  Employing state-of-the-art molecular beam epitaxy techniques to grow thin,
modulation-doped AlAs quantum wells, we have achieved a low temperature
mobility of 5.5 m$^2$/Vs with out-of-plane occupation, an order of magnitude
improvement over previous studies. However, due to the narrow well width,
mobilities are still limited by scattering due to interface roughness disorder.
We demonstrate the successful implementation of a novel technique utilizing
thermally-induced, biaxial, tensile strain that forces electrons to occupy the
out-of-plane valley in thicker quantum wells, reducing interface roughness
scattering and allowing us to achieve mobilities as high as 8.8 m$^2$/Vs.
