Extended States in Correlated-Disorder GaAs/AlGaAs Superlattices
We report  the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as previous theoretical calculations suggested , in contrast to the earlier belief that all eigenstates are localized. This has been done studying the dc vertical transport and photoluminescence (PL) in GaAs-AlGaAs superlattices (SL’s) with intentional correlated disorder. The spectra are compared to those obtained in ordered and uncorrelated disordered superlattices. To verify this theoretical prevision we grew several n-i-n heterostructures (being i the undoped SL”s) by molecular beam epitaxy. All SL’s have 200 period and Al 0.3 Ga 0.7 As barriers 3.2 nm thick. In the Ordered-SL all the 200 wells are identical with thickness 3.2 nm (hereafter referred to as A wells). In the Random-SL, 58 A wells are replaced by wells of thickness 2.6 nm (hereafter referred to as B wells) and this replacement is done randomly. The so-called Random dimer-SL is identical to the Random-SL with the additional constraint that the B wells appear only in pairs. X-ray diffraction experiments confirm that the dimer constraint intentionally introduced during sample growth is the only difference between the Random and Random Dimer-SL.
We measured  the vertical dc resistance of our sample at dark as a function of temperatures. The resistance of the Random Dimer-SL is very similar to the resistance of the Ordered-SL for temperatures below 50K, and the small differences are due to the different miniband-width between the two. On the other hand, Random- SL shows a much higher resistance in this range of temperature. This is due to the presence of extended states in the Random Dimer-SL showing transport properties very similar to a Ordered-SL. According to theoretical studies , these extended states in Random Dimer-SL’s are not Bloch-like, as occurs in Ordered-SL’s. PL experiment confirm this interpretation. The PL peak of the Ordered-SL is at the lower energy among the three SL’s. The PL peak of the Random-SL shifts towards higher energies compared with the other two samples. In this SL the intentional disorder introduced by the random distribution of thinner wells B (2.6 nm) localizes the electronic states . The PL peak of the Random Dimer-SL is red-shifted with respect to the PL peak for the Random-SL. This red-shift of the PL peak is due to the formation of a miniband with tunnel process for carriers between the GaAs wells. The position of the electronic levels were calculated with the Kronig-Penney model and calculation show that the Ordered-SL and the Random Dimer-SL exhibit extended electronic states . The experimental PL positions of the three SL’s are in very good agreement with the calculated ones. This is completely consistent with the above interpretation of the transport experiments.