An optical study of self-assembled InxGa1-xAs/GaAs quantum dots embedded in a two-dimensional electron gas

We studied the low temperature (77 K) photomodulated reflection and transmi ssion as well as the photoluminescence at 2.2 K of a self-assembled InxGa1- xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of t he substrate in order to achieve a gradual variation of the In concentratio n along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a c haracteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra l eads to an estimate of the average value of the Fermi energy in the conduct ion band of the wetting layer (E(F)similar or equal to 13.4 meV). The varia tion of this Fermi energy along the composition gradient can be obtained fr om the spectra, and an estimate of the gradient of the density of quantum d ots along this direction can be made. A careful comparison of the variation of the critical energy of the different lines suggests that the average qu antum dot size depends on the In molar fraction of the alloy, which is seen to vary more or less linearly across the wafer diameter. (C) 2000 American Institute of Physics. [S0021-8979(00)06510-5].