mobility*

Modulation-doped Al_0.33Ga_0.67As/AlAs/Al_0.33Ga_0.67As quantum wells with electron densities of 1.4, 2.2, and 3.3 x 10¹¹ cm^-2 were grown with well widths ranging from 11 nm to 50 nm. In these samples electrons occupy in-plane valleys with large and anisotropic effective masses (longitudinal and transverse effective masses are 1.0 and 0.21, in units of free electron mass). We find that the optimized well width varies according to sample density, most likely because of scattering contributions from the second sub-band. Our highest quality samples show exceptional mobility values as high as 2.4 x 10⁶ cm²V^-1s^-1 at the density of 2.2 x 10¹¹ cm^-2, and we observe high order fractional quantum Hall states up to 8/15 in samples with 1.4 x 10¹¹ cm^-2 density. These advances clearly demonstrate that 2D electrons in AlAs now have quality comparable to modern GaAs, highlighting the system as an excellent candidate to study electron-electron interactions in an anisotropic, multi-valley environment.