Highly photoconductive oxide films functionalized with GeSi nanoparticles

Growth of self-assembled quantum dots is of great interest due to their potential quantum confinement effect and numerous applications in optoelectronics and nano-sized structures. Semiconducting Si, Ge and SiGe nanocrystals (NCs), embedded in a dielectric-oxide matrix have for instance been found to exhibit strong quantum confinement. For SiGe nano-based structures in addition to strong quantum confinement effect they offer the advantage of fine tuneability of energy-band structure via quantum confinement, strain engineering and varying the Si/Ge ratio. Among the most common methods to obtain NCs embedded in oxide systems is deposition with magnetron sputtering, followed by subsequent anneal treatments. However, the device performance obtained are lower in production line than obtained for research devices. This has mainly been attributed to the thermal treatment used, which causes strain accumulation within the structure, dislocations and dangling bonds, clustering and phase separation of Ge in Si1-xGex system, diffusion and formation of unwanted insulating oxide. All of these side-effects cause degradation of optical and electrical properties of the fabricated structures. In this study, structures comprising of SiO2/SiGe/SiO2 and TiO2/SiGe/TiO2 were fabricated by utilizing radio frequency (rfMS), direct current (dcMS) and/or high power impulse magnetron sputtering (HiPIMS). The structures were then subjected to thermal and/or hydrogen (H2) plasma treatment. Their photocurrent intensity was increased by up to several orders of magnitude along with wider spectral coverage into near infra-red regime by controlling the sputter discharge and anneal parameters. Moreover, as a proof of concept, a control over the HiPIMS discharge parameters have exhibited the possibility of obtaining as-grown crystalline structures, consisting of SiGe NCs without the need of annealing, along with a viable control over the size of NCs. The annealing of such structure prepared via HiPIMS method, have shown an interesting self-organization of periodically arranged columnar SiGe NCs. Exposure to hydrogen plasma of both as-grown samples and annealed samples ensued amplification in photoconductivity by neutralization of dangling bonds and passivation of non-radiative defects in the oxide matrix and/or at SiGe/matrix interfaces.Ræktun sjálfsamsettra skammtapunkta er mjög áhugavert rannsóknaverkefni vegna margvíslegra notkunarmöguleika í ljósnæmum rafeindatækjum og ýmsum örsmáum skynjurum. Hálfleiðandi Si, Ge og SiGe öragnir í þunnhúðum úr torleiðiefnum (einkum málmoxíðum) hafa til dæmis reynst hafa sterka skammtaeiginleika. Sökum skammtahrifa má fínstilla þá ljós-öldulengd sem þarf til að gera þá leiðandi með því að stýra stærð öragnanna, hlutfalli milli Si og Ge og álagi sem þeir verða fyrir í þunnhúðinni. Algengasta leiðin til að búa til ofangreind kerfi er þunnhúðun með segulspætun og hitameðhöndlun í kjölfarið. Hefðbundin hitameðferð veldur hinsvegar ákveðnum skemmdum í Si1-xGex kerfinu, s.s. lausum efnatengjum, efnis-aðskilnaði, myndun þyrpinga og útsveimi. Þessar aukaverkanir rýra ljós- og rafeiginleika efnisins. Framleiðslu aðferðir sem valda ekki slíkum skemmdum geta því haft mikla þýðingu. Í þessari rannsókn voru riðspennu (rfMS)-, jafnspennu (dcMS) - og háaflpúlsuð segulspætun (HiPIMS) aðferðir notaðar til að rækta lög af SiO2/SiGe/SiO2 og TiO2/SiGe/TiO2 kerfum. Í kjölfarið var mildum hita- og vetnis rafgasmeðferðum beitt til að framkalla SiGe og Ge öragnir í húðinni sem sýndu breytilega ljósnæmni. Með þessum hætti tókst að auka ljósnæmnina um nokkrar stærðargráður auk þess sem næmnisvið litrófsins var víkkað. Önnur megin niðurstaða er að með notkun HiPIMS aðferðarinnar tóks að útbúa sýni með háa ljósnæmni án þess að hitameðhöndla þau. Meðhöndlun með vetnis-rafgasi leiddi til mikillar (stærðargráðu) aukningar á ljósnæmni húðanna, bæði fyrir og eftir hitaneðhöndlun.Technology Development Fund of the Icelandic Center for Research (RANNIS) grant no. 159006-0611, through the M-ERA NET program

