Irradiation-Induced Photoconductivity in Magnesium Oxide

Abstract

Crystals of magnesium oxide have been colored by irradiation with ultraviolet light and neutrons. The spectral distribution of optical absorption has the form of a gradually increasing tail extending from the visible region to the far ultraviolet upon which are superimposed several absorption bands. Photoconductivity in single crystals of magnesium oxide was measured by a dc method using a vibrating reed electrometer. The spectral distribution of photoconductivity is characterized by a gradually rising tail with superimposed peaks at 2.1, 3.7, and 4.8 ev, corresponding to known optical absorption bands. A photoconductivity band was found at 1.2 ev which has not been detected by optical absorption measurements. Irradiation of the crystals by ultraviolet light causes an enhancement of the photoconductivity subsequently measured in the 1.2- and 2.1-ev bands. The enhancement effect reaches a saturation level which is independent of the intensity of the ultraviolet light and which is a measure of the density of imperfections in the crystal lattice. The ultraviolet activated region can be displaced by an electric field in such a direction as to indicate that the charge carriers are holes in the valence band. Neutron irradiation of the crystals gives rise to a thermally unstable enhancement of photoconductivity throughout the spectrum and also causes an increase in the level of saturation of the ultraviolet activation. The latter increase is stable at room temperature and indicates that the neutron irradiation produces new lattice defects. This effect saturates with increasing neutron flux. An estimate of the density of lattice defects can be made from the photoconductivity. An energy-level model is proposed to explain the various photoconductivity bands and the enhancement and saturation effects.