Rozprawy doktorskie na temat „Layered metal dichalcogenides”

The interplay between different self-organized electronically ordered states and their relation to unconventional electronic properties like superconductivity constitutes one of the most exciting challenges of modern condensed matter physics. In the present thesis this issue is thoroughly investigated for the prototypical layered material 1T-TaS2 both experimentally and theoretically. At first the static charge density wave order in 1T-TaS2 is investigated as a function of pressure and temperature by means of X-ray diffraction. These data indeed reveal that the superconductivity in this material coexists with an inhomogeneous charge density wave on a macroscopic scale in real space. This result is fundamentally different from a previously proposed separation of superconducting and insulating regions in real space. Furthermore, the X-ray diffraction data uncover the important role of interlayer correlations in 1T-TaS2. Based on the detailed insights into the charge density wave structure obtained by the X-ray diffraction experiments, density functional theory models are deduced in order to describe the electronic structure of 1T-TaS2 in the second part of this thesis. As opposed to most previous studies, these calculations take the three-dimensional character of the charge density wave into account. Indeed the electronic structure calculations uncover complex orbital textures, which are interwoven with the charge density wave order and cause dramatic differences in the electronic structure depending on the alignment of the orbitals between neighboring layers. Furthermore, it is demonstrated that these orbital-mediated effects provide a route to drive semiconductor-to-metal transitions with technologically pertinent gaps and on ultrafast timescales. These results are particularly relevant for the ongoing development of novel, miniaturized and ultrafast devices based on layered transition metal dichalcogenides. The discovery of orbital textures also helps to explain a number of long-standing puzzles concerning the electronic self-organization in 1T-TaS2 : the ultrafast response to optical excitations, the high sensitivity to pressure as well as a mysterious commensurate phase that is commonly thought to be a special phase a so-called “Mott phase” and that is not found in any other isostructural modification.

Thesis (Ph.D. (Engineering mechanics)) --University of Limpopo, 2004
This thesis explores the important issues associated with the insertion of Mg2+ and Li+ into the solid materials: molybdenum sulphide and titanium disulphide. This process, which is also known as intercalation, is driven by charge transfer and is the basic cell reaction of advanced batteries. We perform a systematic computational investigation of the new Chevrel phase, MgxMo6S8 for 0 ≤ x ≤ 2, a candidate for high energy density cathode in prototype rechargeable magnesium (Mg) battery systems. Mg2+ intercalation property of the Mo6S8 Chevrel phase compound and accompanied structural changes were evaluated. We conduct our study within the framework of both the local-density functional theory and the generalised gradient approximation techniques. Analysis of the calculated energetics for different magnesium positions and composition suggest a triclinic structure of MgxMo6S8 (x = 1 and 2). The results compare favourably with experimental data. Band-structure calculations imply the existence of an energy gap located ~1 eV above the Fermi level, which is a characteristic feature of the electronic structure of the Chevrel compounds. Calculations of electronic charge density suggest a charge transfer from Mg to the Mo6S8 cluster, which has a significant effect on the Mo-Mo bond length. There is relatively no theoretical work, in particular ab initio pseudopotential calculations, reported in literature on structural stability, cations "site energy" calculations, and pressure work. Structures obtained on the basis from experimental studies of other ternary molybdenum sulphides are examined with respect to pressure-induced structural transformation. We report the first bulk and linear moduli of the new Chevrel phase structures. This thesis also studies the reaction between lithium and titanium disulfide, which is the perfect intercalation reaction, with the product having the same structure over the range of reaction 0  x  1 in LixTiS2. Calculated lattice parameters, bulk moduli, linear moduli, elastic constants, density of states, and Mulliken populations are reported. Our calculations confirm that there is a single phase present with an expansion of the crystalline lattice as is typical for a solid solution, about 10% perpendicular to the basal plane layers. A slight expansion of the lattice in the basal plane is also observed due to the electron density increasing on the sulfur ions. Details on the correlation between the electronic structure and the energetic (i.e. the thermodynamics) of intercalation are obtained by establishing the connection between the charge transfer and lithium intercalation into TiS2. The theoretical determination of the densities of states for the pure TiS2 and Li1TiS2 confirms a charge transfer. Lithium charge is donated to the S (3p) and Ti (3d) orbitals. Comparison with experiment shows that the calculated optical properties for energies below 12 eV agrees well with reflectivity spectra. The structural and electronic properties of the intercalation compound LixTiS2, for x = 1/4, 3/4, and 1, are also investigated. This study indicates that the following physical changes in LixTiS2 are induced by intercalation: (1) the crystal expands uniaxially in the c-direction, (2) no staging is observed. We also focus on the intercalation voltage where the variation of the cell potential with the degree of discharge for LiTiS2 is calculated. Our results show that it can be predicted with these well-developed total energy methods. The detailed understanding of the electronic structure of the intercalation compounds provided by this method gives an approach to the interpretation of the voltage composition profiles of electrode materials, and may now clearly be used routinely to determine the contributions of the anode and cathode processes to the cell voltage. Hence becoming an important tool in the selection and design of new systems. Keywords Magnesium rechargeable battery; Chevrel, Lithium batteries; Li and Mg-ion insertion; TiS2; Mo6S8; Charge transfer; reflectivity, intercalation, elastic constants, voltage, EOS, Moduli.
the National Research Foundation, the Royal Society(U.K),the Council for Scientific and Industrial Research,and Eskom

