Scattering anisotropy in HgTe (013) quantum well

We report on a detailed experimental study of the electron transport anisotropy in HgTe (013) quantum well of 22 nm width in the directions $[100]$ and $[03\bar{1}]$ as the function of the electron density $n$. The anisotropy is absent at minimal electron density near the charge neutrality point. The anisotropy increases with the increase of n and reaches about 10% when the Fermi level is within the first subband H1. There is a sharp increase of the anisotropy (up to 60%) when the Fermi level reaches the second subband E2. We conclude that the first effect is due to the small intra-subband anisotropic interface roughness scattering, and the second one is due to the strongly anisotropic inter-subband roughness scattering, but the microscopical reason of such a strong change in the anisotropy remains unknown.