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2009 CARBON ALLOTROPESImproving graphene’s cleanliness for high field magneto-transportPulsed magnetic field magneto-transport on exfoliatedgraphene has been performed over the last few years andpreliminary results have shown anomalous Integer QuantumHall effect in disordered samples. The use of veryhigh magnetic field is required to investigate on the fundamentalelectronic states of 2D graphene, where Landaulevel degeneracy is expected to be lifted by the field. Foreven stronger magnetic field, the system eventually evolvesinto the fractional quantum Hall effect regime [Bolotin etal. Nature 462, 196 (2009)]. However, to make these investigationspossible, clean graphene devices (that is freeof defects or impurities and displaying very high mobility)are necessary. Actually, graphene devices are inescapablyheavily contaminated through the fabrication and contactingprocesses, as well as by direct exposure to air. Therefore,efficient cleaning procedures have to be found in orderto recover the intrinsic properties of graphene.Figure 16: Resistance of the graphene sample as a function ofgate voltage after several annealing processes.Figure 15: (a) Example of home-made graphene device mountedon a pulsed-field friendly sample holder (b) Optical microscopeimage of the central part of the device.Successive cleaning processes have been initiated in thelaboratory. Samples are first annealed at 250 ◦ C in secondaryvacuum. Investigations have shown that this processdrastically improves the quality of the samples which,however, quickly degrades when exposed to air. The devicesare then connected inside the experimental measurementsetup and annealed at 90 ◦ C under vacuum. This secondannealing helps in recovering high quality samples. Finally,electrical annealing [Moser et al. Appl. Phys. Lett.91, 163513 (2007)] is performed just before the magnetotransportmeasurements take place. All the aforementionedprocedures require specific know-how and often lead tosample’s destruction if not properly performed. These necessarytests (which are both time and sample consuming)have been done with home-made graphene samples (see figure15). Micro-mechanical cleaving of graphite (to obtaingraphene on a substrate) followed by electron-beam lithography,etching and metal deposition have been performedusing clean-room facilities at LAAS (CNRS).Figure 16 illustrates the influence of two of the above mentionedcleaning methods on a graphene sample. The resistanceis measured as a function of gate voltage before andafter annealing. The position of the resistance maximum aswell as the HWFM can be used to estimate the cleanlinessof the device. Further improvements have been achievedon similar home-made devices and high magnetic field experimentsare now under progress. These annealing methodshave also been successfully tested on another graphenesample (provided by the University of Manchester) and adirect comparison of the results could be made between thedisordered and clean regime. In the close vicinity of thecharge neutrality point, in the disordered case, transport isdominated by the formation of electron and hole puddlesin the sample over a large energy range. Accordingly themean Hall resistance is weak (contribution of electrons andholes to the Hall resistance cancel each other) and displayslarge fluctuations. Actually, disorder prevents the occurrenceof Landau level degeneracy lifting which could notbe observed in magnetic field as high as 60 T. A weak annealingof the device drastically changed its transport properties;at very high magnetic field the Hall resistance increases(contrary to the disordered case) although withoutdisplaying a new quantized resistance plateau. This effecttogether with the divergence of the longitudinal resistanceat very high magnetic field are interpreted as a first sign ofLandau level degeneracy lifting. Further experiments withimproved annealing processes are currently in progress toaddress the clean high field magneto-transport regime ingraphene.J.M. Poumirol, R. Ribeiro, A. Kumar, W. Escoffier13