Die untersuchten Ferekristalle sind neuartige Verwachsungs-Schichtverbindungen aus m Monolagen von Niobdiselenid (NbSe2), die wiederholt mit n atomaren Bilagen von Bleiselenid (PbSe) oder Zinnselenid (SnSe) geschichtet sind. Niobdiselenid als Volumenmaterial besitzt eine Schichtstruktur und ist ein Supraleiter. Aufgrund ihrer gezielt einstellbaren atomar geschichteten Struktur können Ferekristalle als Modellsysteme für geschichtete Supraleiter dienen. In dieser Arbeit werden ihre strukturellen und elektrischen Eigenschaften untersucht. Mittels Transmissionselektronenmikroskopie wird ihre turbostratisch ungeordnete, nanokristalline Struktur nachgewiesen. Die atomare Struktur innerhalb der einzelnen Schichten ist ähnlich wie in den Volumenmaterialien NbSe2, PbSe und SnSe, wobei die kristallographischen c-Achsen parallel zur Stapelrichtung der Ferekristalle zeigen. Eine quantitative Analyse unter Verwendung eines Zwei-Schicht-Modells für den spezifischen Widerstand, Hall-Koeffizienten und Magnetwiderstand liefert ähnliche Ladungsträgersorten, -dichten und –beweglichkeiten in den NbSe2-Schichten, wie sie für isolierte Einzellagen von NbSe2 berichtet wurden. Diese unterscheiden sich von denen des Volumenmaterials NbSe2. Erstmals wurde ein Übergang der Ferekristalle in den supraleitenden Zustand nachgewiesen. Die Sprungtemperaturen sind dabei in etwa auf die Hälfte der Sprungtemperaturen der jeweiligen nicht turbostratisch ungeordneten Misfit-Schichtverbindungen reduziert. Diese Reduzierung kann der turbostratischen Unordnung der Ferekristalle zugeordnet werden. Das Verhältnis zwischen der schichtsenkrechten Ginzburg-Landau-Kohärenzlänge und dem Abstand zwischen den supraleitenden Schichten ist bei den Ferekristallen kleiner als bei den nicht ungeordneten Misfit-Schichtverbindungen, was Ferekristalle zu vielversprechenden Kandidaten für (quasi-)zweidimensionale Supraleiter macht.
The investigated ferecrystals are novel layered intergrowth compounds consisting of m monolayers of niobium diselenide (NbSe2) stacked repeatedly with n atomic bilayers of lead selenide (PbSe) or tin selenide (SnSe). Bulk NbSe2 is a layered compound showing superconductivity. Due to their artificially atomic-scale layered structure, which is tunable on the atomic scale, ferecrystals can serve as model systems for layered superconductors. In this study, their structural and electrical properties are investigated. Using transmission electron microscopy their turbostratically disordered, nanocrystalline structure is revealed. The atomic structure within the individual layers is similar as for bulk NbSe2, PbSe and SnSe, with the crystallographic c-axes parallel to the stacking direction in the ferecrystals. A quantitative analysis using a two-layer model fit for the electrical resistivity, Hall coefficient and magnetoresistance yields a similar carrier type, density and mobility in the NbSe2 layers as reported for isolated NbSe2 monolayers. These values differ from those of bulk NbSe2. For the first time, a normal-to-superconducting transition has been detected in ferecrystals. The transition temperatures of the ferecrystals are reduced to about a half of those of analogous non-disordered misfit layer compounds. This reduction in transition temperature can be correlated to the turbostratic disorder in ferecrystals. The ratio between the cross-plane Ginzburg-Landau coherence length and the cross-plane distance between the NbSe2 layers for the ferecrystals is lower than for non-disordered misfit layer compounds, making ferecrystals promising candidates for (quasi-)two-dimensional superconductors.

The ability to convert photons into mechanical motion is of significant importance for many energy conversion and reconfigurable technologies. Establishing an optical-mechanical interface has been attempted since 1881; nevertheless, only few materials exist that can convert photons of different wavelengths into mechanical motion that is large enough for practical import. Recently, various nanomaterials including nanoparticles, nanowires, carbon nanotubes, and graphene have been used as photo-thermal agents in different polymer systems and triggered using near infrared (NIR) light for photo-thermal actuation. In general, most photomechanical actuators based on sp bonded carbon namely nanotube and graphene are triggered mainly using near infra-red light and they do not exhibit wavelength selectivity. Layered transition metal dichalcogenides (TMDs) provide intriguing opportunities to develop low cost, light and wavelength tunable stimuli responsive systems that are not possible with their conventional macroscopic counterparts. Compared to graphene, which is just a layer of carbon atoms and has no bandgap, TMDs are stacks of triple layers with transition metal layer between two chalcogen layers and they also possess an intrinsic bandgap. While the atoms within the layers are chemically bonded using covalent bonds, the triple layers can be mechanically/chemically exfoliated due to weak van der Waals bonding between the layers. Due to the large optical absorption in these materials, they are already being exploited for photocatalytic, photoluminescence, photo-transistors, and solar cell applications. The large breaking strength together with large band gap and strong light- matter interaction in these materials have resulted in plethora of investigation on electronic, optical and magnetic properties of such layered ultra-thin semiconductors. This dissertation will go in depth in the synthesis, characterization, development, and application of two- dimensional (2D) nanomaterials, with an emphasis on TMDs and molybdenum disulfide (MoS2), when used as photo-thermal agents in photoactuation technologies. It will present a new class of photo-thermal actuators based on TMDs and hyperelastic elastomers with large opto-mechanical energy conversion, and investigate the layer-dependent optoelectronics and light-matter interaction in these nanomaterials and nanocomposites. Different attributes of semiconductive nanoparticles will be studied through different applications, and the possibility of globally/locally engineering the bandgap of such nanomaterials, along with its consequent effect on optomechanical properties of photo thermal actuators will be investigated. Using liquid phase exfoliation in deionized water, inks based on 2D- materials will be developed, and inkjet printing of 2D materials will be utilized as an efficient method for fast fabrication of functional devices based on nanomaterials, such as paper-graphene-based photo actuators. The scalability, simplicity, biocompatibility, and fast fabrication characteristics of the inkjet printing of 2D materials along with its applicability to a variety of substrates such as plastics and papers can potentially be implemented to fabricate high-performance devices with countless applications in soft robotics, wearable technologies, flexible electronics and optoelectronics, bio- sensing, photovoltaics, artificial skins/muscles, transparent displays and photo-detectors.

