Rotation of the conduction band valleys in AlAs due to X-X-X-Y mixing

We report resonant magnetotunneling measurements of the energy dispersion n ear the third X symmetry subband edge in 60 and 70 Angstrom thick AlAs quan tum wells with GaAs barriers, grown along z = [001]. An elliptical constant energy surface is observed, oriented parallel to either [110] or [(1) over bar 10]. This rotation of 45 degrees with respect to the bulk AlAs Fermi s urface is explained by interface induced X-X-X-Y mixing. Our results provid e new insight into both Gamma-X-z and X-X-X-Y mixing, showing conclusively that states with both X-1 and X-3 symmetry contribute. This contrasts with several recent theoretical studies in which the X-1 contribution is zero.