Kashet al. reply

Abstract

Kash et al. Reply:In the preceding Comment [1], Baranovskii and Thomas (BT) suggest a different explanation for the striking temperature-dependent changes we observed [2,3] in the exciton luminescence in coupled quantum wells (QWs). In this Reply, we show that the explanation of BT fails to account for the dependence of the luminescence linewidth on the photoexcited exciton density. On the other hand, this dependence is quantitatively explained by our model [3], as will be shown below. It also seems to us that the magnitude of the effect suggested by BT is far too small to account for the changes we observed.BT suggest that, because of compensation, our samples contain -10 14 cm" 3 electrons distributed over roughly twice that concentration of donor atoms. The electron distribution is highly correlated at low temperatures, but is random at high temperatures. This charge redistribution was proposed by Baranovskii et al. [4] to explain earlier photoexcitation data on donors in bulk GaAs. Since the distribution of these uncompensated electrons does not depend upon the photoexcited exciton density, BT would predict that the linewidth of the exciton photoluminescence in a coupled QW should be independent of the exciton density. As we have mentioned [3], there is a strong density dependence to the linewidth, which is explicitly shown in Fig. 1 here. The data (circles) and corresponding theory (solid curve) at a density of 3xl0 8 cm -2 are the same as those in Fig. 2Two additional densities are shown in the present figure. This strong density dependence is quantitatively accounted for by our model of a Fermi-Dirac distribution of excitons; there are no adjustable parameters among the three curves in Fig. 1. The model of BT does not account for this density dependence.The redistribution of the electrons over the donor atoms in Ref.[4] produced a change in linewidth of about 0.02 meV. The linewidth change observed in our experiments is about 4 meV, i.e., several hundred times larger. BT note that the energy change in the donor spectra of Ref.[4] results from the interaction between the quadrupole moment of the donor state and the second derivative of the fluctuating Coulomb potential. Based on a simple scaling argument, BT claim that the variation in the Coulomb potential itself is several hundred times larger, thus accounting for the different scale of the linewidth changes. However, because an exciton is electrically neutral, there is no first-order interaction with the Coulomb potential. Instead, in the case of a coupled QW under an applied electric field, each exciton has an electric dipole, which will interact with the gradient of the Coulomb potential. Using the terminology and scaling argument of -0 I • , i 0 10 20 30 TEMPERATURE (K) FIG. 1. Measured (symbols) and calculated (solid curves) full width at half maximum of the indirect exciton photoluminescence peak for a 50 A-40 A-50 A coupled quantum well at three different exciton densities. The applied electric field is 30 kV/cm. Additional details can be found in...

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