A novel growth method to improve the quality of GaAs nanowires grown by Ga-assisted chemical beam epitaxy

The successful synthesis of high crystalline quality and high aspect ratio GaAs nanowires (NWs) with a uniform diameter is needed to develop advanced applications beyond the limits established by thin film and bulk material properties. Vertically aligned GaAs NWs have been extensively grown by Ga-assisted vapor–liquid–solid (VLS) mechanism on Si(111) substrates, and they have been used as building blocks in photovoltaics, optoelectronics, electronics, and so forth. However, the nucleation of parasitic species such as traces and nanocrystals on the Si substrate surface during the NW growth could affect significantly the controlled nucleation of those NWs, and therefore the resulting performance of NW-based devices. Preventing the nucleation of parasitic species on the Si substrate is a matter of interest, because they could act as traps for gaseous precursors and/or chemical elements during VLS growth, drastically reducing the maximum length of grown NWs, affecting their morphology and structure, and reducing the NW density along the Si substrate surface. This work presents a novel and easy to develop growth method (i.e., without using advanced nanolithography techniques) to prevent the nucleation of parasitic species, while preserving the quality of GaAs NWs even for long duration growths. GaAs NWs are grown by Ga-assisted chemical beam epitaxy on oxidized Si(111) substrates using triethylgallium and tertiarybutylarsine precursors by a two-step-based growth method presented here; this method includes a growth interruption for an oxidation on air between both steps of growth, reducing the nucleation of parasitic crystals on the thicker SiOx capping layer during the second and longer growth step. VLS conditions are preserved overtime, resulting in a stable NW growth rate of around 6 μm/h for growth times up to 1 h. Resulting GaAs NWs have a high aspect ratio of 85 and average radius of 35 nm. We also report on the existence of characteristic reflection high-energy electron diffraction patterns associated with the epitaxial growth of GaAs NWs on Si(111) substrates, which have been analyzed and compared to the morphological characterization of GaAs NWs grown for different times under different conditions

Photodetector Fabrication by Dielectrophoretic Assembly of GaAs Nanowires Grown by a Two-steps Method

GaAs nanowires (NWs) are promising advanced materials for the development of high performance photodetectors in the visible and infrared range. In this work, we optimize the epitaxial growth of GaAs NWs compared to conventional procedures, by introducing a novel two-steps growth method that exhibits an improvement of the resulting NW aspectratio and an enhancement of the NW growth rate. Moreover, we investigate the contactless manipulation of NWs using non-uniform electric fields to assemble a single GaAs NW on conductive electrodes, resulting in assembly yields above 90%/site and an alignment yields of around 95%. The electrical characteristics of the dielectrophoretic contact formed between the NW and the electrode have been measured, observing that the use of n-type Al-doped ZnO (AZO) as electrode material for NW alignment produces Schottky barrier contacts with the GaAs NW body. Moreover, our results show the fast fabrication of diodes with rectifying characteristics due to the formation of a low-resistance contact between the Ga catalytic droplet at the tip of the NW and the AZO electrode. The current-voltage measurements of a single GaAs NW diode under different illumination conditions show a strong light responsivity of the forward bias characteristic mainly produced by a change on the series resistance

Single GaAs nanowire based photodetector fabricated by dielectrophoresis

Mechanical manipulation of nanowires (NWs) for their integration in electronics is still problematic because of their reduced dimensions, risking to produce mechanical damage to the NW structure and electronic properties during the assembly process. In this regard, contactless NW manipulation based methods using non-uniform electric fields, like dielectrophoresis (DEP) are usually much softer than mechanical methods, offering a less destructive alternative for integrating nanostructures in electronic devices. Here, we report a feasible and reproducible dielectrophoretic method to assemble single GaAs NWs (with radius 35–50 nm, and lengths 3–5 μm) on conductive electrodes layout with assembly yields above 90% per site, and alignment yields of 95%. The electrical characteristics of the dielectrophoretic contact formed between a GaAs NW and conductive electrodes have been measured, observing Schottky barrier like contacts. Our results also show the fast fabrication of diodes with rectifying characteristics due to the formation of a low-resistance contact between the Ga catalytic droplet at the tip of the NW when using Al doped ZnO as electrode. The current-voltage characteristics of a single Ga-terminated GaAs NW measured in dark and under illumination exhibit a strong sensitivity to visible light under forward bias conditions (around two orders of magnitude), mainly produced by a change on the series resistance of the device

