Polarity Control of Top Gated Black Phosphorous FETs by Workfunction Engineering of Pre-Patterned Au and Ag Embedded Electrodes

We propose and experimentally demonstrate top-gated complementary n- and p-type black phosphorous field effect devices (FETs) by engineering the workfunction of pre-patterned electrodes embedded in a SiO2 bottom layer. Pre-patterned electrodes offer the advantages of reducing the exposure time of exfoliated flakes to oxidant agents with respect to top-contacted devices and maximizing the accessible area for sensing applications. The presented devices are realized by mechanical exfoliation of multilayer black phosphorous flakes on top of pre-patterned embedded source and drain contacts. A capping layer consisting of 15-nm thick Al2O3 is deposited to prevent flakes degradation and serves as top gate dielectric. The silicon substrate can be exploited as back gate to program the FETs threshold voltage. We deposited both Au and Ag embedded contacts to investigate the impact of electrodes workfunction on BP FETs polarity. Au contacted devices show p-type conduction with ON/OFF current ratio 140 and holes mobility up to 40 cm(2)V(-1)s(-1). Devices with Ag contacts exhibit prevalent n-type conduction with ON/OFF ratio 1700 and electron mobility 2 cm(2)V(-1)s(-1). The reported results represent a substantial improvement with respect to reported alternative implementations of black phosphorous FETs with pre-patterned, non-embedded electrodes. Moreover, we demonstrate that Ag is a promising metal for electron injection in black phosphorous FETs

Complementary black phosphorous FETs by workfunction engineering of pre-patterned Au and Ag embedded electrodes

We propose and experimentally demonstrate topgated complementary n- and p-type black phosphorous FETs by engineering the workfunction of pre-patterned electrodes embedded in a SiO2 layer. Pre-patterned electrodes offer the possibility of reducing the exposure time of exfoliated flakes to oxidant agents with respect to top-contacted devices and maximize the accessible area for sensing applications. The devices are realized by exfoliating multilayer black phosphorous flakes on top of pre-patterned embedded source and drain contacts. A capping layer consisting of 15 nm thick Al2O3 is used to prevent flakes degradation and serves as top gate dielectric. We deposited both Au and Ag contacts to investigate the impact of the electrode workfunctions on BP FETs polarity. Au contacted devices showed p-type conduction with ON/OFF current ratio 140 and holes mobility up to 40 cm2V-1s-1. Devices with Ag contacts showed prevalent n-type conduction with ON/OFF ratio 1700 and electron mobility 2 cm2 V-1s-1. The reported results represent a substantial improvement with respect to reported alternative implementations of black phosphorous FETs with pre-patterned, non-embedded electrodes. Moreover, we demonstrate that Ag is a promising metal for electron injection in black phosphorous FETs

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

2D Steep Transistor Technology: Overcoming Fundamental Barriers in Low-Power Electronics and Ultra-Sensitive Biosensors

