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.

Inorganic Photovoltaics - Planar and Nanostructured Devices

Since its invention in the 1950s, semiconductor solar cell technology has evolved in great leaps and bounds. Solar power is now being considered as a serious leading contender for replacing fossil fuel based power generation. This article reviews the evolution and current state, and potential areas of near future research focus, of leading inorganic materials based solar cells, including bulk crystalline, amorphous thin-films, and nanomaterials based solar cells. Bulk crystalline silicon solar cells continue to dominate the solar power market, and continued efforts at device fabrication improvements, and device topology advancements are discussed. III-V compound semiconductor materials on c-Si for solar power generation are also reviewed. Developments in thin-film based solar cells are reviewed, with a focus on amorphous silicon, copper zinc tin sulfide, cadmium telluride, as well as nanostructured Cadmium telluride. Recent developments in the use of nano-materials for solar power generation, including silicon and gallium arsenide nanowires, are also reviewed.

Quadruple-junction inverted metamorphic concentrator devices

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of (43.8 ± 2.2)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of (42.9 ± 2.1)% at 869 suns, which is the highest concentration measured. The V_oc increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.

Interrogating Capabilities of IoT Devices

This research is supported by the UK Research Councils’ Digital Economy IT as a Utility Network+ (EP/K003569/1) and the dot.rural Digital Economy Hub (EP/G066051/1).

Interrogating Capabilities of IoT Devices

This research is supported by the UK Research Councils’ Digital Economy IT as a Utility Network+ (EP/K003569/1) and the dot.rural Digital Economy Hub (EP/G066051/1).

Trusted Tiny Things : Making Devices in Smart Cities More Transparent

Postprint

Innovative analytical strategies for the development of sensor devices and mass spectrometry methods

Il lavoro presentato in questa tesi di Dottorato è incentrato sullo sviluppo di strategie analitiche innovative basate sulla sensoristica e su tecniche di spettrometria di massa in ambito biologico e della sicurezza alimentare. Il primo capitolo tratta lo studio di aspetti metodologici ed applicativi di procedure sensoristiche per l’identificazione e la determinazione di biomarkers associati alla malattia celiaca. In tale ambito, sono stati sviluppati due immunosensori, uno a trasduzione piezoelettrica e uno a trasduzione amperometrica, per la rivelazione di anticorpi anti-transglutaminasi tissutale associati a questa malattia. L’innovazione di questi dispositivi riguarda l’immobilizzazione dell’enzima tTG nella conformazione aperta (Open-tTG), che è stato dimostrato essere quella principalmente coinvolta nella patogenesi. Sulla base dei risultati ottenuti, entrambi i sistemi sviluppati si sono dimostrati una valida alternativa ai test di screening attualmente in uso per la diagnosi della celiachia. Rimanendo sempre nel contesto della malattia celiaca, ulteriore ricerca oggetto di questa tesi di Dottorato, ha riguardato lo sviluppo di metodi affidabili per il controllo di prodotti “gluten-free”. Il secondo capitolo tratta lo sviluppo di un metodo di spettrometria di massa e di un immunosensore competitivo per la rivelazione di prolammine in alimenti “gluten-free”. E’ stato sviluppato un metodo LC-ESI-MS/MS basato su un’analisi target con modalità di acquisizione del segnale selected reaction monitoring per l’identificazione di glutine in diversi cereali potenzialmente tossici per i celiaci. Inoltre ci si è focalizzati su un immunosensore competitivo per la rivelazione di gliadina, come metodo di screening rapido di farine. Entrambi i sistemi sono stati ottimizzati impiegando miscele di farina di riso addizionata di gliadina, avenine, ordeine e secaline nel caso del sistema LC-MS/MS e con sola gliadina nel caso del sensore. Infine i sistemi analitici sono stati validati analizzando sia materie prime (farine) che alimenti (biscotti, pasta, pane, etc.). L’approccio sviluppato in spettrometria di massa apre la strada alla possibilità di sviluppare un test di screening multiplo per la valutazione della sicurezza di prodotti dichiarati “gluten-free”, mentre ulteriori studi dovranno essere svolti per ricercare condizioni di estrazione compatibili con l’immunosaggio competitivo, per ora applicabile solo all’analisi di farine estratte con etanolo. Terzo capitolo di questa tesi riguarda lo sviluppo di nuovi metodi per la rivelazione di HPV, Chlamydia e Gonorrhoeae in fluidi biologici. Si è scelto un substrato costituito da strips di carta in quanto possono costituire una valida piattaforma di rivelazione, offrendo vantaggi grazie al basso costo, alla possibilità di generare dispositivi portatili e di poter visualizzare il risultato visivamente senza la necessità di strumentazioni. La metodologia sviluppata è molto semplice, non prevede l’uso di strumentazione complessa e si basa sull’uso della isothermal rolling-circle amplification per l’amplificazione del target. Inoltre, di fondamentale importanza, è l’utilizzo di nanoparticelle colorate che, essendo state funzionalizzate con una sequenza di DNA complementare al target amplificato derivante dalla RCA, ne permettono la rivelazione a occhio nudo mediante l’uso di filtri di carta. Queste strips sono state testate su campioni reali permettendo una discriminazione tra campioni positivi e negativi in tempi rapidi (10-15 minuti), aprendo una nuova via verso nuovi test altamente competitivi con quelli attualmente sul mercato.