Development and analyses of innovative thin films for photovoltaic applications

In solar cell current research, innovative solutions and materials are continuously requested for efficiency improvements. Si-based technology rules over 95% of the market, with silicon heterojunction (SHJ) solar cell reaching 26.7% record efficiency. Nonetheless, hydrogenated amorphous silicon (a-Si:H) layers employed in the structure still have challenges, resolvable with oxygen/nitrogen inclusion. In parallel, new technologies based on different materials still lack in the market due to stability issues or low efficiencies. However, a preliminary study of their properties creates a deeper knowledge exploitable in photovoltaic application. In this perspective, we investigated both innovative Si-based materials (nanocrystalline and amorphous silicon oxy-nitride and oxide thin films, nc-SiOxNy, a-SiOxNy and a-SiOx, respectively) and innovative materials (perovskite lanthanum-vanadium oxide LaVO3 thin films, indium gallium nitride InxGa1-xN and aluminium indium gallium nitride AlxInyGa1-x-yN layers) for solar cell concepts. Different deposition conditions have been employed to extract their influence on compositional, optical, and electrical properties. The study on nc-SiOxNy layers by conductive atomic force microscopy (c-AFM) and surface photovoltage (SPV) has allowed to clarify O, N, and B content, and annealing treatment role on microscopic transport properties. On a-SiOx and a-SiOxNy layers, by spectral ellipsometry, Fourier transform infrared spectroscopy, photoconductance decay and SPV, we can conclude that moderate insertions of O/N in a-Si:H lead to a decrease of optical parasitic absorption, preserving the passivation quality of the layers. The measurements by AFM and Kelvin probe force microscopy on LaVO3 have clearly shown that it is a poor charge-transport medium, thus not suitable for photovoltaic applications. The analysis on InGaN and AlGaInN by SPV measurements has shown how low In content, Si doping and no misfit dislocations in InGaN/GaN structure cause less recombination processes at the interface, whereas, the strain relaxation (tensile and compressive) with the formation of pinholes produces better interfaces in the AlGaInN/GaN samples

Growth and characterization of Germanium and Silicon Nanostructures

Ph.DDOCTOR OF PHILOSOPH

Optical Properties of MacEtch-Fabricated Porous Silicon Nanowire Arrays

The increasing demand for complex devices that utilize unique, three-dimensional nanostructures has spurred the development of controllable and versatile semiconductor fabrication techniques. However, there exists a need to refine such methodologies to overcome existing processing constraints that compromise device performance and evolution. Conventional wet etching techniques (e.g., crystallographic KOH etching of Si) successfully generate textured Si structures with smooth sidewalls but lack the capabilities of controllably producing high aspectratio structures. Alternatively, dry etching techniques (e.g., reactive-ion etching), while highly controllable and capable of generating vertically aligned, high aspect-ratio structures for IC technologies, introduce considerable sidewall and lattice damage as a result of high-energy ion bombardment that may compromise device performance. Metal-assisted chemical etching (MacEtch) provides an alternative process that is capable of anisotropically generating high aspect-ratio micro and nanostructures using a room temperature, solution-based technique. This fabrication process employs an appropriate metal catalyst (e.g., Au, Ag, Pt, Pd) to induce etching in several semiconducting materials (e.g., Si, GaAs) submerged in a solution containing an oxidant and an etchant. The MacEtch process resembles a galvanic cell such that cathodic and anodic half reactions take place at the catalyst/solution interface and catalyst/substrate interface, respectively. At the cathode, the metal catalyzes the reduction of the oxidant resulting in the generation and accumulation of charge carriers (e.g., holes, h+) that are subsequently injected into the underlying substrate at the anode. This results in the formation of oxide species that are preferentially dissolved by the etchant. Thus, MacEtch provides a tunable, top-down, catalytic fabrication technique enabling greater process control and versatility for generating high aspect-ratio semiconductor structures. In this thesis, Au and Au/Pd catalyzed MacEtch is used to generate ultradeep Si micropillar structures, and porous SiNW (p-SiNW) arrays with enhanced optical properties. Using a combination of Au-MacEtch and a crystallographic KOH etch, Si micropillars with ~100 μm height were fabricated with up to 70 μm clearance between pillars to allow efficient fluid flow for optical detection of viral particles. Alternatively, porous SiNW arrays fabricated via AuPd- MacEtch demonstrated broadband absorption ≥ 90% from 200 – 900 nm and were shown to outperform RCWA-simulated SiNW arrays with similar morphologies. Additionally, photoluminescence (PL) spectra collected from as prepared p-SiNW showed significant enhancement in intensity centered near 650 nm as etch depth increased from 30 μm to 100 μm, attributed to an increase in the porous volume. Using atomic layer deposition (ALD) the p-SiNW were passivated using alumina (Al2O3) and hafnia (HfO2) thin films in addition to ITO thin films deposited via sputtering. PL intensity also increased after ALD passivation, attributed to a quenching effect on non-radiative SRH recombination sites on the NW surfaces, with a red shift in the peak wavelength as ALD film thickness increased from 10 nm to 50 nm, resulting from strain effects acting on the NW themselves. These results show promise in such micropillar and coated and uncoated p-SiNW structures towards applications in microfluidic devices, and indoor light-harvesting and outdoor solar-based technologies

