Selective area growth of a- and c-plane GaN nanocolumns by molecular beam epitaxy using colloidal nanolithography

Selective area growth of a-plane GaN nanocolumns by molecular beam epitaxy was performed for the first time on a-plane GaN templates. Ti masks with 150 nm diameter nanoholes were fabricated by colloidal lithography, an easy, fast and cheap process capable to handle large areas. Even though colloidal lithography does not provide a perfect geometrical arrangement like e-beam lithography, it produces a very homogeneous mask in terms of nanohole diameter and density, and is used here for the first time for the selective area growth of GaN. Selective area growth of a-plane GaN nanocolumns is compared, in terms of anisotropic lateral and vertical growth rates, with GaN nanocolumns grown selectively on the c-plane

Electrophoretic Deposition of Transparent ZnO Thin Films from Highly Stabilized Colloidal Suspensions

The parameters that control the stability of ZnO-nanoparticles suspensions and their deposition by electrophoretic deposition were studied, so as to organize the assembly and compaction of nanoparticles. The addition of cationic polyelectrolyte - Polyethylenimine (PEI) - with different molecular weights was investigated, in order to study their effectiveness and the influence of the molecular weight of the organic chain on suspensions dispersion. It was found that PEI with the highest molecular weight provided better dispersion conditions. Cathodic EPD was performed under previously optimized suspensions conditions and over electropolished stainless steel substrates. Experimental results showed that the EPD process in these conditions allows obtaining dense transparent ZnO thin films. Deposition times and intensities were optimized by analyzing the resulting thin films characteristics. Finally, the deposits were characterized by FE-SEM, AFM, and different spectroscopic techniques.

Self-organized colloidal quantum dots and metal nanoparticles for plasmon-enhanced intermediate-band solar cells

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude. In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance. The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.

Desarrollo de estructuras nanofotonicas de campo cercano para intensificacion localizada de luz en celulas solares de banda intermedia - Near-field nanophotonic structures for localized light enhancement in intermediate band solar cells

This doctoral thesis explores some of the possibilities that near-field optics can bring to photovoltaics, and in particular to quantum-dot intermediate band solar cells (QD-IBSCs). Our main focus is the analytical optimization of the electric field distribution produced in the vicinity of single scattering particles, in order to produce the highest possible absorption enhancement in the photovoltaic medium in their surroundings. Near-field scattering structures have also been fabricated in laboratory, allowing the application of the previously studied theoretical concepts to real devices. We start by looking into the electrostatic scattering regime, which is only applicable to sub-wavelength sized particles. In this regime it was found that metallic nano-spheroids can produce absorption enhancements of about two orders of magnitude on the material in their vicinity, due to their strong plasmonic resonance. The frequency of such resonance can be tuned with the shape of the particles, allowing us to match it with the optimal transition energies of the intermediate band material. Since these metallic nanoparticles (MNPs) are to be inserted inside the cell photovoltaic medium, they should be coated by a thin insulating layer to prevent electron-hole recombination at their surface. This analysis is then generalized, using an analytical separation-of-variables method implemented in Mathematica7.0, to compute scattering by spheroids of any size and material. This code allowed the study of the scattering properties of wavelengthsized particles (mesoscopic regime), and it was verified that in this regime dielectric spheroids perform better than metallic. The light intensity scattered from such dielectric spheroids can have more than two orders of magnitude than the incident intensity, and the focal region in front of the particle can be shaped in several ways by changing the particle geometry and/or material. Experimental work was also performed in this PhD to implement in practice the concepts studied in the analysis of sub-wavelength MNPs. A wet-coating method was developed to self-assemble regular arrays of colloidal MNPs on the surface of several materials, such as silicon wafers, amorphous silicon films, gallium arsenide and glass. A series of thermal and chemical tests have been performed showing what treatments the nanoparticles can withstand for their embedment in a photovoltaic medium. MNPs arrays are then inserted in an amorphous silicon medium to study the effect of their plasmonic near-field enhancement on the absorption spectrum of the material. The self-assembled arrays of MNPs constructed in these experiments inspired a new strategy for fabricating IBSCs using colloidal quantum dots (CQDs). Such CQDs can be deposited in self-assembled monolayers, using procedures similar to those developed for the patterning of colloidal MNPs. The use of CQDs to form the intermediate band presents several important practical and physical advantages relative to the conventional dots epitaxially grown by the Stranski-Krastanov method. Besides, this provides a fast and inexpensive method for patterning binary arrays of QDs and MNPs, envisioned in the theoretical part of this thesis, in which the MNPs act as antennas focusing the light in the QDs and therefore boosting their absorption