In the last 50 years, the semiconductor industry has been scaling the silicon transistor to achieve faster devices, lower power consumption, and improve device performance. Transistor gate dimensions have become so small that short channel effects and gate leakage have become a significant problem. To address these issues, performance enhancement techniques such as strained silicon are used to improve mobility, while new high-k gate dielectric materials replace silicon oxide to reduce gate leakage. At some point the fundamental limit of silicon will be reached and the semiconductor industry will need to find an alternate solution. The advent of graphene led to the discovery of other layered materials such as the transition metal dichalcogenides. These materials have a layered structure similar to graphene and therefore possess some of the same qualities, but unlike graphene, these materials possess sizeable bandgaps between 1-2 eV making them useful for digital electronic applications. Since initially discovered, most of the research on these films has been from mechanically exfoliated flakes, which are easily produced due to the weak van der Waals force binding the layers together. For these materials to be considered for use in mainstream semiconductor technology, methods need to be explored to grow these films uniformly over a large area. In this research, atomic layer deposition (ALD) was employed as the growth technique used to produce large area uniform thin films of several different transition metal dichalcogenides. By optimizing the ALD growth parameters, it is possible to grow high quality films a few to several monolayers thick over a large area with good uniformity. This has been demonstrated and verified using several physical analytical tests such as Raman spectroscopy, photoluminescence, x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron spectroscopy, and scanning electron microscopy, which show that these films possess the same qualities as those of the mechanically exfoliated films. Back-gated field effect transistors were created and electrical characterization was performed to determine if ALD grown films possess the same electronic properties as films produced from other methods. The tests revealed that the ALD grown films have high field effect mobility and high current on/off ratios. The WSe2 films also exhibited ambipolar electrical behavior making them a possible candidate for complementary metal-oxide semiconductor (CMOS) technology. Ab-initio density functional theory calculations were performed and compared to experimental properties of MoS2 and WSe2 films, which show that the ALD films grown in this research match theoretical predictions. The transconductance measurements from the WSe2 devices used, matched very well with the theoretical calculations, bridging the gap between experimental data and theoretical predictions. Based upon this research, ALD growth of TMD films proves to be a viable alternative for silicon based digital electronics.

In this thesis work we explored the capability of electrochemical gating to reliably control the ground state of several chosen materials, with a specific focus on the engineering of the superconducting state. We also experimented with different electrolyte compositions in order to best match the electrochemical requirements of the various materials under study (e.g., chemical stability). In the presentation of the results, we will move from the thicker, bulk-like materials down to the truly two-dimensional properties of thin exfoliated single crystals. Chapter 1 presents a general analysis of the field-effect technique based on an electrolytic gate. We discuss the basic principle that allows for the existence of ultrahigh electric fields at the device surface, together with the several pratical limitations and criticalities the technique entails. In particular, we consider the critical distinction between purely electrostatic gating and the regimes where various types of electrochemical interactions are activated between the sample and the electrolyte. We also discuss in detail a purely electrochemical measurement that can be performed on the complete devices in order to determine the amount of charge accumulated in the electric double layer. Chapter 2 shows a selection of our results on superconducting thin films. We analyze extensively the response of conventional BCS superconductor niobium nitride to EDL gating as a function of film thickness (∼ 40−10 nm), and we interpret our data in the framework of a bulk control of the superconducting transition mediated by proximity effect. We then extend our analysis to more complex materials. We show preliminary results on state-of-the-art thin films (∼ 20 nm) of two-gap superconductor magnesium diboride. Finally, we consider thin films of iron-based superconductor barium iron arsenide and show how its Tc can be modulated by the electric field only in the smallest thicknesses available by state-of-the-art growth techniques (∼ 10 nm). Chapter 3 presents our results on thin flakes (∼ 5−10 nm) of transition metal dichalcogenides. We explore via EDL gating the valley occupation in the conduction and of semiconducting molybdenum and tungsten disulphides at high carrier densities. We show preliminary evidence linking the emergence of EDL-induced superconductivity with the population of secondary minima in the bandstructure for molybdenum disulphide. We also exploit electrochemical gating beyond the electrostatic regime to perform field-assisted intercalation of molybdenum disulphide with alkali ions, in an effort to demonstrate both surface and bulk gate-controlled superconductivity in the same device architecture. We find preliminary evidence for the onset of a possible Charge-Density-Wave phase at very high ion doping. Chapter 4 is entirely devoted to our results on few-layer graphene. While we did not observe any gate-induced superconductivity (down to T= 3.5 K) even at the highest induced carrier densities ∼ 6 · 1014 cm-2, we were able to extensively study the dominant scattering mechanisms both in the high and low temperature regimes; in particular, we showed that inelastic scattering for T . 90 K is dominated by electron-electron collisions, in contrast with what was found in the literature for single-layer graphene. Moreover, we observed the emergence of quantum coherence phenomena (weak localization) for T . 20 K in these previously unreached conditions of ultrahigh carrier doping. Finally, in the Conclusions we summarize the most significant results obtained during this thesis work together with the questions that are still left open. Furthermore, we consider some perspectives and future lines of research that could be pursued in the framework of electrolyte gating.

