Combined nanoscale KPFM characterization and device simulation for the evaluation of the MOSFET variability related to metal gate workfunction fluctuations

In this work, a more realistic approximation based on 2D nanoscale experimental data obtained on a metal layer is presented to investigate the impact of the metal gate polycrystallinity on the MOSFET variability. The nanoscale data (obtained with a Kelvin Probe Force Microscope, KPFM) were introduced in a device simulator to analyze the effect of a TiN metal gate work functions (WF) fluctuations on the MOSFET electrical characteristics. The results demonstrate that the device characteristics are affected not only by the WF fluctuations, but also their spatial distribution, which is specially relevant in very small devices. The effect on these characteristics of the spatial distribution on the gate area of such fluctuations is also evaluatedThis work has been partially supported by the Spanish AEI and ERDF (TEC2016-75151-C3-1-R, TEC2014-53909-REDT and RYC-2017-23312)S

Influence of colloidal Au on the growth of ZnO nanostructures

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles

Treatments and Analysis of C-sp(2) nanomaterials

[cat] El carboni es pot trobar en molts tipus de configuracions, cadascun amb les seves pròpies propietats. La majoria d’aquestes configuracions tenen com a component base el grafé, que curiosament va ser isolat per primera vegada al 2004 per A.K. Geim i K. Novoselov. Per exemple, els nanotubs i nanofibres de carboni o el propi grafit son exemples de diferents configuracions d’aquestes capes de grafé. Les propietats i aplicacions de cada nanomaterial varia segons aquesta configuració. Per exemple, la relació d’aspecte dels nanotubs/nanofibres de carboni és ideal per crear materials compostos mitjançant polímers fent servir una càrrega mínima, obtenint nous materials amb millors propietats mecàniques, elèctriques o tèrmiques. Per una altre banda, el grafé pot ser molt interessant per la fabricació de capes protectores o elèctrodes transparents. Les nanofibres de carboni son una nanoestructura interessant per aplicacions en sensors de gas. Es ben sabut que les superfícies de carboni sp2 pur son inerts a causa de la naturalesa d’aquest enllaç. És per aquesta raó que els nanotubs de carboni purs necessiten un tractament superficial per generar defectes sobre la seva superfície i així crear centres d’adsorció per les molècules presents a l’ambient. Per una altre banda, les nanofibres de carboni, a causa de la seva estructura, tenen més enllaços sp3 exposats a l’atmosfera que facilita l’adsorció de les molècules. A més a més aquesta propietat permet a les nanofibres de carboni ser estable en solvents polars, facilitant la seva manipulació. En aquest treball s’han estudiat dos tipus de nanofibres de carboni: les obtingudes directament del procés de síntesis (CNF) i les nanofibres de carboni gratificades (CNFG), que son nanofibres tractades en un procés d’alta temperatura amb l’objectiu d’eliminar totes les impureses i reordenar l’estructura cristal•lina. El paper crític de la superfície de les nanofibres de carboni en les propietats del sensor de gas justifica les propietats químiques de la superfície. Per fer-ho, les nanofibres han estat estudiades fent servir la tècnica de Espectroscòpia de Fotoelectrons emesos per raigs X (XPS) per obtenir informació de la composició química de la seva superfície. S’ha descobert que el procés de grafitització elimina totes les impureses de la superfície deixant únicament carboni i oxigen. Aquest últim element és una troballa sorprenent ja que, tot i que la quantitat d’oxigen present és baixa (1%), això vol dir que tot i el procés de grafitització encara hi ha alguns grups funcionals que sobreviuen al tractament amb altes temperatures i atmosfera reductora. Bàsicament, els grups funcionals supervivents son àtoms d’oxigen enllaçats amb un enllaç o hidroxils, mentre que els grups carbonils son els que pateixen més reducció durant el tractament. Per fabricar el sensor de gas es necessari de trobar una forma de manipular les nanofibres de carboni. La forma més fàcil és fer servir un solvent com a portador de les nanofibres, però les CNFG no son estables en solvents polars. Amb les mesures de XPS es pot veure com la superfície de les nanofibres es torna més ordenada després de la grafitització (augment dels enllaços sp2). Una inspecció detallada de la superfície de les nanofibres mitjançant Microscòpia Electrònica de Transmissió (TEM) mostra que les vores de les nanofbires de carboni es tanquen sobre elles mateixes exposant majoritàriament els enllaçoso sp2 que son menys reactius. S’ha descobert que aplicant un tractament tèrmic en una atmosfera rica en oxigen es possible tornar a obrir aquestes voreres exposant els enllaços sp3 tornant més reactiva la superfície i permetent la formació de solucions estables de nanofibres de carboni en solvents polars en concentracions de fins 1 mg/ml. Amb aquestes solucions es possible fabricar els sensors de gas utilitzant les nanofibres de carboni com a capa activa. Com que l’objectiu es fabricar un sensor de gas flexible els elèctrodes han estat fabricats mitjançant la tècnica d’impressió ink-jet amb tinta de plata sobre un substrat de kapton. Les nanofibres de carboni son dipositades directament sobre els elèctrodes mitjançant un sistema modificat