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We report on optical measurements of a two-dimensional electron gas near the metal-insulator transition. We observe the appearance of excitons and negatively charged excitons, X^-, at the onset of the transition. The fact that these excitons appear at a relatively large average electron density shows that transition is induced by localization of single electrons in the electrostatic potential fluctuations of the remote ionized donors.
Finkelstein et al. (Mon,) studied this question.