Dans un contexte de diversification des fonctionnalités sur silicium (more than Moore), les sulfures de transition sont actuellement activement étudiés pour la réalisation de dispositifs optiques originaux. Dans cette famille, certains matériaux présentent une structure lamellaire structurellement semblables au graphène. C'est le cas de certains sulfures de vanadium. La synthèse de ces films lamellaires reste activement dominée par les procédés CVD à haute température (>550°C). Cependant, pour espérer le développement d'une synthèse fiable, il est important de diminuer cette température de dépôt qui conduit à des films souvent peu uniforme et conforme. Dans ce travail nous avons étudié la potentialité d'une approche de dépôt par voie chimique en phase vapeur à basse température (200°C). Cette synthèse a permis l'obtention d'un film de sulfure de vanadium amorphe sur un substrat de 300mm et a montré la capacité de ce film à se réorganiser pour obtenir un film lamellaire de V7S8 après recuit thermique. Un film de 5,2nm présente des propriétés optiques et électriques intéressantes ; ce film est conducteur il possède une densité de porteur de 1,1.1023 cm-3, les porteurs majoritaires sont les trous (type p), une mobilité de 0,2 cm2.(V.s) -1, une conductivité de 1063 S.cm-1, un travail de sortie de 4,8 eV tout en préservant une bonne transparence (transmittance de 75% pour une longueur d’onde de 550nm)
In the context of functional diversification (“More than Moore”), transition sulfides are currently being actively studied for original optical devices production. Some materials in this family have a lamellar structure, similar to graphene like vanadium sulfides. The synthesis of these lamellar films remains actively dominated by high-temperature CVD processes (> 550 ° C). However, in order to hope the development of a reliable synthesis methods, it's important to reduce this deposition temperature which leads to a poor uniformity and a poor conformity. In this work we have studied the potential of a chemical vapor deposition approach at low temperature (200 ° C). This method allow us to obtain an amorphous vanadium sulfide film on a 300 mm wafer and point out theirability to self-reorganize in order to obtain a lamellar film of V7S8 after thermal annealing. A 5.2nm film has interesting optical and electrical properties; this film is conductive with a carrier density of 1.1.1023 cm-3, the holes are the main charges carriers (type p), a mobility of 0.2 cm2. (Vs) -1, a conductivity of 1063 S.cm -1, an output work of 4.8 eV while preserving good transparency (transmittance of 75% for a wavelength of 550nm)

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.

The research reported in the thesis entitled ’Optical properties of thin layers of transition metal dichalcogenides’ focuses on physical phenomena which emerge in the limit of two-dimensional (2D) miniaturisation when the thickness of fabricated films reaches an atomic scale. The importance of such man-made structures has been revealed by the dynamic research on graphene: a single atomic plane of carbon atoms arranged in honeycomb lattice. Graphene is intrinsically gapless and therefore mainly explored with respect to its electric properties. The investigation of semiconducting materials which can also display the hexagonal crystal structure and which can be thinned down to individual layers, bridges the concepts characteristic of graphene-like systems (K-valley physics) with more conventional properties of semiconductors. This has been indeed demonstrated in a number of recent studies of ultra-thin films of semiconducting transition metal dichalcogenides (sc-TMD). Particularly appealing, from the point of view of optical studies, is a transformation of the bandgap alignment of sc-TMD films, from the indirect bandgap bulk crystals to the direct bandgap system in single layers. The presented thesis work provides a comprehensive optical characterisation of thin structures of sc-TMD crystals. The manuscript is divided into five parts: three main chapters with a preceding introduction and the appendix reporting the supplementary studies of another layered material: hexagonal boron nitride. Introduction. The fundamental properties of the investigated crystals are presented, especially those which are important from the point of view of optical studies. The discussion includes information on the crystal structure, Brillouin zone and electronic band structure. Also, the general description of the samples’ preparation process and experimental set-up is provided. Chapter 1. Basic optical characterisation of excitonic resonances in mono- and multi-layers of sc-TMDs. The optical response, as seen in the reflectance and luminescence spectra of thin scTMD is analysed (mostly for MoSe2 and WSe2 materials). The impact of the number of layers and temperature on the optical resonances is studied and interpreted in details. The complementary time-resolved study is also presented. Chapter 2. Zeeman spectroscopy of excitonic resonances in magnetic fields. The evolution of the optical resonances in an external magnetic field, applied perpendicularly to the layers of sc-TMD materials is investigated. Based on these results, a phenomenological model is developed aiming to describe the linear with magnetic field contributions to the energy of individual electronic states in fundamental sub-bands of sc-TMD monolayers. Furthermore, the effects of optical pumping are investigated in WSe2 monolayers, which can be tuned by tiny magnetic fields. Chapter 3. Single photon sources in thin sc-TMD flakes. The uncovering of localised narrow lines emitting centres at the edges of thin exfoliated sc-TMD flakes is discussed. The optical investigations provide information on their fundamental properties. The presented study covers a broad range of topics, such as the impact of temperature and magnetic field on the optical response of the emitting centres, analysis of their polarisation properties and excitation spectra as well as photon correlation measurements. Appendix A. Single photon emitters in boron nitride crystals. Hexagonal boron nitride also belongs to the family of layered materials, but it exhibits much larger band gap than semiconducting transition metal dichalcogenides. Narrow lines emitting centres have been observed in boron nitride structures, which reveal multiple similarities to defect centres in wide gap materials. They are characterised in a similar manner as the emitting centres in WSe2.
Badania opisane w rozprawie pod tytułem „Optyczne własności cienkich warstw dichalkogenków metali przejściowych” dotyczą zjawisk fizycznych, które pojawiają się w granicy dwuwymiarowej miniaturyzacji, gdy grubość struktur osiąga skalę atomową. Znaczenie takich wytworzonych przez człowieka struktur dla zrozumienia podstawowych własności materiałów ujawniło się podczas dynamicznie rozwijających się badań nad grafenem: pojedynczej warstwie atomów węgla ułożonych w strukturę heksagonalną. Grafen, jako materiał bez przerwy energetycznej, był rozpatrywany głównie pod kątem własności elektrycznych. Badania materiałów półprzewodnikowych, również charakteryzujących się strukturą heksagonalną, dla których udało się odizolować pojedyncze warstwy, łączą nowe idee wywodzące się z odkrycia szczególnych cech grafenu (fizyka dolin w punkcie K strefy Brillouina) z wiedzą o bardziej typowych właściwościach półprzewodników. Rzeczywiście, nowego typu zjawiska zostały zademonstrowane w licznych, prowadzonych ostatnio, badaniach ultra-cienkich warstw półprzewodnikowych dichalkogenków metali przejściowych. Szczególnie interesujące, z puntu widzenia badań optycznych, wydaje się odkrycie zmiany charakteru przerwy energetycznej, która jest skośna w kryształach objętościowych, ale staje się prosta dla pojedynczej warstwy materiału. Opisane w tej pracy badania wykorzystują szczegółową charakteryzację optycznych własności cienkich struktur dichalkogenków metali przejściowych jako podstawę do rozważań na temat ich własności elektronowych. Manuskrypt składa się z pięciu części: trzech głównych rozdziałów poprzedzonych wstępem i uzupełnionych dodatkiem, w którym omówione zostały badania dotyczące innego przedstawiciela materiałów warstwowych: heksagonalnego azotku boru. Wstęp. Przedstawione zostały podstawowe własności badanych kryształów, szczególnie istotne z punktu widzenia badań optycznych. Dyskusja obejmuje informacje o strukturze krystalicznej, strefie Brillouina i elektronowej strukturze pasmowej. Ponadto omówiono ogólnie proces wytwarzania próbek i główne cechy układów doświadczalnych. Rozdział 1. Podstawowe własności optyczne rezonansów ekscytonowych w pojedynczych warstwach i wielowarstwach półprzewodnikowych dichalkogenków metali przejściowych. Przeanalizowano optyczną odpowiedź cienkich struktur dwuselenku molibdenu (MoSe2) i dwuselenku wolframu (WSe2), badaną poprzez pomiary widm odbicia i luminescencji. Szczegółowo zinterpretowano dane doświadczalne dotyczące wpływu liczby warstw oraz temperatury na energię i szerokość optycznych rezonansów. Uwzględniono także uzupełniające badania rozdzielone w czasie. Rozdział 2. Spektroskopia Zeemana rezonansów ekscytonowych w polu magnetycznym. Zbadano wpływ pola magnetycznego, przyłożonego prostopadle do powierzchni badanych struktur, na przejścia optyczne. Na podstawie otrzymanych wyników opracowano fenomenologiczny model mający na celu opis liniowych z polem magnetycznym wkładów do energii indywidualnych stanów elektronowych w podstawowych podpasmach pojedynczych warstw dichalkogenków metali przejściowych. Ponadto przeanalizowano efekty związane z pompowaniem optycznym w pojedynczych warstwach WSe2, którego wydajność można zwiększyć poprzez przyłożenie niewielkiego pola magnetycznego. Rozdział 3. Źródła pojedynczych fotonów w cienkich warstwach półprzewodnikowych dichalkogenków metali przejściowych. Przedyskutowano odkrycie centrów emitujących światło w postaci cienkich linii widmowych w eksfoliowanych strukturach dichalkogenków metali przejściowych. Optyczne badania dostarczyły informacji o ich podstawowych własnościach. Przedstawione badania dotyczą wpływu temperatury i pola magnetycznego na optyczną odpowiedź emitujących centrów, własności polaryzacyjnych oraz widm pobudzania jak również pomiarów korelacji fotonów. Dodatek A. Emitery pojedynczych fotonów w kryształach azotku boru. Heksagonalny azotek boru również należy do rodziny materiałów warstwowych, lecz charakteryzuje się znacznie większą przerwą energetyczną niż dichalkogenki metali przejściowych. Centra emitujące wąskie linie widmowe także zostały zaobserwowane w strukturach azotku boru. Wykazują one cechy upodabniające je do barwnych centrów w innych materiałach szeroko-przerwowych. Emitery w azotku boru zostały scharakteryzowane podobnie jak emitery w kryształach WSe2.

