Changes in the photoconductivity of sputtered films of cadmium sulphide resulting from oxygen chemisorption

The effect which O₂ has on the photocurrent flowing parallel to the surface in sputtered thin films of cadmium sulphide is investigated. The photocurrent was observed to be inversely related to the partial pressure of O₂ in a flowing N₂ environment. By using Wolkenstein's theory of “weak” and “strong” chemisorption, an expression describing the rate at which gas introduced surface states become filled is derived, $\text{d}\text{N′}{}_{\mathrm{<mtext>s</mtext>}}\text{d}\text{t}\text{= aN}{}^1{}_{\mathrm{<mtext>s</mtext>}}\text{exp}\text{( ? bN′}{}_{\mathrm{<mtext>s</mtext>}}\text{) ? c × ×}\text{exp}\text{(bN′}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$, where N′_s is the density of surface states introduced by the gas being detected. Relating N′_s(t) to the photocurrent I_p(t) allows comparison of experimental and theoretical curves. The chemisorption time constant τ_c is found to be inversely related to gas partial pressure, light intensity, and temperature.