The lead salt quantum dot intermediate band solar cell

We propose a new kind of quantum dot (QD) materials for the implementation of the intermediate band solar cell (IBSC) [1]. The materials are formed by lead salt QDs of the family IV-VI (PbTe, PbSe or PbS) embedded in a semiconductor of the family II-VI (Cd1-xMgxTe, CdxZn1-xTe, and CdS1-xSex or ZnSe1-xTex, respectively). These QDs are not nucleated due to lattice mismatch, as it is the case of the InAs/GaAs QD material system grown by the Stranski-Krastanov (S-K) mode. In these materials, the QDs precipitate due to the difference in lattice type: the QD lead salt material crystallizes in the rocksalt structure, while the II-VI host material has the zincblende structure [2]. Therefore, it is possible to use lattice-matched QD/host combinations, avoiding all the strain-related problems found in previous QD-IBSC developments. In this paper we discuss the properties of the lead salt QD materials and propose that they are appropriate to overcome the fundamental drawbacks of present III-V-based QD-IBSC prototypes. We also calculate the band diagram for some examples of IV-VI/II-VI QD materials. The detailed balance efficiency limit of QD-IBSCs based on the studied materials is found to be over 60% under maximum concentration.

Critical aspects of substrate nanopatterning for the ordered growth of GaN nanocolumns.

Precise and reproducible surface nanopatterning is the key for a successful ordered growth of GaN nanocolumns. In this work, we point out the main technological issues related to the patterning process, mainly surface roughness and cleaning, and mask adhesion to the substrate. We found that each of these factors, process-related, has a dramatic impact on the subsequent selective growth of the columns inside the patterned holes. We compare the performance of e-beam lithography, colloidal lithography, and focused ion beam in the fabrication of hole-patterned masks for ordered columnar growth. These results are applicable to the ordered growth of nanocolumns of different materials.

Fabrication of GaN nanorods by focused ion beam

Ordered arrays of III-Nitride nanocolumns are excellent candidates for the fabrication of nano-optoelectronic devices. Different technologies such as e-beam lithography or colloidal lithography, have been used to obtain ordered arrays. All these technologies have in common several processing steps that can affect the crystalline growth of the nanocolumns. In this work, we present a single lithographic step that permits to grow ordered GaN nanocolumns with different geometries. The patterning is based in the use of a focusedionbeam with different doses. With this method has been possible to create GaN nanopillars and nanocylinders

Crecimiento, fabricación y caracterización de heteroestructuras y nanocolumnas ordenadas basadas en nitruros del grupo III para aplicaciones sensoras

