The foundation of vacuum nanoelectronics was laid as early as in 1961 when Kenneth Shoulders proposed the development of vertical field-emission micro-triodes. After years of conspicuous stagnancy in the field much interest has reemerged for the vacuum nanoelectronics in recent years. Electron field emission under high electric field from conventional and exotic nanoemitters, which have now been made possible with the use of modern day technology, has been the driving force behind this renewal of interest in vacuum nanoelectronics. In the research reported in this thesis self-assembled silicon nanostructures were studied as a potential source of field emission for vacuum nanoelectronic device applications. Whiskerlike protruding silicon nanostructures were grown on untreated n- and p-type silicon surfaces using electron-beam annealing under high vacuum. The electrical transport characteristics of the silicon nanostructures were investigated using conductive atomic force microscopy (C-AFM). Higher electrical conductivities for the nanostructured surface compared to that for the surrounding planar silicon substrate region were observed. Non-ideal diode behaviour with high ideality factors were reported for the individual nanostructure-AFM tip Schottky nanocontacts. This demonstration, indicative of the presence of a significant field emission component in the analysed current transport phenomena was also detailed. Field emission from these nanostructures was demonstrated qualitatively in a lift-mode interleave C-AFM study. A technique to fabricate integrated field emission diodes using silicon nanostructures in a CMOS process technology was developed. The process incorporated the nanostructure growth phase at the closing steps in the process flow. Turn-on voltages as low as ~ 0.6 V were reported for these devices, which make them good candidates for incorporation into standard CMOS circuit applications. Reproducible I V characteristics exhibited by these fabricated devices were further studied and field emission parameters were extracted. A new consistent and reliable method to extract field emission parameters such as effective barrier height, field conversion factor, and total emitting area at the onset of the field emission regime was developed and is reported herein. The developed parameter extraction method used a unified electron emission approach in the transition region of the device operation. The existence of an electron-supply limited current saturation region at very high electric field was also confirmed. Both the C-AFM and the device characterization studies were modelled and simulated using the finite element method in COMSOL Multiphysics. The experimental results – the field developed at various operating environments – are explained in relation to these finite element analyses. Field enhancements at the atomically sharp nanostructure apexes as suggested in the experimental studies were confirmed. The nanostructure tip radius effect and sensitivity to small nanostructure height variation were investigated and mathematical relations for the nanostructure regime of our interest were established. A technique to optimize the cathode-opening area was also demonstrated. Suggestions related to further research on field emission from silicon nanostructures, optimization of the field emission device fabrication process, and fabrication of field emission triodes are elaborated in the final chapter of this thesis. The experimental, modelling, and simulation works of this thesis indicate that silicon field emission devices could be integrated into the existing CMOS process technology. This integration would offer goods from both the worlds of vacuum and solid-sate nanoelectronics – fast ballistic electron transport, temperature insensitivity, radiation hardness, high packing density, mature technological backing, and economies of scale among other features.

Carbon nanotubes (CNTs), discovered by Ijima in 1991, are one of the allotropes of carbon, and can be described as cylinders of graphene sheet capped by hemifullerenes. CNTs have excellent electrical, mechanical, thermal and optical properties and very small size. Due to their unique properties and small size, CNTs have a great potential for use in electronics, medical applications, field emission devices (displays,scanning and electronprobes/microscopes) and reinforced composites. CNTs can be grown by different methods from a number of carbon sources such as graphite, CO,C2H4, CH4 and camphor. Under certain conditions, a metallic catalyst is used to initiate the growth. The three main methods used to grow CNTs are: Arc-discharge, laser ablation (LA) and chemical vapour deposition (CVD). In the present work CNTs were grown from a mixture of camphor (C10H16O) and ferrocene (C10H10Fe) using Chemical Vapour Deposition (CVD) and argon was used as a carrier gas. The iron particles from ferrocene acted as catalysts for growth. The substrates used for the growth of CNTs were crystalline Si and SiO2 (Quartz) placed in a quartz tube in a horizontal furnace. Several parameters have been found to affect the CNT growth process. The effects of three parameters: growth temperature, carrier gas (Ar) flow rate and catalyst concentration were investigated in the present work in order to optimize the growth conditions with a simple and economical CVD setup. The samples were characterized using electron microscopy (EM), thermogravimetirc analysis (TGA), Raman and FTIR spectroscopy techniques. It was found that the quality and yield of the CNTs were best at 800°C growth temperature, 80sccm flow rate and 4% catalyst concentration.

Orientador: Prof. Dr. Michel Oliveira da Silva Dantas / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, 2016. / Sensores de vacuo sao amplamente utilizados tanto no ambito industrial como no da pesquisa cientifica, pois possuem aplicacoes em diversas tecnicas de fabricacao e de analise, como a microscopia eletronica de varredura (MEV), a litografia por feixe de eletrons, e a espectrometria de massa, entre outras. Dentre os diversos tipos de sensores de vacuo destacam-se os baseados em efeito de campo (FE - Field Emission Device), que sao dispositivos que emitem eletrons em vacuo na presenca de um elevado campo eletrico. A literatura destaca diversas vantagens destes dispositivos: operacao em temperatura ambiente, reducao de consumo de potencia e tensao de operacao, obtencao de altas densidades de correntes em areas reduzidas, e rapido tempo de resposta. Existem diversas tecnicas de microfabricacao que podem ser utilizadas para obtencao de dispositivos FE, destacando-se a tecnica HI-PS (gHydrogen Implantation . Porous Siliconh), que proporciona baixa complexidade e custo. No entanto, para obtencao de FEs com sistema anodo-catodo integrado, a tecnica HI-PS apresenta algumas limitacoes, como o elevado numero de etapas de processo, a necessidade de elevada temperatura e tempo de oxidacao, e principalmente a isolacao eletrica deficiente entre as estruturas do anodo e do catodo, propiciando a existencia de correntes de fuga pelo gcorpoh do dispositivo. Frente a estes problemas, este trabalho apresenta estrategias estudadas para aprimorar a tecnica HI-PS de microfabricacao de dispositivos de emissao de campo integrados. Visando a reducao do numero de etapas de processo e a eliminacao de defeitos, inicialmente, foi estudada a utilizacao de fotorresiste como mascara a implantacao ionica de hidrogenio. Esta estrategia se mostrou viavel, resultando na formacao seletiva de silicio poroso e na obtencao de micropontas (catodos) com altura em torno de 10 ¿Êm e diametro dos apices em torno de dezenas de nanometro, dimensoes estas atestadas por MEV. Tambem foi pesquisada a utilizacao de fotorresiste como camada dieletrica, que se mostrou inviavel para a aplicacao proposta devido aos valores de correntes de fuga relativamente elevados. Para melhorar a isolacao eletrica entre as estruturas do anodo e do catodo, a estrategia pesquisada foi a utilizacao de oxido de silicio poroso (Ox-PS) como camada dieletrica entre as referidas estruturas. Para obtencao do Ox-PS, foram estudados diferentes parametros de oxidacao, como temperatura, tempo de processo, gradiente de temperatura de oxidacao (pre-oxidacao), e processo de recozimento termico pos-oxidacao em ambiente Forming Gas. Para as caracterizacoes morfologicas do Ox-PS, foram analisados, por meio de microscopia otica, parametros como espessura, estabilidade estrutural, taxa de corrosao e oxidacao total da camada PS, sendo este ultimo realizado atraves da tecnica Fourier Transform Infrared Spectroscopy (FTIR). Para a caracterizacao eletrica da corrente de fuga, foram confeccionados dispositivos MOS, caracterizados eletricamente por aparato constituido por um analisador de parametros semicondutores. O Ox-PS obtido com T = 1000 ¿C, t = 1 h, e com recozimento termico pos-oxidacao em ambiente Forming Gas apresentou significativa reducao da corrente de fuga (de 30 nA para 0,125 nA), comprovando, deste modo, sua potencialidade para a aplicacao proposta. Ja na fabricacao do FE integrado, o Ox-PS obtido nestas condicoes apresentou elevada instabilidade estrutural, gerando a necessidade de implementar processos de pre-oxidacao para obtencao da estrutura anodo-catodo integrada. Atraves dos parametros adequados, foi finalmente comprovada a viabilidade da otimizacao da tecnica HI-PS atraves das estrategias estudadas, possibilitando a fabricacao do dispositivo FE integrado contendo micropontas de alturas de aproximadamente 10 micrometros e apices da ordem de dezenas de nanometros circundadas pela estrutura do anodo com distancias de separacao de aproximadamente 20 micrometros. Com a otimizacao dos processos de fabricacao, almeja-se futuramente implementar o dispositivo FE integrado obtido por HI-PS no desenvolvimento de sensores compactos e de baixo custo e complexidade de fabricacao. / Vacuum sensors are widely used in industry and in scientific research, because they can be applied in several fabrication and analysis techniques, such as Scanning Electron Microscopy (SEM), electron beam lithography and mass spectrometry, for example. Among the large number of vacuum sensors, we can highlight the Field Emission Devices (FE), which are devices that emit electrons in vacuum environment when submitted to a high electric field. The literature reports several advantages of these devices: operation at room temperature, low power consumption, high current densities in small areas, and fast response times. Several