Spectroscopic Search for Fractional Charge in Ultrapure Semiconductors

Abstract

A search for fractional-charge impurities in semiconductors, using photothermal ionization spectroscopy, is described. It is shown that, depending on the concentration of compensating impurities, the detection sensitivity for fractional-charge impurities in ultrapure germanium is ${10}^{7}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in a conventional configuration and, after modification ${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}3}$---i.e., one fractional charge per ${10}^{19}$ nucleons. From measured spectra of an ultrapure Ge sample it was deduced that it contained less than 1.5\ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ acceptorlike fractional-charge impurities.