Anomalous Properties of Sub-10-nm Magnetic Tunneling Junctions

Magnetic Logic Devices have the advantage of non-volatility, radiation hardness, scalability down to the sub-10nm range, and three-dimensional (3D) integration capability. Despite these advantages, magnetic applications for information processing remain limited. The main stumbling block is the high energy required to switch information states in spin-based devices. Recently, the spin transfer torque (STT) effect has been introduced as a promising solution. STT based magnetic tunneling junctions (MTJs), use a spin polarized electric current to switch magnetic states. They are theorized to bring the switching energy down substantially. However, the switching current density remains in the order of 1 MA/cm2 in current STT-MTJ devices, with the smallest device reported to date around 10nm. This current density remains inadequately high for enabling a wide range of information processing applications. For this technology to be competitive in the near future it is critical to show that it could be favorably scaled into the sub-10-nm range. This is an intriguing size range that currently remains unexplored. Nanomagnetic devices may display promising characteristics that can make them superior to their semiconductor counterparts. Below 10nm the spin physics from the vii surface become dominate versus those due to volume. The goal is to understand the size dependence versus the switching current

Design, Fabrication, Characterization and Modeling of CMOS-Compatible PtSe2 MOSFETs

For the last 50 years, Si metal-oxide-semiconductor field-effect transistors (MOSFETs) have undergone tremendous development under the Moore’s law. However, it has become more and more difficult to continue the scaling due to the limitation of Si quantum confinement when the gate length is less than 5 nm. Two-dimensional (2D) atomic-layered materials may replace Si in future-generation ultra-thin-body, low-power, and high-performance MOSFETs. However, for any 2D material to replace Si, it must not only have high mobility and sizable bandgap, but also be manufacturable. Graphene has high mobility but no bandgap. MoS2 has sizable bandgap but low mobility. Black phosphorus (BP) has high mobility and sizable bandgap but is unstable in air and has not been grown under reasonable pressure and temperature. By contrast, monolayer PtSe2 has high mobility, sizable bandgap, air stability, and can be synthesized below 450 °C ‒ −the thermal budget of CMOS back-of-the-line (BEOL) processes (BEOL). Additionally, bulk PtSe2 is semi-metallic to facilitate low resistance contact, a critical issue for all 2D devices. Experimentally, PtSe2 MOSFETs have been demonstrated by exfoliated flakes, chemical vapor deposited film, and thermal assisted converted film. In this dissertation, both material preparation and device fabrication are done below 450 °C. Transistors based on molecular beam epitaxy grown PtSe2 are fabricated and characterized for the first time. With 3-monolayer (ML) PtSe2 grown by molecular beam, transistor with n-type carrier has been demonstrated with on/off ratio of 43, which increases to 1600 at 80 K and is the best among n-type PtSe2 transistors fabricated on grown films. The MOSFETs are batch-fabricated by a CMOS-compatible process based on 200-mm-diameter Si substrates prepared by a state-of-the-art BiCMOS foundry. Dozens of rounds of fabrication were carried out to ensure the yield of large-scale fabrication. Photoresist residue formed on 2D material were reduced by reduced dry etching time. The poor adhesion between 2D material and the substrate was also addressed. Despite the thin PtSe2 layer, doping by contact bias lowers the contact resistance significantly and boosts the on current and on/off ratio. Temperature-dependent current-voltage characteristics show the bandgap is approximately 0.2 eV, which confirms that the semiconductor-semimetal transition of PtSe2 is not as abrupt as originally predicted. By the chip maps, performances of 66 devices are presented, which show reasonable uniformity across the 10 mm × 10 mm chip. Better MOSFET performance can be expected by growing even thinner PtSe2 uniformly and by thickening the PtSe2 in the contact regions

Advanced Graphene Microelectronic Devices

The outstanding electrical and material properties of Graphene have made it a promising material for several fields of analog applications, though its zero bandgap precludes its application in digital and logic devices. With its remarkably high electron mobility at room temperature, Graphene also has strong potential for terahertz (THz) plasmonic devices. However there still are challenges to be solved to realize Graphene’s full potential for practical applications. In this dissertation, we investigate solutions for some of these challenges. First, to reduce the access resistances which significantly reduces the radio frequency (RF) performance of Graphene field effect transistors (GFETs), a novel device structure consisting of two additional contacts at the access region has been successfully modeled, designed, microfabicated/integrated, and characterized. The additional contacts of the proposed device are capacitively coupled to the device channel and independently biased, that induce more carriers and effectively reduce access resistance. In addition to that, in this dissertation, bandgap has been experimentally introduced to semi-metallic Graphene, by decorating with randomly distributed gold nano-particles and zinc oxide (ZnO) nano-seeds, where their interaction breaks its sublattice symmetry and opens up bandgap. The engineered bandgap was extracted from its temperature dependent conductivity characteristics and compared with reported theoretical estimation. The proposed method of device engineering combined with material bandgap engineering, on a single device, introduces a gateway towards high speed Graphene logic devices. Finally, THz plasmon generation and propagation in Graphene grating gate field effect transistors and Graphene plasmonic ring resonators have been investigated analytically and numerically to explore their potential use for compact, solid state tunable THz detectors

