Limit of Electron Mobility in AlGaAs/GaAs Modulation-doped Heterostructures

We have achieved electron mobilities as high as 1.05×107 cm2/Vs at 1.5 K with an electron density of approximately 3×1011/ cm2 for modulation-doped AlGaAs/GaAs by using high purity layers with a residual acceptor concentration of 1×1013/ cm3, and relatively thick spacer layers ∼(75 nm). We found the electron scattering process caused by spatially separated ionized donors to be most important in limiting the observed low-temperature electron mobility, even in these thick-spacer-layer samples. Theoretical calculation predicts that the mobility caused by this scattering mechanism is approximately 1.6×107 cm2/Vs. The observed electron mobility exhibits an anisotropy with respect to the principal axes ([110] and [*BAR*1*BAR*10] directions) on the (001) surface. The anisotropy is such that the mobility in the [*BAR*1*BAR*10] direction is always higher than that in the [110] direction. Theoretical calculations reasonably explained this anisotropy by assuming the existence of islands at the interface which are longer in the [*BAR*1*BAR*10] direction than in the [110] direction, and revealed that the scattering caused by interface roughness was as important as that caused by ionized donors. Thus, these two major components mainly determine the observed low-temperature electron mobility. We also discuss the mobility expected for residual impurity free limit.