Achieving balance of valley occupancy in narrow AlAs quantum wells

The terahertz photoconductivity of $100μm$ and $20μm$ Hall bars fabricated from narrow AlAs quantum wells (QWs) of different widths is investigated in this paper. The photoresponse is dominated by collective magnetoplasmon excitations within the body of the Hall structure. We observed a radical change of the magnetoplasma spectrum measured precisely for AlAs QWs of widths ranging from 4 nm to 15 nm. We have shown that the observed behavior is a vivid manifestation of valley transition taking place in the two-dimensional electron system. Remarkably, we show that the photoresponse for AlAs QWs with a width of 6 nm features two resonances, indicating simultaneous occupation of strongly anisotropic $Xx−y$ valleys and isotropic $Xz$ valley in the QW plane. Our results pave the way for realizing valley-selective layered heterostructures, with potential applications in valleytronics.