News
Feb 11, 2009
Argon atmosphere enhances graphene-on-SiC
Innovative carbon monolayer production method avoids high-vacuum annealing and opens the subject up to a wider range of researchers.
German and US researchers have driven forward methods for producing graphene by annealing SiC substrates, producing the largest homogeneous epitaxial domains of the carbon material yet described.
The team, led by scientists from the University of Erlangen-Nuremberg, found that heating SiC in argon produces significantly better quality graphene than methods using ultra-high vacuum conditions.
Analyses of the resulting samples at Erlangen and at the Fritz-Haber-Institute, Berlin, and Lawrence Berkeley National Laboratories, California, showed graphene terraces up to 3 µm wide and 50 µm long. This compares with 30 to 200 nm for the vacuum approach.
The group's progress towards effectively producing the material comprised of a single monolayer of graphite was published online in Nature Materials on February 8. “We can unambiguously conclude that the large atomically flat macro-terraces are homogeneously covered with a graphene monolayer,” wrote author Thomas Seyller in the paper.
“We were looking for practical ways to avoid the ultra-high vacuum environment for graphene growth, which is almost impossible to scale up to mass production,” Seyller told compoundsemiconductor.net. Using a type of vertical cold wall reactor that has previously performed post-growth annealing, it seems that the goal has been attained.
“It appears much simpler than the growth under ultra-high vacuum,” commented Pierre Mallet, a leading graphene researcher at Institut Néel in France. “I believe many labs and companies will try to synthesize their own epitaxial graphene-on-SiC using this method.”
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Graphene is formed as silicon evaporates from the SiC substrate, and the 900 mbar argon atmosphere the researchers use reduces the rate at which this occurs. The argon deflects silicon atoms back towards the substrate, meaning that silicon desorption doesn't begin until 1500°C, compared to 1150°C for the vacuum method.
Erlangen's annealing method therefore occurs at 1680°C, enhancing diffusion of silicon and carbon atoms and improving surface morphology compared to the 1280°C vacuum method.
A homogeneous layer is important because a single monolayer of graphene is a gapless semiconductor, but additional carbon monolayers change the film's electronic structure.
Previously, large epitaxial graphene films have been formed on the transition metal ruthenium, but for this to be useful electronically the fragile graphene layer must be transferred to an insulating substrate. By using the insulating, on-axis, Si-terminated, Si(0001) wafers produced by substrate vendor SiCrystal, Seyller and his colleagues had a clear advantage.
Electron mobility measurements showed a highest value of 2000 cm2V-1s-1 for graphene grown in argon, compared to 710 cm2V-1s-1 for vacuum-grown material. This is well short of the 200,000 cm2V-1s-1 reported by the discoverers of graphene at the University of Manchester.
“Compared to exfoliated graphene, the mobility of our epitaxial graphene films is still much below expectations,” Seyller conceded. “We need to understand the reasons for this and develop strategies for improvement.”
Mallet suggests that the price of SiC substrates and the vertical cold-wall reactor used could also be offputting to some, but he is generally enthusiastic about the method.
“The impact will be very high,” Mallet said. “Not only the graphene-on-SiC community should be interested, but the entire condensed matter community.”
