Triangular-Pulse Measurement for Hysteresis of High-Performance and Flexible Graphene Field-Effect Transistors

We use a triangular-pulse measurement technique to obtain the hysteretic electrical characteristics of flexible graphene field-effect transistors (GFETs). To minimize charge trapping, the gate-voltage scanning rate was controlled (up to 2 V/mu s) by varying the triangular-pulse rise and fall times. This method makes it possible to measure the intrinsic-like transfer characteristics of chemical vapor deposition graphene devices. The maximum electron (hole) mobility measured by a dc measurement is similar to 4800 (5200) cm(2)/Vs, whereas the maximum electron (hole) mobility measured by the triangular-pulse technique with a gate-voltage scanning rate of 0.4 V/mu s is similar to 10 600 (8500) cm(2)/Vs. For measurements with a triangular gate pulse, the shift of the Dirac voltage is less than that measured by the dc method. These results indicate that the triangular-gate-pulse measurement is a promising technique with which to determine the intrinsic properties and ambipolar transfer characteristics of GFETs.X112sciescopu

MoS2 functionalization for ultra-thin atomic layer deposited dielectrics

The effect of room temperature ultraviolet-ozone (UV-O3) exposure of MoS2 on the uniformity of subsequent atomic layer deposition of Al2O3 is investigated. It is found that a UV-O3 pre-treatment removes adsorbed carbon contamination from the MoS2 surface and also functionalizes the MoS2 surface through the formation of a weak sulfur-oxygen bond without any evidence of molybdenum-sulfur bond disruption. This is supported by first principles density functional theory calculations which show that oxygen bonded to a surface sulfur atom while the sulfur is simultaneously back-bonded to three molybdenum atoms is a thermodynamically favorable configuration. The adsorbed oxygen increases the reactivity of MoS2 surface and provides nucleation sites for atomic layer deposition of Al2O3. The enhanced nucleation is found to be dependent on the thin film deposition temperature

Atomic Layer Deposition of Dielectrics on Graphene Using Reversibly Physisorbed Ozone

Integration of graphene field-effect transistors (GFETs) requires the ability to grow or deposit high-quality, ultrathin dielectric insulators on graphene to modulate the channel potential. Here, we study a novel and facile approach based on atomic layer deposition through ozone functionalization to deposit high-K dielectrics (such as Al2O3) without breaking vacuum. The underlying mechanisms of functionalization have been studied theoretically using ab initio calculations and experimentally using in situ monitoring of transport properties. It is found that ozone molecules are physisorbed on the surface of graphene, which act as nucleation sites for dielectric deposition. The physisorbed ozone molecules eventually react with the metal precursor, trimethylaluminum to form Al2O3. Additionally, we successfully demonstrate the performance of dual-gated GFETs with Al2O3 of sub-5 nm physical thickness as a gate dielectric. Back-gated GFETs with mobilities of similar to 19 000 cm(2)/(V.s) are also achieved after Al2O3 deposition. These results indicate that ozone functionalization is a promising pathway to achieve scaled gate dielectrics on graphene without leaving a residual nucleation layer.close535

Partially Fluorinated Graphene: Structural and Electrical Characterization

Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF₄ as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF₄ plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C–F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C–F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C–F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C

Controlling the Atomic Layer Deposition of Titanium Dioxide on Silicon: Dependence on Surface Termination

Patterned fabrication depends on selective deposition that can be best achieved with atomic layer deposition (ALD). For the growth of TiO₂ by ALD using TiCl₄ and H₂O, X-ray photoelectron spectroscopy reveals a marked difference in growth on oxidized and hydrogen-terminated silicon surfaces, characterized by typical and predictable deposition rates observed on SiO₂ surfaces that can be 185 times greater than the deposition rates on hydrogen-terminated Si(100) and Si(111) surfaces. Large-scale patterning is demonstrated using wet chemistry, and nanometer-scale patterned TiO₂ growth is achieved through scanning tunneling microscopy (STM) tip-based lithography and ALD. The initial adsorption mechanisms of TiCl₄ on clean, hydrogen-terminated, and OH-terminated Si(100)-(2 × 1) surfaces are investigated in detail through density functional theory calculations. Varying the reactive groups on the substrate is found to strongly affect the probability of precursor nucleation on the surface during the ALD process. Theoretical studies provide quantitative understanding of the experimental differences obtained for the SiO₂, hydrogen-terminated, and clean Si(100) and Si(111) surfaces

Rapid Selective Etching of PMMA Residues from Transferred Graphene by Carbon Dioxide

During chemical-vapor-deposited graphene transfer onto target substrates, a polymer film coating is necessary to provide a mechanical support. However, the remaining polymer residues after organic solvent rinsing cannot be effectively removed by the empirical thermal annealing in vacuum or forming gas. Little progress has been achieved in the past years, for little is known about the chemical evolution of the polymer macromolecules and their interaction with the environment. Through in situ Raman and infrared spectroscopy studies of PMMA transferred graphene annealed in nitrogen, two main processes are uncovered involving the polymer dehydrogenation below 200 degrees C and a subsequent depolymerization above 200 degrees C. Polymeric carbons over the monolayer graphitic carbon are found to constitute a fundamental bottleneck for a thorough etching of PMMA residues. The dehydrogenated polymeric chains consist of active C=C bonding sites that are readily attacked by oxidative gases. The combination of Raman spectroscopy, X-ray photoemission spectroscopy, and transmission electron microscopy reveals the largely improved carbon removal by annealing in oxidative atmospheres. CO2 outperforms other oxidative gases (e.g., NO2, O-2) because of its oxidative strength to remove polymeric carbons efficiently at 500 degrees C in a few minutes while preserving the underlying graphene lattice. The strategy and mechanism described here open the way for a significantly improved oxidative cleaning of transferred graphene sheets, which may require optimization tailored to specific applications