Enhancement of the in-plane effective mass of electrons in modulation-doped In(x)Ga(1-x)As quantum wells due to confinement effects

We present investigations on the in-plane effective mass of conduction electrons in pseudomorphic, strained GaAs/InsubxGasub1minusxAs/AlsubyGasub1minusyAs quantum wells. The samples are modulation doped by silicon leading to electron sheet densities in the range of 10high12cmhighminus2 in the Insub1GasubqminusxAs layers. In photoluminescence experiments at low temperature we observe that all electrons of the two-dimensional electron gas up to the Fermi energy contribute to the luminescence. This leads to an asymmetric broadening of the luminescence line shape and indicates a breakdown of the k-conservation rule. This offers the possibility of determining the Fermi energy from the low-temperature spectra. From contactless microwave Shubnikov-de Haas measurements we determine a quantity correlated to the sheet carrier density. By combining both methods we deduce the in-plane effective electronic mass and investigate its dependence on confinement. We observe a slight increase of the mass due to the built-in strain of the pseudomorphic layers and a strong increase due to confinement effects by up to 40% for 2-nm wells. Self-consistent calculations of the electronic-energy levels, the wave functions, and the perpendicular effective mass show that the observed dependence of the effective mass on the confinement is supported from a theoretical point of view. We compare the in-plane effective mass with the perpendicular one.