Search for fractional-charge impurities in silicon using infrared photoionization and field ionization

A search for unconfined fractional charges in Si based on fractional-charge impurity energy-level predictions of Chaudhuri, Coon, and Derkits [Phys. Rev. Lett. 45, 1374 (1980)] is described. An upper limit (95% C.L.) of 2.3x10-20 fractional charges per atom is obtained using a combination of an infrared photoionization and a field-ionization technique. This optoelectronic approach has the advantage of giving an estimate for the concentration of fractional charges, being repeatable and without mechanical techniques. By comparison, drop type experiments detect the presence of fractional-charge modulo one unit of fractional charge