Calix[6]arene Derivatives as Pivotal Components of Working Devices and Molecular Machine Prototypes

This Ph.D. thesis describes the synthesis, characterization and study of calix[6]arene derivatives as pivotal components for the construction of molecular machine prototypes. Initially, the ability of a calix[6]arene wheel to supramolecularly assist and increase the rate of a nucleophilic substitution reaction was exploited for the synthesis of two constitutionally isomeric oriented rotaxanes. Then, the synthesis and characterization of several hetero-functionalised calix[6]arene derivatives and the possibility to obtain molecular muscle prototypes was reported. The ability of calix[6]arenes to form oriented pseudorotaxane towards dialkyl viologen axles was then exploited for the synthesis of two calixarene-based [2]catenanes. As last part of this thesis, studies on the electrochemical response of the threading-dethreading process of calix[6]arene-based pseudorotaxanes and rotaxanes supported on glassy carbon electrodes are reported.

Oxide Semiconductors For Organic Opto-electronic Devices

Development of transparent oxide semiconductors (TOS) from Earth-abundant materials is of great interest for cost-effective thin film device applications, such as solar cells, light emitting diodes (LEDs), touch-sensitive displays, electronic paper, and transparent thin film transistors. The need of inexpensive or high performance electrode might be even greater for organic photovoltaic (OPV), with the goal to harvest renewable energy with inexpensive, lightweight, and cost competitive materials. The natural abundance of zinc and the wide bandgap ($sim$3.3 eV) of its oxide make it an ideal candidate. In this dissertation, I have introduced various concepts on the modulations of various surface, interface and bulk opto-electronic properties of ZnO based semiconductor for charge transport, charge selectivity and optimal device performance. I have categorized transparent semiconductors into two sub groups depending upon their role in a device. Electrodes, usually 200 to 500 nm thick, optimized for good transparency and transporting the charges to the external circuit. Here, the electrical conductivity in parallel direction to thin film, i.e bulk conductivity is important. And contacts, usually 5 to 50 nm thick, are optimized in case of solar cells for providing charge selectivity and asymmetry to manipulate the built in field inside the device for charge separation and collection. Whereas in Organic LEDs (OLEDs), contacts provide optimum energy level alignment at organic oxide interface for improved charge injections. For an optimal solar cell performance, transparent electrodes are designed with maximum transparency in the region of interest to maximize the light to pass through to the absorber layer for photo-generation, plus they are designed for minimum sheet resistance for efficient charge collection and transport. As such there is need for material with high conductivity and transparency. Doping ZnO with some common elements such as B, Al, Ga, In, Ge, Si, and F result in n-type doping with increase in carriers resulting in high conductivity electrode, with better or comparable opto-electronic properties compared to current industry-standard indium tin oxide (ITO). Furthermore, improvement in mobility due to improvement on crystallographic structure also provide alternative path for high conductivity ZnO TCOs. Implementing these two aspects, various studies were done on gallium doped zinc oxide (GZO) transparent electrode, a very promising indium free electrode. The dynamics of the superimposed RF and DC power sputtering was utilized to improve the microstructure during the thin films growth, resulting in GZO electrode with conductivity greater than 4000 S/cm and transparency greater than 90 %. Similarly, various studies on research and development of Indium Zinc Tin Oxide and Indium Zinc Oxide thin films which can be applied to flexible substrates for next generation solar cells application is presented. In these new TCO systems, understanding the role of crystallographic structure ranging from poly-crystalline to amorphous phase and the influence on the charge transport and optical transparency as well as important surface passivation and surface charge transport properties. Implementation of these electrode based on ZnO on opto-electronics devices such as OLED and OPV is complicated due to chemical interaction over time with the organic layer or with ambient. The problem of inefficient charge collection/injection due to poor understanding of interface and/or bulk property of oxide electrode exists at several oxide-organic interfaces. The surface conductivity, the work function, the formation of dipoles and the band-bending at the interfacial sites can positively or negatively impact the device performance. Detailed characterization of the surface composition both before and