Esta memoria está basada en el crecimiento y caracterización de heteroestructuras Al(Ga)N/GaN y nanocolumnas ordenadas de GaN, y su aplicación en sensores químicos. El método de crecimiento ha sido la epitaxia de haces moleculares asistida por plasma (PAMBE). En el caso de las heteroestructuras Al(Ga)N/GaN, se han crecido barreras de distinto espesor y composición, desde AlN de 5 nm, hasta AlGaN de 35 nm. Además de una caracterización morfológica, estructural y eléctrica básica de las capas, también se han fabricado a partir de ellas dispositivos tipo HEMTs. La caracterización eléctrica de dichos dispositivos (carga y movilidad de en el canal bidimensional) indica que las mejores heteroestructuras son aquellas con un espesor de barrera intermedio (alrededor de 20 nm). Sin embargo, un objetivo importante de esta Tesis ha sido verificar las ventajas que podían tener los sensores basados en heteroestructuras AlN/GaN (frente a los típicos basados en AlGaN/GaN), con espesores de barrera muy finos (alrededor de 5 nm), ya que el canal de conducción que se modula por efecto de cambios químicos está más cerca de la superficie en donde ocurren dichos cambios químicos. De esta manera, se han utilizado los dispositivos tipo HEMTs como sensores químicos de pH (ISFETs), y se ha comprobado la mayor sensibilidad (variación de corriente frente a cambios de pH, Ids/pH) en los sensores basados en AlN/GaN frente a los basados en AlGaN/GaN. La mayor sensibilidad es incluso más patente en aplicaciones en las que no se utiliza un electrodo de referencia. Se han fabricado y caracterizado dispositivos ISFET similares utilizando capas compactas de InN. Estos sensores presentan peor estabilidad que los basados en Al(Ga)N/GaN, aunque la sensibilidad superficial al pH era la misma (Vgs/pH), y su sensibilidad en terminos de corriente de canal (Ids/pH) arroja valores intermedios entre los ISFET basados en AlN/GaN y los valores de los basados en AlGaN/GaN. Para continuar con la comparación entre dispositivos basados en Al(Ga)N/GaN, se fabricaron ISFETs con el área sensible más pequeña (35 x 35 m2), de tamaño similar a los dispositivos destinados a las medidas de actividad celular. Sometiendo los dispositivos a pulsos de voltaje en su área sensible, la respuesta de los dispositivos de AlN presentaron menor ruido que los basados en AlGaN. El ruido en la corriente para dispositivos de AlN, donde el encapsulado no ha sido optimizado, fue tan bajo como 8.9 nA (valor rms), y el ruido equivalente en el potencial superficial 38.7 V. Estos valores son más bajos que los encontrados en los dispositivos típicos para la detección de actividad celular (basados en Si), y del orden de los mejores resultados encontrados en la literatura sobre AlGaN/GaN. Desde el punto de vista de la caracterización electro-química de las superficies de GaN e InN, se ha determinado su punto isoeléctrico. Dicho valor no había sido reportado en la literatura hasta el momento. El valor, determinado por medidas de “streaming potential”, es de 4.4 y 4 respectivamente. Este valor es una importante característica a tener en cuenta en sensores, en inmovilización electrostática o en la litografía coloidal. Esta última técnica se discute en esta memoria, y se aplica en el último bloque de investigación de esta Tesis (i.e. crecimiento ordenado). El último apartado de resultados experimentales de esta Tesis analiza el crecimiento selectivo de nanocolumnas ordenadas de GaN por MBE, utilizando mascaras de Ti con nanoagujeros. Se ha estudiado como los distintos parámetros de crecimiento (i.e. flujos de los elementos Ga y N, temperatura de crecimiento y diseño de la máscara) afectan a la selectividad y a la morfología de las nanocolumnas. Se ha conseguido con éxito el crecimiento selectivo sobre pseudosustratos de GaN con distinta orientación cristalina o polaridad; templates de GaN(0001)/zafiro, GaN(0001)/AlN/Si, GaN(000-1)/Si y GaN(11-20)/zafiro. Se ha verificado experimentalmente la alta calidad cristalina de las nanocolumnas ordenadas, y su mayor estabilidad térmica comparada con las capas compactas del mismo material. Las nanocolumnas ordenadas de nitruros del grupo III tienen una clara aplicación en el campo de la optoelectrónica, principalmente para nanoemisores de luz blanca. Sin embargo, en esta Tesis se proponen como alternativa a la utilización de capas compactas o nanocolumnas auto-ensambladas en sensores. Las nanocolumnas auto-ensambladas de GaN, debido a su alta razón superficie/volumen, son muy prometedoras en el campo de los sensores, pero su amplia dispersión en dimensiones (altura y diámetro) supone un problema para el procesado y funcionamiento de dispositivos reales. En ese aspecto, las nanocolumnas ordenadas son más robustas y homogéneas, manteniendo una alta relación superficie/volumen. Como primer experimento en el ámbito de los sensores, se ha estudiado como se ve afectada la emisión de fotoluminiscencia de las NCs ordenadas al estar expuestas