Polymer Pen Printing: A Tool for Studying 2D Enzymatic Lithography and Printing 3D Carbon Features

Polymer Pen Lithography (PPL) is a promising molecular printing approach which combines the advantages of both microcontact printing (low cost, high-throughput) and the dip pen lithography (DPN) (arbitrary writing, high-resolution) into one cohesive lithography method to create 2 dimensional (2-D) patterns with micro/nano-features on different substrates. The goal of this dissertation is to design and develop a new tool based upon PPL, which is not limited to forming 2D parallel patterns, but can also create 3D complex microstructures, finding applications in both biotechnology and Micro-Electro-Mechanical systems (MEMS) technology. This novel approach is named Polymer Pen Printing. Different from PPL using traditional dry-ink printing methods, an inking step is added to each printing repetition in the polymer pen printing process. Thus a wide range of ink materials with diverse viscosities can be transferred to substrates to create functional 2D and 3D microstructures. The polymer pen printing apparatus used in this thesis has been accomplished and introduced in Chapter 2. As a preliminary attempt, the single polymer pen printing approach was developed by simply attaching a solid polydimethylsiloxane (PDMS) pen tip to a multi-axis robot for small microarray fabrication. Compared to the single pen printing method, multi-pen printing can create large arrays of features. Therefore, an improved apparatus for polymer pen printing with high-throughput was discussed and built. Silicon molds, which consist of hundreds of uniform pyramidal openings, were photolithographically defined and etched using hydrofluoric acid (HF) followed by potassium hydroxide solution; after surface-modification with fluorosilane, these silicon molds were used to cast arrays of PDMS pyramidal pen tip. The cast PDMS pen array was mounted to a hollow holder with a 45° mirror inside. Therefore, each PDMS pen can be observed and monitored from the microscope on the side. To achieve prints less than 1 micron across, a Z axis stage with nanometer resolution was incorporated; and to control the compression of PDMS pen tips, a force gauge was also incorporated to detect 1 mg of applied force from the tips. The printing process for the multi-pen system is almost the same as single pen system. PDMS pens are coated with ink solution before each printing cycle by dipping into an inkwell and then brought into contact with the substrate surface. Thus multiple patterns, one from each tip, are created in parallel simultaneously. Furthermore, with control of the printing force, feature sizes could be controlled over the range submicron to tens of microns. Three ink candidates have been printed by polymer pen printing approach to fabricate 2D&3D microstructures. The first ink material is Barium Strontium Titanate (BST) nanocrystallites dispersed in a furfuryl alcohol (FA), which was printed by the single PDMS pen with 100 μm tip diameter (Chapter 3). After printing, samples were heated to crosslink FA monomers, forming a stable polymeric matrix with embedded BST nanocrystallites. Without shear-thinning properties, BST/FA ink cannot be used to build 3D posts, but it has the capability to create circular patterns with different thickness by the single or multi-tier deposition method. It was found that the thickness of film increased linearly with the number of deposits without changing the diameter significantly. This encouraging result could enable the formation of microcapacitors with multi-tiered structure. Moreover, the study of printing parameters, including printing height and ink pick-up position, shows that changes to the pen positions in the ink reservoir or substrate have essentially no impact on deposit thickness or diameter. Beyond that, the effect of surface chemistry of PDMS pen and silicon wafer have also been studied. The plasma treated