Inhomogeneous microstrain in cylindrical semiconductor heterostructures and its influence on the adiabatic motion of electrons

Abstract

We analyze fluctuation of the layer thicknesses and its influence on the strain state of (In,Ga)As/(Al,Ga)As microtubes containing quantum well structures. In those structures a curved high-mobility two-dimensional electron gas (HM2DEG) is established. The layer thickness fluctuation studied by atomic force microscopy, x-ray scattering, and spatially resolved cathodoluminescence spectroscopy occurs on two different lateral length scales. On the shorter length scale of about 0.01 μm, we found from atomic force micrographs and the broadening of the satellite maxima in x-ray diffraction curves a very small value of the mean square roughness of 0.1 nm. However, on a longer length scale of about 1.0 μm, step bunching during epitaxial growth resulted in layer thickness inhomogeneities of up to 2 nm. The resulting fluctuation of the strain in the microtubes leads to a local variation of the chemical potential, which results in the fluctuation of the carrier density as well. This leads to a phase cancellation of the Shubnikov-de Haas oscillations in the curved HM2DEG and a reduction of the single-electron scattering time, while the electron mobility in the structures remains high. The estimated fluctuation of the carrier density agrees well with the energy fluctuation measured in the cathodoluminescence spectra of the free-electron transition of the quantum well.