Laser-induced forward transfer (LIFT) of water soluble polyvinyl alcohol (PVA) polymers for use as support material for 3D-printed structures

The additive microfabrication method of laser-induced forward transfer (LIFT) permits the creation of functional microstructures with feature sizes down to below a micrometre [1]. Compared to other additive manufacturing techniques, LIFT can be used to deposit a broad range of materials in a contactless fashion. LIFT features the possibility of building out of plane features, but is currently limited to 2D or 2½D structures [2–4]. That is because printing of 3D structures requires sophisticated printing strategies, such as mechanical support structures and post-processing, as the material to be printed is in the liquid phase. Therefore, we propose the use of water-soluble materials as a support (and sacrificial) material, which can be easily removed after printing, by submerging the printed structure in water, without exposing the sample to more aggressive solvents or sintering treatments. Here, we present studies on LIFT printing of polyvinyl alcohol (PVA) polymer thin films via a picosecond pulsed laser source. Glass carriers are coated with a solution of PVA (donor) and brought into proximity to a receiver substrate (glass, silicon) once dried. Focussing of a laser pulse with a beam radius of 2 µm at the interface of carrier and donor leads to the ejection of a small volume of PVA that is being deposited on a receiver substrate. The effect of laser pulse fluence , donor film thickness and receiver material on the morphology (shape and size) of the deposits are studied. Adhesion of the deposits on the receiver is verified via deposition on various receiver materials and via a tape test. The solubility of PVA after laser irradiation is confirmed via dissolution in de-ionised water. In our study, the feasibility of the concept of printing PVA with the help of LIFT is demonstrated. The transfer process maintains the ability of water solubility of the deposits allowing the use as support material in LIFT printing of complex 3D structures. Future studies will investigate the compatibility (i.e. adhesion) of PVA with relevant donor materials, such as metals and functional polymers. References: [1] A. Piqué and P. Serra (2018) Laser Printing of Functional Materials. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA. [2] R. C. Y. Auyeung, H. Kim, A. J. Birnbaum, M. Zalalutdinov, S. A. Mathews, and A. Piqué (2009) Laser decal transfer of freestanding microcantilevers and microbridges, Appl. Phys. A, vol. 97, no. 3, pp. 513–519. [3] C. W. Visser, R. Pohl, C. Sun, G.-W. Römer, B. Huis in ‘t Veld, and D. Lohse (2015) Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer, Adv. Mater., vol. 27, no. 27, pp. 4087–4092. [4] J. Luo et al. (2017) Printing Functional 3D Microdevices by Laser-Induced Forward Transfer, Small, vol. 13, no. 9, p. 1602553

Beyond solid-state lighting: Miniaturization, hybrid integration, and applications og GaN nano- and micro-LEDs

Gallium Nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting and ubiquitous white light sources has developed around the inception of the Nobel-price-winning blue GaN LEDs. Today GaN optoelectronics is developing beyond lighting, leading to new and innovative devices, e.g. for micro-displays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (microLEDs and nanoLEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN microLEDs with silicon microelectronics in a fully hybrid approach. GaN LED technology today is even spreading into the realm of display technology, which has been occupied by organic LED (OLED) and liquid crystal display (LCD) for decades. In this review, the technological transition towards GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art

The micro-LED roadmap: status quo and prospects

Micro light-emitting diode (micro-LED) will play an important role in the future generation of smart displays. They are found very attractive in many applications, such as maskless lithography, biosensor, augmented reality (AR)/mixed reality etc, at the same time. A monitor that can fulfill saturated color rendering, high display resolution, and fast response time is highly desirable, and the micro-LED-based technology could be our best chance to meet these requirements. At present, semiconductor-based red, green and blue micro-LED chips and color-conversion enhanced micro-LEDs are the major contenders for full-color high-resolution displays. Both technologies need revolutionary ways to perfect the material qualities, fabricate the device, and assemble the individual parts into a system. In this roadmap, we will highlight the current status and challenges of micro-LED-related issues and discuss the possible advances in science and technology that can stand up to the challenges. The innovation in epitaxy, such as the tunnel junction, the direct epitaxy and nitride-based quantum wells for red and ultraviolet, can provide critical solutions to the micro-LED performance in various aspects. The quantum scale structure, like nanowires or nanorods, can be crucial for the scaling of the devices. Meanwhile, the color conversion method, which uses colloidal quantum dot as the active material, can provide a hassle-free way to assemble a large micro-LED array and emphasis the full-color demonstration via colloidal quantum dot. These quantum dots can be patterned by porous structure, inkjet, or photo-sensitive resin. In addition to the micro-LED devices, the peripheral components or technologies are equally important. Microchip transfer and repair, heterogeneous integration with the electronics, and the novel 2D material cannot be ignored, or the overall display module will be very power-consuming. The AR is one of the potential customers for micro-LED displays, and the user experience so far is limited due to the lack of a truly qualified display. Our analysis showed the micro-LED is on the way to addressing and solving the current problems, such as high loss optical coupling and narrow field of view. All these efforts are channeled to achieve an efficient display with all ideal qualities that meet our most stringent viewing requirements, and we expect it to become an indispensable part of our daily life

