Microscopic thickness variation of macroscopically uniform quantum wells

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Abstract
Recently, GaAs/AlxGa1−xAs quantum wells were grown, which display remarkable uniformity, over macroscopic distances, in the splitting of their photoluminescence lines. The negligible variation of the peak splitting was interpreted as being consistent with a number of possible interface configurations, the simplest of which was abrupt, atomically smooth interfaces with discrete thickness changes of integer multiples of monolayers. However, here, further analysis of the splittings show them to be slightly below one monolayer, supporting a modification of the ‘‘atomically smooth island’’ picture to one of microroughness, defined as that having a length scale below the exciton diameter (≊15 nm), superimposed on islands larger than the exciton diameter. In addition, by extending these measurements to the ‘‘mesoscopic’’ regime by a high resolution cathodoluminescence technique, we directly show that these microrough islands can have sizes smaller than the spatial resolution of our scanning cathodoluminescence microscope, ≊60 nm.