Uzyskanie pojedynczej warstwy atomów węgla - grafenu - zapoczątkowało nowy trend w nauce skupiający się na dwuwymiarowych strukturach dobrze znanych materiałów objętościowych. Dodatkowo szybki postęp technik wytwarzania próbek przyczynił się do zwiększonego zainteresowania materiałami warstwowymi, między innymi dichalkogenkami metali przejściowych. Różnorodność własności chemiczno-fizycznych wyróżnia te materiały ze względu na szeroką gamę możliwych zastosowań w elektronice, optoelektronice, fotowoltaice. Mogą też zostać wykorzystane w fotoogniwach, medycynie oraz czujnikach biologicznych. Głównym celem tej pracy była charakteryzacja własności optycznych przykładowego półprzewodnikowe dichalkogenku metalu przejściowego w zależności od jego grubości. Spośród wielu przedstawicieli rodziny tych związków wybrano tellurek molibdenu (MoTe2), ponieważ pozostaje on stosunkowo słabiej poznany w porównaniu do innych związków, jak np. siarczku molibdenu (MoS2). Badania opisane w pracy wykonano za pomocą spektroskopii rozpraszania Ramana. Technika ta skupia się na pomiarach światła rozproszonego w krysztale, dających informację na temat dynamiki sieci krystalicznej, propagacji ciepła oraz mechanicznej wytrzymałości kryształów. Widma rozpraszania Ramana pozostają też ważnym źródłem informacji na temat grubości badanych płatków. Rozprawa doktorska zawiera dziesięć rozdziałów. Rozdział 1 to przegląd ogólnych informacji na temat materiałów dwuwymiarowych, w szczególności półprzewodnikowych dichalkogenków metali przejściowych. Ze względu na obszerność tematu, skupiono się na wybranych aspektach, szczególnie istotnych dla pracy. Rozdział 2 skupia się na własnościach MoTe2 w postaci materiału objętościowego oraz jego cienkich warstw. Zaprezentowano między innymi jego strukturę krystaliczną i pasmową oraz zależnościach dyspersyjnych fononów. Elementy te stanowią podstawę zrozumienia prezentowanych wyników. Rozdział 3 demonstruje szeroką gamę metod uzyskiwania omawianych materiałów oraz techniki ich charakteryzacji. Rozdział 4 stanowi teoretyczny opis podstaw spektroskopii rozpraszania Ramana oraz notacji teorii grup potrzebny do zrozumienia analizy przeprowadzonej w kolejnych rozdziałach. Rozdział 5 zawiera szczegółowy opis metody wytwarzania próbek badanych w ramach pracy oraz prezentuje układy eksperymentalne. W rozdziale 6 przeprowadzono identyfikację modów fononowych, która umożliwia określenie grubości cienkich warstw MoTe2 na podstawie widm rozpraszania Ramana. Rozdział 7 zawiera szereg wyników uzyskanych w czasie pomiarów próbek w zależności od liczby warstw. Na ich podstawie przeprowadzono analizę niskoenergetycznych modów drgań w cienkich warstwach MoTe2 oraz heterostruktur, co umożliwiło wyznaczenie stałych siłowych oddziaływań między-warstwowych oraz między materiałem a podłożem. Dodatkowo opisane zostały obserwacje rozszczepień modów poza-płaszczyznowych wywołanych efektami Davydova oraz pomiary oddziaływania z podłożem. Rozdział 8 stanowi podsumowanie wyników rozpraszania Ramana w zależności od energii pobudzania. Badania te ujawniły szereg efektów rezonansowych związanych z gęstościami stanów w charakterystycznych punktach strefy Brillouina oraz interferencjami kwantowymi. Rozdział 9 opisuje pomiary prowadzone w funkcji temperatury. Wyniki te stanowią uzupełnienie poprzedniego rozdziału. Ostatnia część rozprawy stanowi podsumowanie pracy. W rozdziale tym zawarto także wykaz osiągnięć Autorki. Pracę zamyka bibliografia.
Obtaining a single layer of carbon atoms - graphene - initiated a new trend in science focusing on two-dimensional structures of well-known bulk materials. In addition, rapid advances in mechanical exfoliation techniques have contributed to increased interest in layered materials, including transition metal dichalcogenides (TMD). The variety of chemico-physical properties distinguishes these materials with an extremely wide range of possible applications in the fields of electronics, optoelectronics, photovoltaics. They can also be used in medicine and biological sensors. The aim of this work is the characterization of optical properties as a function of the thickness of a typical representative from the semiconducting TMD family. Molybdenum ditelluride (MoTe2) was chosen since it remains relatively poorly known as compared to other TMDs like MoS2. The research conducted as part of the thesis is focused on Raman scattering spectroscopy. This technique relies on measurements of light scattered on a crystal, yielding information on crystal lattice dynamics, heat propagation and mechanical strength. Raman scattering spectroscopy primarily remains an important method to determine the thickness of the examined flakes. The doctoral dissertation contains ten chapters. Chapter 1 is a review of general information on 2D materials, in particular, semiconducting TMDs. Due to the breadth of the topic, the focus is on selected aspects that are particularly relevant to the work. Chapter 2 focuses on the properties of MoTe2 in the bulk form and its thin layers. Its crystal and bandwidth structure and dependencies of phonon dispersion are presented. These characteristics are the basis for understanding the presented results. Chapter 3 demonstrates a wide range of methods of obtaining the materials in question and techniques of their characterization. Chapter 4 provides a theoretical description of the basics of Raman scattering spectroscopy and group theory notation needed to understand the analysis performed in the following chapters. Chapter 5 contains a detailed description of the method of fabrication of the samples used in the study. Experimental setups are also presented. In Chapter 6, the identification of phonon modes has been carried out, which enables determination of the thin layer thickness based on Raman scattering spectra. Chapter 7 contains results obtained during the measurement of the samples with a different number of layers. Based on these results, low energy phonons in thin layers MoTe2 and its heterostructures were analyzed, which made it possible to determine the force constants of layer-layer and layer-substrate interactions. Additionally, splitting of phonon modes caused by the Davydov effect and the interaction with the substrate were described. Chapter 8 summarizes the results of Raman scattering in relation to the energy of excitation. These studies revealed several resonance effects related to the maxima in the density of states at characteristic points of the Brillouin zone and quantum interference. Chapter 9 describes the temperature-dependent measurements. These results complement the previous chapter. The last part of the dissertation is a summary of the work. This chapter also contains a list of the author's achievements. The work concludes with a bibliography.

Metal-based electronics remain one of the longstanding goals of researchers to achieve ultra-fast and radiation-hard electronic circuits. Generally, metals are primarily used as passive conductors in modern electronics and do not play an active role. Nanoscale materials with distinctive size-dependent properties provide opportunities to achieve new device functionalities. Ta-based di-chalcogenides, particularly 1T-TaS2 and 2H-TaSe2, which form layered structures and exhibit charge density waves (CDW), are promising in this context. CDW is a macroscopic state shown by materials with reduced dimensions, for example, one-dimensional and layered two-dimensional crystals. It results from the modulation in the electronic charge arising due to a periodic modulation in the crystal lattice. 1T-TaS2 exhibits one of the strongest known CDW characteristics enabling temperature-dependent distinct resistivity phases. The nearly commensurate (NC) to the incommensurate (IC) CDW phase transition that usually occurs at 353 K and can be driven electrically at room temperature is of high practical interest. However, resistivity switching during this phase transition is weak (< 2) and cannot be modulated by an external gate voltage – limiting its widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, we show resistivity switching up to 17.3, which is ~14.5-fold higher than standalone TaS2. We demonstrate a low barrier electrical contact between a TaS2 source and a MoS2 channel, promising “all-2D” flexible electronics. Additionally, we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied by a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface – providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) CDW phase transition increases the SBH owing to a collapse of the Mott gap in TaS2. The change in SBH allows us to estimate an electrical Mott gap opening of ~71 ± 7 meV in the C phase of TaS2. The results show a promising pathway to externally control and amplify the CDW induced resistivity switching. Further, we achieve gate- and light-controlled negative differential resistance (NDR) characteristics in an asymmetric 1T-TaS2/2H-MoS2 T-junction by exploiting the electrically driven CDW phase transition of TaS2. The device operation is purely governed by majority charge carriers, making it distinct from typical tunneling-based NDR devices, thus avoiding the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density over 10^5 nA μm^(-2), which is about two orders of magnitude higher than that obtained in typical layered material-based NDR implementations. An external gate voltage and photo-gating can effectively tune the peak current density. The device characteristics show a peak-to-valley current ratio (PVCR) of 1.06 at 290 K, increasing to 1.59 at 180 K. To exploit the low thermal conductivity of 1T-TaS2 and 2H-TaSe2 in a local heater structure, we insert 2H-TaSe2 in between TaS2 and MoS2 layers, thereby forming a triple-layered 1T-TaS2/2H-TaSe2/2H-MoS2 T-junction. TaSe2 acts as a buffer layer preventing the CDW-induced SBH modulation at TaS2/MoS2 interface. This will allow efficient thermionic switching of carriers resulting from sharp temperature rise in the junction due to electrically driven TaS2 phase transitions. Interestingly, the device can toggle between the current increment and NDR characteristics by simply changing the biasing conditions. At TaS2 biasing, the heterostructure device shows a current increment by a factor of 3 at 300 K, which gets enhanced up to ~10^3 at 77 K, beneficial for various switching circuits and sensing applications. However, under TaSe2 biasing, the device exhibits NDR characteristics with a PVCR of 1.04 and 1.10 at 300 K and 77 K, respectively. The external back-gate voltage can effectively tune the current enhancement factor and NDR. The devices mentioned above are robust against ambiance-induced degradation, and the characteristics repeat in multiple measurements over more than six months. Conventional metals, in general, do not exhibit strong photoluminescence. However, we found that 2H-TaSe2 exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a seven-fold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS2 through non-radiative resonant energy transfer from TaSe2 transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe2 have a relatively long lifetime, unlike conventional metals, making TaSe2 an excellent hot-electron injector. Finally, we show a vertical TaSe2/MoS2/graphene photodetector demonstrating a responsivity greater than 10 AW^(-1) at 0.1 MHz - one of the fastest reported photodetectors using MoS2. The findings will boost device applications that exploit CDW phase transitions, such as ultra-broadband photodetection, negative differential conductance, thermal sensors, fast oscillator, and threshold switching in neuromorphic chips. These functionalities will enable the implementation of active metal-based circuits.