hydrophilic PDMS pen can pen transfer more BST/FA than untreated one; and the larger diameters with smaller thickness were obtained on a hydrophilic silicon wafer. The second ink candidate is a dilute aqueous solution of enzyme Candia antartica lipase B (CALB), which is known to catalyze the decomposition of poly (ε-caprolactone) (PCL) films. By bringing enzymes into contact with pre-defined regions of a surface, a polymer film can be selectively degraded to form patterned features that are requited for applications in biotechnology and electronics. This so-called enzymatic lithography is an environmentally friendly process as it does not require any actinic radiation or synthetic chemicals to develop required features. But the need to restrict the mobility of the enzyme in order to maintain control of feature sizes poses a significant challenge. In Chapter 4, after writing 2D enzyme patterns onto a spin-cast PCL film by single pen printing, samples with CALB were incubated at 37 ℃ and 95% relative humidity (RH) for up to 7 days to develop features. The CALB selectively degraded the PCL film during incubation, forming openings through the film. The size of these features (10 to 50 μm diameter) is well suited for use as biocompatible micro-reactors. Previous study of patterning CALB by single polymer pen printing technique resulted in slow etch rates, low throughput and poor image quality. In Chapter 5, I present an improved enzymatic lithography approach, still based on enzyme CALB and PCL system, which can resolve fine-scale features (\u3c 1 μm across) in thick (0.1 - 2.0 μm) polymer films after 5 minutes to 2 hours of incubation at 37 ℃ and 87% RH. Immobilization of the enzyme on the polymer surface was monitored using fluorescence microscopy by labeling CALB with FITC. The crystallite size in the PCL films was systematically varied; small crystallites resulted in significantly faster etch rates (20 nm/min) and the ability to resolve smaller features (as fine as 1 μm). The effect of printing conditions and RH during incubation is also presented. Patterns formed in the PCL film were transferred to an underlying copper foil demonstrating a Green approach to the fabrication of printed circuit boards. In parallel, the third ink material is a mixture of 25 wt% graphite dispersed in a high viscosity phenolic resin n-methyl-2-pyrrolidone (NMP) solution, which can be converted into carbon/carbon composites after a pyrolysis process. The 3D polymeric posts were created by depositing multilayers of thixotropic phenolic ink on a silicon substrate by single polymer pen printing method with a 10 μm radius PDMS pen tip (Chapter 6). After pyrolysis at 1000 ℃ in a nitrogen (N2) atmosphere, the polymeric features were converted to the glassy carbon/graphite features with a high aspect ratio (\u3e2). These features may be used as microelectrodes. Last, arrays of needle-shaped glassy carbon have been developed by a drawing approach using multi-pen printing technique followed by simple pyrolysis process (Chapter 7). To build polymeric needles with ultra-high aspect ratio, the polymeric ink was prepared by dissolving phenolic resin in the high boiling point (204 ℃) solvent NMP without fillers to achieve good printability and suitable viscosity. By slowly lifting up the print head from substrate, liquid needle structures were formed and then solidified on silicon substrates or gold electrodes due to the solvent evaporation. In addition, suspended resin fibers connected to two electrodes have also been fabricated by precisely controlling the movement of the PDMS pen. After pyrolysis, these resin features were converted to glassy carbon and the 3D structures remained. The electrical characterization results showed that glassy carbon made by this method had relatively low resistivity (2.5 x 10-5 Ωm). Therefore the glassy carbon based microneedles are well-suited to be electrodes for electrochemical sensors for biological applications

