  The effect of modulation doping by Be on the ferromagnetic properties of
Ga(1-x)Mn(x)As is investigated in Ga(1-x)Mn(x)As/Ga(1-y)Al(y)As heterojunctions
and quantum wells. Introducing Be acceptors into the Ga(1-y)Al(y)As barriers
leads to an increase of the Curie temperature T_C of Ga(1-x)Mn(x)As, from 70 K
in undoped structures to over 100 K with the modulation doping. This increase
is qualitatively consistent with a multi-band mean field theory simulation of
carrier-mediated ferromagnetism. An important feature is that the increase of
T_C occurs only in those structures where the modulation doping is introduced
after the deposition of the magnetic layer, but not when the Be-doped layer is
grown first. This behavior is expected from the strong sensitivity of Mn
interstitial formation to the value of the Fermi energy during growth.