Innovative patternable materials for micro- and nano- fabrication

The research activity of this thesis is focused on the development and optimization of new directly patternable organically modified TiO2, Al2O3 and ZrO2 based sol-gel materials whose peculiar characteristics and performances were optimized and exploited for the final specific application. In particular, the main strategy that lies at the basis of all the thesis work is the combination of top down and- bottom up approach for the final device realization. In fact, special attention has been set to materials design and synthesis (bottom up) and subsequently to the micro- and nano- fabrication of patterns on the corresponding film surface with different lithographic techniques (top down) in order to achieve the required properties, according to the final application. As it concerns the bottom up approach, the sol-gel has been assumed as the main synthetic method since, by mixing different organic-inorganic precursors, new materials with unique properties and microstructures can be created. In fact, by using organically modified precursors (such as trimethoxyphenylsilane, 3-glycidoxypropyltrimethoxysilane, 3-(Trimethoxysilyl)propyl methacrylate) or organic monomers it was possible to produce hybrid materials with the organic and inorganic components intimately mixed at a molecular scale, with the twofold effect of obtaining new properties and conferring them the patternability. Moreover, the addition of tetrafunctional precursors (Titanium isopropoxide, Zirconium butoxide, Aluminum-tri-sec-butoxide) allowed to increase the reticulation degree, taking part to the inorganic network formation, to improve the material mechanical properties (such as scratch, abrasion, plasma etching resistance) and to confer particular characteristics to the final materials, i.e. to modulate the refractive index. On the other hand, as it regards the top down approach, different lithographic techniques (photolithography, X-ray lithography, electron beam lithography and nanoimprint lithography) have been exploited in the realization of high refractive index patterns, high selective etching masks features, adaptive-optics devices and stamps for microinjection moulding directly with the synthesized materials. The structural and chemical changes induced inside the material by the interactions with the source used in the lithographic process have been deeply investigated in order to optimize both the synthesis of the best sol-gel systems and the final lithographic procedures. In conclusion the development of all the above mentioned advanced materials and innovative processing was pushed by the main target of improving, simplifying and decreasing costs and time of the overall micro- and nano- fabrication process in order to obtain better final features quality, with respect to traditional lithographic procedures

Graphene based high frequency electronics

Ankara : The Department of Physics and the Institute of Engineering and Sciences of Bilkent University, 2010.Includes bibliographical references leaves 61-66.Recent advances in chemical vapor deposition of graphene on large area substrates stimulate a significant research effort in order to search for new applications of graphene in the field of unusual electronics such as macroelectronics. The primary aim of this work is to use single layer of graphene for applications of high frequency electronics. This thesis consists of both theoretical and experimental studies of graphene transistors for the use of radio frequency electronics. We have grown graphene layer using chemical vapor deposition technique on large area copper substrates. The grown graphene layers are then transferred onto dielectric substrates for the fabrication of graphene transistors. The theoretical part of the thesis is focused on the understanding the performance limits of the graphene transistor for high frequency operation. We investigate the intrinsic high frequency performance of graphene field effect transistors using a self consistent transport model. The self-consistent transport model is based on a nonuniversal diffusive transport that is governed by the charged impurity scattering. The output and transfer characteristics of graphene field effect transistors are characterized as a function of impurity concentration and dielectric constant of the gate insulator. These experimental and theoretical studies shape the basis of our research on the graphene based radio frequency electronics.Pinçe, ErçağM.S

Optically Induced Nanostructures

Nanostructuring of materials is a task at the heart of many modern disciplines in mechanical engineering, as well as optics, electronics, and the life sciences. This book includes an introduction to the relevant nonlinear optical processes associated with very short laser pulses for the generation of structures far below the classical optical diffraction limit of about 200 nanometers as well as coverage of state-of-the-art technical and biomedical applications. These applications include silicon and glass wafer processing, production of nanowires, laser transfection and cell reprogramming, optical cleaning, surface treatments of implants, nanowires, 3D nanoprinting, STED lithography, friction modification, and integrated optics. The book highlights also the use of modern femtosecond laser microscopes and nanoscopes as novel nanoprocessing tools

Improving the Spectral Bandwidth of Superconducting Nanowire Single-Photon Detectors (SNSPDs)

This work presents a comprehensive investigation of the influence of geometry-dependent factors on performance metrics of superconducting single-photon detectors. With fundamental knowledge, main investigations are focused to extend the spectral bandwidth and to enhance the detection efficiency, especially in infrared range. The developed technology of single-spiral detectors and unconventional electron-beam lithography allows to improve the performance of superconducting detectors