Bilayer Graphene Transistors Research Articles

Transfer-free fabrication of graphene transistors

We invented a method to fabricate graphene transistors on oxidized silicon wafers without the need to transfer graphene layers. To stimulate the growth of graphene layers on oxidized silicon a catalyst system of nanometer thin aluminum/nickel double layer is used. This catalyst system is structured via liftoff before the wafer enters the catalytic chemical vapor deposition (CCVD) chamber. In the subsequent methane based growth process monolayer graphene field-effect transistors (MoLGFETs) and bilayer graphene transistors (BiLGFETs) are realized directly on oxidized silicon substrate, whereby the number of stacked graphene layers is determined by the selected CCVD process parameters, e.g. temperature and gas mixture. Subsequently, Raman spectroscopy is performed within the channel region in between the catalytic areas and the Raman spectra of fivelayer, bilayer and monolayer graphene confirm the existence of graphene grown by this silicon-compatible, transfer-free and in-situ fabrication approach. These graphene FETs will allow a simple and low-cost integration of graphene devices for nanoelectronic applications in a hybrid silicon CMOS environment.

Mechanical cleaning of graphene

Contamination of graphene due to residues from nanofabrication often introduces background doping and reduces electron mobility. For samples of high electronic quality, post-lithography cleaning treatments are therefore needed. We report that mechanical cleaning based on contact mode atomic force microscopy removes residues and significantly improves the electronic properties. A mechanically cleaned dual-gated bilayer graphene transistor with hexagonal boron nitride dielectrics exhibited a mobility of ∼36 000 cm2/Vs at low temperature.

Silicon-CMOS compatible in-situ CCVD grown graphene transistors with ultra-high on/off-current ratio

By means of catalytic chemical vapor deposition (CCVD) in-situ grown monolayer graphene field-effect transistors (MoLGFETs) and bilayer graphene transistors (BiLGFETs) are realized directly on oxidized silicon substrate without the need to transfer graphene layers. In-situ grown MoLGFETs exhibit the expected Dirac point together with the typical low on/off-current ratios. In contrast, BiLGFETs possess unipolar p-type device characteristics with an extremely high on/off-current ratio up to 1×107. The complete fabrication process is silicon CMOS compatible. This will allow a simple and low-cost integration of graphene devices for nanoelectronic applications in a hybrid silicon CMOS environment.

Toward Tunable Band Gap and Tunable Dirac Point in Bilayer Graphene with Molecular Doping

The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and the band gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices, such as complementary inverters.

Polymer-electrolyte gated graphene transistors for analog and digital phase detection

We present an alternating current (ac) circuit based on a misoriented bilayer graphene device for analog and digital phase detection. We exploit the ambipolar nature of the transfer characteristics of a misoriented bilayer graphene transistor. The transistor action here is realized using an electrochemical gate integrated into a solid polymer electrolyte layer. This unique combination provides a voltage gain close to unity under ambient conditions, which is one order of magnitude higher than that attainable in back-gated devices. The achieved gain provides sufficient sensitivity to detect phase differences between pairs of analog or digital signals.

1/f Noise in Graphene Field-Effect Transistors: Dependence on the Device Channel Area

ABSTRACTWe carried out a systematic experimental study of the low-frequency noise characteristics in a large number of single and bilayer graphene transistors. The prime purpose was to determine the dominant noise sources in these devices and the effect of aging on the current-voltage and noise characteristics. The analysis of the noise spectral density dependence on the surface area of the graphene channel indicates that the dominant contributions to the 1/f electronic noise come from the graphene channel region itself. Aging of graphene transistors due to exposure to ambient for over a month resulted in substantially increased noise, which was attributed to the decreasing mobility of graphene and increasing contact resistance. The noise spectral density in both single and bilayer graphene transistors shows a non-monotonic dependence on the gate bias. This observation confirms that the 1/f noise characteristics of graphene transistors are qualitatively different from those of conventional silicon metal-oxide-semiconductor field-effect transistors.

Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition

The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm(2) size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices.