microfabrication techniques allow obtaining FE devices, including the HI-PS (Hydrogen Implantation ¿ Porous Silicon) technique, which is remarkable due to its low complexity and cost. However, HI-PS presents some limitations when applied to obtain FE with integrated anode-cathode system: high number of process steps, high temperature and oxidation times, and mainly the poor electrical insulation between anode-cathode structures, which results in leakage currents through the bulk of these devices. In this context, this work shows strategies to improve the HI-PS technique for microfabrication of integrated FE devices. First, we use photoresist as mask for hydrogen ion implantation aiming at defects elimination and reduction of process steps. This strategy resulted in the selective formation of porous silicon and in obtaining microtips (cathodes) with 10 ìm height and apex around tens of nanometers, as verified by Scanning Electron Microscopy (SEM). In addition, photoresist was tested as dielectric between anodecathode structures, but the high leakage current measured hindered the use of this material for the proposed application. The main strategy researched to improve the electrical insulation between anode-cathode structures was the use of oxidized porous silicon (Ox-PS) as dielectric. To obtain Ox-PS, we studied oxidation parameters such as temperature, time, pre-oxidation, and post-oxidation annealing. Optical Microscopy and Fourier Transform Infrared Spectroscopy (FTIR) were applied to analyze morphological aspects such as thickness, stability, etch rates and full oxidation of PS layers. A semiconductor parameter analyzer was used to characterize the leakage current from fabricated MOS devices. The Ox-PS obtained with T = 1000 °C, t = 1 h, and post-oxidation annealing in Forming Gas environment showed remarkable decrease of leakage current in comparison to the other oxidation conditions (from 30 nA to 0,125 nA), which demonstrates potentiality for the proposed application. Additionally, a pre-oxidation process was introduced to improve structural stability of Ox-PS layers. After this implementation, the optimization viability of HI-PS technique was finally proved, allowing obtaining an integrated FE device with microtips with 10 micrometers height and apex about tens of nanometers surrounded by the anode structure. The separation distance between anode-cathode structures was about 20 micrometers. With the optimization of fabrication process, we intend to implement hereafter the integrated FE device obtained by HI-PS technique in the development of compact sensors with low cost and low fabrication complexity.

Zinc oxide (ZnO) is perhaps one of the most widely studied material in the last two decades. It has received so much of attention because of its incredible potential for wide ranging applications. ZnO is a wide band gap semiconductor (Eg = 3.37 eV at 300 K) with a rather large excitonic binding energy (~60 meV). This combination of properties makes it an ideal choice for several optoelectronic devices that can easily work at room temperature. ZnO is a truly multifunctional material possessing several desirable electrical, optical, optoelectronic, and piezoelectric properties. In addition, it is highly amenable to production of various kinds of nanostructures such as nanorods, nanotubes, nanoribbons, nanoneedles, etc., which makes it even more desirable for nanoscale devices. Examples of ZnO based nanodevices could include photodiodes, photodetectors, nano-lasers, field-emission devices and memristors. In order to make such devices, one could need device quality nanostructures that must be reproducible and cost effective. Naturally, one has to look for a synthesis process that has great controls and is relatively inexpensive. The study provided here shows that among the various methods available for ZnO synthesis, the microwave-assisted chemical synthesis offers outstanding advantages in terms of rapid growth of nanostructures, economical use of energy and excellent controls of process parameters. In order to produce device quality ZnO nanostructures using microwave-assisted synthesis, one has to study the effect of various process parameters and optimise them for the desired growth. Therefore, in the current study, first, a systematic study was undertaken to synthesize ZnO nanostructures both in a aqueous and non-aqueous medium and their characterization was carried out in order to understand the effect of microwave power, time of irradiation, pressure, solvent and salt concentration, etc. The goal was to develop synthesis protocols for various kinds of nanostructures that could guarantee reproducibility, good yield, and device quality structures. This study has led to successful growth of ZnO nanostructures on various substrates, vertically aligned ZnO nanorods and templated arrays of desired structures, all