# Presentation: 484743

## Strong Field-Effect in Graphene on SiC(0001) Randall Feenstra, Carnegie Mellon University, DMR-0503748

- Graphene, consisting of monolayer thick carbon, can be produced as an isolated layer (exfoliated graphite) or as an epitaxial layer on a SiC(0001) surface. We have used the latter method to produce graphene on 4H-SiC by heating at 1300°C in ultra-high vacuum with in-situ monitoring by electron diffraction. The graphene formation is performed at CMU, while transistor fabrication (with 1.5x0.5 mm2 channel area) is performed at Sarnoff Corp. A strong field-effect is found in the transistors, with mobility of 535 cm2/Vs, exceeding that of prior room temperature studies. Further improvement is likely, by modification of the graphene formation procedure to eliminate surface pits [seen in panel (c)] that are believed to form because of residual surface contamination present during the heating.

*Gu, Nie, Feenstra, Devaty, Choyke, Chan, Kane, *Appl. Phys. Lett. **90**, 23507 (2007)

- (a) Schematic cross-section of the semimetal graphene based transistor, (b) AFM image of the 4H SiC surface after H-etching prior to graphitization, and (c) AFM image after graphitization.

**Notes:**
Graphene has a unique 2-D band structure, with vanishingly small effective masses at the Fermi-level and hence a high expected mobility. Indeed in low-temperature studies of exfoliated graphite (monolayers that have been peeled off of a piece of bulk graphite), field-effect mobilities above 104 cm2/Vs have been reported.
The aim of our work is to fabricate high mobility transistors using epitaxial graphite on a SiC surface. We form the graphene by heating the sample in ultra-high vacuum as silicon is desorbed at a high temperature, leaving carbon behind. This method, although known for many decades, has been used and popularized recently by deHeer and co-workers at Georgia Tech [C. Berger, Z. Song, T. Li, X. Li, A. Y. Ogbazghi, R. Feng, Z. Dai, A. N. Marchenkov, E. H. Conrad, P. N. First, and W. A. de Heer, J. Phys. Chem. B 108, 19912 (2004)]. Using epitaxial graphene as opposed to exfoliated is advantageous for eventual device application (since the large-scale homogeneity of the exfoliated pieces is poor). The 535 cm2/Vs mobility at room temperature that we achieve is a significant advance over prior work, most likely because of improvements in the transistor design to minimize leakage and shunt resistances. Nevertheless, we hope in the future to achieve even higher mobilities which would then open up a range of new circuit applications for such devices.
To achieve higher mobilities, improvements in surface morphology are needed. Prior to heating our samples in UHV, we first hydrogen-etch them at 1700C in 1 atm molecular hydrogen in order to remove polishing damage from the SiC wafer. The result is an array of monoatomic steps (due to unintentional miscut of the wafer), as seen in panel (b). The hydrogen-etching is done using a simple strip heater (running current through a thin Ta foil on which the sample sits). The sample is then transferred through air to a UHV system where the graphitization is done by heating the sample to 1300°C by electron-bombardment on the back of the wafer on which a Mo film has been predeposited. 
Air exposure occurs between the H-etching and the graphitization steps. The final morphology of the sample is shown in panel (c), where shallow monolayer-deep pits are evident on the surface. These pits will act as scattering centers, reducing the mobility through the graphene. The pits are believed to arise from the unintentional oxidation during transfer through air of the sample between the H-etching and the UHV chambers. We are developing a single chamber system for performing both steps, discussed on the next slide, to hopefully alleviate this problem.

## Undergraduate Research Project for Improved Graphene Formation Kenji Oman and Randall Feenstra, Carnegie Mellon University

- A two-step procedure is used for forming graphene on SiC: first the SiC is heated to 1700°C in hydrogen to etch the surface thereby removing polishing damage, and second it is heated to 1300°C in vacuum to sublimate Si and leave C (graphene) on the surface. The two steps are currently performed in separate vacuum systems in Feenstra’s laboratory. A new vacuum system capable to performing both processing steps without intervening air exposure was developed by Kenji Oman, an undergraduate in Feenstra’s laboratory. The system contains two thin Ta strips that are heated resistively; one strip is used for the H-etching (and it disintegrates after that step) and the second for the graphitization. The photo shows a picture of Kenji with the system that he built up.