Rozprawy doktorskie na temat „2D material technology”

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, May, 2020
Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages 198-209).
Over the past 50 years, electronics has truly revolutionized our lives. Today, many everyday objects rely on electronic circuitry from gadgets such as wireless earbuds, smartphones and laptops to larger devices like household appliances and cars. However, the size range of electronic devices is still rather limited from the millimeter to meter scale. Being able to extend the reach of electronics from the size of a red blood cell to a skyscraper would enable new applications in many areas including energy production, entertainment, environmental sensing, and healthcare. 2D-materials, a new class of atomically thin materials with a variety of electric properties, are promising for such electronic systems with extreme dimension due to their flexibility and ease of integration. On the macroscopic side, electronics produced on thin films by roll-to-roll fabrication has great potential due to its high throughput and low production cost. Towards this end, this thesis explores the transfer of 2D-materials onto flexible EVA/PET substrates with hot roll lamination and electrochemical delamination using a custom designed roll-to-roll setup. The transfer process is characterized in detail and the lamination of multiple 2D material layers is demonstrated. As exemplary large-scale electronics application, a flexible solar cell with graphene transparent electrode is discussed. On the microscopic side, this thesis presents a 60x60 [mu]m² microsystem platform called synthetic cells or SynCells. This platform offers a variety of building blocks such as chemical sensors and transistors based on molybdenum disulfide, passive germanium timers, iron magnets for actuation, as well as gallium nitride LEDs and solar cells for communication and energy harvesting. Several system-level applications of SynCells are explored such as sensing in a microfluidic channel or spray-coating SynCells on arbitrary surfaces.
by Marek Hempel.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

Questa tesi analizza la possibile implementazione di due tipologie di dispositivi elettronici con funzionalità innovative: dispositivi per la computazione quantistica e transistors a film sottile. Negli ultimi decenni l’industria dei semiconduttori ha portato alla realizzazione di circuiti integrati con milioni di transistors e performance sempre migliori a costi contenuti. Tuttavia, questo processo di miniaturizzazione è giunto a un punto tale che i dispositivi elettronici sono ora composti da pochissimi atomi e ridurne ulteriormente le dimensioni sta diventando sempre più difficile. L’International Technology Roadmap of Semiconductors (ITRS) suggerisce due vie alternative per migliorare le caratteristiche dei dispositivi a partire dalla Front-End-Of-Line. La prima si avvale di nuovi dispositivi sulla base di architetture innovative o dell’utilizzo di diverse variabili di stato (Emerging Research Devices), mentre la seconda punta all’utilizzo di nuovi materiali (Emerging Research Materials). Questa tesi esamina due possibili candidati in questa ottica: i dispositivi per la computazione quantistica su architettura Complementary Metal-Oxide-Semiconductor (CMOS) e i transistors a film sottile basati su un semiconduttore bidimensionale come il MoS2. Da un lato, l’integrazione della computazione quantistica su Si sfrutterebbe il background tecnologico dell’industria dei semiconduttori per implementare su larga scala un nuovo protocollo di computazione dotato di un potenziale enorme e ancora inesplorato. D’altra parte il di-solfuro di molibdeno (MoS2) è intrinsecamente scalabile, in quanto può essere esfoliato fino allo spessore di un singolo strato atomico. Per questo motivo potrebbe essere un semiconduttore ideale per dispositivi elettronici ultrascalati, così come per applicazioni nella sensoristica, nell’optoelettronica e nell’elettronica flessibile. Questo lavoro mostra l’attività svolta al Laboratorio MDM-IMM-CNR nell’ambito del corso di dottorato in Nanostrutture e Nanotecnologie all’Università di Milano Bicocca. Lo sviluppo e l’utilizzo di processi di fabbricazione della nanoelettronica, in particolare la litografia a fascio elettronico (EBL), sono stati parte integrante dell’attività sperimentale dedicata alla realizzazione di dispositivi CMOS-compatibili per la computazione quantistica e per l’integrazione di film sottili di MoS2 in strutture Metal-Oxide-Semiconductor Field-Effect-Transistor (MOS FET). I necessari passi di processo sono stati adeguatamente calibrati e ottimizzati in modo da ottenere dispositivi quantistici basati su Quantum Dots (QD) con dimensioni caratteristiche inferiori a 50 nm. Tali dispositivi sono stati sviluppati con tecnologia Silicon-On-Insulator (SOI), mantenendo così la compatibilità con lo standard della tecnologia CMOS. Dispositivi a singolo donore e con QD di silicio sono stati poi caratterizzati elettricamente a temperature criogeniche (fino a 300 mK). Impulsando i potenziali di gate in modo controllato, è stato possibile studiare fenomeni di tunneling di singoli elettroni su un donore in alti campi magnetici (8T). In modo analogo è stato dimostrato il controllo dello stato di carica di QDs di Si. In particolare, si è osservato l’insorgere di rumore telegrafico associato al movimento di un singolo elettrone tra due QDs. Infine è stato condotto uno studio di fattibilità per l’integrazione su larga scala di un’architettura di computazione quantistica (il cosiddetto hybrid spin qubit) basata su doppi QDs di Si. Sul secondo fronte sono stati realizzati dei MOS FETs a film sottile basati su frammenti di MoS2, ottenuti per esfoliazione meccanica e contattati elettricamente tramite litografia EBL. Tali transistors sono poi stati caratterizzati elettricamente, con particolare attenzione alle proprietà di trasporto di carica e alla spettroscopia delle trappole all’interfaccia con l’ossido.
This work of thesis explores two emerging research device concepts as possible platforms for novel integrated circuits with unconventional functionalities. Nowadays integrated circuits with advanced performances are available at affordable costs, thanks to the progressive miniaturization of electronic components in the last decades. However, bare geometrical scaling is no more a practical way to improve the device performances and alternative strategies must be considered to achieve an equivalent scaling of the functionalities. The introduction of conceptually new devices and paradigms of information processing (Emerging Research Devices) or new materials with unconventional properties (Emerging Research Materials) are viable approaches, as indicated by the International Technology Roadmap of Semiconductors (ITRS), to enhance the functionalities of integrated circuits at the Front-End-Of-Line. The two options investigated to this respect are silicon devices for quantum computation based on a classical Complementary Metal-Oxide-Semiconductor (CMOS) platform and standard Metal-Oxide-Semiconductor Field-Effect-Transistors (MOSFETs) based on MoS2 thin film. In particular, the integration of Quantum Information Processing (QIP) in Si would take advantage of Si-based technology to introduce a completely new paradigm of information processing that has the potential to outperform classical computers in some computational tasks, like prime number factoring and the search in a big database. MoS2, conversely, can be exfoliated up to the single layer thickness. Such intrinsic and extreme scalability makes this material suitable for end-of-roadmap ultrascaled electronic devices as well as for other applications in the fields of sensors, optoelectronics and flexible electronics. This work reports on the experimental activity carried out at Laboratory MDM-IMM-CNR in the framework of the PhD school on Nanostructures and Nanotechnology at Università di Milano Bicocca. Electron Beam Lithography (EBL) and mainstream clean-room processing techniques have been intensively utilized to fabricate CMOS devices for QIP on the one hand and to integrate mechanically exfoliated MoS2 flakes in a conventional FET structure on the other hand. After a careful calibration and optimization of the process parameters, several different Quantum Dot (QD) configurations were designed and fully realized, achieving critical dimensions under 50 nm. Such device architectures were developed on a Silicon-On-Insulator (SOI) platform, in order to eventually access a straightforward integration into the CMOS mainstream technology. Si-QDs and donor-based devices have been then tested by electrical characterization techniques at cryogenic temperatures down to 300 mK. In detail, single electron tunneling events on a donor atom have been controlled by pulsed-gate techniques in high magnetic fields up to 8T, providing a preliminary characterization for the initialization procedure of donor qubits. The control of the charge states of Si-QDs have been also demonstrated by means of stability diagrams as well as the analysis of random telegraph noise arising from single electron tunneling between two QDs. Finally, a feasibility study for the large scale integration of quantum information processing was done based on a double QD hybrid qubit architecture. On the other side, MoS2 thin film transistors have been made by mechanical exfoliation of crystalline MoS2 and electrodes definition by EBL. Electrical characterization was performed on such devices, with a particular focus on the electrical transport in a FET device and on the spectroscopy of interface traps, that turns out to be a limiting factor for the logic operation.

