Organic Gate Dielectrics for Tetracene Field Effect Transistors

Dans les trois dernières décennies, les couches minces de semiconducteurs organiques ont fait l’objet de recherches intensives. Ces couches peuvent être utilisées dans une grande variété de dispositifs optoélectroniques de nouvelle génération, tels que les diodes électroluminescentes (OLED), les transistors à effet de champ (OFET), et les cellules photovoltaïques. Récemment, des couches minces poly-cristallines de tétracène, sublimées sous vide, ont été utilisées pour réaliser le premier transistor organique à effet de champ émetteur de lumière (OLEFET), qui intègre dans un seul dispositif la fonction de modulation du courant typique d’un transistor avec la capacité de production de lumière d’une diode électroluminescente. La démonstration des OLEFETs n’est pas simple. Tout d’abord, une intégration efficace des fonctions optiques et électroniques nécessite l’utilisation d’un semiconducteur électroluminescent ayant aussi des propriétés de transport de charge intéressantes. Deuxièmement, un transport de charge ambipolaire (i.e. de trous et d’électrons) doit être réalisé pour produire des OLEFETs à haute performance. Dans ce contexte, le contrôle de la chimie de surface du substrat diélectrique s’est révélé être une stratégie efficace pour limiter la suppression du transport des électrons induite par les états électroniques pièges à l'interface diélectrique/semiconducteur. En même temps, la modification de la nature chimique et physique du substrat diélectrique influence la morphologie/structure des couches minces organiques, qui à son tour influence la performance du dispositif. Dans le cadre de ce projet, des couches minces poly-cristallines de tétracène – pour applications dans les OLEFETs – ont été sublimées sous vide sur différents substrats organiques diélectriques, y compris des polymères (parylène C, poly méthacrylate de méthyle, polystyrène) et des monocouches auto-assemblées d’hexaméthyldisilazane (HMDS) et octadécyltrichlorosilane (OTS). Le taux de dépôt était de 3.5 Å/s, la pression à l'intérieur de la chambre à vide était de 2.5×10-6 Torr, les substrats ont été maintenus à température ambiante. Le processus de germination et croissance a été étudié à partir des premières étapes de la croissance jusqu’au recouvrement complet de la surface, au moyen de la microscopie à force atomique (AFM) ex-situ. La densité de nucléation, la forme des grains cristallins et leur inter-----------Abstract Over the last three decades, thin films of organic semiconductors (OS) have been the object of intense research. These films can be used in a wide variety of new-generation optoelectronic devices, such as Organic Light Emitting Diodes (OLED), Organic Field Effect Transistors (OFET) and photovoltaic cells. Recently, vacuum sublimed tetracene films were used to realize the first Organic Light Emitting Field Effect Transistor (OLEFET), which integrates in a single device the current modulation function of a FET with the light generation capability of a LED. The demonstration of OLEFETs is not straightforward. First of all, an efficient integration of optical and electronic functionalities requires the use of a semiconductor with both efficient electroluminescence and good charge transport properties. Secondly, an ambipolar charge transport has to be achieved to produce high performance OLEFETs. Within this context, controlling the dielectric substrate surface chemistry has proven to be an efficient strategy, since it contributes to avoid the suppression of the electron transport induced by the electronic trap states at the dielectric/semiconductor interface. At the same time, the modification of the chemical and physical nature of the dielectric substrate influences the morphology/structure of the organic thin-films, in turn influencing the final device performance. In this work, polycrystalline tetracene thin films – to be incorporated in OLEFETs – were vacuum sublimed on different organic dielectric substrates, including polymers (parylene C, polymethylmethacrylate, polystyrene) and self-assembled monolayers of hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The scope of the work was indeed to shed light on the role of the organic dielectric surface in influencing the charge transport properties of tetracene OLEFETs. The tetracene deposition rate was 3.5 Å/s, the substrates were kept at room temperature and the pressure inside the vacuum chamber was 2.5×10-6 Torr. The growth process was studied from sub-monolayer to complete coverage by means of ex-situ Atomic Force Microscopy (AFM). The nucleation density, the grain size and the connectivity between the grains were observed to be strongly dependent on the physical and chemical properties of the dielectric substrate surface