In order to sustain the unprecedented growth of the Information Technology, it is necessary to achieve dimensional scalability along with power reduction, which is a daunting challenge. In this dissertation, two-dimensional (2D) materials are explored as promising materials for future electronics since they can, not only enable dimensional scaling without degradation of device electrostatics but it is also shown here, that they are highly potential candidate for interconnects and passive devices. 2D semiconductors are investigated for transistor applications, and novel approach for doping using nanoparticle functionalization is developed. It is also demonstrated that these materials can lead to ideal transfer characteristics. Aimed towards on-chip interconnect and inductor applications, the first detailed methodology for the accurate evaluation of high-frequency impedance of graphene is presented. Using the developed method the intricate high-frequency effects in graphene such as Anomalous Skin Effect (ASE), high-frequency resistance and inductance saturation, intercoupled relation between edge specularity and ASE and the influence of linear dimensions on impedance are investigated in detail for the first time. While 2D materials can address the issue of dimensional scalability, power reduction requires scaling of power supply voltage, which is difficult due to the fundamental thermionic limitation in the steepness of turn-ON characteristics or subthreshold swing (SS) of conventional Field-Effect-Transistors (FETs). To address this issue, a detailed theoretical and experimental analysis of fundamentally different carrier transport mechanism, based on quantum mechanical band-to-band tunneling (BTBT) is presented. This dissertation elucidates an underlying physical concept behind the BTBT process and provides clear insight into the interplay between electron and hole characteristics of carriers within the forbidden gap during tunneling. Moreover, a novel methodology for increasing the BTBT current through incorporation of metallic nanoparticles at the tunnel junction is proposed and theoretically analyzed, followed by experimental demonstration as proof of concept, which can open up new avenues for enhancing the performance of Tunneling-Field-Effect-Transistors (TFETs). This dissertation, also establishes, for the first time, that the material and device technology which have evolved mainly with an aim of sustaining the glorious scaling trend of Information Technology, can also revolutionize a completely diverse field of bio/gas-sensor technology. The unique advantages of 2D semiconductor for electrical sensors is demonstrated and it is shown that they lead to ultra-high sensitivity, and also provide an attractive pathway for single molecular detectability- the holy grail for all biosensing research. Moreover, it is theoretically illustrated that steep turn-ON characteristics, obtained through novel technology such as BTBT, can result in unprecedented performance improvement compared to that of conventional electrical biosensors, with around 4 orders of magnitude higher sensitivity and 10-fold lower detection time. With a view to building ultra-scaled low power electronics as well as highly efficient sensors, new generation of van-der Waal's BTBT junctions combining 2D with 3D materials is proposed and experimentally demonstrated, which not only retain the advantages of 2D films but also leverages the matured doping technology of 3D materials, thus harnessing the best of both worlds. These attributes are instrumental in the achievement of unprecedented BTBT current, which is more than 3 orders of magnitude higher than that of best reported 2D heterojunctions till date. Finally, with the optimization of the novel heterojunctions, this dissertation also achieves a significant milestone, furnishing the first experimental demonstration of TFETs based on 2D channel material to beat the fundamental limitation in subthreshold swing (SS). This device is the first ever TFET, in a planar architecture to achieve sub-thermionic SS over 4 decades of drain current, a necessary characteristic prescribed by the International Technology Roadmap for Semiconductors and in fact, the only TFET to date, to achieve so, in any architecture and in any material platform, at a low power-supply voltage of 0.1 V. It also represents the world's thinnest channel sub-thermionic transistor, thus, cracking the long-standing issue of simultaneous dimensional and power supply scalability and hence, can lead to a paradigm shift in information technology as well as healthcare

21st Century Nanostructured Materials

Nanostructured materials (NMs) are attracting interest as low-dimensional materials in the high-tech era of the 21st century. Recently, nanomaterials have experienced breakthroughs in synthesis and industrial and biomedical applications. This book presents recent achievements related to NMs such as graphene, carbon nanotubes, plasmonic materials, metal nanowires, metal oxides, nanoparticles, metamaterials, nanofibers, and nanocomposites, along with their physical and chemical aspects. Additionally, the book discusses the potential uses of these nanomaterials in photodetectors, transistors, quantum technology, chemical sensors, energy storage, silk fibroin, composites, drug delivery, tissue engineering, and sustainable agriculture and environmental applications

Carbon Nanotubes for Electronics and Energy

Ever since their discovery, carbon nanotubes have been touted as a new material for the future and a correspondingly lengthy list of possible applications are often cited in the literature. This excitement for carbon nanotubes is a result of their richly varying physical, electronic and optical properties, where it is possible to have single, double and multiple carbon walls with each wall potentially being either semiconducting or metallic and possessing unique optical transitions covering the ultraviolet to infrared spectral range. However, to date the realization of many of the proposed applications has been hindered by exactly the characteristic that made carbon nanotubes so attractive in the first place, namely the inherent inhomogeneity and varying properties of as-prepared or grown material. In order to become a true advanced material of the future, methods to prepare carbon nanotubes with defined length, wall number, diameter, electronic and optical property are necessary. Additionally, such methods to sort carbon nanotubes must afford high purity levels, be amenable to large-scale preparation and be compatible with subsequent integration into device architectures. In this work these issues are addressed with the use of gel based sorting techniques, which with the use of an automated gel permeation system allows for the routine preparation of milligram quantities of metallic and semiconducting carbon nanotubes, chirality pure single walled carbon nanotubes and even double walled carbon nanotubes sorted by their outer-wall electronic type. Having developed techniques to prepare large quantities, methodologies to control the order and orientation of this 1 D nanomaterial on the macro scale are developed. Inks of carbon nanotubes with liquid crystal concentrations and aligned films thereof are developed and this newfound control over the electronic and structural property opened the door for energy related applications. For example the use of thin films as the transparent electrodes in silicon:carbon nanotube solar cells or as the light harvesting layer in combination with fullerenes with the goal of creating an all carbon solar cell. Likewise on the few nanotube level the unique optical transitions of different nanotube chiralities are used in the fabrication of nanoscale photosensitive elements

