Stefan Blawid

High-Frequency Ballistic Transport Phenomena in Schottky Barrier CNTFETs

The effective-mass Schrödinger equation is solved directly for the wave functions to explore the steady state and the time-dependent quantum-ballistic transport in Schottky barrier carbon nanotube (CNT) field-effect transistors (FETs). The employed contact parameters allow for discontinuities of the effective mass at the metal-CNT interface, and carefully chosen boundary conditions minimize spurious reflections at the simulation domain boundaries. Two-port Y-parameters of a selected device structure are computed and qualitatively explained with the time dependence of coherent quantum-ballistic charge injection. The finite escape times of the charge carriers in an open quantum system are identified for determining the inertial response to high-frequency terminal signals. Since the escape times depend on the shape of the Schottky barriers, the latter contribute to the dynamic behavior of ballistic CNTFETs.

Towards a multiscale modeling framework for metal-CNT interfaces

This paper gives a short overview on our recent investigations towards a multiscale modeling and simulation framework for metal-CNT interfaces. We employ three simulation approaches with well defined interfaces. For the simulation at device level we make use of a recently developed wave-function based effective-mass Schrödinger-Poisson solver which employs a hetero-junction like contact model to capture the physics in the contact region where the CNT is embedded into metal. The required model parameters are adjusted to TB and DFT simulation results. A comparison with experimental data for a short channel device shows the applicability of the proposed approach.

Impact of near-contact barriers on the subthreshold slope of short-channel CNTFETs

Recent experimental investigations of sub-10nm carbon nanotube (CNT) field-effect transistors (FETs) promote CNT based 1D-electronics as a candidate for a future aggressively scaled transistor technology. However, the ballistic transport within the 1D semiconducting CNT channel is largely determined by charge injection from the contacts rendering reliable theoretical predictions of the transistor performance difficult. Based on a simplified heterojunction like contact model, we demonstrate by solving the Schrödinger-Poisson equations that aggressive scaling will rely on a careful balance between two components of the injected charges, one responsible for the formation of near-contact barriers and the other carrying the current. Excellent electrostatic gate control (e.g. employing thin gate oxides) may then enable transistor scaling until the onset of direct source-drain tunneling.

Evaluation of reconfigurable tunnel FETs for low power and high performance operation

Undoped channel materials and multiple independent gates allow reconfiguration of the transistor operation between n- and p-type behavior. Materials with low effective masses allow in addition to switch on demand between a highperformance (HP) and a low-power (LP) mode, operating the transistor beyond and below the 60 mV/dec subthreshold slope limit, respectively. Based on 22 nm double-pattering design rules, a reconfigurable tunnel nanoFET device architecture is proposed, which allows n/p- as well as HP/LP-mode reconfiguration. Important key performance indicators for the technology evaluation at the device level are extracted by means of an augmented driftdiffusion simulator and at the inverter level by means of an empirical compact model.

High-frequency benchmark circuit design for a sub 50 nm CNTFET technology

The acceptance of an emerging revolutionary process technology for circuit and system design depends significantly on (i) unique device features provided by the technology, (ii) a reliable fabrication process, and (iii) a suitable transistor compact model for circuit design and simulation. Carbon nanotube (CNT) field-effect transistors (FETs) belong to a group of emerging technologies for 1D-electronics which might have the potential to replace an existing semiconductor process technology due to their unique intrinsic properties, especially in the field of analog high-frequency (HF) circuit applications such as amplifiers, oscillators and mixers. The recent progress in CNTFET process technologies has increased the demand for suitable compact models. This paper focuses on the application of a recently developed physics-based compact model TCAM for the design of analog HF circuits. In addition, the impact of important CNTFET technology parameters on the behavior of selected HF benchmark circuits is studied.

Experimental and theoretical description of the optical properties of Myrcia sylvatica essential oil

We present an extensive study of the optical properties of Myrcia sylvatica essential oil with the goal of investigating the suitability of its material system for uses in organic photovoltaics. The methods of extraction, experimental analysis, and theoretical modeling are described in detail. The precise composition of the oil in our samples is determined via gas chromatography, mass spectrometry, and X-ray scattering techniques. The measurements indicate that, indeed, the material system of Myrcia sylvatica essential oil may be successfully employed for the design of organic photovoltaic devices. The optical absorption of the molecules that compose the oil are calculated using time-dependent density functional theory and used to explain the measured UV-Vis spectra of the oil. We show that it is sufficient to consider the α-bisabolol/cadalene pair, two of the main constituents of the oil, to obtain the main features of the UV-Vis spectra. This finding is of importance for future works that aim to use Myrcia sylvatica essential oil as a photovoltaic material.

Combined UMC— DFT prediction of electron-hole coupling in unit cells of pentacene crystals

Pentacene is an organic semiconductor that draws special attention from the scientific community due to the high mobility of its charge carriers. As electron-hole interactions are important aspects in the regard of such property, a computationally inexpensive method to predict the coupling between these quasi-particles is highly desired. In this work, we propose a hybrid methodology of combining Uncoupled Monte Carlo Simulations (UMC) and Density functional Theory (DFT) methodologies to obtain a good compromise between computational feasibility and accuracy. As a first step in considering a Pentacene crystal, we describe its unit cell: the Pentacene Dimer. Because many conformations can be encountered for the dimer and considering the complexity of the system, we make use of UMC in order to find the most probable structures and relative orientations for the Pentacene-Pentacene complex. Following, we carry out electronic structure calculations in the scope of DFT with the goal of describing the electron-hole coupling on the most probable configurations obtained by UMC. The comparison of our results with previously reported data on the literature suggests that the methodology is well suited for describing transfer integrals of organic semiconductors. The observed accuracy together with the smaller computational cost required by our approach allows us to conclude that such methodology might be an important tool towards the description of systems with higher complexity.

Optical and electronic structure description of metal-doped phthalocyanines

AbstractPhthalocyanines represent a crucial class of organic compounds with high technological appeal. By doping the center of these systems with metals, one obtains the so-called metal-phthalocyanines, whose property of being an effective electron donor allows for potentially interesting uses in organic electronics. In this sense, investigating optical and electronic structure changes in the phthalocyanine profiles in the presence of different metals is of fundamental importance for evaluating the appropriateness of the resulting system as far as these uses are concerned. In the present work, we carry out this kind of effort for phthalocyanines doped with different metals, namely, copper, nickel, and magnesium. Density functional theory was applied to obtain the absorption spectra, and electronic and structural properties of the complexes. Our results suggest that depending on the dopant, a different level of change is achieved. Moreover, electrostatic potential energy mapping shows how the charge distribution can be affected by solar radiation. Our contribution is crucial in describing the best possible candidates for use in different organic photovoltaic applications. Graphical AbstractRepresentation of meta-phthalocyanine systems. All calculations of this work are based on varying metal position along z axis, considering the z-axis has its zero point matching with the center of phthalocyanine cavityconsidering.

Multi-scale modeling of metal-CNT interfaces

The authors studied the impact of contact materials on CNTFET behavior using multiscale modeling and simulation framework. A strong correlation between metal-CNT coupling strength, contact length and contact resistance was found. The atomistic simulation was used to adjust the contact model used within the transport studies at the device level.