solar cells for photovoltaics with high efficiency and reduced cost

IBPOWER is a Project awarded under the 7th European Framework Programme that aims to advance research on intermediate band solar cells (IBSCs). These are solar cells conceived to absorb below bandgap energy photons by means of an electronic energy band that is located within the semiconductor bandgap, whilst producing photocurrent with output voltage still limited by the total semiconductor bandgap. IBPOWER employs two basic strategies for implementing the IBSC concept. The first is based on the use of quantum dots, the IB arising from the confined energy levels of the electrons in the dots. Quantum dots have led to devices that demonstrate the physical operation principles of the IB concept and have allowed identification of the problems to be solved to achieve actual high efficiencies. The second approach is based on the creation of bulk intermediate band materials by the insertion of an appropriate impurity into a bulk semiconductor. Under this approach it is expected that, when inserted at high densities, these impurities will find it difficult to capture electrons by producing a breathing mode and will cease behaving as non-radiative recombination centres. Towards this end the following systems are being investigated: a) Mn: In1-xGax N; b) transition metals in GaAs and c) thin films.

Ion-beam damage induced in SiO2: Excitonic mechanisms under heavy electronic excitation

The Centro de Micro-Análisis de Materiales (CMAM) in the Universidad Autónoma de Madrid is carrying out an extensive research program on the processes induced by high energy heavy mass ions (SHI) on dielectric materials and their photonic applications [1?21]. A significant part of this activity constitutes a relevant contribution to the scientific program associated to the TECHNOFUSION project. It is performed in collaboration with the Instituto de Fusion Nuclear at the UPM, the CIEMAT, the Departamento de Física de Materiales at UAM and several other national institutions (INTA) and international laboratories (GANIL, France), Legnaro Italy, Grenoble?. The program has led to a large number of publications in reputed international journals.

Estudio teórico de propiedades termodinámicas y optoelectronicas de nuevos materiales fotovoltaicos de banda intermedia = Theoretical study of thermodynamic and optoelectronic properties of new intermediate band photovoltaic materials

Los materiales de banda intermedia han atraido la atención de la comunidad científica en el campo de la energía solar fotovoltaica en los últimos años. Sin embargo, con el objetivo de entender los fundamentos de las células solares de banda intermedia, se debe llevar a cabo un estudio profundo de la características de los materiales. Esto se puede hacer mediante un modelo teórico usando Primeros Principios. A partir de este enfoque se pueden obtener resultados tales como la estructura electrónica y propiedades ópticas, entre otras, de los semiconductores fuertemente dopados y sus precursores. Con el fin de desentrañar las estructuras de estos sistemas electrónicos, esta tesis presenta un estudio termodinámico y optoelectrónico de varios materiales fotovoltaicos. Específicamente se caracterizaron los materiales avanzados de banda intermedia y sus precursores. El estudio se hizo en términos de caracterización teórica de la estructura electrónica, la energética del sistema, entre otros. Además la estabilidad se obtuvo usando configuraciones adaptadas a la simetría del sistema y basado en la combinatoria. Las configuraciones de los sitios ocupados por defectos permiten obtener información sobre un espacio de configuraciones donde las posiciones de los dopantes sustituidos se basan en la simetría del sólido cristalino. El resultado puede ser tratado usando elementos de termodinámica estadística y da información de la estabilidad de todo el espacio simétrico. Además se estudiaron otras características importantes de los semiconductores de base. En concreto, el análisis de las interacciones de van der Waals fueron incluidas en el semiconductor en capas SnS2, y el grado de inversión en el caso de las espinelas [M]In2S4. En este trabajo además realizamos una descripción teórica exhaustiva del sistema CdTe:Bi. Este material de banda-intermedia muestra características que son distintas a las de los otros materiales estudiados. También se analizó el Zn como agente modulador de la posición de las sub-bandas prohibidas en el material de banda-intermedia CuGaS2:Ti. Analizándose además la viabilidad termodinámica de la formación de este compuesto. Finalmente, también se describió el GaN:Cr como material de banda intermedia, en la estructura zinc-blenda y en wurtztite, usando configuraciones de sitios ocupados de acuerdo a la simetría del sistema cristalino del semiconductor de base. Todos los resultados, siempre que fue posible, fueron comparados con los resultados experimentales. ABSTRACT The intermediate-band materials have attracted the attention of the scientific community in the field of the photovoltaics in recent years. Nevertheless, in order to understand the intermediate-band solar cell fundamentals, a profound study of the characteristics of the materials is required. This can be done using theoretical modelling from first-principles. The electronic structure and optical properties of heavily doped semiconductors and their precursor semiconductors are, among others, results that can be obtained from this approach. In order to unravel the structures of these crystalline systems, this thesis presents a thermodynamic and optoelectronic study of several photovoltaic materials. Specifically advanced intermediate-band materials and their precursor semiconductors were characterized. The study was made in terms of theoretical characterization of the electronic structure, energetics among others. The stability was obtained using site-occupancy-disorder configurations adapted to the symmetry of the system and based on combinatorics. The site-occupancy-disorder method allows the formation of a configurational space of substitutional dopant positions based on the symmetry of the crystalline solid. The result, that can be treated using statistical thermodynamics, gives information of the stability of the whole space of symmetry of the crystalline lattice. Furthermore, certain other important characteristics of host semiconductors were studied. Specifically, the van der Waal interactions were included in the SnS2 layered semiconductor, and the inversion degree in cases of [M]In2S4 spinels. In this work we also carried out an exhaustive theoretical description of the CdTe:Bi system. This intermediate-band material shows characteristics that are distinct from those of the other studied intermediate-band materials. In addition, Zn was analysed as a modulator of the positions of the sub-band gaps in the CuGaS2:Ti intermediate-band material. The thermodynamic feasibility of the formation of this compound was also carried out. Finally GaN:Cr intermediate-band material was also described both in the zinc-blende and the wurtztite type structures, using the symmetry-adapted-space of configurations. All results, whenever possible, were compared with experimental results.

Electronic and optical properties of substitutional V, Cr and Ir impurities in Cu2ZnSnS4

The substitution of cation atoms by V, Cr and It in the natural and synthetic quaternary Cu2ZnSnS4 semiconductor is analyzed using first-principles methods. In most of the substitutions, the electronic structure of these modified CZTS is characterized for intermediate bands with different occupation and position within of the energy band gap. A study of the symmetry and composition of these intermediate bands is carried out for all substitutions. These bands permit additional photon absorption and emission channels depending on their occupation. The optical properties are obtained and analyzed. The absorption coefficients are split into contributions from the different absorption channels and from the inter- and intra-atomic components. The sub bandgap transitions are significant in many cases because the anion states contribute to the valence, conduction and intermediates bands. These properties could therefore be used for novel optoelectronic devices.