Phase Transformations and Switching of Chalcogenide Phase-change Material Films Prepared by Pulsed Laser Deposition

The thesis deals with the preparation, characterization and, in particular, with the switching properties of phase-change material (PCM) thin films. The films were deposited using the Pulsed Laser Deposition (PLD) technique. Phase transformations in these films were triggered by means of thermal annealing, laser pulses, and electrical pulses. The five major physical aspects structure transformation, crystallization kinetics, topography, optical properties, and electrical properties have been investigated using XRD, TEM, SEM, AFM, DSC, UV-Vis spectroscopy, a custom-made nanosecond UV laser pump-probe system, in situ resistance measurements, and conductive-AFM. The systematic investigation of the ex situ thermally induced crystallization process of pure stoichiometric GeTe films and O-incorporating GeTe films provides detailed information on structure transformation, topography, crystallization kinetics, optical reflectivity and electrical resistivity. The results reveal a significant improvement of the thermal stability in PCM application for data storage. With the aim of reducing the switching energy consumption and to enhance the optical reflectivity contrast by improving the quality of the produced films, the growth of the GeTe films with simultaneous in situ thermal treatment was investigated with respect to optimizing the film growth conditions, e.g. growth temperature, substrate type. For the investigation of the fast phase transformation process, GeTe films were irradiated by ns UV laser pulses, tailoring various parameters such as pulse number, laser fluence, pulse repetition rate, and film thickness. Additionally, the investigation focused on the comparison of crystallization of GST thin films induced by either nano- or femtosecond single laser pulse irradiation, used to attain a high data transfer rate and to improve the understanding of the mechanisms of fast phase transformation. Non-volatile optical multilevel switching in GeTe phase-change films was identified to be feasible and accurately controllable at a timescale of nanoseconds, which is promising for high speed and high storage density of optical memory devices. Moreover, correlating the dynamics of the optical switching process and the structural information demonstrated not only exactly how fast phase change processes take place, but also, importantly, allowed the determination of the rapid kinetics of phase transformation on the microscopic scale. In the next step, a new general concept for the combination of PCRAM and ReRAM was developed. Bipolar electrical switching of PCM memory cells at the nanoscale can be achieved and improvements of the performance in terms of RESET/SET operation voltage, On/Off resistance ratio and cycling endurance are demonstrated. The original underlying mechanism was verified by the Poole-Frenkel conduction model. The polarity-dependent resistance switching processes can be visualized simultaneously by topography and current images. The local microstructure on the nanoscale of such memory cells and the corresponding local chemical composition were correlated. The gained results contribute to meeting the key challenges of the current understanding and of the development of PCMs for data storage applications, covering thin film preparation, thermal stability, signal-to-noise ratio, switching energy, data transfer rate, storage density, and scalability

Controlled Synthesis and Characterization of Metal Oxide Nanowires by Chemical Vapor Deposition on Silicon and Carbon Substrates

Nanotechnology and nanomaterials have attracted considerable interest and are predicted to revolutionize many materials and technologies that we use in everyday life. In the past few years, significant research has focused on one dimensional metal oxide nanostructures due to their unique properties and potential applications in various fields from nanoelectronics to energy. However, controlled synthesis of these nanostructures is still a challenge. The objective of this thesis is to synthesize metal oxide nanowires by chemical vapour deposition directly on various substrates. The nanostructures include (i) silicon oxide nanostructures on silicon substrate, (ii) manganese oxide nanostructures on silicon substrate, and (iii) manganese oxide nanostructures on carbon paper substrate. Firstly, silicon oxide nanowires were synthesized on silicon substrate by a VO2 assisted chemical vapor deposition. Networked features of silicon oxide nanowires were found. Systematic study on the nanowire growth has indicated that morphology and composition of the final products are considerably sensitive to the catalyst components, reaction atmosphere and temperature. These results will help in better understanding the growth process of silicon oxide nanowires. Secondly, manganese oxide nanostructures were synthesized on silicon substrate by chemical vapor deposition method. It was found that MnO nanowires are high density and single crystalline with average diameter of 150 nm. These nanowires were characterized using FESEM, EDX, TEM and XRD. The synthesis process and effects of growth parameters such as temperature, heating rate and source/substrate distance on the morphology, composition and structure of the products were systematically studied. Finally, manganese oxide nanostructures were synthesized on carbon paper substrate by chemical vapor deposition method. It was revealed that manganese oxide nanowires and nanobelts can be selectively grown on carbon paper substrate by using a catalyst (gold) assisted or catalyst free thermal evaporation of manganese powder under an argon gas atmosphere. Various effects of growth parameters such as temperature, catalyst and buffered substrate on the growth product were also systematically investigated by using SEM, TEM and XPS