Nonvolatile Memory Cells Based on MoS2/Graphene Heterostructures

Memory cells are an important building block of digital electronics. We combine here the unique electronic properties of semiconducting monolayer MoS2 with the high conductivity of graphene to build a 2D heterostructure capable of information storage. MoS2 acts as a channel in an intimate contact with graphene electrodes in a field-effect transistor geometry. Our prototypical all-2D transistor is further integrated with a multilayer graphene charge trapping layer into a device that can be operated as a nonvolatile memory cell. Because of its band gap and 2D nature, monolayer MoS2 is highly sensitive to the presence of charges in the charge trapping layer, resulting in a factor of 10000 difference between memory program and erase states. The two-dimensional nature of both the contact and the channel can be harnessed for the fabrication of flexible nanoelectronic devices with large-scale integration.Comment: Submitted versio

Stretching and Breaking of Ultrathin MoS2

We report on measurements of the stiffness and breaking strength of monolayer MoS2, a new semiconducting analogue of graphene. Single and bilayer MoS2 is exfoliated from bulk and transferred to a substrate containing an array of microfabricated circular holes. The resulting suspended, free-standing membranes are deformed and eventually broken using an atomic force microscope. We find that the in-plane stiffness of monolayer MoS2 is 180 ± 60 Nm-1, corresponding to an effective Young’s modulus of 270 ± 100 GPa which is comparable to that of steel. Breaking occurs at an effective strain between 6-11% with the average breaking strength of 15 ± 3 Nm-1 (23GPa). The strength of strongest monolayer membranes is 11% of its Young’s modulus, corresponding to the upper theoretical limit which indicates that the material can be highly crystalline and almost defect-free. Our results show that monolayer MoS2 could be suitable for a variety of applications such as reinforcing elements in composites and for fabrication of flexible electronic devices

Single-Layer MoS2 Electronics

CONSPECTUS: Atomic crystals of two-dimensional materials consisting of single sheets extracted from layered materials are gaining increasing attention. The most well-known material from this group is graphene, a single layer of graphite that can be extracted from the bulk material or grown on a suitable substrate. Its discovery has given rise to intense research effort culminating in the 2010 Nobel Prize in physics awarded to Andre Geim and Konstantin Novoselov. Graphene however represents only the proverbial tip of the iceberg, and increasing attention of researchers is now turning towards the veritable zoo of so-called "other 2D materials". They have properties complementary to graphene, which in its pristine form lacks a bandgap: MoS2, for example, is a semiconductor, while NbSe2 is a superconductor. They could hold the key to important practical applications and new scientific discoveries in the two-dimensional limit. This family of materials has been studied since the 1960s, but most of the research focused on their tribological applications: MoS2 is best known today as a high-performance dry lubricant for ultrahigh-vacuum applications and in car engines. The realization that single layers of MoS2 and related materials could also be used in functional electronic devices where they could offer advantages compared with silicon or graphene created a renewed interest in these materials. MoS2 is currently gaining the most attention because the material is easily available in the form of a mineral, molybdenite, but other 2D transition metal dichalcogenide (TMD) semiconductors are expected to have qualitatively similar properties.In this Account, we describe recent progress in the area of single-layer MoS2-based devices for electronic circuits. We will start with MoS2 transistors, which showed for the first time that devices based on MoS2 and related TMDs could have electrical properties on the same level as other, more established semiconducting materials. This allowed rapid progress in this area and was followed by demonstrations of basic digital circuits and transistors operating in the technologically relevant gigahertz range of frequencies, showing that the mobility of MoS2 and TMD materials is sufficiently high to allow device operation at such high frequencies.Monolayer MoS2 and other TMDs are also direct band gap semiconductors making them interesting for realizing optoelectronic devices. These range from simple phototransistors showing high sensitivity and low noise, to light emitting diodes and solar cells. All the electronic and optoelectronic properties of MoS2 and TMDs are accompanied by interesting mechanical properties with monolayer MoS2 being as stiff as steel and 30x stronger. This makes it especially interesting in the context of flexible electronics where it could combine the high degree of mechanical flexibility commonly associated with organic semiconductors with high levels of electrical performance. All these results show that MoS2 and TMDs are promising materials for electronic and optoelectronic applications

