▎ 摘　　要

In this letter, we report on transport measurements of epitaxial graphene on SiC(0001) with oxygen adsorption. In a 50 x 50 mu m(2) size Hall bar, we observe the half-integer quantum Hall effect with a transverse resistance plateau quantized at filling factor around 2, an evidence of monolayer graphene. We find low electron concentration of 9 x 10(11) cm(-2) and we show that a doping of 10(13) cm(-2) which is characteristic of intrinsic epitaxial graphene can be restored by vacuum annealing. The effect of oxygen adsorption on carrier density is confirmed by local angle-resolved photoemission spectroscopy measurements. These results are important for understanding oxygen adsorption on epitaxial graphene and for its application to metrology and mesoscopic physics where a low carrier concentration is required. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729824]