Defect-Rich Size-Selected Nanoclusters and Nanocrystalline Films of Titanium (IV) Oxide and Tantalum (IV) Oxide for Efficient Photocatalyst and Electroforming-Free Memristor Applications

Transition metal oxides, TiO2 and Ta2O5, are two of the most extensively studied wide bandgap semiconductor materials (with high work functions). Due to their suitable band edge positions for hydrogen evolution and exceptional stability against photocorrosion upon optical excitation, their application in heterogeneous photocatalysis has attracted a lot of attention. These oxides are also great components in the field of electronic devices such as field effect transistors, solar cells, and more recently advanced memory devices. Here, we focus on ultrasmall nanoclusters (< 5 nm) and nanocrystalline thin films of defect-rich TiO2 and Ta2O5 and their applications as high-performance photocatalysts in photoelectrochemical water splitting reactions and as resistive switching materials in memory applications. The present work is divided into two main parts. In the first part, ultrasmall nanoclusters (below 10 nm) of defect-rich TiO2 and Ta2O5 are synthesized using a gas phase aggregation technique in a nanocluster generation source based on DC magnetron sputtering. With a careful optimization of the deposition parameters such as aggregation zone length (condensation volume), Ar gas flow rate, deposition temperature and source power, we are able to produce metal/metal oxide nanoclusters with a narrow size distribution. As most of these as-grown nanoclusters are negatively charged, it is possible to conduct size-selection according to their mass-to-charge ratio. Using a quadrupole mass filter (directly coupled to the magnetron source), we achieve precise size-selection of nanoclusters, with the size distribution reduced to below 2% mass resolution. The nearly monosized nanoclusters so produced are deposited onto appropriate substrates to serve as the photoanodes for photoelectrochemical water splitting application. We demonstrate, for the first time, that the precisely size-selected TiO2 nanoclusters can be deposited on H-terminated Si(001) in a soft-landing condition and they can be used as highperformance photocatalysts for solar harvesting, with greater enhancement in the photoconversion efficiency. Three different sizes of TiO2 nanoclusters (4, 6 and 8 nm) are synthesized with appropriate combinations of aggregation length and Ar flow rate. Despite the low amount of material loading (of ~20% of substrate coverage), these supported TiO2 nanoclusters exhibit remarkable photocatalytic activities during photoelectrochemical water splitting reaction under simulated sunlight (50 mW/cm^2). Higher photocurrent densities (up to 0.8 mA/cm^2) and photoconversion efficiencies (up to 1%) with decreasing nanocluster size (at the applied voltage of –0.22 V vs Ag/AgCl) are observed. We attribute this enhancement to the presence of surface defects, providing a large amount of active surface sites, in the amorphous TiO2 nanoclusters as-grown at room temperature. We have further shown that the incorporation of metallic nanoclusters with the semiconductor photocatalysts can enhance the photoconversion efficiency. In this work, we have co-deposited surface oxygen deficient Ta2O5 or TaOx nanoclusters along with Pt nanoclusters of similar nanocluster size (~5 nm), the latter used as a promoter. The electron-hole pairs generated in the water splitting reaction can be effectively separated and stored with the presence of Pt nanoclusters, while the increase in Pt loading as a promoter can enhance the reaction by providing a large number of electrons for H2 evolution. However, loading too much Pt nanoclusters could actually reduce the photoresponse, which is due to blocking of photosensitive TaOx surface by excess Pt nanoclusters. In both cases, the photoconversion efficiency could potentially be enhanced at least 5 times by increasing the amount of nanocluster loading from 20% coverage to a monolayer coverage (e.g., by increasing the amount of deposition time for TiO2 and TaOx nanoclusters). Even higher photoconversion efficiency can be obtained with multiple layers of nanoclusters and by employing nanoclusters with even smaller size and/or with modification by chemical functionalization. These potential improvements could dramatically increase the photoconversion efficiency, making these nanocluster samples to be among the top photoelectrochemical catalysis performers. In the second part of the present work, we employ defect-rich nanocrystalline TiOx and TaOx thin films as active materials for resistive switching for memory application. Based on resistive switching principle, memristive devices (or memristors) provide the unique capability of multistep information storage. The development of memristors has often been hailed as the next evolution in non-volatile memories, low-power remote sensing, and adaptive intelligent prototypes including neuromorphic and biological systems. One major obstacle in achieving high switching performance is the irreversible electroforming step that is required to create oxygen vacancies for resistive switching. Using magnetron sputtering film deposition technique, we have fabricated the heterojunction memristor devices based on nanocrystalline TiOx and TaOx thin films (10-60 nm thick) with a high density of built-in oxygen vacancies, sandwiched between a pair of metallic Pt electrodes (30 nm thick). To avoid the destructive electroforming process and to achieve a high switching performance in the memristor device, we carefully manipulate the chamber pressure and ambient in deposition chamber during deposition to generate the required highly oxygen deficient semiconducting films. The films, as-deposited at room temperature, exhibit a crystallite size of 4-5 nm. In the fabricated Pt/TiOx/Pt memristors, a high electric field gradient can be generated in the TiOx film at a much lower electroforming voltage of +1.5 V, due in part to the nanocrystalline nature, which causes localization of this electric field and consequently enhanced reproducibility and repeatability in the device performance. After the first switching, consecutive 250 switching cycles can be achieved with a low programing voltage of ±1.0 V, along with a high ON/OFF current ratio, and long retention (up to 10^5 s). We further improve this TiOx memristor device by totally removing the electroforming step by fabricating an electroforming-free memristive device based on a heterojunction interface of TiOx and TaOx layers. In the Pt/TiOx/TaOx/Pt architecture structure (with Pt serving as the top and bottom electrodes), a high-κ dielectric TaOx layer is used to facilitate trapping and release of the electronic carriers, while a TiOx layer provides low-bias rectification as an additional oxygen vacancy source. With the incorporation of TaOx layer, the need for the electroforming step can be eliminated. More importantly, the resistance states of the device can be tuned such that switching between the high resistance state and the low resistance state can be achieved even smaller programming voltage of +0.8 V. With the low leakage current properties of TaOx, the high endurance (10^4 repeated cycles) and high retention capabilities (up to 10^8 s) can be enhanced manifold with highly stable ON/OFF current ratio. In both memristor devices, four different junction sizes (5×5, 10×10, 20×20 and 50×50 μm^2) have been evaluated according to their ON/OFF current ratio. We observe that the smaller is the junction size is, the higher is the current ratio. For the Pt/TiOx/TaOx/Pt memristor, we have also analyzed the thickness dependent effect of the switching behavior of devices with four different TaOx layer thicknesses (10, 20, 40 and 60 nm) and a TiOx layer thickness constant at 10 nm. The device with 10 nm thick TaOx (being amorphous in nature) shows unipolar switching with two SETs and two RESETs in one sweep cycle. This is in contrast to the bipolar resistive switching found in devices with the thicker TaOx films with a SET in the positive sweep and a RESET during the negative sweep. We further demonstrate that resistive switching can also occur at very low programming voltage (~50 mV), thus qualifying it as an ultralow power consumption device (~nW). The stable non-volatile bipolar switching characteristics with high ON/OFF current ratio and low power consumption make our devices best suitable for various analog and discrete programmable electric pulses. With the simplicity in the construction, the performance achieved for our memristors represents the best reported to date. This new class of defect-rich metal oxides nanomaterials with an ultrananocrystalline nature shows solid promises for various catalytic and electronic applications and, also, the simple, scalable roomtemperature device fabrication process makes this approach easily migratable further to transparent and/or flexible devices