al aire o al vacio. Se observa una fuerte caída en la intensidad de la fotoluminiscencia cuando las nanocolumnas están expuestas al aire (probablemente por la foto-adsorción de oxigeno en la superficie), como ya había sido documentado anteriormente en nanocolumnas auto-ensambladas. Este experimento abre el camino para futuros sensores basados en nanocolumnas ordenadas. Abstract This manuscript deals with the growth and characterization of Al(Ga)N/GaN heterostructures and GaN ordered nanocolumns, and their application in chemical sensors. The growth technique has been the plasma-assisted molecular beam epitaxy (PAMBE). In the case of Al(Ga)N/GaN heterostructures, barriers of different thickness and composition, from AlN (5 nm) to AlGaN (35 nm) have been grown. Besides the basic morphological, structural and electrical characterization of the layers, HEMT devices have been fabricated based on these layers. The best electrical characteristics (larger carriers concentration and mobility in the two dimensional electron gas) are those in AlGaN/GaN heterostructures with a medium thickness (around 20 nm). However, one of the goals of this Thesis has been to verify the advantages that sensors based on AlN/GaN (thickness around 7 nm) have compared to standard AlGaN/GaN, because the conduction channel to be modulated by chemical changes is closer to the sensitive area. In this way, HEMT devices have been used as chemical pH sensors (ISFETs), and the higher sensitivity (conductance change related to pH changes, Ids/pH) of AlN/GaN based sensors has been proved. The higher sensibility is even more obvious in application without reference electrode. Similar ISFETs devices have been fabricated based on InN compact layers. These devices show a poor stability, but its surface sensitivity to pH (Vgs/pH) and its sensibility (Ids/pH) yield values between the corresponding ones of AlN/GaN and AlGaN/GaN heterostructures. In order to a further comparison between Al(Ga)N/GaN based devices, ISFETs with smaller sensitive area (35 x 35 m2), similar to the ones used in cellular activity record, were fabricated and characterized. When the devices are subjected to a voltage pulse through the sensitive area, the response of AlN based devices shows lower noise than the ones based on AlGaN. The noise in the current of such a AlN based device, where the encapsulation has not been optimized, is as low as 8.9 nA (rms value), and the equivalent noise to the surface potential is 38.7 V. These values are lower than the found in typical devices used for cellular activity recording (based on Si), and in the range of the best published results on AlGaN/GaN. From the point of view of the electrochemical characterization of GaN and InN surfaces, their isoelectric point has been experimentally determined. Such a value is the first time reported for GaN and InN surfaces. These values are determined by “streaming potential”, being pH 4.4 and 4, respectively. Isoelectric point value is an important characteristic in sensors, electrostatic immobilization or in colloidal lithography. In particular, colloidal lithography has been optimized in this Thesis for GaN surfaces, and applied in the last part of experimental results (i.e. ordered growth). The last block of this Thesis is focused on the selective area growth of GaN nanocolumns by MBE, using Ti masks decorated with nanoholes. The effect of the different growth parameters (Ga and N fluxes, growth temperature and mask design) is studied, in particular their impact in the selectivity and in the morphology of the nanocolumns. Selective area growth has been successful performed on GaN templates with different orientation or polarity; GaN(0001)/sapphire, GaN(0001)/AlN/Si, GaN(000- 1)/Si and GaN(11-20)/sapphire. Ordered nanocolumns exhibit a high crystal quality, and a higher thermal stability (lower thermal decomposition) than the compact layers of the same material. Ordered nanocolumns based on III nitrides have a clear application in optoelectronics, mainly for white light nanoemitters. However, this Thesis proposes them as an alternative to compact layers and self-assembled nanocolumns in sensor applications. Self-assembled GaN nanocolumns are very appealing for sensor applications, due to their large surface/volume ratio. However, their large dispersion in heights and diameters are a problem in terms of processing and operation of real devices. In this aspect, ordered nanocolumns are more robust and homogeneous, keeping the large surface/volume ratio. As first experimental evidence of their sensor capabilities, ordered nanocolumns have been studied regarding their photoluminiscence on air and vacuum ambient. A big drop in the intensity is observed when the nanocolumns are exposed to air (probably because of the oxygen photo-adsortion), as was already reported in the case of self-assembled nanocolumns. This opens the way to future sensors based on ordered III nitrides nanocolumns.