Van der Waals Layered Materials: Surface Morphology, Interlayer Interaction, and Electronic Structure

Over the past decades, new materials have formed the backbone in shaping the landscape of technology. From the Si-based transistors in our smart devices to the carbon fibers that have redefined air-transportation, the pursuit for a stronger, lighter, and cost-effective material has never ceased, as well as the attempt to fully understand their physics and material properties. Moore's law just turned 50th this year. Moore's law seems harder and harder to hold as the industry has reached a point where the dimensions of those Si-based transistors are getting too small and thin to proceed quickly and without incurring substantial additional cost. Also, the transistor dimensions have been getting closer and closer to the physical limitation of the Si. Today, the most advanced node is merely "7 nm" – less than 15 layers of Si, silicon oxides, or other metal oxides. At this point, every layers of atoms counts. The search for new ultrathin materials as the "new silicon" has begun. In this regard, graphene, which is a single sheet of carbon atoms arranged in a hexagonal honeycomb lattice, has led to a huge interest in the science and technology communities due to its exotic physics that arises from low-dimensional confinement. This interest soon extended to two-dimensional (2D) electronic materials systems, especially semiconducting van der Waals layered-materials such as MoS₂ that, unlike graphene, has a direct bandgap material in its monolayer form. There are also many other promising candidates such as transition metal dichalcogenides (TMDCs), black phosphorene, and perovskites on this rapid growing "2D materials" family tree. From condensed matter physics' point of view, studying the electronic behavior of these 2D systems can provide insight into a variety of phenomena, including epitaxial growth, interfacial charge transfer, energy-momentum relation, and carrier mobility, that leads to advanced device fabrication and engineering. In this dissertation, I examine (1) the surface structure, including the growth, the crystal quality, and thin film surface corrugation of a monolayer sample and a few layers of MoS₂ and WSe₂, and (2) their electronic structure. The characteristics of these electronic systems depend intimately on the morphology of the surfaces they inhabit, and their interactions with the substrate or within layers. These physical properties will be addressed in each chapter. This thesis has dedicated to the characterization of mono- and a few layers of MoS₂ and WSe₂ that uses surface-sensitive probes such as low-energy electron microscopy and diffraction (LEEM and LEED). Prior to our studies, the characterization of monolayer MoS₂ and WSe₂ has been generally limited to optical and transport probes. Furthermore, the heavy use of thick silicon oxide layer as the supporting substrate has been important in order to allow optical microscopic characterization of the 2D material. Hence, to the best of our knowledge, this has prohibited studies of this material on other surfaces, and it has precluded the discovery of potentially rich interface interactions that may exist between MoS₂ and its supporting substrate. Thus, in our study, we use a so-called SPELEEM system (Spectroscopic Photo-Emission and Low Energy Electron Microscopy) to address these imaging modalities: (1) real-space microscopy, which would allow locating of monolayer MoS₂ samples, (2) spatially-resolved low-energy diffraction which would allow confirmation of the crystalline quality and domain orientation of MoS₂ samples, and, (3) spatially-resolved spectroscopy, which would allow electronic structure mapping of MoS₂ samples. Moreover, we have developed a preparation procedure for samples that yield, a surface-probe ready, ultra-clean, and can be transferred on an arbitrary substrate. In this thesis, to fully understand the physics in MoS₂ such as direct-to-indirect band gap transition, hole mobility, strain, or large spin-orbit splitting, we investigate our sample using micro-probe angle-resolved photoemission (µ-ARPES), which is a powerful tool to directly measure the electronic structure. We find that the valence bands of monolayer MoS₂, particularly the low-binding-energy bands, are distinctly different from those of bulk MoS₂ in that the valence band maximum (VBM) of a monolayer is located at Κ ̅ of the first Brillouin zone (BZ), rather than at Γ ̅, as is the case in bilayer and thicker MoS₂ crystals. This result serves as a direct evidence, if complemented with the photoluminescence studies of conduction bands, which shows the direct-to-indirect transition from mono- to milti-layer MoS₂. We also confirmed this same effect in WSe₂ in our later studies. Also, by carefully studying the uppermost valence band (UVB) of both exfoliated and CVD-grown monolayer MoS₂, we found a compression in energy in comparison with the calculated band, an effect, which were also observed in suspended sample with minimum-to-none substrate interaction. We tentatively attribute it to an intrinsic effect of monolayer MoS₂ owning to lattice relaxation. The degree of compression in CVD-grown MoS₂ is larger than that in exfoliated monolayer MoS₂, likely due to defects, doping, or stress. Furthermore, we find that the uppermost valence band near Κ ̅ of monolayer MoS₂ is less dispersive than that of the bulk, which leads to a striking increase in the hole effective-mass and, hence, the reduced carrier mobility of the monolayer compared to bulk MoS₂. Beyond monolayer MoS₂, we have studied the evolution of bandgap as a function of interlayer twist angles in a bilayer MoS₂ system. Our µ-ARPES measurements over the whole surface-Brillouin zone reveal the Γ ̅ state is, indeed, the highest lying occupied state for all twist angles, affirming the indirect bandgap designation for bilayer MoS₂, irrespective of twist angle. We directly quantify the energy separation between the high symmetry points Γ ̅ and K ̅ of the highest occupied states; this energy separation is predicted to be directly proportional to the interlayer separation, which is a function of the twist angle. We also confirm that this trend is a result of the energy shifting of the top-most occupied state at Γ ̅, which is predicted by DFT calculations. Finally, we also report on the variation of the hole effective mass at Γ ̅ and K ̅ with respect to twist angle and compare it with theory. Our study provides a direct measurement and serves as an example for how the interlayer coupling can affect the band structure and electron transitions, which is crucial in designing TMDs devices. To the end of this thesis, I briefly sum up our angle-resolve two-photon photoemission (2PPE) studies on self-assembly molecules, organic molecules, and graphene on highly-crystalline metal systems, and our investigation of their interfacial charge transfer/trapping, image potential states, and coverage-dependent dipole moments, as well as their work functions by using a tunable ultra-fast femtosecond laser

