Photoconductivity in Anodic Oxide Films on Bismuth

Abstract

Photoconduction of bismuth anodic films in different contacting and forming electrolytes is studied as a function of formation voltage, Uf, (film thickness, lf); electrode potential, Ua; light intensity, Lo; and wavelength, λ. Oscillations of the photocurrent, Iph, were observed when the film is obtained by galvanostatic anodization, these oscillations being eliminated by prolonged constant voltage anodization. Photoconduction is observed only under anodic polarization of the oxide‐covered electrode. Iph was found to be proportional to Ua and Lo, and to increase with Uf (or lf). The spectral distribution Iph (λ) is independent of the contacting electrolyte and for films anodized in a glycolborate electrolyte or in dilute aqueous solutions shows an optical activation energy Wopt ∼ 2.8 eV. For films anodized in a phosphate electrolyte Wopt decreases to ca. 2.5 eV. This is attributed to the incorporation of ions from the electrolyte into the film. An attempt to explain the properties of the photoconduction is made on the assumption that a high density of trapping levels exists in the forbidden band of the structurally disordered anodic film.