Light-induced optical changes in amorphous red phosphorus

Abstract

Optical changes induced by band gap illumination were examined in amorphous red phosphorus films prepared by r.f. sputtering technique. The optical band gaps of as-sputtered films and the well-annealed films were evaluated to be 1.91 and 2.07 eV, respectively. Amorphous red phosphorus films were found to show two types of light-induced optical changes, i.e., reversible photodarkening and photodoping of noble metals, both of which are commonly observed in amorphous chalcogenide alloys; on illumination at ambient temperature, the optical band gap of the well-annealed films shifted by 0.04 eV to a lower energy, and this shift was restored by heating to a higher temperature above 200°C. The dissolution of Ag or Cu into the films was observed distinctly by light illumination. These were entirely analogous to those reported in amorphous chalcogenide alloys. A tentative model involving photodissociation was proposed for the photodarkening observed here.