In this article we review Raman studies of defects and dopants in graphene as well as the importance of both for device applications. First a brief overview of Raman spectroscopy of graphene is presented. In the following section we discuss the Raman characterization of three defect types: point defects, edges, and grain boundaries. The next section reviews the dependence of the Raman spectrum on dopants and highlights several common doping techniques. In the final section, several device applications are discussed which exploit doping and defects in graphene. Generally defects degrade the figures of merit for devices, such as carrier mobility and conductivity, whereas doping provides a means to tune the carrier concentration in graphene thereby enabling the engineering of novel material systems. Accurately measuring both the defect density and doping is critical and Raman spectroscopy provides a powerful tool to accomplish this task.

https://iopscience.iop.org/article/10.1088/0953-8984/27/8/083002/pdf

Raman characterization of defects and dopants in graphene

Reduction of graphite oxide to graphene with laser irradiation

Device instabilities of graphene metal-oxide-semiconductor field effect transistors such as hysteresis and Dirac point shifts have been attributed to charge trapping in the underlying substrate, especially in SiO2. In this letter, trapping time constants around 87 μs and 1.76 ms were identified using a short pulse current-voltage method. The values of two trapping time constants with reversible trapping behavior indicate that the hysteretic behaviors of graphene field effect transistors are due to neither charge trapping in the bulk SiO2 or tunneling into other interfacial materials. Also, it is concluded that the dc measurement method significantly underestimated the performance of graphene devices.

Fast transient charging at the graphene/SiO2 interface causing hysteretic device characteristics

Air-stable, n-doped or p-doped graphene sheets on a chip were achieved by modifying the substrates with self-assembled layers of silane and polymer. The interfacial effects on the electronic properties of graphene were investigated using micro-Raman and Kelvin probe force microscopy (KPFM). Raman studies demonstrated that the phonon vibrations were sensitive to the doping level of graphene on the various substrates. Complementary information on the charge transfer between the graphene and substrate was extracted by measuring the surface potential of graphene flakes using KPFM, which illustrated the distribution of carriers in different graphene layers as well as the formation of dipoles at the interface. The Fermi level of single layer graphene on the modified substrates could be tuned in a range from −130 to 90 mV with respect to the Dirac point, corresponding to the doped carrier concentrations up to 1012 cm−2.

Control of carrier type and density in exfoliated graphene by interface engineering.

https://www.materialstoday.com/download/51085/

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0 0 0 1) by oxygen intercalation upon annealing in air

We have fabricated graphene devices on lightly doped Si substrates and show that pronounced changes in resistance versus gate voltage, R(Vg), characteristics of these devices at 77 K are induced by the variation in the charge distribution in substrate with both gate voltage and illumination. The R(Vg) of the graphene devices in the dark shows remarkable changes as the carriers in the underlying substrate go through accumulation, depletion, and inversion regimes. We demonstrate the possibility of using a graphene device as an optical-latch.

Hysteresis in the resistance of a graphene device induced by charge modulation in the substrate

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

In this work the authors establish the use of the photolithography technique by direct laser writing for fabrication of devices on bilayer graphene coated with a photoresist. This technique is simple to use, versatile, reliable, and capable of achieving good throughput. The alignment of the patterns with the graphene flakes and between different lithography steps can be performed with an accuracy of about 0.5 μm allowing the placement of electric contacts and the definition of the Hall-bar geometries in an effective way. The devices fabricated were characterized by four-terminal resistance measurements as a function of the back gate and the Hall effect. The devices show initially p-type doping, but after annealing inside the cryostat at 127 °C in a He atmosphere, the samples become n-type. Different temperature dependence resistivity behaviors are found in bilayer graphene samples with high and low carrier densities. This approach offers a high degree of flexibility for fabrication of graphene devices.

Rapid fabrication of bilayer graphene devices using direct laser writing photolithography

In this work, we establish the use of lithography technique by laser direct writing for fabricating bilayer graphene devices. This technique, which is based on direct laser writing on graphene coated with a photoresist is simple to implement, versatile, and capable of achieving good throughput. Double-layer graphene flakes were obtained by micromechanical cleavage of graphite producing large graphene samples up to 40μm in size. The presence of a bilayer of graphene on SiO 2 /Si substrate was verified by optical microscopy and resonant Raman spectroscopy. We have measured the four-terminal resistance as a function on the back-gate voltage and found initially p-type doping in graphene, but annealing inside cryostat at 127C° in He atmosphere, the samples become n-type. Our measurements show electron mobility reached values around ˜1,900 cm 2 /V.s at high electron concentration.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400025896

Jorge A. Leon

Papers

Hysteresis in the resistance of a graphene device induced by charge modulation in the substrate

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

Rapid fabrication of bilayer graphene devices using direct laser writing photolithography

Graphene Devices Processing Using Direct Writing Laser Lithography and Electrical Properties Characteristics