Symmetry of the conduction-band minimum in AlP-GaP quantum wells

We investigate the properties of quasi-two-dimensional electrons in AlP quantum wells by measuring cyclotron resonance, quantum Hall effect, and Shubnikov-de Haas oscillations in modulation-doped AlP-GaP type-II quantum wells. We find that in wide AlP wells, the lowest conduction band states are in the X-t valleys transverse to the growth direction, that the valley degeneracy of this state is g(nu)=2, and that the cyclotron effective mass root m(t)m(l)=(0.52 +/- 0.01)xm(0). These results indicate that the biaxial strain resulting from the lattice mismatch of AlP quantum well with respect to the GaP substrate and barrier layers causes the longitudinal X-l valley to be lifted above the transverse X-t valley. Further, the twofold degeneracy of the X-t valley indicates that the conduction band minimum in AlP is located exactly at the X point of the Brillouin zone.