Large yield production of high mobility freely suspended graphene electronic devices on a polydimethylglutarimide based organic polymer
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(Color online) Fabrication procedure of a suspended graphene device on a LOR. (a) The Scotch tape technique is used to deposit graphene on a LOR layer. (b) For the electron beam lithography (EBL) step, we spin coat two polymethylmethacrylate (PMMA) polymer resists on top of the graphene layer, the low molecular weight resist PMMA 50 K and the high molecular weight resist PMMA 450 K. Exposure is done at 30 keV with an area dose of 180 μC/cm2. (c) Development of the exposed areas is done using xylene at 21 °C. The undercut obtained in the EBL exposed structures is necessary for successful lift-off. (d) Evaporation of Ti/Au. (e) Lift-off is done in hot xylene (T = 80 °C). (f) The parts of the graphene layer that should be suspended are exposed with the EBL at 30 keV and area dose 1050 μC/cm2. Note that instead of an EBL step, deep-UV exposure also can be used to expose the LOR [see Fig. 4(b)]. (g) Suspended graphene is obtained after removal of the exposed LOR with ethyllactate developer at 21 °C.
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(Color online) Suspended graphene device on a LOR polymer layer. (a) Optical image of a 40 μm long graphene layer with 80 nm thick Ti/Au electrodes and suspended graphene parts of (from left to right) 2, 10, 5, 4, 10, and 2 μm. (b) Picture taken with a scanning electron microscope under an angle of 70°. A zoomed view of the 10 μm long suspended graphene layer shows a small amount of LOR-A resist underneath the graphene layer that was not removed completely after 1 min of development in ethyllactate. This issue can be resolved by increasing the development time from 1 min (used for this device) to 1.5 to 2 min. The dissolution rate of the LOR layers is around 1.5 μm/min when exposed with 30 keV electrons at an area dose of 1050 μC/cm2. (c) The resistivity of the 2 μm long suspended graphene part (region A) shown already before a current annealing step; there is a clear Dirac neutrality point at Vg = 1 V applied gate voltage. All other suspended graphene regions show strong p-doping, and a current annealing step is needed to clean up the polymer remains from the suspended graphene in order to observe the Dirac neutrality point at ∼0 V gate voltage. (d) The mobility of the suspended graphene layer in region A is around 37.000 cm2/Vs at nh = 4.7 × 1010 cm−2, for which no current annealing was used. Region B shows a mobility of 600.000 cm2/Vs at ne = 5.0 × 109 cm−2 after current annealing.
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(Color online) Electronic measurements on a suspended graphene device prepared on a 1.4 μm thick LOR (300 nm thick SiO2). (a) Optical image of the sample. The outer Au electrodes are connected to a current source, and the inner electrodes are connected to a voltage probe. (b) The device was initially heavily p-doped, and after current annealing at room temperature (RT) in vacuum (10−5 mbar) we obtained a clear Dirac neutrality point at Vg ∼ 8 V. The Dirac linewidth becomes very narrow at 77 K. The resistivity increases by a factor of 5 from RT to 4.2 K. (c) The RT mobility at nh = 2.2 × 1010cm−2 is 70.000 cm2/Vs and increases to 250.000 cm2/Vs at 77 K and nh = 1.0 × 109cm−2. (d) Application of an external magnetic field B perpendicular to the suspended graphene layer at 4.2 K. The characteristic quantum resistance plateaus for graphene at 2G 0, 6G 0, and 10G 0 are clearly visible at a magnetic field of 1 T. The 2G 0 plateau is already well developed at B = 0.5 T, which is a clear indication of the excellent electronic quality of our suspended graphene device.
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Scanning electron microscope pictures showing the potential of the new technique. (a) Top view of a freely suspended graphene Hall bar at a distance of 1.15 μm from the SiO2 substrate. We introduced two extra fabrication steps in between steps e and f (Fig. 1) in order to etch a Hall bar (see Sec. V). (b) Side view under a 70° angle. Here, instead of the final EBL exposure, we introduced a short deep-UV exposure followed by development in ethyllactate in order to obtain suspended graphene (step f, Fig. 1). Note that the LOR below the gold contacts is not exposed, because the 75 nm thick gold functions as a mask layer for the deep-UV light. This shows the potential of using optical lithography for the mass production of suspended graphene devices fabricated with this technique.
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