碩士
國立交通大學
光電工程研究所
106
Ultrafast dynamic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) molybdenum diselenide (MoSe2) films were investigated using femtosecond pump-and-probe technique. Strong in-plane covalent bonding and weak van der Waals coupling between TMDC layers enables MoSe2 to have layered structure. Bulk and multilayer MoSe2 is known to have an indirect bandgap, while monolayer MoSe2 is a direct bandgap semiconductor. Its unique property of strong quantum confinement leads to the formation of tightly bound excitons with extremely large binding energy for atomically thin TMDCs. In this work, we have studied the ultrafast dynamic evolution of A-excitons in multilayer (2–4 layers) MoSe2 grown by chemical vapor deposition (CVD). The transient transmission shows the initial negative signals around time zero for both far below and above the A-exciton absorption edge, whereas it shows the positive signals near the exciton transition peak. The photoexcited carriers relax quickly within 0.7 ps, which can be attributed to either carrier cooling via carrier-phonon scattering or defect capturing. The fast relaxing negative (positive) signals change its sign to positive (negative) instead of simple exponential decay to equilibrium and slowly relax within 20 – 30 ps. This secondary absorption may be defect-induced absorption related to the CVD deposition process. The band-broadening due to carrier collision in closely neighboring A and B excitons may be responsible for the sign flipping of initial photo-induced absorption.

碩士
國立臺灣大學
光電工程學研究所
105
In this thesis, we have demonstrated that large-area molybdenum disulfide (MoS2) can be prepared by sulfurizing the pre-deposited transition metal films. Good layer number controllability up to 10 layers of the MoS2 film is also achieved by controlling the sputtering times of the pre-deposited transition metal films. For the sample with thicker Mo films, although MoS2 films with the layer number larger than 10 can be obtained, clusters of multi-layer 2D crystals covering Mo oxides are obtained for the sample. The results suggest that two growth mechanisms of planar MoS2 formation and Mo oxide segregation would take place simultaneously during the sulfurization procedure. After sequential transition metal deposition and sulfurization procedures of Mo and tungsten (W), MoS2/WS2 2D crystal hetero-structures can be established. After transferring the hetero-structure film to a 300 nm SiO2/Si substrate, a bottom-gate transistor with enhanced field-effect mobility is obtained. The results have revealed that the establishment of different hetero-structures is a promising approach to overcome the limit of individual 2D crystals and still maintain their advantage. The atomic layer etchings of MoS2 and WS2 are demonstrated in this paper. By repeated oxygen plasma etchings and a final re-sulfurization procedure, multi-layer WS2 can be selectively etched off from the WS2/MoS2 hetero-structure. A WS2/MoS2 hetero-structure transistor is fabricated with source/drain electrodes contacted directly to the MoS2 channel by using the repeated atomic layer etching technique. The results have revealed that the equivalent selective etching effect for two-dimensional crystal hetero-structures can be achieved by repeating the atomic layer etching procedure, which is an important step for the device fabrication of 2D crystal hetero-structures.

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.