with outstanding properties of the structures as confirmed by XRD, MicroRaman, photoluminescence, cathodoluminescence, FESEM, TEM, PFM studies and pole figure analysis. Piezoelectric force microscopy (PFM) and physical property measurement system (PPMS, Quantum Design), have been used to study the multifunctional properties of ZnO nanostructures. The PFM is a powerful technique to measure the local piezoelectric coefficient of nanostructures and nanoscale thin films. PFM works on the converse piezoelectric effect in which electric potential is applied and mechanical strain is measured using a cantilever deflection. The PFM (Brucker’s AFM dimension Scan Assist) was used to characterize individual ZnO nanorods. Extensive studies were carried out with PFM measurements and it was observed that the nanorods consistently showed high piezoelectric coupling coefficients (d33~50-154 pm/V). It was also found that the variation in d33 depended on morphology and size of nanostructure. The multifunctional properties were observed in small ZnO nanocrystals (NCs). Such high values of piezoelectric coupling coefficients open the door for novel ZnO based nanoscale sensors and actuators. The synthesized ZnO nanostructures were further optimized and characterized keeping in view three device applications namely Field emission, Memristors and Gas Sensors. The fabrication and characterization of these three devices with ZnO nanostructure was carried out using electron beam lithography and direct laser writing micromachining. Device fabrication using lithography involved several steps such as substrate cleaning, photoresist spin coating, pre-baking, post-baking, pattern writing, developing, sputtering/deposition of material for lift-off, ZnO growth, and overlay lithography. For field emission devices, high quality, well aligned, c-axis oriented ZnO nano-needles were grown on sputter coated Ti/Pt (20nm/100nm) on SiO2/Si substrate by rapid microwave-assisted method in aqueous medium. The diameter of the tip was found to be 1~2 nm and the length of the rod was approximately 3~5μm. For a particular batch the tip size, morphology, and lengths were found to be the same and highly repeatable. Pole figure analysis revealed that nanorods were highly oriented towards <002> direction. Field-emission measurements using the ZnO nanoneedles arrays as cathode showed very low turn-on electric field of 0.9 V/μm and a very high field enhancement factor ~ 20200. Such a high emission current density, low turn-on electric field, and high field enhancement factor are attributed to the high aspect ratio, narrow tip size, high quality and single crystallinity of the nanoneedles. The high emission current density, high stability, low threshold electric field (0.95 V/μm) and low turn-on field make the ZnO nanoneedle arrays one of the ideal candidates for field-emission displays and field emission sensors. In the suitability of ZnO nanostructures for memristor application it was found that the single crystalline ZnO nanorods were not suitable as they did not show memristive behaviour but the ZnO nanorods with native defects exhibited considerable memristive behaviour. Therefore the microwave-assisted grown ZnO nanorods with defects were used to fabricate memristive devices. Single and multiple ZnO nanorods based memristors were fabricated using electron beam lithography. These devices were characterized electrically by measuring the hysteresis in the I/V characteristics. A high degree of repeatability has been established in terms of growth, device fabrication, and measurements. The switching in single nanorod based devices was found to have “ON-to- OFF” resistance ratio of approximately 104 and current switching ratio (ION/IOFF) of 106. Gas sensing based on electrical resistance change depends on absorption and desorption rate of gases on the analyte which is governed by surface properties, morphologies and activation energy. Therefore, various morphologies of nanostructure were grown for gas sensing application. Through experimentation, the emphasis shifted to c-axis oriented ZnO nanostructures on SiO2 substrate for gas sensing. The c-axis orientation of ZnO nanostructures was preferred mainly due to its huge surface area. The measurements showed that the c-axis oriented ZnO nanorods were excellent hydrogen sensors, able to detect H2 as low concentration as 2 ppm, even when the sensing temperature is as low as 200 ˚C. However, oxygen sensing was achieved at a higher temperature (300 ˚C). Thus, the study undertaken in this thesis presents a microwave based rapid and economical method for synthesizing high quality, device grade ZnO nanostructures, their extensive characterization that shows the multifunctional properties of these structures, and there examples of varied device applications of the synthesized nanostructures as field emitters, memristors, and gas sensors.