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

Abstract

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 ${g}_{\ensuremath{\nu}}=2$, and that the cyclotron effective mass $\sqrt{{m}_{t}{m}_{l}}=(0.52\ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}{m}_{0}$. 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.