  Thanks to their multi-valley, anisotropic, energy band structure,
two-dimensional electron systems (2DESs) in modulation-doped AlAs quantum wells
(QWs) provide a unique platform to investigate electron interaction physics and
ballistic transport. Indeed, a plethora of phenomena unseen in other 2DESs have
been observed over the past decade. However, a foundation for sample design is
still lacking for AlAs 2DESs, limiting the means to achieve optimal quality
samples. Here we present a systematic study on the fabrication of
modulation-doped AlAs and GaAs QWs over a wide range of AlxGa1-xAs barrier
alloy compositions. Our data indicate clear similarities in modulation doping
mechanisms for AlAs and GaAs, and provide guidelines for the fabrication of
very high quality AlAs 2DESs. We highlight the unprecedented quality of the
fabricated AlAs samples by presenting the magnetotransport data for low density
(~1X1011 cm2) AlAs 2DESs that exhibit high-order fractional quantum Hall
signatures.
