Zezhao He

Buffer layer induced band gap and surface low energy optical phonon scattering in epitaxial graphene on SiC(0001)

The temperature dependent electrical properties of epitaxial graphene grown on Si-face and C-face SiC substrates were investigated by Hall measurements, respectively. Quasi-free-standing epitaxial graphene by H2 intercalation was adopted as the control. Due to a ∼200 meV band gap, the electrical conductivities of graphene on Si-face SiC showed a great increase at temperatures above 350 K compared to the other two. The opened band gap was found attributed to the existent buffer layer. The fitting results of Hall mobility indicates that the buffer layer also limits the carrier transportation of graphene grown on Si-face SiC, as it introduced low energy optical phonon scattering to its epilayer.

Preparation and electrical transport properties of quasi free standing bilayer graphene on SiC (0001) substrate by H intercalation

We investigate the temperature dependent electrical transport properties of quasi-free standing bilayer graphene on 4H-SiC (0001) substrate. Three groups of monolayer epitaxial graphene and corresponding quasi-free standing bilayer graphene with different crystal quality and layer number homogeneity are prepared. Raman spectroscopy and atomic-force microscopy are used to obtain their morphologies and layer number, and verify the complete translation of buffer layer into graphene. The highest room temperature mobility reaches 3700 cm2/V·s for the quasi-free standing graphene. The scattering mechanism analysis shows that poor crystal quality and layer number inhomogeneity introduce stronger interacting of SiC substrate to the graphene layer and more impurities, which limit the carrier mobility of the quasi-free standing bilayer graphene samples.

Quasi-free-standing bilayer epitaxial graphene field-effect transistors on 4H-SiC (0001) substrates

Quasi-free-standing epitaxial graphene grown on wide band gap semiconductor SiC demonstrates high carrier mobility and good material uniformity, which make it promising for graphene-based electronic devices. In this work, quasi-free-standing bilayer epitaxial graphene is prepared and its transistors with gate lengths of 100 nm and 200 nm are fabricated and characterized. The 100 nm gate length graphene transistor shows improved DC and RF performances including a maximum current density Ids of 4.2 A/mm, and a peak transconductance gm of 2880 mS/mm. Intrinsic current-gain cutoff frequency fT of 407 GHz is obtained. The exciting DC and RF performances obtained in the quasi-free-standing bilayer epitaxial graphene transistor show the great application potential of this material system.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates*

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors (GFETs) fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density IDS, improved transconductance g m , reduced sheet resistance R on , and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasi-free-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

High-frequency noise characterization of graphene field effect transistors on SiC substrates

Considering its high carrier mobility and high saturation velocity, a low-noise amplifier is thought of as being the most attractive analogue application of graphene field-effect transistors. The noise performance of graphene field-effect transistors at frequencies in the K-band remains unknown. In this work, the noise parameters of a graphene transistor are measured from 10 to 26 GHz and noise models are built with the data. The extrinsic minimum noise figure for a graphene transistor reached 1.5 dB, and the intrinsic minimum noise figure was as low as 0.8 dB at a frequency of 10 GHz, which were comparable with the results from tests on Si CMOS and started to approach those for GaAs and InP transistors. Considering the short development time, the current results are a significant step forward for graphene transistors and show their application potential in high-frequency electronics.

High temperature characteristics of bilayer epitaxial graphene field-effect transistors on SiC Substrate*

In this paper, high temperature direct current (DC) performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25 °C to 200 °C. At a gate voltage of −8 V (far from Dirac point), the drain-source current decreases obviously with increasing temperature, but it has little change at a gate bias of +8 V (near Dirac point). The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies. Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures. The devices exhibit almost unchanged DC performances after high temperature measurements at 200 °C for 5 hours in air ambience, demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices.