Effects of substrate misorientation and background impurities on electron transport in molecular-beam-epitaxial-grown GaAs/AlGaAs modulation-doped quantum-well structures

Abstract

Single GaAs quantum wells, clad with Al0.3Ga0.7As, and modulation doped with silicon introduced in the Al0.3Ga0.7As after the quantum wells are grown have been grown by molecular-beam epitaxy on GaAs substrates tilted a few degrees from the nominal (001) plane towards either of the (111) planes. The low-field two-dimensional electron gas mobility is observed to be a function of the tilt angle (0°, 2°, 4°, 6.5°) and of the direction of tilt [towards (111)A or (111)B]. The two-dimensional electron gas mobilities in quantum-well structures grown on substrates tilted towards (111)A are larger than those in structures grown on nominally flat (001) substrates. The improvement in two-dimensional electron gas transport is attributed to an improvement in the quality of the inverted interface (i.e., GaAs grown on AlGaAs). Quantum wells grown on substrates tilted toward (111)A also exhibit larger two-dimensional electron gas mobilities than quantum wells grown on substrates tilted toward (111)B for a given angle of tilt. For quantum-well structures where interface scattering from the inverted interface is significant, the two-dimensional electron gas mobility is observed to be anisotropic and larger in the [110] direction in comparison to the [1̄10] direction. The anisotropy in electron transport in the GaAs quantum well is observed to be larger for structures where the substrate tilt is towards (111)B in comparison to (111)A. For quantum wells grown on substrates tilted toward (111)A the anisotropy in two-dimensional electron gas mobility gets progressively larger as the tilt angle gets smaller. Larger molecular-beam epitaxy machine background impurity concentrations are observed to significantly increase the magnitude of the anisotropy in two-dimensional electron gas mobility suggesting that impurities and/or defects introduced during MBE growth are the origin of the anisotropic transport.