d’electrospray, formant la capa activa. A part de tindre una major reactivitat i estabilitat en solució, també s’ha estudiat la possibilitat de decorar la superfície de les nanofibres de carboni amb nanopartícules metàl•liques per millorar la seva resposta a determinats gasos. S’ha seguit la estratègia de la mescla directa de sals metàl•liques (AuCl3 i PdCl2), fent servir un procés extra de ball-milling per promoure l’adhesió de la sal a la superfície de la nanofibra. Després del procés d’impregnació el material es tractat en un procés de recuit en una atmosfera d’oxigen per descompondre la sal i obtenir les nanopartícules metàl•liques (Au i Pd). La resposta pels diferents tipus de nanofibres (sense modificar i decorades amb Au i Pd) han estat estudiades. En general, les nanofibres responen bé a la humitat inclús a temperatura ambient. A més, amb una humitat relativa del 50 % es poden detectar gasos tal com NH3 o NO2 amb un petit efecte de enverinament de la superfície. Si s’aplica temperatura aquest efecte desapareix i es troba que la temperatura òptima d’operació es 110ºC. Per la seva banda, el grafé està demostrant sent un material prometedor per una gran varietat d’aplicacions. Gràcies a la seva estructura planar i transparència es molt adequat per la fabricació d’elèctrodes transparents, però també pot ser útil per transistors o capes protectores. Tot i això, un dels principals problemes és trobar un mètode estandarditzat per sintetitzar i dipositar grans àrees amb grafé d’alta qualitat. Una de les possibilitats és seguir una ruta química, en que s’oxida fortament el grafit i després es desmunten les plaques mitjançant sonicació, obtenint el que s’anomena grafé oxidat (GO). Després es té que reduir el material per obtenir grafé oxidat reduït (rGO), un material similar al grafé. És necessari que tots aquests tractaments siguin monitoritzats per obtenir el millor material possible. A més, aquest sistema d’anàlisi tindria que ser compatible amb el possible procés industrial de síntesi i dipòsit. L’espectroscòpia Raman compleix aquests requisits , ja que és una tècnica ràpida i fàcil d’utilitzar, a part de que és no destructiva i dona la possibilitat de analitzar directament els dispositius finals. En aquest estudi s’ha proposat fer servir el pic D’’ per analitzar el GO i els seus productes reduïts a partir d’un procés de reducció tèrmic. Hem descobert que el pic D’’ pot ser usat com a marcador per analitzar el estat de la reducció a ja que la seva posició depèn d’aquesta característica. A més a més, s’ha demostrat que fent servir el pic D’’ els altres pics (D, G i D’) segueixen les relacions presentades per altres autors per carbonis desordenats o activats, volent dir que el pic D’ ajuda a obtenir el grau de desordre real de la mostra.[eng] Carbon can be found or synthesised in different arrangements, each one with its particular properties. Curiously, the main building block of the majority of these structures (graphene) was first isolated in 2004 by A.K Geim and K. Novoselov. Using graphene layers as a base is possible to derive almost all the other carbon structures. Bucky balls, carbon nanotubes and nanofibers or graphite, all these are examples of different configurations of graphene layers. The properties and applications of each nanomaterial vary depending of this configuration. For example, carbon nanotubes/nanofibers aspect ratio are ideal for create polymer composites using low charges, obtaining new materials with increased mechanical, electrical or thermal properties. On the other side, graphene may be very useful for the fabrication of protective coatings or transparent electrodes. Carbon nanofibers are an interesting nanostructure for using in gas sensing applications. It is known that pure carbon sp2 surfaces are chemically inert because the nature of the bonding. This is the reason that pure carbon nanotubes need a surface treatment for generate defects over its surface in order to create bonding sites for achieving the efficient adsorption of the environmental molecules. On the other hand, the carbon nanofibers have more sp3 bonds exposed due its natural structures allowing an easier natural adsorption of molecules. Moreover, this property allows to the nanofibers being stable in different polar solvents making easier its manipulation. In this work two types of stacked-up carbon nanofibers were used: the bare carbon nanofibers (CNF) and the graphitized carbon nanofibers (CNFG), that are nanofibers treated with a high temperature process in order to eliminate all its impurities and obtain a rearrangement of the crystal structure. The critical role of the surface of the nanofiber in its sensing characteristics justifies the study of the surface chemical properties. In order to do so, the nanofibers were studied using the X-ray Photoelectron Spectroscopy (XPS) technique in order to obtain information of the chemical composition of its surface. It has been found that the graphitication process eliminates all the impurities of the surface, leaving only carbon and oxygen. Oxygen is a surprising finding, as although the amount is low (around 1%), this means that despite the graphitization process there are some functional groups that can survive the treatments although the high temperatures and the reducing atmosphere. Basically, seems that the surviving functional groups are single bonded oxygen