  We report distinctive magnetotransport properties of a graphene p-n-p
junction prepared by controlled diffusion of metallic contacts. In most cases,
materials deposited on a graphene surface introduce substantial carrier
scattering, which greatly reduces the high mobility of intrinsic graphene.
However, we show that an oxide layer only weakly perturbs the carrier
transport, which enables fabrication of a high-quality graphene p-n-p junction
through a one-step and resist-free method. The measured conductance-gate
voltage $(G-V_G)$ curves can be well described by a metal contact model, which
confirms the charge density depinning due to the oxide layer. The graphene
p-n-p junction samples exhibit pronounced quantum Hall effect, a well-defined
transition point of the zeroth Landau level (LL), and scaling behavior. The
scaling exponent obtained from the evolution of the zeroth LL width as a
function of temperature exhibits a relatively low value of
$\kappa=0.21\pm0.01$. Moreover, we calculate the energy level for the LLs based
on the distribution of plateau-plateau transition points, further validating
the assignment of the LL index of the QH plateau-plateau transition.
