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Register a new userResidual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene
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Integration of graphene with Si microelectronics is very appealing by offering potentially a broad range of new functionalities. New materials to be integrated with Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etch and electrochemical delamination methods with respect to residual sub-monolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection x-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10$^{13}$ atoms/cm$^{2}$. These metal impurities appear to be partly mobile upon thermal treatment as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics these results reveal that further progress in synthesis, handling, and cleaning of graphene is required on the way to its advanced electronic and optoelectronic applications.
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We report high room-temperature mobility in single layer graphene grown by Chemical Vapor Deposition (CVD) after wet transfer on SiO$_2$ and hexagonal boron nitride (hBN) encapsulation. By removing contaminations trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to$sim70000cm^2 V^{-1} s^{-1}$ at room temperature and$sim120000cm^2 V^{-1} s^{-1}$ at 9K. These are over twice those of previous wet transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room temperature mobilities$sim30000 cm^2 V^{-1} s^{-1}$. These results show that, with appropriate encapsulation and cleaning, room temperature mobilities well above $10000cm^2 V^{-1} s^{-1}$ can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
Chemical vapor deposited (CVD) graphene is often presented as a scalable solution to graphene device fabrication, but to date such graphene has exhibited lower mobility than that produced by exfoliation. Using a boron nitride underlayer, we achieve mobilities as high as 37,000 cm^2/Vs, an order of magnitude higher than commonly reported for CVD graphene and better than most exfoliated graphene. This result demonstrates that the barrier to scalable, high mobility CVD graphene is not the growth technique but rather the choice of a substrate that minimizes carrier scattering.
Graphene is a material with enormous potential for numerous applications. Therefore, significant efforts are dedicated to large-scale graphene production using a chemical vapor deposition (CVD) technique. In addition, research is directed at developing methods to incorporate graphene in established production technologies and process flows. In this paper, we present a brief review of available CVD methods for graphene synthesis. We also discuss scalable methods to transfer graphene onto desired substrates. Finally, we discuss potential applications that would benefit from a fully scaled, semiconductor technology compatible production process.
We demonstrate a high-yield fabrication of non-local spin valve devices with room-temperature spin lifetimes of up to 3 ns and spin relaxation lengths as long as 9 $mu$m in platinum-based chemical vapor deposition (Pt-CVD) synthesized single-layer graphene on SiO$_2$/Si substrates. The spin-lifetime systematically presents a marked minimum at the charge neutrality point, as typically observed in pristine exfoliated graphene. However, by studying the carrier density dependence beyond n ~ 5 x 10$^{12}$ cm$^{-2}$, via electrostatic gating, it is found that the spin lifetime reaches a maximum and then starts decreasing, a behavior that is reminiscent of that predicted when the spin-relaxation is driven by spin-orbit interaction. The spin lifetimes and relaxation lengths compare well with state-of-the-art results using exfoliated graphene on SiO$_2$/Si, being a factor two-to-three larger than the best values reported at room temperature using the same substrate. As a result, the spin signal can be readily measured across 30 $mu$m long graphene channels. These observations indicate that Pt-CVD graphene is a promising material for large-scale spin-based logic-in-memory applications.
We realize and investigate ionic liquid gated field-effect transistors (FETs) on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under electron accumulation, the performance of these devices is comparable to that of FETs based on exfoliated flakes. FETs on CVD-grown material, however, exhibit clear ambipolar transport, which for MoS2 monolayers had not been reported previously. We exploit this property to estimate the bandgap {Delta} of monolayer MoS2 directly from the device transfer curves and find {Delta} $approx$ 2.4-2.7 eV. In the ambipolar injection regime, we observe electroluminescence due to exciton recombination in MoS2, originating from the region close to the hole-injecting contact. Both the observed transport properties and the behavior of the electroluminescence can be consistently understood as due to the presence of defect states at an energy of 250-300 meV above the top of the valence band, acting as deep traps for holes. Our results are of technological relevance, as they show that devices with useful optoelectronic functionality can be realized on large-area MoS2 monolayers produced by controllable and scalable techniques.
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