Flexographic printed nanogranular LBZA derived ZnO gas sensors: Synthesis, printing and processing

Within this document, investigations of the processes towards the production of a flexographic printed ZnO gas sensor for breath H2 analysis are presented. Initially, a hexamethylenetetramine (HMTA) based, microwave assisted, synthesis method of layered basic zinc acetate (LBZA) nanomaterials was investigated. Using the synthesised LBZA, a dropcast nanogranular ZnO gas sensor was produced. The testing of the sensor showed high sensitivity towards hydrogen with response (Resistanceair/ Resistancegas) to 200 ppm H2 at 328 °C of 7.27. The sensor is highly competitive with non-catalyst surface decorated sensors and sensitive enough to measure current H2 guideline thresholds for carbohydrate malabsorption (Positive test threshold: 20 ppm H2, Predicted response: 1.34). Secondly, a novel LBZA synthesis method was developed, replacing the HMTA by NaOH. This resulted in a large yield improvement, from a [OH-] conversion of 4.08 at% to 71.2 at%. The effects of [OH-]/[Zn2+] ratio, microwave exposure and transport to nucleation rate ratio on purity, length, aspect ratio and polydispersity were investigated in detail. Using classical nucleation theory, analysis of the basal layer charge symmetries, and oriented attachment theory, a dipole-oriented attachment reaction mechanism is presented. The mechanism is the first theory in literature capable of describing all observed morphological features along length scales. The importance of transport to nucleation rate ratio as the defining property that controls purity and polydispersity is then shown. Using the NaOH derived LBZA, a flexographic printing ink was developed, and proof-of-concept sensors printed. Gas sensing results showed a high response to 200 ppm H2 at 300 °C of 60.2. Through IV measurements and SEM analysis this was shown to be a result of transfer of silver between the electrode and the sensing layer during the printing process. Finally, Investigations into the intense pulsed light treatment of LBZA were conducted. The results show that dehydration at 150 °C prior to exposure is a requirement for successful calcination, producing ZnO quantum dots (QDs) in the process. SEM measurements show mean radii of 1.77-2.02 nm. The QDs show size confinement effects with the exciton blue shifting by 0.105 eV, and exceptionally low defect emission in photoluminescence spectra, indicative of high crystalline quality, and high conductivity. Due to the high crystalline quality and amenity to printing, the IPL ZnO QDs have numerous potential uses ranging from sensing to opto-electronic devices

EUROSENSORS XVII : book of abstracts

Fundação Calouste Gulbenkien (FCG).Fundação para a Ciência e a Tecnologia (FCT)