The screening of charged impurities in bilayer graphene

Positively charged impurities were introduced into a bilayer graphene (BLG) transistor by n-doping with dimethylformamide. Subsequent exposure of the BLG device to moisture resulted in a positive shift of the Dirac point and an increase of hole mobility, suggesting that moisture could reduce the scattering strength of the existing charged impurities. In other words, moisture screened off the 'effective density' of charged impurities. At the early stage of moisture screening the scattering of hole carriers is dominated by long-range Coulomb scatter, but an alternative scattering mechanism should also be taken into consideration when the effective density of impurities is further lowered on moisture exposure.

Electrical and noise characteristics of graphene field-effect transistors: ambient effects, noisesources and physical mechanisms

We fabricated a large number of single and bilayer graphene transistors and carriedout a systematic experimental study of their low-frequency noise characteristics.Special attention was given to determining the dominant noise sources in thesedevices and the effect of aging on the current–voltage and noise characteristics. Theanalysis of the noise spectral density dependence on the area of graphene channelshowed that the dominant contributions to the low-frequency electronic noise comefrom the graphene layer itself rather than from the contacts. Aging of graphenetransistors due to exposure to ambient conditions for over a month resulted insubstantially increased noise, attributed to the decreasing mobility of graphene andincreasing contact resistance. The noise spectral density in both single and bilayergraphene transistors either increased with deviation from the charge neutralitypoint or depended weakly on the gate bias. This observation confirms that thelow-frequency noise characteristics of graphene transistors are qualitatively different fromthose of conventional silicon metal–oxide–semiconductor field-effect transistors.

Intrinsic limits of subthreshold slope in biased bilayer graphene transistor

In this work, we investigate the intrinsic limits of subthreshold slope in a dual gated bilayer graphene transistor using a coupled self-consistent Poisson-bandstructure solver. We benchmark the solver by matching the bias dependent band gap results obtained from the solver against published experimental data. We show that the intrinsic bias dependence of the electronic structure and the self-consistent electrostatics limit the subthreshold slope obtained in such a transistor well above the Boltzmann limit of 60 mV/decade at room temperature, but much below the results experimentally shown till date, indicating room for technological improvement of bilayer graphene.

Graphene Field-Effect Transistors with High On/Off Current Ratio and Large Transport Band Gap at Room Temperature

Graphene is considered to be a promising candidate for future nanoelectronics due to its exceptional electronic properties. Unfortunately, the graphene field-effect transistors (FETs) cannot be turned off effectively due to the absence of a band gap, leading to an on/off current ratio typically around 5 in top-gated graphene FETs. On the other hand, theoretical investigations and optical measurements suggest that a band gap up to a few hundred millielectronvolts can be created by the perpendicular E-field in bilayer graphenes. Although previous carrier transport measurements in bilayer graphene transistors did indicate a gate-induced insulating state at temperatures below 1 K, the electrical (or transport) band gap was estimated to be a few millielectronvolts, and the room temperature on/off current ratio in bilayer graphene FETs remains similar to those in single-layer graphene FETs. Here, for the first time, we report an on/off current ratio of around 100 and 2000 at room temperature and 20 K, respectively, in our dual-gate bilayer graphene FETs. We also measured an electrical band gap of >130 and 80 meV at average electric displacements of 2.2 and 1.3 V nm(-1), respectively. This demonstration reveals the great potential of bilayer graphene in applications such as digital electronics, pseudospintronics, terahertz technology, and infrared nanophotonics.

Flicker Noise in Bilayer Graphene Transistors

We present results of the experimental investigation of the low-frequency noise in bilayer graphene transistors. The back-gated devices were fabricated using the electron beam lithography and evaporation. The charge neutrality point for the transistors was around +10 V. The noise spectra at frequencies f > 10-100 Hz were of the 1/f type with the spectral density on the order of S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-23</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-22</sup> A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Hz at the frequency of 1 kHz. The deviation from the 1/f spectrum at f < 10-100 Hz suggests that the noise is of the carrier-number fluctuation origin due to the carrier trapping by defects. The Hooge parameter was determined to be as low as ~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . The gate dependence of the normalized noise spectral density indicates that it is dominated by the contributions from the ungated parts of the device and can be reduced even further. The obtained results are important for graphene electronic and sensor applications.