Effective mass and ground state of AlAs quantum wells studied by magnetoresistance measurements

Transport properties of electrons confined in AlAs quantum wells were studied using low temperature magnetoresistance measurement. The structure used is quantum wells consisting of AlAs channel layers sandwiched by Si-doped Al_0.45Ga_0.55As barrier layers. This structure enables us to confine electrons in the X valleys of AlAs. The electron confinement in the AlAs quantum wells were confirmed by a mobility enhancement at low temperatures. Magnetoresistance measurements showed clear Shubnikov-de Haas oscillations for the magnetic field parallel to the growth direction. A large effective mass of (0.55±0.05) m₀, which is close to √m_{tm l} (m_t:transverse effective mass, m_l: longitudinal effective mass), was obtained from the temperature dependence of the oscillations. This indicates that the ground state is X_x, X _y, even though X_z was expected to be the ground state due to its large effective mass parallel to the growth direction. This changeover of AlAs X valley states can most likely be attributed to a strain-induced energy shift caused by the small lattice mismatch between GaAs and AlAs.