after various chemicals treatment of various oxide electrode can therefore provide insight into optimization of device performance. Some of the work related to controlling the interfacial chemistry associated with charge transport of transparent electrodes are discussed. Thus, the role of various pre-treatment on poly-crystalline GZO electrode and amorphous indium zinc oxide (IZO) electrode is compared and contrasted. From the study, we have found that removal of defects and self passivating defects caused by accumulation of hydroxides in the surface of both poly-crystalline GZO and amorphous IZO, are critical for improving the surface conductivity and charge transport. Further insight on how these insulating and self-passivating defects cause charge accumulation and recombination in an device is discussed. With recent rapid development of bulk-heterojunction organic photovoltaics active materials, devices employing ZnO and ZnO based electrode provide air stable and cost-competitive alternatives to traditional inorganic photovoltaics. The organic light emitting diodes (OLEDs) have already been commercialized, thus to follow in the footsteps of this technology, OPV devices need further improvement in power conversion efficiency and stable materials resulting in long device lifetimes. Use of low work function metals such as Ca/Al in standard geometry do provide good electrode for electron collection, but serious problems using low work-function metal electrodes originates from the formation of non-conductive metal oxide due to oxidation resulting in rapid device failure. Hence, using low work-function, air stable, conductive metal oxides such as ZnO as electrons collecting electrode and high work-function, air stable metals such as silver for harvesting holes, has been on the rise. Devices with degenerately doped ZnO functioning as transparent conductive electrode, or as charge selective layer in a polymer/fullerene based heterojunction, present useful device structures for investigating the functional mechanisms within OPV devices and a possible pathway towards improved air-stable high efficiency devices. Furthermore, analysis of the physical properties of the ZnO layers with varying thickness, crystallographic structure, surface chemistry and grain size deposited via various techniques such as atomic layer deposition, sputtering and solution-processed ZnO with their respective OPV device performance is discussed. We find similarity and differences in electrode property for good charge injection in OLEDs and good charge collection in OPV devices very insightful in understanding physics behind device failures and successes. In general, self-passivating surface of amorphous TCOs IZO, ZTO and IZTO forms insulating layer that hinders the charge collection. Similarly, we find modulation of the carrier concentration and the mobility in electron transport layer, namely zinc oxide thin films, very important for optimizing device performance.

Giant magnetoresistance in ultrasmall graphene based devices

By computing spin-polarized electronic transport across a finite zigzag graphene ribbon bridging two metallic graphene electrodes, we demonstrate, as a proof of principle, that devices featuring 100% magnetoresistance can be built entirely out of carbon. In the ground state a short zigzag ribbon is an antiferromagnetic insulator which, when connecting two metallic electrodes, acts as a tunnel barrier that suppresses the conductance. The application of a magnetic field makes the ribbon ferromagnetic and conductive, increasing dramatically the current between electrodes. We predict large magnetoresistance in this system at liquid nitrogen temperature and 10 T or at liquid helium temperature and 300 G.

Incorporating global positioning data in real time P2P audio/video streams for mobile devices

We propose an original method to geoposition an audio/video stream with multiple emitters that are at the same time receivers of the mixed signal. The achieved method is suitable for those comes where a list of positions within a designated area is encoded with a degree of precision adjusted to the visualization capabilities; and is also easily extensible to support new requirements. This method extends a previously proposed protocol, without incurring in any performance penalty.

P2P audio/video protocol with global positioning data in real time for mobile devices

In this paper, we propose an original method to geoposition an audio/video stream with multiple emitters that are at the same time receivers of the mixed signal. The obtained method is suitable when a list of positions within a known area is encoded with precision tailored to the visualization capabilities of the target device. Nevertheless, it is easily adaptable to new precision requirements, as well as parameterized data precision. This method extends a previously proposed protocol, without incurring in any performance penalty.