La microscopie optique sur substrats antireflets est un outil de caractérisation simple et puissant qui a notamment permis l'isolation du graphène en 2004. Depuis, le domaine d'étude des matériaux bidimensionnels (2D) s'est rapidement développé, tant au niveau fondamental qu'appliqué. Ces matériaux ultraminces présentent des inhomogénéités (bords, joints de grains, multicouches, etc.) qui impactent fortement leurs propriétés physiques et chimiques. Ainsi leur caractérisation à l'échelle locale est primordiale. Cette thèse s'intéresse à une technique récente de microscopie optique à fort contraste, nommée BALM, basée sur l'utilisation originale de couches antireflets très minces (2-5 nm) et fortement absorbantes (métalliques). Elle a notamment pour but d'évaluer les mérites de cette technique pour l'étude des matériaux 2D et de leur réactivité chimique. Ainsi, les différents leviers permettant d'améliorer les conditions d'observation des matériaux 2D ont tout d'abord été étudiés et optimisés pour deux matériaux modèles : l'oxyde de graphène et les monocouches de MoS₂. L'étude de la dynamique de dépôt de couches moléculaires a notamment permis de montrer à la fois l'extrême sensibilité de BALM pour ce type de mesures et l'apport significatif des multicouches antireflets pour l'augmentation du contraste lors de l'observation des matériaux 2D. L'un des atouts principaux de BALM venant de sa combinaison à d'autres techniques, nous nous sommes particulièrement intéressés au couplage de mesures optiques et électrochimiques pour lesquelles le revêtement antireflet sert d'électrode de travail. Nous avons ainsi pu étudier optiquement la dynamique de réduction électrochimique de l'oxyde de graphène (GO), l'électro-greffage de couches minces organiques par réduction de sels de diazonium sur le GO et sa forme réduite (r-GO), ainsi que l'intercalation d'ions métalliques entre feuillets de GO. En combinant versatilité et fort-contraste, BALM est ainsi établi comme un outil prometteur pour l'étude des matériaux 2D et en particulier pour la caractérisation locale et in situ de leur réactivité chimique et électrochimique
Optical microscopy based on anti-reflective coatings is a simple yet powerful characterization tool which notably allowed the first observation of graphene in 2004. Since then, the field of two-dimensional (2D) materials has developed rapidly both at the fundamental and applied levels. These ultrathin materials present inhomogeneities (edges, grain boundaries, multilayers, etc.) which strongly impact their physical and chemical properties. Thus their local characterization is essential. This thesis focuses on a recent enhanced-contrast optical microscopy technique, named BALM, based on ultrathin (2-5 nm) and strongly light-absorbing (metallic) anti-reflective layers. The goal is notably to evaluate the benefits of this technique for the study of 2D materials and their chemical reactivity. The various levers to improve 2D materials observation were investigated and optimized for two model materials: graphene oxide and MoS₂ monolayers. The investigation of molecular layer deposition dynamic notably showed the extreme sensitivity of BALM for such measurements and the significant contribution of multilayers anti-reflective coatings to enhance contrast during the observation of 2D materials. One of the main assets of BALM comes from its combination to other techniques. We particularly considered the coupling between optical measurements and electrochemistry for which the anti-reflective layer serves as working electrode. We investigated optically the dynamic of electrochemical reduction of Graphene Oxide (GO), the electrografting of organic layers by diazonium salts reduction on GO and its reduced form (rGO), as well as the intercalation of metallic ions within GO sheets. By combining versatility and high-contrast, BALM is established as a promising tool for the study of 2D materials, especially for the local and in situ characterization of their chemical and electrochemical reactivity

The terahertz (THz) frequency range of the electromagnetic spectrum is usually defined in the range between 0.1 THz and 10 THz, corresponding to wavelengths in the interval from 3 mm to 30 µm, lying in-between the infrared and the microwave spectral regimes. In recent years, the progress of THz technology has fostered interdisciplinary research in spectroscopy and tomography to map macroscopic systems, (chemical detection and imaging, amongst others) or microscopic ones, such as nanoparticles and nanowires on either static or dynamic timescales. THz radiation is commonly generated with photoconductive emitters, semiconductor diodes, free-electron lasers, photomixing, and beating of a pump and idler signal from non-linear crystals. These approaches are often bulky, expensive or with limited optical powers. The breakthrough demonstration of quantum cascade lasers operating in the far-infrared, and based on quantum engineered heterostructures, paved the way to the development of much more compact, efficient and powerful semiconductor THz sources. Thanks to the atomic-layer resolution ensured in the heterostructure growth by molecular beam epitaxy (MBE), very accurate designs can be implemented via a proper sequence of quantum barriers and quantum wells. In this way, sharp discontinuities in the conduction and valence bands edges are created, in order to manipulate the electron energy levels and wavefunction localization, and to provide optical intersubband transitions at the desired frequencies. [...]

As part of the EU SmartVista project to develop a multi-modal wearable sensor for health diagnostics, field-effect transistor (FET) based biosensors were explored, with glucose as the analyte, and carbon nanotubes (CNTs) or monolayer MoS2 as the semiconducting sensing layer.  Numerous arrays of CNT-FETs and MoS2-FETs were fabricated by photolithographic methods and packaged as integrated circuits.  Functionalization of the sensing layer using linkers and enzymes was performed, and the samples were characterized by atomic force microscopy, scanning electron microscopy, optical microscopy, and electrical measurements. ON/OFF ratios of 102 p-type and < 102 n-type were acheived, respectively, and the work helped survey the viability of realizing such sensors in a wearable device.
EU Horizon 2020 - SmartVista (825114)

In this work, we explored one material from the broad family of 2D semiconductors, namely WSe2 to serve as an enabler for advanced, low-power, high-performance nanoelectronics and optoelectronic devices. A 2D WSe2 based field-effect-transistor (FET) was designed and fabricated using electron-beam lithography, that revealed an ultra-high mobility of ~ 625 cm2/V-s, with tunable charge transport behavior in the WSe2 channel, making it a promising candidate for high speed Si-based complimentary-metal-oxide-semiconductor (CMOS) technology. Furthermore, optoelectronic properties in 2D WSe2 based photodetectors and 2D WSe2/2D MoS2 based p-n junction diodes were also analyzed, where the photoresponsivity R and external quantum efficiency were exceptional. The monolayer WSe2 based photodetector, fabricated with Al metal contacts, showed a high R ~502 AW-1 under white light illumination. The EQE was also found to vary from 2.74×101 % - 4.02×103 % within the 400 nm -1100 nm spectral range of the tunable laser source. The interfacial metal-2D WSe2 junction characteristics, which promotes the use of such devices for end-use optoelectronics and quantum scale systems, were also studied and the interfacial stated density Dit in Al/2D WSe2 junction was computed to be the lowest reported to date ~ 3.45×1012 cm-2 eV-1. We also examined the large exciton binding energy present in WSe2 through temperature-dependent Raman and photoluminescence spectroscopy, where localized exciton states perpetuated at 78 K that are gaining increasing attention for single photon emitters for quantum information processing. The exciton and phonon dynamics in 2D WSe2 were further analyzed to unveil other multi-body states besides localized excitons, such as trions whose population densities also evolved with temperature. The phonon lifetime, which is another interesting aspect of phonon dynamics, is calculated in 2D layered WSe2 using Raman spectroscopy for the first time and the influence of external stimuli such as temperature and laser power on the phonon behavior was also studied. Furthermore, we investigated the thermal properties in 2D WSe2 in a suspended architecture platform, and the thermal conductivity in suspended WSe2 was found to be ~ 1940 W/mK which was enhanced by ~ 4X when compared with substrate supported regions. We also studied the use of halide-assisted low-pressure chemical vapor deposition (CVD) with NaCl to help to reduce the growth temperature to ∼750 °C, which is lower than the typical temperatures needed with conventional CVD for realizing 1L WSe2. The synthesis of monolayer WSe2 with high crystalline and optical quality using a halide assisted CVD method was successfully demonstrated where the role of substrate was deemed to play an important role to control the optical quality of the as-grown 2D WSe2. For example, the crystalline, optical and optoelectronics quality in CVD-grown monolayer WSe2 found to improve when sapphire was used as the substrate. Our work provides fundamental insights into the electronic, optoelectronic and quantum properties of WSe2 to pave the way for high-performance electronic, optoelectronic, and quantum-optoelectronic devices using scalable synthesis routes.

In this this thesis, I have studied dynamics of the two-dimensional (2D) material based NEMS resonators with resonant frequency ranging typically from 10 MHz to 100 MHz. The experiment involved fabrication of the suspended nano-scale devices both with global and local gate architectures. The experiments focused on parametric manipulation of MoS2 drum resonator using electrical actuation and detection schemes. This study demonstrated parametric ampli cation in the NEMS at non-cryogenic temperature and discussed effects of During non-linearity on the parametric gain. Further, multimodal coupling among the mechanical modes in the drum resonator was also demonstrated

In this thesis, we presented different contributions towards the development of 2D material technology. Firstly the realization of desired dimensions over singlecrystal high-quality MoS2 material through dry etching techniques. SF6 plasma induces large residue over the material, inhibiting the application despite its advantage over SiO2 etch selectivity. On the other hand, CHF3 plasma is shown to give a well-controlled etching process with its relatively lower etch rate than SF6 plasma. However, under over-etch conditions, plasma is observed to introduce two significant challenges. The first is the doping induced by high-energy fluorine radicals diffused through resist and the TMD material. The second one is the crystal damage caused by plasma from the side walls elimination of these two challenges required highly controlled etching. Optimized and controlled etching using CHF3 plasma resulted in transistors’ fabrication without compromising the performance compared to reference transistors. The same controlled etching process is observed to apply to other TMDs as well. Transistors implemented with such an approach have shown no degradation in performance metrics than standard devices, thus generalizing the process applicability to all TMDs.

Atomically thin 2D materials have ushered in a new era in the fi eld of nano-science and tech- nology and have been translated to notable advancements in the design of sensors, optoelectronic devices, exible electronics. These atomically thin materials are predicted to replace conven- tional bulk materials, Si and Ge, for transistor channels and extend the complementary metal oxide semiconductor technology road-map beyond the deca-nanometer regime. Constant efforts are being made to fabricate devices based on some of the recently discovered van der Waal's materials such as graphene, hexagonal boron nitride, MoS2, phosphorene. Apart from these, a large number of novel 2D materials and their derivatives are being constantly explored through both experiments and density functional theory analysis. In order to narrow down the mate- rial and design selection space for time- and cost-heavy experimental device fabrication, atomic level density functional theory (DFT) calculations need to be coupled with device-level physics models. Thus, we propose a multiscale computational framework bridging first principles based DFT calculations with device physics simulations. Under this framework, we start with crys- tallographic information of a 2D material and perform DFT simulations to extract important electronic parameters, such as effective mass, band gap, real and complex band dispersion, and phonon spectrum. This is followed by construction of the material hamiltonian based on the DFT extracted parameters. Next, the hamiltonian is used to perform self-consistent solution of the Schrodinger and the Poisson's equations through the non-equilibrium Green's function approach in order to describe the complex, spatially heterogeneous intrinsic carrier transport and resulting device performance in both ballistic and dissipative regimes. Modeling studies on three devices: (i) monolayer germanane metal oxide semiconductor fi eld effect transistors (MOSFETs), (ii) monolayer GeSe based tunneling field effect transistor (TFET), and (iii) phosphorene based MOSFET and TFET, will be presented in the thesis and their design and performance limits will be evaluated to guide future material selection and device fabrication.

In 2020, Apple introduced its most advanced laptop that has the A14 Bionic processor. The very first processor, Intel’s 4004, was launched in 1971 and had a transistor (the basic building block of a processor) density of ~ 205 transistors/mm2. Compared to that, Apple’s A14 Bionic processor comes with an astonishingly large ~ 125 million transistors/mm2. Such a remarkable evolution in silicon transistor technology has occurred in the last five decades. This has arguably been the most rapidly growing field in the 300,000 thousand years old history of modern humans. Computational power, functionality and speed of a processor largely depend on the size of the transistor. Aggressive transistor scaling, based on Dennard’s law, in all aspects has resulted in today’s transistors with minimum feature size of 5 nm which is almost 2000 times smaller than that of the most primitive, commercially available transistors. Perpetual demand to perform high speed data processing and cost minimization has led to dramatic advancement in transistor design and process technology which has clearly served the purpose, so far. Our hunger for further improvement in data processing, especially in the era of Internet of Things (IoT) and big data, has kept us looking into the future of transistor scaling. Unfortunately, transistor size has started to hit a fundamental limit and highly scaled transistors with tens or hundreds of atoms are not able to satisfactorily process data. In other words, a transistor fabricated from the current state-of-the-art 5 nm process would fail to even operate like a transistor if it were further scaled down. Apparently, for further improvement in processing speed and functionality, it seems tough to move into the atomic regimes of transistor size as long as silicon is in use. Another way to achieve better performance of our systems is to stay with the already established silicon technology and focus on better system level integration of processors and/or come-up with a paradigm shift from using the current von-Neumann computer architecture to a more energy efficient one. It turns out that atomically thin 2-dimensional (2D) materials can prove to successfully replace silicon in transistors, owing to their robustness against short channel effects which otherwise degrade performance of ultra-scaled silicon devices. In order to qualify 2D materials for a possible future technology, it is important to develop insights on them through a constant effort in form of fundamental as well as technological research. Although, moving from bulk silicon to 2D appears to be the right approach to realize further transistor scaling, it has been way too challenging to address and eventually solve three major challenges – large contact resistance, lack of industrially viable doping techniques and lack of large area single crystal growth methods, that prevent quick technology development on 2D materials for transistor applications. More than 200 2D materials naturally occur, from which transition metal dichalcogenides (TMDCs) are so far considered to be most suitable for ultra-scaled transistors, owing to their non-zero finite electronic bandgap and large electron effective mass. Since the fabrication of first TMDC transistor in 2011, enormous effort has been put in to identify ways to solve concerns related to large contact resistance, polarity control through doping and wafer-scale single crystal growth. As a result, device-level optimizations on TMDCs have enabled highly effective contact, channel and dielectric engineering techniques to address underlying problems associated with TMDCs. One of these contact engineering techniques is H2S treatment of TMDCs at temperature much lower than that during growth process. Unlike most other techniques to degenerately dope the transistor which lead to improvement in drive current along with significantly degraded gate control and OFF state performance, the method to expose contacts and channel with H2S, developed during this work, turns out to have improved almost all aspects of transistor performance, besides being a dry and scalable process [1]. Remarkable ON state performance improvement has been realized along with orders of magnitude improvement in the drain current modulation of the transistor [1]. It is found that presence of unique chalcogen impurities at the contact and channel lead to much better overlap of atomic orbitals of TMDC and metal atoms which results in drastically improved carrier injection across the interface [1]. When the same contact engineering technique is used with tungsten diselenide (WSe2), the interface becomes greatly dependent on the metal contact in use [2]. Choice of contact turns out to be critical in the presence of Sulfur interstitial atom-induced unique gap states (DIGS), as these impurity atoms alter the nature of metal-induced gap states (MIGS) [2]. It is found that contacts to WSe2 can be realized selectively to obtain desirable transistor polarity. Owing to lack of implantation techniques for 2D TMDCs, doping and polarity control has remained a bottleneck for long. Realization of n and p-type transistors on the same substrate using this method could result in a complete CMOS fabrication process development on WSe2. The defect states added by H2S exposure, universally result in hole current improvement for all conventional TMDCs [3]. Extracting hole current from all TMDCs is not trivial especially in CVD-grown monolayer MoS2 due to its large bandgap and sulfur vacancy concentration. H2S induced sulfur interstitial defects are found to lie close to the valence band edge due to which fermi-level is pinned closer to valence band thereby reducing barrier height for hole conduction [3]. While one part of this work is focused on developing CMOS circuits on TMDCs using H2S treatment of contacts, exploring long-term electrical reliability of TMDC systems has also been an important part. So far, reliability studies on TMDCs have remained confined to studying ambient induced effects and standard high field phenomena like velocity saturation, avalanche breakdown, negative differential resistance etc. Such reports on reliability physics of TMDCs are not only limited but also restricted to observing breakdown and high field phenomena, instead of identifying progressive response to high-field and root cause of device performance degradation to eventual failure. In this work, TMDC FETs are subjected to long-term DC electrical stress to identify electrothermal transport induced material perturbations [4, 5]. Insights developed on low-field and high-field transport in TMDCs suggest that channel conductance improves when devices are stressed for long durations due to introduction of sulfur vacancy-like defects in the channel. Negative shift in threshold voltage and decrease in channel potential after electrical stress along with red shift in Raman out-of-plane mode and increase in trion formation, observed in photoluminescence spectra, point towards the fact that a prolonged transistor operation results in a piezoelectric response due to which weak hopping transport and enhanced screening of charged impurities are observed [4, 5, 6]. Classical interpretation of Raman spectra and channel potential measurements using kelvin probe force microscopy (KPFM) reveal that these defects are in fact perturbed Mo-S bonds [6]. Besides electrical transport, sufficient evidence of improved thermal transport through the channel is found [6]. Our findings trigger an interesting visualization of how TMDCs would naturally adapt, through piezoelectric response, to high-field electrical stress in a way that makes them better electrical and thermal conductors [6]. Remarkable similarities between long-term electrical stress and hysteresis induced resistance switching have been particularly encouraging. Both are found to have induced resistance switching in MoS2 [7]. Based on polarity dependent response to electrical stress, it is found that, MoS2 based RRAMs subjected to certain electrical stress conditions start exhibiting improved binary switching performance [7]. As a result, an electroforming method has been developed that triggers better binary memory switching characteristics. Moreover, charge transport mechanism in MoS2 based gated RRAMs along with forming voltage are found to influence resistance switching performance, substantially [7]. This work has broadly focused on improving contact and channel properties of TMDC based transistors, polarity control and hole current improvement followed by development of CMOS circuit fabrication process on TMDCs, especially WSe2, and developing insights on physics of material-device reliability and resistance switching in TMDCs. For TMDC transistor technology to flourish, it is important not only to fabricate and characterize devices and circuits but also to assess the reliability quotient. Long term device and circuit reliability of TMDCs has remained un-addressed and must be thoroughly understood if TMDC based electronics is to be pursued in future. Besides adding to technological advancement of 2D material electronics through realization of polarity specific devices and CMOS circuits using a scalable process, this work would significantly contribute to fundamental knowledge base for TMDCs by familiarizing the 2D community with the role of sulfur interstitial defects, self-adapting behavior of MoS2 against electrical stress and transport dependent resistance switching mechanism.