Can 2D-Nanocrystals Extend the Lifetime of Floating-Gate Transistor Based Nonvolatile Memory?

Conventional floating-gate (FG) transistors (made with Si/poly-Si) that form the building blocks of the widely employed nonvolatile flash memory technology face severe scaling challenges beyond the 12-nm node. In this paper, for the first time, a comprehensive evaluation of the FG transistor made from emerging nanocrystals in the form of 2-dimensional (2D) transition metal dichalcogenides (TMDs) and multilayer graphene (MLG) is presented. It is shown that TMD based 2D channel materials have excellent gate length scaling potential due to their atomic scale thicknesses. On the other hand, employing MLG as FG greatly reduces cell-to-cell interference and alleviates reliability concerns. Moreover, it is also revealed that TMD/MLG heterostructures enable new mechanism for improving charge retention, thereby allowing the effective oxide thickness of gate dielectrics to be scaled to a few nanometers. Thus, this work indicates that judiciously selected 2D-nanocrystals can significantly extend the lifetime of the FG-based memory cell

Nano-Subsidence Assisted Precise Integration of Patterned Two-Dimensional Materials for High-Performance Photodetector Arrays

The spatially precise integration of arrays of micro-patterned two-dimensional (2D) crystals onto three-dimensionally structured Si/SiO$_2$ substrates represents an attractive strategy towards the low-cost system-on-chip integration of extended functions in silicon microelectronics. However, the reliable integration of the arrays of 2D materials on non-flat surfaces has thus far proved extremely challenging due to their poor adhesion to underlying substrates as ruled by weak van der Waals interactions. Here we report on a novel fabrication method based on nano-subsidence which enables the precise and reliable integration of the micro-patterned 2D materials/silicon photodiode arrays exhibiting high uniformity. Our devices display peak sensitivity as high as 0.35 A/W and external quantum efficiency (EQE) of ca. 90%, outperforming most commercial photodiodes. The nano-subsidence technique opens a viable path to on-chip integrate 2D crystals onto silicon for beyond-silicon microelectronics.Comment: 41 pages, 5 figures, with S

Parvovírus canino: uma abordagem evolutiva e clínica

A parvovirose canina é causada pelo parvovírus canino tipo 2 (CPV-2) e consiste em uma enfermidade mundialmente conhecida na medicina de cães, uma vez que é altamente contagiosa, caracterizada principalmente por episódios de hematoquezia, vômitos e desidratação. No Brasil, milhares de animais são infectados todo ano, sendo a via oronasal a principal forma de contágio. A mortalidade é relativamente grande já que a doença possui apenas tratamento sintomático e os animais chegam ao ambulatório em estágio crítico. Sabe-se que a vacinação reduz drasticamente a incidência da doença, porém, a evolução do vírus ainda levanta questões sobre a eficácia de algumas vacinas já que alguns animais, mesmo vacinados, acabam desenvolvendo a virose. A presente revisão aborda aspectos relacionados à parvovirose canina como: seu histórico, grupos de risco, fontes de infecção, sinais clínicos, dados epidemiológicos, métodos de diagnóstico, vacinação e tratamento

Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2

Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si3N4 substrate-supported monolayer and few-layer MoS2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS2. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS2 along with controlling their dimensions

Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E-2g(1) and the out-of-plane A(1g) modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity kappa = (34.5 +/- 4) W/mK at room temperature agrees well with the first-principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical