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We often realize the value of an object, or even a friendship, only after we have lost it. This is also true of electrons in a semiconductor. When such electrons move to a higher energy level, they leave behind a "hole" - the mark of a missing electron. This hole is the "alter ego" of the electron: it carries a positive electrical charge, as opposed to the electron's negative charge. Under certain conditions, the electron that is excited to a higher energy level may move around the hole it left behind. In other words, the positively charged hole functions as a nucleus for an atom-like complex, called an exciton.
%2C Omer (15)%2C and Asaf (18).jpg "Prof. Israel Bar-Joseph with Amos (9), Omer (15), and Asaf (18)")
When a single electron moves around this hole, the exciton that forms is equivalent to a neutral hydrogen atom. Prof. Israel Bar-Joseph of the Weizmann Institute's Joseph H. and Belle R. Braun Center for Submicron Research discovered that under certain conditions a second electron may join the exciton, creating a charged exciton equivalent to a hydrogen ion. More recently, he used the special properties of these charged excitons to map with great precision the position of electrons in a semiconductor. This achievement may aid the design of advanced electronic and electrooptic components.
Bar-Joseph's method was to illuminate a segment of a semiconductor, causing the formation of excitons, which then emitted light and gradually dimmed. He and his research team followed this process using a near-field microscope they built themselves. Containing a needle-fine optical fiber probe, this microscope moves very close to the surface of the sampled matter so that it perceives light emitted by different, infinitesimally small regions on the surface. By determining the strength of the spectral line emitted by the charged exciton in each region, Bar-Joseph succeeded in finding the local electron density, and consequently mapped the electron distribution in the semiconductor.
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