Direct measurement of the effective-mass renormalization in n-type modulation-doped ${\mathrm{Al}}_{0.23}$${\mathrm{Ga}}_{0.77}$As/${\mathrm{In}}_{0.08}$${\mathrm{Ga}}_{0.92}$As/GaAs quantum wells

We have made the first direct measurement of the electron effective-mass renormalization in n-type modulation-doped ${\mathrm{Al}}_{0.23}$${\mathrm{Ga}}_{0.77}$As/${\mathrm{In}}_{0.08}$${\mathrm{Ga}}_{0.92}$As/GaAs quantum wells. The technique takes advantage of the presence, in such asymmetric structures, of parity-allowed optical transitions between the two lowest conduction subbands and the lowest valence subband. We have found that the electron mass is enhanced by 17% in a well with a sheet carrier concentration of 1.25×${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}2}$.