Electron mobilities in modulation-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs and pseudomorphic ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{In}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As quantum-well structures

Electron mobilities in modulation-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As and pseudomorphic ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{In}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum-well structures with high electron concentration have been measured in order to clarify their dependences on the quantum-well thickness. It has been found that the 77-K mobilities show their highest values for quantum-well widths ranging from 80 to 100 Å. To understand the experimental results, mobility calculations were performed using a two-subband transport model. The experimental dependences of electron mobility on the quantum-well thickness were reproduced in the theoretical calculations and it has been found that the conditions for the highest mobilities at 77 K correspond to the case when the Fermi energies of electrons in the ground subband become maximum with almost all the electrons populating in the ground subband. For wider quantum-well thicknesses, electrons begin to populate in the upper subband, which results in sudden reduction in electron mobilities due to the sudden increase of intersubband scattering rate and reductions in Fermi energies of electrons in the ground subband.