We report an experimental investigation of the residual (n-type) and intentional (p-type) doping level of < 11-20> epitaxial layers grown on a-cut 4H-SiC substrates. Using SIMS, C(V) measurements, low temperature photoluminescence and Hall effect investigations, we show that nitrogen incorporates 3 times more than usually found for < 0001> surfaces. Conversely, aluminum incorporates 8 times less. Altogether, this is in excellent agreement with previous results from stepcontrolled epitaxy.