This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

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The electronic properties of graphene

Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

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Single-layer MoS2 transistors

Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.

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Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

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Graphene photonics and optoelectronics

Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.

2D transition metal dichalcogenides

Abstract Human African trypanosomiasis (HAT) is a neglected tropical disease (NTD) for which adequate therapeutic and diagnostic measures are still lacking. Causative agent of HAT is the African trypanosome, a single-cell parasite, which propagates in the blood and cerebrospinal fluid of infected patients. Although different testing methods for the pathogen exist, none is robust, reliable and cost-efficient enough to support large-scale screening and control programs. Here we propose the design of a new sensor-type for the detection of infective-stage trypanosomes. The sensor exploits the highly selective binding capacity of nucleic acid aptamers to the surface of the parasite in combination with passive sensor structures to allow an electromagnetic remote read-out using terahertz (THz)-radiation. The short wavelength provides a superior interaction with the parasite cells than longer wavelengths, which is essential for a high sensitivity. We present two different sensor structures using both, micro- and nano-scale elements, as well as different measurement principles.

Towards the Development of THz-Sensors for the Detection of African Trypanosomes

We study the impact of hot-carrier degradation (HCD) on the performance of graphene field-effect transistors (GFETs) for different polarities of HC and bias stress Our results show that the impact of HCD consists in a change of both charged defect density and carrier mobility At the same time, the mobility degradation agrees with an attractive/repulsive scattering asymmetry and can be understood based on the analysis of the defect density variation

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Impact of hot carrier stress on the defect density and mobility in double-gated graphene field-effect transistors

Investigation of NiAlN as gate-material for submicron CMOS technology

Graphene is a possible candidate for advanced channel materials in future field effect transistors. This presentation gives a brief overview about recent experimental results in the field of graphene transistors for future electronic applications.

Device architectures based on graphene channels

Scaling of complementary metal oxide semiconductor (CMOS) integrated circuits has led to continuously increasing complexity and performance of electronic applications [1]. Further scaling, however, requires new dielectric materials and substitution of polysilicon gates by metal electrodes to reduce gate leakage currents and decrease equivalent oxide thickness as required by 2007 and beyond [2]. Crystalline rare earth oxides with high dielectric constants (high-k) have been demonstrated as promising candidates to replace silicon oxide or oxynitrides [3][4][5]. Titanium nitride (TiN) and fully silicided (FUSI) nickel silicide (NiSi) metal gates are receiving increased attention for their process compatibility and the potential for dual workfunctions [6][7]. In this work the compatibility of CMOS process steps with crystalline high-k dielectric layers is investigated. The influence of wet chemical cleaning, resist stripping, reactive ion etching (RIE), lithographic steps, and thermal treatments is studied on the basis of MOS structures with Gd2O3. Furthermore, different gate electrodes of TiN, NiSi and polysilicon are compared. Crystalline Gd2O3 high-k dielectric layers have been grown on p-Si (001) 6” wafers by a modified epitaxy process using granular Gd2O3 [8]. Wafers have been covered in vacuo with amorphous Si capping layer. NiSi electrodes have been fabricated by forming sputtered Ni dots with a lift-off technique and subsequent full silicidation of the amorphous Si layer at 500°C. Excessive Ni has been removed and electrodes have been completed by RIE of amorphous Si surrounding the NiSi dots. A process overview is shown in Fig. 1. Capacitance-voltage (CV) characteristics of MOS structures with tox = 7.1 nm are shown in Fig. 2. A capacitance equivalent oxide thickness of CET = 2.6 nm is calculated, yielding an effective dielectric constant of about k = 11. Leakage current density is extremely low as according to Fig. 3. Various process steps have been applied to these MOS structures for evaluation of CMOS compatibility. The resulting CV characteristics are shown in Fig. 4 and summarized in Tab. 1. Lithographic steps using resist and developer and wet chemical cleaning steps with acetone, propanol, H2SO4 (not shown in graph) have no influence on the Gd2O3. Further results on standard clean chemistry will be presented at the conference. RIE plasma processes based on HBr chemistry show no changes in the CV graph while O2-plasma stripping leads to a shift of the flatband voltage of ∆VFB = -140 mV. Thermal process compatibility of TiN, NiSi and polysilicon gate electrodes with crystalline Gd2O3 is addressed by experiments on forming gas annealing steps as well as rapid thermal activation up to 950°C. The results of this work contribute to the understanding of crucial process compatibility issues with novel crystalline Gd2O3 gate dielectrics.

http://ieeexplore.ieee.org/iel5/10640/33577/01596092.pdf

Max C. Lemme

Papers

Towards the Development of THz-Sensors for the Detection of African Trypanosomes

Impact of hot carrier stress on the defect density and mobility in double-gated graphene field-effect transistors

Investigation of NiAlN as gate-material for submicron CMOS technology

Device architectures based on graphene channels

CMOS Compatibility of Crystalline Gd/sub 2/Osub 3/ High-K / Metal Gate Stacks