The transistor scaling is witness to many extraordinary inventions during its consecutive miniaturization. The journey began from Dennard’s classical constant field scaling, crossing through the milestones like strain engineering, high ‘k’ gate dielectric, ultrathin body transistor (UTB), silicon on insulator (SOI), and multi-gate 3D architectures, and continues in the form of advanced FinFET technology. However, further downscaling is sensing a dead-end because of the various challenges due to fundamental limitations of silicon, the building material of the transistor. Among these, two significant challenges are mobility degradation due to boundary scattering by surface dangling bonds and loss of gate control due to quantum confinement. To keep downscaling alive, the research community is looking for an alternate material that can mitigate these issues and consist of better fundamental properties from silicon like intrinsic mobility, thermal conductivity, optical response, and mechanical strength. Two-dimension material (2D material) shows great potential for next-generation electronic material and provides multiple avenues for further exploration. The material is one or a few atomic-layer 2D thin sheets of covalently bonded atoms stacked using weak van der Walls (vdW) forces in the third dimension. The lack of surface dangling bond and atomic-scale thickness mitigates the significant challenges of low mobility and inadequate gate control of the silicon material, respectively. Presently, more than 150 materials exist in the 2D material family. Graphene, Transition Metal Dichalcogenides (TMDs), and Phosphorene are well ahead of other family members due to their extraordinary properties, thereby plenty of investigations. Despite these properties, the materials have several roadblocks to their technological application. Opening bandgap and minimizing contact resistance are significant challenges in graphene, and reducing contact resistance and mature growth and reliability are big concerns for the TMDs. Phosphorene, which has hybrid properties of graphene and TMDs, is relatively less explored due to its spontaneous degradation in the ambient environment. Understanding and mitigating its spontaneous ambient degradation is still an open challenge for the electronics and material research communities across the globe. Keeping in mind these limitations, we explore the problems one by one and find their reasonable solutions. Based on DFT investigations, the discussion begins with a proposal for a reliable direct bandgap opening technique in graphene. Graphene possesses zero bandgap due to its highly symmetric hexagonal structures, which touch its π and π* orbitals’ energy states near the Fermi level, known as the Dirac point. Breaking this symmetry by carbon vacancy or Stone-Wales (SW) defects opens the bandgap at the Dirac point. However, the carbon vacancy creates unwanted mid-gap (trap) states, attributed to unbound orbitals of the nearest unsaturated carbon atoms at the vacant site. Moreover, the unsaturated carbon atoms react with ambient gases like oxygen, making graphene unstable. Interestingly, hydrogenation or fluorination of the unsaturated carbon atoms near the vacant site helps prevent the trap states while contributing to promising direct band gaps in graphene. The opened bandgap is tunable in the infrared regime and persists for different sizes and densities of hydrogenated or fluorinated patterns. The proposed approach is thermodynamically favorable as well as stable. The next work demonstrates the contact resistance reduction of graphene with palladium (Pd) by carbon vacancy engineering. The discussion begins with fundamental insights into the Pd-graphene interface and carbon vacancy-assisted contact resistance reduction using Density Functional Theory (DFT), followed by its experimental validation by various processes. Our study reveals significant interaction of Pd with graphene. Their orbitals overlap leads to potential barrier lowering at the interface, which can be reduced further by bringing graphene closer to the bulk Pd using carbon vacancy engineering at the contacts. Thus, the carbon vacancy-assisted barrier modulation reduces contact resistance by increasing carrier transmission probabilities at the interface. The theoretical findings have been emulated experimentally by carbon vacancy engineering at the graphene Field Effect Transistors (FETs). Different contact engineered graphene devices with Pd contacts shows significant contact resistance reduction, measuring as low as ~78 Ω-µm at room temperature. The contact resistance shows a ‘V’ shape curve as a function of defect density. The optimum contact resistance achieved is significantly lower than their pristine counterpart, as predicted by the theoretical estimates. Subsequently, the journey turns towards an atomic level investigation of phosphorene ambient degradation using the first-principles Molecular Dynamics (MD) simulations in the following work. The study reveals that the oxygen molecule dissociates spontaneously over pristine phosphorene in the ambient environment resulting in an exothermic reaction, which is boosted further by increasing partial pressure, temperature, and the presence of oxygen free radicals. The surface reaction is mainly due to lone pair electrons of phosphorous atoms, making the degradation directional and spontaneous under oxygen atoms. Furthermore, water molecules play a vital role in the degradation process by changing the reaction dynamics path of phosphorene-oxygen interaction and reducing activation energy and reaction energy due to its catalyzing action. In addition, phosphorous vacancy acts as an epicenter for oxidation. The oxygen attacks directly over the vacant site and reacts faster than its pristine counterpart. As a result, phosphorene edges resembling extended vacancy are prominent reaction sites that oxidize anisotropically due to different bond angle strains. The edge initiated spontaneous degradation, and rapid oxidation under the free radicals are validated using consistent probing under an optical microscope and Transmission Electron microscope (TEM), respectively. After material exploration, the next work reveals a unique reliability issue in the Phosphorene FETs. Here, we investigate the role of channel excess holes (due to inversion) in phosphorene degradation using the first-principles MD computations and electrical and Raman characterization. The results show that phosphorene degrades faster under negative gate bias (excess hole) than in pristine conditions (unbiased). The rapid degradation is mainly due to the enhanced chemical interaction of oxygen with the available hole in the channel. The computational findings are experimentally verified over phosphorene FETs. Compared to the unbiased condition, the devices show a faster change in drain current and fast decay of all primary Raman peaks in the ambient environment under negative gate bias. At the risk of ambient degradation, phosphorene thin flakes are to be identified quickly using a non-destructive technique like Raman to make their FETs for further exploration. The next work shows that the Raman signature of a low-frequency interlayer out-of-plane phonon mode, known as breathing mode, helps in identifying the thin flake quickly. Further, the work talks about thermal evolution and estimates the first-order temperature coefficient of different breathing modes. All the captured modes show a negative temperature coefficient around -0.002-0.003 cm-1/K across different flake thicknesses. Moreover, a closer look at the thermal evolution reflects that the modes follow three-phonon and four-phonon process dominant scattering phenomena at low and high-temperature ranges. The three-phonon process scattering is dominant below ~100 K, shifting to four-phonon process dominant scattering beyond ~150 K. Besides, the work discusses pristine instrumental error in the Raman shift characterization and suggests a mitigation method using Stokes and Anti-stokes scattering lines. Finally, the last work discusses the interactions of different metals (Au, Cr, Ni, and Pd) with TMDs (MoS2, MoSe2, WS2, and WSe2). The work reflects that Au has a weak interaction with all the TMDs. Thus, it stays more than 2 Å away from the TMDs surfaces. However, other metals show strong chemistry with TMDs. Due to weak interaction, Au offers very few metal-induced gap states (MIGS) in all the TMDs. On the other hand, metals like Cr, Ni, and Pd flood many MIGS in the bandgap region of the TMDs. During interactions, all the metals offer n-type doping to TMDs. Chalcogen vacancy enhances the interaction of the metals with all the TMDs. The vacancy leaves the unbounded orbitals, which bond strongly with the approaching metals. The bonding enhancement reduces the metal-TMDs distances that can be used in contact resistance engineering in their bulk counterparts. Chalcogen interstitial impurity also enhances the bond strength of some metal-TMDs interfaces. Our journey helps in overall technological advancement in the leading 2D materials. The work digs into the leading roadblocks like contact resistance reduction and method of bandgap opening in graphene, understanding the degradation issue of phosphorene at the material and device level, and exploring metal-TMDs interactions for their contact resistance engineering.

Wearable sensors, as the name implies, are devices that can be donned onto the body in order to continuously detect, monitor and analyze various signals generated by the subject and the immediate surroundings. Applications of these sensors span over the vast domains of healthcare, athletics, automation and robotics. Conventional wafer-based electronics are brittle and rigid. Wearable devices demand new materials that provide mechanical liberty in terms of flexibility and stretchability with superior functionalities. When the physical dimensions of materials are reduced to the nano scale regime, they exhibit remarkable change in their properties compared to their bulky counterparts. Most widely explored nano materials include 0D, 1D and 2D structures synthesized via advanced processing and chemical routes. The recent progress in nano materials and fabrication methodologies provide new routes to develop sensors that can be bent, stretched, twisted, compressed, or deformed into arbitrary shapes. My research work is focused on creative utilization 2D materials to develop wearable sensors with the aim of providing seamless user experience. Functionalized inks of 2D materials offer versatile fabrication methods like coating, printing, stamping and patterning for development of flexible sensors that are industrially scalable. Present thesis aims to provide insights into use of graphene and MXene inks for realization of novel wearable devices. Specific focus has been set on integration of solution processed graphene on fabric for e-textile applications, ultrathin graphene-based tattoo sensors for proximity sensing studies and skin conformal MXene tattoo for physiological sensing. As fabrication of next generation sensors for wearable applications pose their own unique challenges, my research work aims to deliver innovative methods to address these issues.

Copper (Cu) has been used as the main conductor in interconnects due to its low resistivity. However, because of its high diffusivity, diffusion barriers/liners (tantalum nitride/tantalum; TaN/Ta) must be incorporated to surround Cu wires. Otherwise, Cu ions/atoms will drift/diffuse through the inter-metal dielectric (IMD) that separates two distinct interconnects, resulting in circuit shorting and chip failures. The scaling limit of conventional Cu diffusion barriers/liners has become the bottleneck for interconnect technology, which in turn limits the IC performance. The interconnect half-pitch size will reach ~20 nm in the coming sub-5 nm technology nodes. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be > 4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultra-scaled interconnect will be compromised by the thick bilayer. Therefore, two dimensional (2D) layered materials have been explored as diffusion barrier alternatives owing to their atomically thin body thicknesses. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued.

The objective of the thesis is to develop a 2D barrier/liner that overcomes the issues mentioned. Therefore, we first visit various 2D layered materials to understand their fundamental capability as barrier candidates through theoretical calculations. Among the candidates, hexagonal-boron-nitride (h-BN) and molybdenum disulfide (MoS₂) are selected for experimental studies. In addition to studying their fundamental properties to know their potential, we have also developed techniques that can realize low-temperature-grown 2D layered materials. Metal-organic chemical vapor deposition (MOCVD) is adopted for the synthesis of BEOL-compatible MoS₂. The electrical test results demonstrate the promises of integrating 2D layered materials to the state-of-the-art interconnect technology. Furthermore, by considering not only diffusion barrier properties but also liner properties, we develop another 2D layered material, tantalum sulfide (TaS_x), using plasma-enhanced chemical vapor deposition (PECVD). The TaS_x is promising in both barrier and liner aspects and is BEOL-compatible. Therefore, we believed that the conventional TaN/Ta bilayer stack can be replaced with an ultra-thin TaS_x layer to maximize the Cu volume for ultra-scaled interconnects and improve the performance. Furthermore, Since via resistance has become the bottleneck for overall interconnect performance, we study the vertical conduction of TaS_x. Both the intrinsic and extrinsic properties of this material are investigated and engineering approaches to improve the vertical conduction are also tested. Finally, we explore the possibilities of benefiting from 2D materials in other applications and propose directions for future studies.

Nowadays, two-dimensional (2D) materials have stimulated intensive research due to their intriguing physical properties and excellent electronic application. van der Waals (vdW) semiconductors are attractive for electrically controllable carrier confinement, combined with the diverse nature of 2D materials that enable superior electrostatic control. Molecular interaction in two-dimensional van der Waals interfaces has drawn tremendous attention for extraordinary materials characteristics. This work encompasses molecular responses study of various atomically thin heterostructures made of molybdenum disulfide (MoS2), graphene, and hexagonal boron nitride (h-BN). The defect induced interfacial states are created in an atomically thin two-dimensional MoS2 channel by underlying a narrow pattern of a graphene layer in a field-effect transistor. The presence of interfacial states in the channel leads to a conductance fluctuation. Its magnitude is modulated nearly three-order of magnitude at room temperature using the nitrogen dioxide gas molecules in the subthreshold region. The study provides a systematic approach to establishing a correlation between modulated conductance fluctuation and the molecular concentration up to parts-per-billion. First-principles density functional theory further explains the role of unique interfacial configuration on conductance fluctuation. Therefore, our study demonstrates an experimental approach to induce charge-state for the modulation of carrier concentration and exploits the role of defect induced interfacial states in atomically thin interfaces for the molecular interaction. So far, sensing molecular interaction characteristics have been exploited extensively to reach detection limit to a few parts-per-billion (ppb) of molecules. Far less attention is given to the evolution of persistent current state due to molecular exposure. Our study focuses on the molecular memory operation of MoS2-graphene heterostructure based field-effect transistor. The metastable resistance state of the device due to external perturbation of molecules is tuned to get a near relaxation free current state at a much lower molecular concentration of 10 ppb to facilitate non-volatile memory features for molecular memory operation. An ultrafast switching operation in milli-second order was achieved at room temperature for the fastest recovery obtained so far in any molecular sensor. The process is co-controlled both by molecular as well as external charge density. Along with the interface property, the proper stacking of the vdW materials can be adapted for real-time room temperature applications. Here, we investigate transport properties of a multilayer MoS2/h-BN heterojunction via a tunable electrostatic barrier using artificially designed different local gates width. A systematic transport characteristic revealed that the charge transfer switching (CTS) is a bias dependent conductance phenomenon with highly depends on local gate width and bias in the channel due to the gating constriction with an ON-OFF ratio of ~103. Furthermore, the CTS can be precisely controlled upon molecular interaction through electrotuneable gated constriction. Interestingly, the large-conductance change (102) due to the 100 ppb level of gas concentration leads to a complete switching off the channel can act as a molecular switch. Further, the mechanism of molecular CTS in the device was explained by the Fermi level shift using the first-principle calculations with nitrogen dioxide molecules adsorbed in MoS2. This precise tunability of CTS has not been previously reported in any atomically thin 2D materials. Results of molecular interaction study in van der Waals materials contribute to the research of various other types of heterostructures and can be further applied for mesoscopic transport phenomena for molecular memory, switching operation at room temperature.

Atomically thin two-dimensional (2D) materials have attracted extensive research interest since the journey started with the successful isolation of graphene in 2004. 2D materials have shown remarkable advancement in the design of the sensor, optoelectronic devices, and flexible electronics. Continuous efforts are being made to fabricate the electronic devices based on these two-dimensional layered materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides (TMD), phosphorene etc. Recently a new 2D material, tellurene has joined the 2D material family and its potential application is demonstrated through the fabrication of metal-oxide-semiconductor field effect transistor. This new 2D material has similar properties that of black phosphorene (BP) however shows better environment stability and lower synthesis cost. Bulk tellurium (Te) is one of the chalcogen group-VI materials possessing a unique helical chain structure with a narrow bandgap of 0.35eV, however the bandgap increases to 1.23eV at monolayer limit. Understanding electronic properties of 2D material interfaces with metal is crucial for designing any electronic devices. Although most of the 2D materials offer dangling-bond free, naturally passivated surface, unusual Fermi-level pinning is observed while interfacing with metals with wide range of work functions. First principles-based calculations based on density functional theory (DFT) provide us atomistic insight to the electronic properties of such 2D material-metal interface, which is difficult to probe even with most sophisticated experimental setup. In this work using DFT calculations we show that tellurene exhibits a very unusual Ohmic nature while interfacing with metals commonly used in experiments. The origin of such violation of classical Schottky-Mott rule is found to be hidden in its electronic localization function. Since a Schottky diode is also a useful device which finds several applications in an electronic system, we explore the possibility of realizing a tellurene based Schottky diode. We surprisingly observed that a Schottky barrier between tellurene and metal could be induced by insertion of graphene, a technique earlier used to reduce the Schottky barrier height between Molybdenum-di-sulphide and various metals. Tellurene is physiosorbed in the graphene interface, insertion of graphene screens the extent of hybridization between the tellurene and metal and thus a Schottky barrier is formed. A Schottky barrier can be useful in the design of tellurene based photodetector, sensor, and in high frequency switching response

Two-dimensional materials are now being considered as viable options for CMOS (complementary metal-oxide-semiconductor) technology extension due to their diverse electronic and opto-electronic properties. However, introduction of any new material in the process integration phase of technology development in the semiconductor industry is an expensive and time-consuming affair. It is also difficult to select an appropriate 2D material from the plethora without assessing their performance at circuit level. Thus, first-principles-based multiscale models that enable systematic performance evaluation of emerging 2D materials at device and circuit levels solely from their crystallographic information are in great demand. In this thesis, such an atom-to-circuit modeling framework, addressing three different levels of abstraction (viz. material, device, and circuit), has been demonstrated. Firstly, the model was developed for a van der Waal’s heterostructure (vdWH) based all-2D metal-insulator-semiconductor field-effect transistor (MISFET), comprising of vertically stacked semi-metallic graphene, insulating hexagonal boron nitride (hBN) and semiconducting monolayer molybdenum disulphide (MoS2). Our physics-based compact model demonstrates the effects of band gap opening in graphene due to its sublattice symmetry breaking interactions with underlying hBN layer. This apart, proposed model captures the effects of semiconductor doping and the band gap variation of graphene at device and circuit levels. The model equations were thereafter implemented in a professional circuit simulator using its Verilog-A interface to facilitate design and simulation of integrated circuits. Secondly, the scope of the proposed model was further extended to capture the inertia of the charge carrier in 2D transistors operating at very high frequencies, typically greater than its intrinsic cut-off frequency. Taking phosphorene as a prototypical example, a multiscale model was developed for 2D transistors that can predict the channel-orientation-dependent high-frequency performance of devices and circuits solely from the crystallographic information of their constituent materials. The material-specific parameters obtained from density functional theory calculations were used to develop a continuity equation based non-quasi-static model to gain insight into the high-frequency behaviours. It was found that channel orientation has strong impact on both the low and high frequency transconductance parameters, however it affects only the high-frequency component of transcapacitances. The model was then implemented in industry-standard circuit simulator using the relaxation-time-approximation technique and simulations of analog and digital circuits were carried out to demonstrate its applicability for near cut-off frequency circuit operation. Finally, the idea was also exercised for modeling novel quantum materials like 2D topological insulators and it was shown that the proposed analytical approach could be useful for developing compact models of topological insulator field effect transistors. A Hamiltonian based continuum model was used to unveil the bandgap opening in the edge-state spectra of spatially confined monolayer 1T' molybdenum disulphide (MoS2), molybdenum diselenide (MoSe2), tungsten disulphide (WS2) and tungsten diselenide (WSe2). It was shown that the application of a perpendicular electric field effectuates a topological phase transition in these materials, and it can simultaneously modulate the band gaps of both bulk and edge-states for finite-width samples. The tuneable edge conductances, as obtained from the Landauer-Büttiker formalism, exhibit a monotonous increasing trend with applied electric field for deca nanometer MoS2, whereas the trend is opposite for other cases. The proposed multiscale modeling framework is ‘core’ in nature and various nonideal effects can further be included using suitable pre-correction techniques to establish a full-fledged industry-standard compact model. Since this model effectively bridges between atomistic material modeling tools and industrial design automation tools, it thereby promises solution to the design-technology co-optimization challenges for new materials.

Efficient preparation and characterization of layered materials and their van der Waals heterojunctions lay the foundation for various opportunities in both fundamental studies and device applications. The vast library of 2D materials displays a range of electronic properties, including conductors, semiconductors, insulators, semimetal, and superconductors, and shows strong light-matter interaction. The fact that each layer in the layered material is bonded via van der Waals interaction opens up the possibility of assembling different layers arbitrarily without any consideration over the precision of lattice match- ing. This unique stacking with one-atomic-plane precision can unfold diverse van der Waals heterostructure devices by efficiently engineering its energy band alignment. This paves a path to design novel devices such as solar cells, photodetectors, light-emitting diodes and transistors. In this thesis, our motivation is to explore the electronic and optoelectronic characteristics of 2D materials and their heterojunctions. We focus on designing 2D heterostructures for the multi-functional devices including electronic (diode/transistor) and optoelectronic (highly sensitive photodetection) applications. As the initial step, we realized SnSe2 based photoconductor which shows a very high responsivity of 10^3 A/W at 1 mV voltage bias. We investigated the role of trap states present at the channel- substrate interface on the observed gain mechanism in typical planar 2D photoconductors. Next, in order to improve the speed for a photodetector, we designed a heterostructure composed of ITO/WSe2/SnSe2 vertical heterojunction. This novel design helped us to achieve a large responsivity at near IR region while maintaining high operational speed. We achieved a high responsivity of more than 1100 A/W and fast transient response time in the order of 10 us. Considering the interest of broad band detection, we then fabricated a graphene-absorption-based photodetector where graphene can act as the absorbing medium, utilizing its zero-band gap nature. The absorbed photo-carriers are vertically transported in a fast time scale to a floating MoS2 quantum well, providing photo-gating. This structure exhibited the responsivity of 4.4 * 10^6 A/W at 30 fW incident power which is higher than that of any reported graphene absorption-based photodetectors. As a continuation of the study of heterostructure transport characteristics, we realized a backward diode with WSe2/SnSe2 structure which exhibits an ultra-high reverse recti cation ratio of 2.1 *10^4 with an impressive curvature coefficient of 37 V^(-1). Finally, we proposed a novel methodology for the extraction of Schottky Barrier Height (SBH) using a vertical heterojunction of multilayer transition metal dichalcogenide with asymmetric contacts which allow easy and direct quantitative evaluation of SBH for two contacts simultaneously.
Visvesvarayya PhD Scheme

There has been a giant leap in technological advancement with the introduction of graphene and its remarkable properties after 2005. Since the inception of graphene, the new class of materials called 2D materials are actively being focused on for their potential use case. The recent introduction of magnetism in 2D materials has sparked a new interest among researchers due to the potential use of magnetic properties in spintronics, which is highly admired in storage devices. The extensive library of newly predicted or even synthesized 2D materials made it impossible to screen them experimentally. Therefore, theoretical and computational tools like Density Functional Theory (DFT), Monte Carlo and Molecular dynamics simulations have been the tool of choice for high-throughput screening and insight finding. Even though computational methods worked well, but they generally demand substantial computational resources. The expanding grasp of machine learning algorithms has been overreaching for material engineering. The idea to club ML algorithms with the rising 2D crystal structures and their DFT calculated properties along with other material data has enabled us to create predictive models encompassing underlying physics using machine learning which can screen the materials much faster with relatively similar accuracies in limited resources. Many materials have been investigated using machine learning algorithms to predict their properties, such as crystal structures, curie temperatures, bandgaps, Fermi energies, and charge density wave phases. In this work, we use a graph-based neural network model (CGCNN) and several highly customized hybrid ML models to identify the magnetic materials from three different databases with heavily skewed data topology. We have employed several supervised ML algorithms to determine how accurate they are in predicting the magnetic state or the amount of anisotropy using the crystal structure as the only source of information. A further effort to develop a complementary regression model for the prediction of magnetic anisotropy

The feasibility of isolation of layered materials and arbitrary stacking of different materials provide plenty of opportunities to realize van der Waals heterostructures (vdWhs) with desired characteristics. In this thesis, we experimentally demonstrate the tunability of optical and electrical characteristics of transition metal dichalcogenides (TMDs), a class of layered materials, using their vdWhs. Monolayer (1L) TMDs exhibit remarkable light-matter interaction by hosting direct bandgap, strongly bound excitonic complexes, ultra-fast radiative decay, many-body states, and coupled spin-valley degrees of freedom. However, their sub-nm thickness limits light absorption, impairing their viability in photonic and optoelectronic applications. The physical proximity of layers in vdWhs drives strong interlayer dipole-dipole coupling resulting in nonradiative energy transfer (NRET) from one layer (donor) to another (acceptor) under spectral resonance. Motivated by the high efficiency of NRET in vdWhs, we study the prospect of enhancement of optical properties of a 1L-TMD stacked on top of strongly absorbing, non-luminescent, multilayer SnSe2 whose direct bandgap is close to exciton emission of 1L-TMDs – MoS2 and WS2. We show that NRET enhances both single-photon and two-photon luminescence by one order of magnitude in such vdWhs. We also demonstrate a new technique of Raman enhancement driven by NRET in vdWhs. We achieve a 10-fold enhancement in the Raman intensity, enabling the observation of the otherwise invisible weak Raman modes. We establish the evidence for NRET-aided photoluminescence (PL) and Raman enhancement by modulating the degree of enhancement by systematically varying multiple parameters - donor material, acceptor material, their thickness, physical separation between donor and acceptor by insertion of spacer layer (hBN), sample temperature, and excitation wavelength. We also use the above parameters to decouple the effects of charge transfer and optical interference from NRET and establish a lower limit of the NRET-driven enhancement factor. We significantly modulate the strength of NRET by controlling the spectral overlap between 1L-TMD and SnSe2 through temperature variation. We show a remarkable agreement between such temperature-dependent Raman enhancement and the NRET-driven Raman polarizability model. We emphasize the advantages of using SnSe2 as a donor and elucidate the impact of various parameters on the PL enhancement using a rate equation framework. This NRET-driven enhancement can be used in tandem with other techniques and thus opens new avenues for improving quantum efficiency, coupling the advantages of uniform enhancement accessible across the entire junction area of vdWhs. Further, we study the role of NRET in photocurrent generation across vdWhs by designing a vertical junction of SnSe2/multilayer-MoS2/TaSe2. We report the observation of an unusual negative differential photoconductance (NDPC) behaviour arising from the existence of NRET across the SnSe2/MoS2 junction. The modulation of NRET-driven NDPC characteristics with incident optical power results in a striking transition of the photocurrent's power law from sublinear to a superlinear regime. These observations highlight the nontrivial impact of NRET on the photoresponse of vdWhs and unfold possibilities to harness NRET in synergy with charge transfer. The stacking angle between the individual layers in vdWhs provides another knob to tune their properties. The emergence of moiré patterns in twisted vdWhs creates superlattices where electronic bands fold into a series of minibands, inducing new phenomena. We experimentally demonstrate the PL emission from the moiré superlattice-induced intralayer exciton minibands in twisted TMD homobilayers using artificially stacked 1L-MoS2 layers at minimal twist angles. We also show the electrical tunability of these moiré excitons and the evolution of distinct moiré trions. We experimentally discern the localized versus delocalized nature of individual moiré peaks through different regimes of gating and optical excitation. Further, we discuss the gate-controlled valley coherence and resonant Raman scattering of moiré excitons. These experimental results provide unique insights into the moiré modulated optical properties of twisted bilayers. Next, we focus on tuning the electrical characteristics of vdWhs to realize ambipolar injection, which is useful for LED and CMOS applications. vdW contacts offer atomically smooth and pristine interfaces without dangling bonds, coupled with a weak interaction at the interface. Such contacts help to achieve a completely de-pinned contact close to the Schottky-Mott limit. We demonstrate the weakly pinned nature of a vdW contact (TaSe2) by realizing improved ambipolar carrier injection into few-layer WS2 and WSe2 channels (compared to Au). Backward diodes offer a superior high-frequency response, temperature stability, radiation hardness, and 1/f noise performance than a conventional diode. We demonstrate a vdWh based backward diode by exploiting the giant staggered band offsets of the WSe2/SnSe2 junction. The diode exhibits an ultra-high reverse rectification ratio of ~2.1*10^4 up to a substantial bias of 1.5 V, with an excellent curvature coefficient of ~37 V^{-1}, outperforming existing backward diode reports. We efficiently modulate the carrier transport by varying the thickness of the WSe2 layer, the type of metal contacts employed, and the external gate and drain bias. We also show that the effective current transfer length at the vertical junction in vdWhs can be as large as the whole interface, which is in sharp contrast to the smaller transfer length (~100 nm) in typical metal-layered semiconductor junctions. The results from this thesis widen the horizon for practical electronic, photonic, and optoelectronic applications of vdWhs.

During the past few decades, photodetectors (PDs) are being regarded as the crucial components of many photonic devices which are being used in various important applications. However, the PDs based on the traditional bulk semiconductors still face a lot of challenges in terms of the device performance such as low responsivities, high response/recovery times, high power consumption, narrow detection range, and so forth. To overcome these limitations, a novel class of two-dimensional materials known as layered metal dichalcogenides (LMDCs) has shown great promise and the LMDCs-based PDs have been reported to exhibit competitive figures of merit to the state-of-the-art PDs. Moreover, the combination of LMDCs with conventional 3D semiconductors such as silicon and group III-Nitrides could extend the current technology towards novel device applications, and self-powered, broadband and ultrafast PDs can be realized. Among LMDCs, MoS2 and SnS2 are two semiconductors which show nearly extreme kind of behavior in terms of their electrical and optical properties. Therefore, a lot of room still exists to tailor the electronic and optoelectronic properties of MoS2 and SnS2-based PDs. Moreover, unlike other members of the LMDC family such as SnSe2, MoTe2, MoSe2, WSe2, and so on, MoS2 and SnS2 are free from toxic elements, and thus, environment-friendly semiconductors. Therefore, the present work focuses on the applications of the MoS2 and SnS2-based hybrid devices. In the present investigation, MoS2 has been grown on the different group III-Nitride semiconductors (AlN, GaN, InN) and the band alignment studies have been done for these three heterojunctions using the technique of high-resolution X-ray photoelectron spectroscopy. This has been followed by the implementation of one of these configurations i.e., MoS2/AlN for the realization of a self-powered, broadband and ultrafast PD. Further, the trade-off that usually exists between the broadband and wavelength-selective photodetection has been overcome via the phenomenon of polarity inversion exhibited by MoS2/GaN/Si-based PD. The device shows a positive photoresponse for the photons of ultra-violet region and exhibits negative photoresponse when the incident light changes to near infrared. After obtaining an excellent device performance by MoS2-based PDs, the optoelectronic properties of the less explored SnS2-based device have been investigated and the SnS2/p-Si-based device shows a high photoresponse with broadband photodetection. And finally, we have extended this work towards investigation of the humidity sensing behavior by SnS2 thin films of different thicknesses. All the devices exhibit a highly responsive behavior in self-powered mode, and a correlation between the sensitivity of the device with film thickness has been established. We believe that the present work can provide new routes towards the basic understanding of 2D/3D-based electronic and optoelectronic devices.

Excitons are quasiparticles formed due to electrostatic attraction between the electrons and the holes in a semiconductor. This Coulomb attraction is very strong in the mono- layers of Transition Metal Dichalcogenides (TMDs) mainly because of strong quantum confinement, reduced dielectric screening, and high effective mass of electrons and holes in these material systems. A 2D hydrogen atom is a simple model to describe confined excitons in these monolayer films. A more formal way to describe excitons in thin semi- conductors is through the Bethe-Salpeter formalism which describes these excitons as a superposition of the electronic states in momentum space. In order to understand exci- tons further, we explore the following excitonic features in this thesis: Probing intrinsic exciton linewidth: Monolayer TMDs are highly luminescent materials despite being sub-nanometer thick. This is due to the ultrashort radiative life- time of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line as the generalized lower bound for allowed radia- tive recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the nonradiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of 1:54 0:17 meV, owing purely to finite radiative lifetime in the absence of nonradiative dephasing processes. As a direct consequence of the large radiative limit, we nd a surprisingly large ( 0:27 meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the nonradiative linewidth broadening at T=0 K. Modulating exciton binding energy: Screening due to the surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in decid- ing the binding energies of strongly bound exciton complexes in quantum confined TMD monolayers. However, owing to strong quasiparticle band-gap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. By changing the dielectric environment, we show a conspicuous photoluminescence peak shift at low temperature for higher energy excitons (2s,3s,4s,5s) in monolayer MoSe2, while the 1s exciton peak position remains unaltered a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasiparticle band-gap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be 58.6% (72.8% for 2s, 75.85% for 3s, and 85.6% for 4s) by coating an Al2O3 layer on top, while the correspond- ing reduction in quasiparticle band-gap is estimated to be 246 meV. Such direct evidence of large tunability of the binding energy of exciton complexes as well as the band-gap in monolayer TMDs holds promise of novel device applications. Enhancing exciton valley coherence time: In monolayer TMDs, valley coher- ence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manip- ulation of exciton a highly formidable task. Using monolayer MoS2 sandwiched between top and bottom graphene, we demonstrate perfect valley coherence by observing 100% degree of linear polarization (DOLP) of excitons in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a com- bination of calculation (solutions of Bethe-Salpeter and steady-state Maialle-Silva-Sham equations) and choice of systematic design of experiments using four different stacks with varying screening and exciton lifetime. To the best of our knowledge, this is the first time where the valley coherence timescale has been significantly enhanced in monolayer semiconductors. Probing the role of motional narrowing in exciton valley coherence: We observe a strong effect of motional narrowing (regime of random phase cancellation) by observing a high DOLP from a defected MoS2 sample, as compared to a clean MoS2 sam- ple which shows relatively lower exciton DOLP. Similar observations hold good for both monolayer and bilayer MoS2 samples, which results from random phase cancellation in the exciton pseudospin in the motional narrowing regime. This highlights the counter- intuitive role of sample quality in the exciton DOLP measurements: a clean sample does not necessarily guarantee large exciton DOLP and vice versa. Generating highly luminescent, highly-polarized, ultra-narrow exciton peak : On generation, the excitons relax to the lowest energy 1s state by scattering with phonons through multiple possible pathways. We use a simple technique in which, by tuning the excitation laser wavelength, the excitons resonantly come down to the 1s state in a single- shot manner through scattering with a specific phonon mode. Using this technique in a monolayer WS2 sample sandwiched between few-layer graphene flakes, we obtain exciton peaks that are: (1) highly luminescent, (2) highly linearly polarized - demonstrating near- perfect valley coherence, and (3) ultra-narrow - due to a reduction in the inhomogeneous broadening. The lowest exciton linewidth obtained using this technique is 1:5 meV, which after deconvolution with the excitation laser gives an upper bound of 0:23 meV on the homogeneous linewidth of the exciton peak. We demonstrate the above features all the way from cryogenic temperature to room-temperature.