AlxIn1−xN on Si (100) Solar Cells (x = 0–0.56) Deposited by RF Sputtering

We investigate the photovoltaic performance of solar cells based on n-AlxIn1−xN (x = 0–0.56) on p-Si (100) hetero-junctions deposited by radio frequency sputtering. The AlxIn1−xN layers own an optical bandgap absorption edge tuneable from 1.73 eV to 2.56 eV within the Al content range. This increase of Al content results in more resistive layers (≈10−4–1 Ω·cm) while the residual carrier concentration drops from ~1021 to ~1019 cm−3 . As a result, the top n-contact resistance varies from ≈10−1 to 1 MΩ for InN to Al0.56In0.44N-based devices, respectively. Best results are obtained for devices with 28% Al that exhibit a broad external quantum efficiency covering the full solar spectrum with a maximum of 80% at 750 nm, an open-circuit voltage of 0.39 V, a short-circuit current density of 17.1 mA/cm2 and a conversion efficiency of 2.12% under air mass 1.5 global (AM1.5G) illumination (1 sun), rendering them promising for novel low-cost III-nitride on Si photovoltaic devices. For Al contents above 28%, the electrical performance of the structures lessens due to the high top-contact resistivityThis research was funded by the national projects from the Ministry of Research and Innovation TEC2017-84378-R and NERA (RTI2018-101037-B-I00); the projects from the Comunidad de Madrid SINFOTON2-CM (P2018/NMT-4326), MADRID-PV2 (P-2018/EMT-4308) and SOLA (CM/JIN/2019-013); the projects from the University of Alcalá ANIS (CCG2018/EXP-042) and PISA (CCG19/IA-005); and by the FEDER program. R. Blasco acknowledges the financial support of his contract associated with the Ramon y Cajal Fellowship RYC-2013-1408

Magnetotransporte en el gas de electrones bidimensional: heteroestructuras semiconductoras de InGaAs y GaN

Tesis doctoral inédita leida en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 18-07-2001El estudio de las propiedades de magnetotransporte son una herramienta muy pode rosa para caracterizar los parámetros.del gas de electrones bidimensional (GEZD) y los mecanismos de dispersión presentes en las heteroestructuras en las que los GE2D se encuentran inmersas. Haciendo uso de las medidas experimentales de la magnetorresistencia transversal henios obtenido estos parámetros para tres tipos de heteroestructuras, dos de InGaAs y una de GaN, que presentan características peculiares y distintas de la heteroestructura de GaAs que ha sido ampliamente estudiada en la literatura. Se presentan en esta Tesis los resultados para la masa efectiva, el tiempo de vida cuántico y el tiempo de vida de Drude, así como los efectos de la intensidad de la corriente para tres lieteroestructuras semiconductoras III~V.L a primera (que llamaremos SI) es un pozo cuántico de AlGaAs/InGaAs/GaAs. A partir de los resultados experimentaies de la rnagnetorresistencia transversal se observa que la masa efectiva presenta un comportamiento oscilatorio y del cociente entre los dos tiempos de vida anteriormente citados se puede inferir que el mecanismo de dispersión presente en nuestra Iieteroestructura es la dispersión por impurezas ionizadas. Además, de los resultados en función de la intensidad.de corriente se deduce el acoplamiento de los estados de borde y de volumen que se produce en el efecto Hallcuántico (EHC). La segunda de las muestras estudiadas (que llamaremos S3) es un pozo cuántico de AlGaAs/InGaAs/AIGaAs que presenta dos subbandas ocupadas, lo que nos permite hacer un estudio detallado de los mecanismos de dispersión presentes en la estructura por medio del cociente entre el tiempo de vida cuántico y el tiempo de vida de Drude para cada una de las dos subbandas. La tercera de las muestras (que llamaremos J1) es una tieteroestructiira de Al- GaN/GaN. Estas estriicturas presentan la mayor densidad de electro~ies conocida para un GE2D. A partir del estudio de las componentes diagonal y no-diagonal de la magnetorresistencia transversal obtenemos dos tiempos de vida de momento y uno de energía, los cuales proporcionan información muy valiosa sobre los mecanismos de dispersión presentes en estas heteroestriicturas. Estos mecanismos caracterizan el GE2D y aportan tina información más precisa que los resultados obtenidos en la literatura para la movilidad

