  In this Report we show the role of charge defects in the context of the
formation of electrostatically defined quantum dots. We introduce a barrier
array structure to probe defects at multiple locations in a single device. We
measure samples both before and after an annealing process which uses an
Al$_2$O$_3$ overlayer, grown by atomic layer deposition. After passivation of
the majority of charge defects with annealing we can electrostatically define
hole quantum dots up to 180 nm in length. Our ambipolar structures reveal
amphoteric charge defects that remain after annealing with charging energies of
~10 meV in both the positive and negative charge state.
