Extrinsic Photoconductivity of Fast‐Electron‐Irradiated Undoped CdS Single Crystals

The extrinsic photoconductivity of undoped platelet‐type CdS crystals irradiated with 280 keV electrons was investigated at liquid nitrogen and room temperatures in the photon energy range 0.1 to 2.5 eV, using polarized light in the range 0.6 to 2.5 eV. A sensitive ac method was used, superimposing a chopped measuring beam on a constant bias illumination level. At least six photoconductivity bands were found. The bands in the range 0.6 to 2.5 eV are weakly polarized, the higher intensity occurring for E ⟂ c. The fine structure of the low‐energy quenching band previously observed by Hemila and Bube [28] was reproduced but was found to be caused by atmospheric absorption bands. Most of the bands in the photoconductivity spectrum correspond to absorption bands found by Boyn [8] in thick crystals. — The present results, assuming the threshold energies for atomic displacement in CdS determined by Kulp and Kelley [2, 3] to be valid, together with the results of trap analysis made also in this work, absorption measurements [8], and our earlier investigations of the trap spectrum, electron lifetime, and dark conductivity of deuteron‐bombarded CdS crystals [27] suggest an assignment of the photoconductivity bands to intrinsic lattice defects and to donor‐acceptor associates involving intrinsic defects and residual impurities.