The semiconductor-metal interface is universal for any electron device. Two-dimensional semiconductors have the advantages of free dangling bonds and atomically flat surfaces, making them promising materials to substitute bulk-Silicon in next-generation transistors. However, two-dimensional material like transition metal dichalcogenides (TMD) makes highly resistive contact with metallic electrodes in electronic devices. A material with intrinsic dipole can optimize this effect. 2D Janus TMD MoSSe has structural symmetry like MoS2 and contains intrinsic dipoles that strongly modify the metal contact properties. A study of MoSSe with potential electrode materials has already seen where both the S and Se sides of MoSSe tend to have ohmic behavior. Along with MoSSe, WSSe is also available commercially for experimental efforts. The study of Janus WSSe material shows that it is an excellent photocatalyst for water splitting, and doped WSSe nanosheet is an efficient nanosensor. However, the electronic nature of this material’s interface with metals is not investigated yet. In this work, we have examined the interfacial properties of monolayer WSSe with bulk metal electrodes. Using density functional theory-based electronic structure calculation, we evaluated the structural and electronic properties of top contacts of WSSe with six metals Ag, Au, Ru, Pd, Pt, and Ti, considering both the S and Se sides. For the side contacts, we have selected three metals Ag, Au, and Ti, and investigated the electronic properties using ab-initio quantum transport simulation. Band structures of the Janus material contacted with Ru, Pd, Pt, and Ti are highly hybridized, leading to no Schottky barrier height in the vertical direction. However, with Au and Ag, Schottky contacts are formed in both lateral and vertical directions. The contacts' nature and barrier heights differ for Au and Ag. This investigation gives insight into the interfacial properties of Janus materials to use in future nanoelectronics devices.

Les modèles de réflexion complexes, avec leurs nombreux paramètres dont certains restent non intuitifs, sont difficiles à contrôler pour obtenir une apparence désirée. De plus, même si un artiste peut plus aisément comprendre la forme de la micro-géométrie d'une surface, sa modélisation en 3D et sa simulation en 4D demeurent extrêmement fastidieuses et coûteuses en mémoire. Nous proposons une solution intermédiaire, où l'artiste représente en 2D une coupe dans un matériau, en dessinant une micro-géométrie de surface en multi-couches. Une simulation efficace par lancer de rayons en seulement 2D capture les distributions de lumière affectées par les micro-géométries. La déviation hors-plan est calculée automatiquement de façon probabiliste en fonction de la normale au point d'intersection et de la direction du rayon incident. Il en résulte des BRDFs isotropes complètes et complexes, simulées à des vitesses interactives, et permettant ainsi une édition interactive de l'apparence de réflectances riches et variées.
Complex reflection models, with their many parameters, some of which are not intuitive at all, are difficult to control when trying to achieve a desired appearance. Moreover, even if an artist can more easily understand the shape of the surface micro-geometry, its 3D modeling and 4D simulation remain extremely tedious and expensive in memory. We propose an intermediate solution, where the artist represents a 2D cross section of a material, by drawing a multi-layered surface micro-geometry. An efficient 2D ray tracing simulation captures the light distribution specific to those micro-geometries. Off plane deflection is automatically calculated in a probabilistic way, based on the surface normal at the intersection point and the incident ray direction. This results in complete and complex isotropic BRDFs, simulated at interactive rates, and allowing interactive editing of rich and varied materials.

Nanomaterials are playing an increasing role in novel technologies, and it is important to develop optical methods to characterize them in situ.  To that end, backside absorbing layer microscopy (BALM) has emerged as a powerful tool, being capable to resolve sub-nanometer height profiles, with video-rate acquisition speeds and a suitable geometry to couple live experiments.  In the internship, several techniques involving BALM were developed, and applied to study optical and electrical properties of the transition metal dichalcogenide (TMD) monolayer MoS2, a type of 2-dimensional (2D) crystalline semiconductor.  A simulations toolkit was created in MATLAB to model BALM, a workflow to reliably extract linear intensities from the CMOS detector was realized, and 2D MoS2 was synthesized by chemical vapor deposition followed by transfer to appropriate substrates.  BALM data of the 2D MoS2 was acquired and combined with simulations, giving a preliminary result for its complex refractive index at 5 optical wavelengths.  In addition, the first steps towards coupling BALM with a gate biased 2D MoS2 field-effect transistor were explored.  To complement BALM measurements, the grown samples were also characterized by conventional optical microscopy, scanning electron microscopy, atomic force microscopy, photoluminescence spectroscopy, and Raman spectroscopy.  This work provides new additions to an existing platform of BALM techniques, enabling novel BALM experiments with nanomaterial systems.  In particular, it introduces a new alternative for local extraction of optical parameters and for probing of electrical charging effects, both of which are vital in the research and development of nano-optoelectronics.

Low-dimensional systems are an exciting platform for exploring new physics and realizing novel devices. The intriguing features, such as the existence of strongly bound multiparticle complexes and thickness-dependent band structures, enable us to utilize them to overcome many challenges faced by bulk materials and conceive new technologies. Since the isolation of graphene, the class of two-dimensional materials has grown tremendously. The array of materials one can choose from for implementing an idea is vast. Nevertheless, understanding the underlying physics is essential for utilizing these properties for real-life applications. Here, we explore the optical, electrical, and optoelectrical characteristics of heterostructures based on 2D layered systems. The strongly bound excitonic complexes hosted by monolayer transition metal dichalcogenide semiconductors (TMDC) are an excellent platform for probing many-body physics. The strong luminescence and a plethora of exciting properties make them a good candidate for applications such as single photon emitters and light-emitting diodes. In the first work, we explore new ways to tune the emission from these particles without compromising their luminescence. Using a high-quality graphene/hBN/WS2/hBN/Au vertical heterojunction, we demonstrate for the first time an out-of-plane electric field-driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. We also find that the encapsulating environment of the monolayer TMDC plays a vital role in wave function spreading and hence in determining the magnitude of the blue Stark shift. We also provide a theoretical framework to understand the underlying physics better. The findings have important implications in probing many-body interaction in the two dimensions and developing layered semiconductor-based tunable optoelectronic devices. A significant advantage of the 2D material system is its robustness against lattice mismatch between the successive layers and the ability to extract exciting characteristics from the resultant system. The final system's behavior greatly depends on how the energy bands of the individual materials line up and can result in drastically different properties. In the second work, we demonstrate how an additional ultra-thin barrier layer modifies the properties of a black phosphorus (BP)/SnSe2 tunnel diode. While the system without the barrier layer showed a linear relationship between current and voltage, the additional barrier layer modified it to a highly nonlinear relation and exhibited negative differential resistance (NDR). Moreover, the tunnel diodes exhibited highly repeatable, ultra-clean, and gate tunable NDR characteristics with a signature of intrinsic oscillation and a large peak-to-valley current ratio (PVCR) of 3.6 at 300 K (4.6 at 7 K), making them suitable for practical applications. We then show that the thermodynamic stability of the van der Waals (vdW) tunnel diode circuit can be tuned from astability to bistability by altering the constraint by choosing a voltage or a current bias, respectively. After exploring the dynamics of the device, we assess its viability for designing systems with real-life applications. In the astable mode under voltage bias, we demonstrate a compact, voltage-controlled oscillator without needing an external tank circuit. In the bistable mode under current bias, we demonstrate a highly scalable, single element, a one-bit memory cell promising for dense random access memory applications in memory-intensive computation architectures. In the third work, we explore the usage of vdW materials for generating a cryptographically secure true random number generator. Such generators rely on external entropy sources for their indeterminism. Physical processes governed by the laws of quantum mechanics are excellent sources of entropy available in nature. However, extracting enough entropy from such systems for generating truly random sequences is challenging while maintaining the feasibility of the extraction procedure for real-world applications. Here, we design a compact and an all-electronic vdW heterostructure-based device capable of detecting discrete charge fluctuations for extracting entropy from physical processes and use it for the generation of independent and identically distributed (IID) true random sequences. Using the proposed scheme, we extract a record high value (> 0.98 bits/bit) of min-entropy. We demonstrate an entropy generation rate tunable over multiple orders of magnitude and show the persistence of the underlying physical process for temperatures ranging from cryogenic to ambient conditions. We verify the random nature of the generated sequences using tests such as the NIST SP 800-90B standard and other statistical measures and verify the suitability of our random sequence for cryptographic applications using the NIST SP 800-22 standard. The generated random sequences are then used to implement various randomized algorithms in real life without preconditioning steps. We then investigate how knowledge of the dynamics of optically generated carriers, ability to sense discrete charge fluctuation, and transport of carriers across vdW heterostructure can be combined to design a comprehensive system to detect single photons. Single-photon detectors (SPDs) are crucial in applications ranging from space and biological imaging to quantum communication and information processing. The SPDs operating at room temperature are particularly interesting to broader application spaces as the energy overhead introduced by cryogenic cooling can be avoided. Although silicon-based single photon avalanche diodes (SPADs) are well matured and operate at room temperature, the bandgap limitation restricts their operation at telecommunication wavelength (1550 nm) and beyond. On the other hand, InGaAs-based SPADs are sensitive to 1550 nm photons but suffer from relatively lower efficiency, high dark count rate, afterpulsing probability, and pose hazards to the environment from the fabrication process. By coupling a low bandgap (~350 meV) absorber (black phosphorus) to a sensitive van der Waals probe capable of detecting discrete electron fluctuation, we demonstrate a room-temperature single-photon detector. While the device is capable of covering up to a wavelength of ~3.5 um, we optimize the device for operation at 1550 nm and demonstrate an overall quantum efficiency of 21.4% (estimated as 42.8% for polarized light) and a minimum dark count of ~720 Hz at room temperature.