On the Dielectrophoretic Assembly of GaAs Nanowires in Electronic Devices

GaAs nanowires have been grown by Ga-assisted chemical beam epitaxy (CBE) on Si(111) substrates using triethylgallium (TEGa) and tertiarybutylarsine (TBAs). In this work, we present a reproducible and reliable way to prepare Si(111) substrates allowing for the Ga droplet formation on top of the oxidized Si(111) surface, and enabling to the nucleation of GaAs nanowires by CBE through a Ga-assisted vapor-liquid-solid (VLS) mechanism. In addition, an alternative growth procedure has been developed to prevent the formation of GaAs surface structures, including traces and nanocrystals, which are the main responsible for limiting the maximum nanowire length because they act as a sink for atomic species. This procedure consists in a two-steps growth, inserting a self-aligned native SiOx layer after the first growth step, increasing the maximum nanowire length up to reaching values of 5 microns, and improving the nanowire aspect ratio. Resulting nanowires present a pure zinc blende structure free of rotational twins as determined by reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). Dielectrophoresis (DEP) is presented here as a potential way to trap and to align GaAs nanowires, connecting a pair of conductive electrodes with a spacing below the nanowire length. In this regard, nanowires are transferred to a liquid medium by sonication forming a suspension of nanowires. A micro-droplet extracted from the nanowire suspension is drop-casted on a sample with pre-defined conductive electrodes while an alternating-current (AC) electric field is applied between them, leading to nanowire trapping at high electric field regions (positive DEP). The assembling efficiency has been analyzed as a function of the DEP parameters, involving amplitude and frequency of the AC signal, relative orientation of the nanowire with respect to the electric field, etc. DEP conditions were optimized, allowing for the fabrication of a single GaAs nanowire based electronic devices. Photoresponse of these nanowires was analyzed in depth, observing high sensitivity to the visible illumination even for the microscope lamp intensity, and showing near two orders of magnitude higher current when comparing to the dark current level

On the Dielectrophoretic Assembly of GaAs Nanowires in Electronic Devices

GaAs nanowires have been grown by Ga-assisted chemical beam epitaxy (CBE) on Si(111) substrates using triethylgallium (TEGa) and tertiarybutylarsine (TBAs). In this work, we present a reproducible and reliable way to prepare Si(111) substrates allowing for the Ga droplet formation on top of the oxidized Si(111) surface, and enabling to the nucleation of GaAs nanowires by CBE through a Ga-assisted vapor-liquid-solid (VLS) mechanism. In addition, an alternative growth procedure has been developed to prevent the formation of GaAs surface structures, including traces and nanocrystals, which are the main responsible for limiting the maximum nanowire length because they act as a sink for atomic species. This procedure consists in a two-steps growth, inserting a self-aligned native SiOx layer after the first growth step, increasing the maximum nanowire length up to reaching values of 5 microns, and improving the nanowire aspect ratio. Resulting nanowires present a pure zinc blende structure free of rotational twins as determined by reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). Dielectrophoresis (DEP) is presented here as a potential way to trap and to align GaAs nanowires, connecting a pair of conductive electrodes with a spacing below the nanowire length. In this regard, nanowires are transferred to a liquid medium by sonication forming a suspension of nanowires. A micro-droplet extracted from the nanowire suspension is drop-casted on a sample with pre-defined conductive electrodes while an alternating-current (AC) electric field is applied between them, leading to nanowire trapping at high electric field regions (positive DEP). The assembling efficiency has been analyzed as a function of the DEP parameters, involving amplitude and frequency of the AC signal, relative orientation of the nanowire with respect to the electric field, etc. DEP conditions were optimized, allowing for the fabrication of a single GaAs nanowire based electronic devices. Photoresponse of these nanowires was analyzed in depth, observing high sensitivity to the visible illumination even for the microscope lamp intensity, and showing near two orders of magnitude higher current when comparing to the dark current level

