  Utilizing a novel carbon doping source, we prepared two-dimensional hole
gases in a symmetric quantum well structure in the GaAs/AlGaAs heterosystem.
Low temperature hole mobilities up to 1.2 x 10^6 cm^2/Vs at a density of 2.3 x
10^11 cm^-2 were achieved on GaAs (001) substrates. In contrast to electron
systems, the hole mobility sensitively depends on variations of the quantum
well width and the spacer thickness. In particular an increase of the quantum
well width from an optimal value of 15 nm to 18 nm is accompanied by a 35 %
reduction of the hole mobility. The quality of ultrahigh-mobility electron
systems is not affected by the employed carbon doping source.
