  In this paper we describe a graphene p-n junction created by chemical doping.
We find that chemical doping does not reduce mobility in contrast to
top-gating. The preparation technique has been developed from systematic
studies about influences on the initial doping of freshly prepared graphene. We
investigated the removal of adsorbates by vacuum treatment, annealing and
compensation doping using NH3. Hysteretic behavior is observed in the electric
field effect due to dipolar adsorbates like water and NH3. Finally we
demonstrate spatially selective doping of graphene using patterned PMMA.
4-terminal transport measurements of the p-n devices reveal edge channel mixing
in the quantum hall regime. Quantized resistances of h/e^2, h/3e^2 and h/15e^2
can be observed as expected from theory.