and hydroxyl groups, meanwhile the carbonyl groups are the ones that suffers a higher degree of reduction. In order to fabricate the sensor is necessary to find a way to manipulate the carbon nanofibers. The easiest way is using a solvent as a carrier, but the treated CNFG aggregates rapidly in polar solvents. XPS measurements show that the surface becomes more ordered (increase of the sp2 bonds). A close inspection using Transmission Electron Microscopy (TEM) shows that the edges of the carbon nanofibers close itself during the graphitization process, exposing mainly the sp2 bonds thus making the surface less reactive. It has been found that applying a thermal treatment in a oxygen atmosphere reopen these edges without increasing the surface oxygen functional groups, making the surface more reactive and allowing the formation of stable solutions of carbon nanofibers in polar solvents in concentrations as high as 1 mg/ml. With these solutions was possible to fabricate a gas sensor using the carbon nanofibers as a sensing layer. As the objective is to fabricate a flexible gas sensor, the interdigitated electrodes were ink-jet printed using Ag ink over a kapton substrate. Then, using a modified electrospray method the carbon nanofibers were deposited over the interdigitated electrodes, forming the sensing layer. In addition of increase the reactivity and stability in solution of the nanofibers, the possibility of decorate its surface with noble metal nanoparticles is studied. The direct mixing of precursor salts (AuCl3 and PdCl2) is used, taking advantage of a ball milling process in order to increase the wetting of the nanofiber surface by the metal salts. After the impregnation of the surface, the product is annealed in an oxygen atmosphere for decomposing the salt and obtaining the metal nanoparticles. Depending the salt used it was found different behaviour on the formation of the clusters with the same thermal treatment. For the Pd case, for low PdCl2 concentrations results in no visible clusters albeit the XPS measurements show the presence of Pd in the surface. This means that its surface is covered by Pd atoms that did not reach the critical surface concentration for coalesce. Increasing the concentration of PdCl2 promotes the growing of Pd nanoparticles that with greater diameter with higher concentrations of salt. For the case of AuCl3 the behaviour is more or less the contrary. For low salt concentrations disperse relatively big clusters of Au can be seen. If the salt concentration is increased, the size of the clusters decrease meanwhile its number increase. Finally, at a critical salt concentration (around 50% respect the carbon nanofiber concentration) the surface of the nanofibers is covered by 1nm Au nanoparticles. The response of the different nanofibers (bare, Au decorated and Pd decorated) to humidity, NH3 and NO2 is studied. It has been found that the bare carbon nanofibers responses quite well to humidity even at room temperature. In addition, is possible to detect NH3 and NO2 gases in a 50% RH ambient at room temperature with good responses and recovery times, although some poisoning is detected. The decoration with metal nanoparticles modifies the response, in particular for NH3 the Au enhance the response meanwhile Pd decreases it, meanwhile for NO2 the two types of cluster decreases the response. The properties of the sensor (response time, recovery time,…) can be enhanced by applying heat in order to clean the surface and increase the adsorption and desorption rates. It was found that the best operating temperature is around 110ºC in wich there is a compromise between the best response time and lower recovery time. Graphene is one of the promising materials that could be applied in a wide range of applications. Because its planar structure and transparency is very adequate for the fabrication of transparent electrodes, although also is planned to be used for example in transistors, supercapacitors, protective coatings. One of the main issues is its synthesis and manipulation at industrial level. Although the great efforts invested still there no is a standardized method to synthesise and coat wide surfaces with good quality graphene. One of the possibilities is the chemical route, where carbon layers are separated by a strong oxidation and separated by sonication. Then the resulting graphene oxide (GO) is reduced for obtain reduced graphene oxide (rGO), a material similar to graphene. All these treatments need to be monitored in order to obtain the best material possible and if is it possible to be compatible in an industrial process. Raman spectroscopy meets with these requirements, as is a fast technique and easy to use. Moreover, is non-destructible, opening the possibility to use this characterization technique in an industrial process directly over the final devices. In this study is proposed the use of the rarely used D’’ peak to analyse the GO and its reduction products in a thermal reduction process. We found that the D’’ can be used as a marker of the reduction state of the product as its position depend on it. Moreover, is demonstrated that with the use of the D’’ peak the other peaks (D, G, and D’) follows the relations presented for different authors for disordered or activated carbons, meaning that the D’’ also helps to obtain the real disorder degree of the sample

Recombination dynamics in ZnO nanowires: Surfaces states versus mode quality factor

In this work, we investigate the influence of finite size on the recombinations dynamics of ZnO nanowires. We demonstrate that diameter as well as lenght of nanowires determine the lifetime of the neutral donor bound excitons. Our findings suggest that while the length is mainly responsible for different mode quality factors of the cavity-like nanowires, the diameter determines the influence of surface states as alternative recombinations channels for the optical modes trapped in the nanocavity. In addition, comparing nanowires grown using different catalyst we show that the surfaces states strongly depend on each precursor characteristics

Workfunction fluctuations in polycrystalline TiN observed with KPFM and their impact on MOSFETs variability

A more realistic approach to evaluate the impact of polycrystalline metal gates on the MOSFET variability is presented. 2D experimental workfunction maps of a polycrystalline TiN layer were obtained by Kelvin Probe Force Microscopy with a nanometer resolution. These data were the input of a device simulator, which allowed us to evaluate the effect of the workfunction fluctuations on MOSFET performance variability. We have demonstrated that in the modelling of TiN workfunction variability not only the different workfunctions of the grains but also the grain boundaries should be included

A CAFM and device level study of MIS structures with graphene as interfacial layer for ReRAM applications

Capacitive Metal-Insulator-Semiconductor structures with graphene as interfacial layer between the HfO dielectric and the top electrode have been fabricated and investigated at device level and at the nanoscale with Conductive Atomic Force Microscope. In particular, their electrical properties and variability have been compared to devices without graphene to evaluate their feasibility as ReRAM devices. At device level, we observe that, when graphene is present as an intercalated layer, several resistive switching cycles can be measured, meanwhile the standard structures without graphene do not show resistive switching behavior. Nanoscale analysis showed that the graphene layer prevents the microstructural irreversible damage of the oxide material during a forming process. Therefore, graphene somehow protects the structure during the CF formation. This protection would explain the observation of RS of the devices with intercalated graphene

A Smart Measurement System for the Combined Nanoscale and Device Level Characterization of Electron Devices : Implementation Using Ink-Jet Printing Technologies

In this article, the integration into a single measurement system of device level and nanoscale measurement equipment is presented and applied to the electrical characterization of emerging electron devices. This system is a smart solution to simplify the test procedure, since it allows a fast switching between measurement modes (device or nanoscale level), also featuring an enlarged testing capability. Key in the system is a custom-made inkjet-printed circuit board (I-PCB) that connects the device terminals to the proper instrumentation. The flexibility offered by inkjet-printing technologies is a clear advantage, since many kinds of devices can be tested, without the need of expensive hardware modifications. As a particular case of study, the proposed strategy is demonstrated by implementing a system that alternates between standard electrical measurements with a Semiconductor Parameter Analyzer (SPA) and Conductive Atomic Force Microscopy (CAFM) nanoscale measurements on back-gate graphene field-effect transistors

Recombination dynamics in ZnO nanowires: Surfaces states versus mode quality factor

In this work, we investigate the influence of finite size on the recombinations dynamics of ZnO nanowires. We demonstrate that diameter as well as lenght of nanowires determine the lifetime of the neutral donor bound excitons. Our findings suggest that while the length is mainly responsible for different mode quality factors of the cavity-like nanowires, the diameter determines the influence of surface states as alternative recombinations channels for the optical modes trapped in the nanocavity. In addition, comparing nanowires grown using different catalyst we show that the surfaces states strongly depend on each precursor characteristics