Photoconductive response of compensating impurities in photothermal ionization spectroscopy of high-purity silicon and germanium

Abstract

In photothermal ionization spectroscopy both positive and negative photoconductivity responses have been reported from compensating centers neutralized by minority carriers generated by band-edge light. Here, the response of compensating impurities in both n-type and p-type high-purity (‖NA−ND‖ ≊1010–1012 cm−3) nuclear-detector-grade silicon and germanium is reported. Negative photoconductive responses from compensating impurities were observed only when the distance the photothermally generated majority carriers traveled before recapture by shallow levels was longer than the sample length (contact to contact). We propose that in high-purity semiconductors, such as used in this study, it is the contact configuration that is responsible for the apparent rapid recombination of minority carriers which causes negative minority-carrier photoconductivity. n+nn+ or p+pp+ structures allow multiple traversals through the sample by only majority carriers. The dependence of the band-edge light generated excess carrier density on applied electric field supports this mechanism.