Long-range distributed solar irradiance sensing using optical fibers

15 pags., 9 figs.Until recently, the amount of solar irradiance reaching the Earth surface was considered to be a steady value over the years. However, there is increasing observational evidence showing that this quantity undergoes substantial variations over time, which need to be addressed in different scenarios ranging from climate change to solar energy applications. With the growing interest in developing solar energy technology with enhanced efficiency and optimized management, the monitoring of solar irradiance at the ground level is now considered to be a fundamental input in the pursuit of that goal. Here, we propose the first fiber-based distributed sensor able of monitoring ground solar irradiance in real time, with meter scale spatial resolutions over distances of several tens of kilometers (up to 100 km). The technique is based on an optical fiber reflectometry technique (CP-φOTDR), which enables real time and long-range high-sensitivity bolometric measurements of solar radiance with a single optical fiber cable and a single interrogator unit. The method is explained and analyzed theoretically. A validation of the method is proposed using a solar simulator irradiating standard optical fibers, where we demonstrate the ability to detect and quantify solar irradiance with less than a 0.1 W/m resolution.This research was funded in part by: the European Commission (FINESSE, MSCA-ITN-ETN-722509; Ocean-DAS 875302); Ministerio de Ciencia, Innovación y Universidades (IJCI-2017-33856, RTI2018-097957-B-C31, RTI2018-097957-B-C33); Comunidad de Madrid and FEDER program (SINFOTON2-CM: P2018/NMT-432

Chemical Beam Epitaxy of Dilute Nitrides for Intermediate Band Solar Cells

Efficient utilization of the full solar spectrum extending from near infrared to ultraviolet is one of the primary challenges for solar power conversion technologies. Intermediate band solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. Using the unique features of the electronic band structure of highly mismatched alloys GaAs1-xNx we have implemented a single junction intermediate band solar cell. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical intermediate band semiconductor with a well-defined energy band separated from the conduction band. The device demonstrates an optical activity of three energy bands that absorb the crucial part of the solar spectrum and convert it into electrical current. Currently, using chemical beam epitaxy we have fabricated intermediate band solar cell structures. The structures were characterized by a variety of structural and optical methods to optimize their properties for intermediate band photovoltaic devices. The results are in a good agreement with the predictions of the band anticrossing model for the electronic band structure of dilute GaAsN alloys

Chemical Beam Epitaxy of Dilute Nitrides for Intermediate Band Solar Cells

Efficient utilization of the full solar spectrum extending from near infrared to ultraviolet is one of the primary challenges for solar power conversion technologies. Intermediate band solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. Using the unique features of the electronic band structure of highly mismatched alloys GaAs1-xNx we have implemented a single junction intermediate band solar cell. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical intermediate band semiconductor with a well-defined energy band separated from the conduction band. The device demonstrates an optical activity of three energy bands that absorb the crucial part of the solar spectrum and convert it into electrical current. Currently, using chemical beam epitaxy we have fabricated intermediate band solar cell structures. The structures were characterized by a variety of structural and optical methods to optimize their properties for intermediate band photovoltaic devices. The results are in a good agreement with the predictions of the band anticrossing model for the electronic band structure of dilute GaAsN alloys