We report preliminary results of precision measurements of the graphene quantum Hall resistance plateau value with respect to magnetic field, temperature and applied current. Resistance value of SiC graphene Hall device was modulated by chemical doping, heating, and UV irradiation at the ambient condition. Precision measurements for R_H (filing factor v = 2) were then performed with a cryogenic current comparator resistance bridge. Relative deviation from R_H (v = 2) less than 30 parts in 10~9 can be achieved down to 7 T of magnetic field and up to 6 K of temperature with driving current of 19. 37 μA.
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