Observation of an electron-wave-function coherence length approaching the theoretical limit in a nearly ideal semiconductor-alloy superlattice.

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Differential photoconductance measurements are used to observe oblique optical transitions between Stark-localized states spanning up to 14 periods in a 30-\AA{} ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/20-\AA{} ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As superlattice. The conduction-band wave-function coherence length is thus deduced to be 1450 \AA{}, which is consistent with Stark-ladder formation when the well-to-well potential drop is about the size of the alloy-broadened linewidth. The 2-K photoluminescence linewidth agrees with the minimum calculated for a perfectly random distribution of atoms on the group III sublattice.