博士
國立交通大學
電子研究所
105
This dissertation provides an extensive assessment of the scalability of the exploratory ultra-thin-body (UTB) III-V/Ge hetero-channel MOSFETs and the performance/stability of 2-D transition-metal-dichalcogenide (TMD) based logic circuits and SRAM cells. Device-circuit interactions and co-optimizations are considered to demonstrate the potential and concerns of the emerging TMD hetero-channel devices from the device/circuit point of view. Through our analysis, the impacts of quantum confinement, backgate biasing, and device variability are investigated to offer insights for future low-voltage device/circuit designs. A new intrinsic mechanism “built-in effective body-bias (VBS,eff) effect” related to the vertical backgate coupling in UTB hetero-channel GeOI and III-V-OI MOSFETs is reported and quantified to be responsible for the anomalous electrostatic integrity (EI) behaviors which violate the expectation of permittivity. For hetero-channel n-MOSFETs, this effect results from the conduction band offset (composed of discrepancies in electron affinity and the effective density-of-states of conduction band) between the high-mobility channel and conventional Si channel. For hetero-channel p-MOSFETs, this effect stems from the valence band offset (which mainly comes from the discrepancies in channel band-gap for Ge pFETs). From the perspective of electrostatic integrity, the built-in effective body-bias effect is shown to be a detrimental effect (whose impact can be comparable to that of permittivity) for most III-V-OI nFETs and the GeOI pFET, and thus the device electrostatics can be worse than what permittivity predicts. In addition, it is shown that the In0.53Ga0.47As-OI nFET and GeOI pFET may possess worse threshold-voltage (VT) variability than the GeOI nFET counterparts due to the aggravated EI by the built-in forward VBS,eff effect. The L-, EOT-, Tch-, TBOX-dependences of the impact of built-in VBS,eff on device electrostatics are also examined and discussed. This intrinsic effect has to be considered when one-to-one comparisons among various UTB hetero-channel MOSFETs regarding the electrostatic integrity are made. The quantum confinement effect becomes critical as channel-thickness keeps scaling down, and its impacts on the device electrostatic integrity and the intrinsic VT variability are theoretically investigated through an analytical solution of Schrödinger equation corroborated with TCAD numerical simulation. Besides, the backgate-bias modulated electrostatic integrity (including drain-induced-barrier-lowering (DIBL), subthreshold swing, and VT roll-off) and VT variability considering qunatum confinement are also analyzed. Our study indicates that albeit the carrier density distribution of the hetero-channel device can be far from the frontgate interface due to the high channel permittivity and the built-in forward body-bias effect, the quantum-confinement effect can move the carrier centroid toward the frontgate, and therefore the device EI such as DIBL and subthreshold swing can be improved and becomes comparable to the Si device. Moreover, the quantum confinement effect lessens the backgate-bias dependences of device electrostatic integrity and the intrinsic VT variability to process and temperature variations for UTB III-V/Ge hetero-channel devices. In other words, the backgate-bias dependence of the within-die VT variation can be suppressed by the quantum confinement effect as the backgate bias is used for power-performance optimization or global variability compensation. Since III-V, Ge and Si channels exhibit different degree of quantum confinement due to different quantization effective mass, the impact of quantum confinement has to be considered when one-to-one comparisons among the hetero-channel devices regarding electrostatic and intrinsic variability are made. Our study may provide insights for multi-VT device/circuit designs using advanced UTB technologies. 2-D layered TMD materials have emerged as promising channel materials for future ultimately-scaled CMOS devices due to the atomic-scale body thickness. We extensively evaluate the performance of logic circuits and the stability/performance of SRAM cells using mono-layer and bi-layer TMD devices based on ITRS 2028 (5.9nm) technology node. For the static CMOS logic family, albeit the bi-layer TMD devices possess higher mobility than the mono-layer counterpart, the mono-layer and bi-layer static CMOS logic circuits may show comparable delay time. On the other hand, for the pass-transistor logic family, the bi-layer pass-transistor logic circuits may exhibit much slower delay time that the mono-layer ones counterparts, particularly for those using single nMOS pass-gate transistors instead of transmission gate as signal propagation switches (e.g., the programmable routing switches in FPGAs). In the SRAM evaluations, the mono-layer MoS2-n/WSe2-p SRAM, with superior device electrostatics, is shown to exhibit larger read static noise margin (RSNM), smaller write static noise margin (WSNM), and comparable read/write performance compared with the bi-layer counterparts. Besides the nominal evaluations, the impacts of intrinsic random variations on the cell stability of 6T/8T TMD based SRAM cells for super-threshold and near-/sub-threshold operations are also conducted. Our study indicates that, for 6T SRAM, due to severe metal gate work function variation (WFV) stemming from the tiny gate area, the mono-layer SRAMs may offer sufficient immunity under super-threshold operation, while both the mono-layer/bi-layer near-/sub-threshold SRAMs exhibit unacceptable RSNM variability in spite of the excellent electrostatics of mono-layer TMD devices. Besides, high source/drain series resistance (RSD) as a major concern of TMDs may degrade the / ratios for mono-layer and bi-layer super-threshold SRAMs, whereas it should be less of an issue for near-/sub-threshold SRAMs for ultra-low power internet-of-things (IoT) applications. The standard 8T SRAM cell with the capability of elimination of read disturb may be utilized to improve the noise margin for variation tolerance. Our results show that the RSNM variations due to WFV of both mono-layer and bi-layer near-/sub-threshold SRAMs can be significantly improved by using 8T cell structure, and thus the 6 RSNM yield requirement can be met. Based on our evaluation, due to the excellent device electrostatics stemming from its single atomic layer, the mono-layer TMD devices are favored for low-power logic and SRAM applications; while the bi-layer devices, with higher carrier mobility, are more suitable for relaxed channel length and high-performance logic and SRAM applications. Our research in TMD based logic circuits and SRAM cells is also extended from planar technology to monolithic 3-D integration. The performance of 3-D logic circuits and performance/stability of 3-D 6T SRAM cells using mono- and few-layer TMD devices are comprehensively evaluated and benchmarked against the planar technology. With the possibility of adopting mono-layer or few-layer TMDs for nFET- and pFET-tiers enabled by monolithic 3-D integration, our study indicates that using the tri-layer devices for nFET- or pFET-tiers may substantially degrade the performance of logic circuits (compared with the planar technology and other 3-D combinations) due to worse subthreshold swing and DIBL even though their mobilities are much higher. For monolithic 3-D 6T SRAMs, stacking the mono-layer pFET-tier over the bi-layer nFET-tier can provide superior stability and read/write performance for 6T super-threshold SRAM cells compared with the planar technology. However, the optimum 3-D configuration for 6T near-/sub-threshold SRAM cell appears to be the mono-layer pFET-tier over the mono-layer nFET-tier. Besides the 6T cell structure, Monolithic 3-D 8T SRAM cells are also investigated under near-/sub-threshold operation. The mono-layer nFET-tier over the bi-layer pFET-tier configuration has been shown to be the optimum 3-D 8T near-/sub-threshold cell design.