Search for fractional-charge impurities in semiconductors with photothermal ionization spectroscopy

The detection of possible fractional-charge impurities (FCI's) in semiconductors with photo-thermal ionization spectroscopy (PTIS) at low temperatures is analyzed. Existing formulas for the PTIS signal strength and for the minimal concentration of normal shallow majority impurities, detectable with PTIS, are extended to the case of majority FCI's. Account has been taken of semiconductor material constants, as well as temperature, background radiation, and degree of compensation. A conventional experimental setup for the detection of normal shallow impurities with PTIS is described. It is shown that this configuration can detect down to ${10}^7$ FCI's ${\mathrm{cm}}^{-3\mathrm{}}$—if present—in ultrapure germanium, depending on the concentration of minority impurities. Modifications to the experiment are proposed which release this dependence and which lower this limit down to ${10}^5$ ${\mathrm{cm}}^{-3\mathrm{}}$, i.e., one fractional charge per ${10}^{19}$ nucleons. From measured PTIS spectra of an ultrapure Ge sample it is deducted that the sample contained less than about $1.5\times {10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$ acceptorlike FCI's.