Fabrication of GaN nanorods by focused ion beam

Ordered arrays of III-Nitride nanocolumns are excellent candidates for the fabrication of nano-optoelectronic devices. Different technologies such as e-beam lithography or colloidal lithography, have been used to obtain ordered arrays. All these technologies have in common several processing steps that can affect the crystalline growth of the nanocolumns. In this work, we present a single lithographic step that permits to grow ordered GaN nanocolumns with different geometries. The patterning is based in the use of a focused ion beam with different doses. With this method has been possible to create GaN nanopillars and nanocylinders.

Thermodynamic aproach to particle size distribution in granular media

The study of particulate systems is of great interest in many fields of science and technology. Soil, sediments, powders, granular materials, colloidal and particulate suspensions are examples of systems involving many size particles. For those systems, the statistical description of the particle size distribution (PSD), that is, the mathematical distribution that defines the relative amounts of particles present, sorted according to size, is a crutial issue. The PSD can be important in understanding soil hydraulic properties, the geological origin or sediments or the physical and chemical properties of granular materials and ceramics, among others.

Silicon nanopillar arrays with SiO2 overlayer for biosensing application

We present the fabrication of silicon dioxide (SiO2) coated silicon nanopillar array structures and demonstrate their application as sensitive optical biosensors. Colloidal lithography, plasma dry etching and deposition processes are used to fabricate SiO2 coated Si nanopillar arrays with two different diameters and periods. Proof of concept bio recognition experiments are carried out with the bovine serum albumin (BSA)/antiBSA model system using Fourier transform visible and IR spectrometry (FT-VIS-IR) in reflection mode. A limit of detection (LoD) value of 5.2 ng/ml is estimated taking in to account the wavenumber uncertainty in the measurements.

Detection of Biological CO2 and 1,3-Pentadiene Using Non-refrigerated Low-Cost MWIR Detectors

The early detection of spoiling metabolic products in contaminated food is a very important tool to control quality. Some volatile compounds produce unpleasant odours at very low concentrations, making their early detection very challenging. This is the case of 1,3-pentadiene produced by microorganisms through decarboxylation of the preservative sorbate. In this work, we have developed a methodology to use the data produced by a low-cost, compact MWIR (Mid-Wave IR) spectrometry device without moving parts, which is based on a linear array of 128 elements of VPD PbSe coupled to a linear variable filter (LVF) working in the spectral range between 3 and 4.6 ?m. This device is able to analyze food headspace gases through dedicated sample presentation setup. This methodology enables the detection of CO2 and the volatile compound 1,3-pentadiene, as compared to synthetic patrons. Data analysis is based on an automated multidimensional dynamic processing of the MWIR spectra. Principal component and discriminant analysis allow segregating between four yeast strains including producers and no producers. The segregation power is accounted as a measure of the discrimination quality.