Zinc Oxide Photoconduction, an Oxygen Adsorption Process

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Abstract
Photoconductive rise and decay curves of porous sintered zinc oxide are studied as a function of time (0.3 to 107 sec), ambient oxygen pressure (5×10—5 to 760 mm), temperature (110° to 373°K), incident light intensity (factor of 103), and various cycling procedures in regard to illumination. The photoconductivity is shown to be entirely due to the desorption of chemically adsorbed oxygen atoms from the surfaces of the porous sample. Light of wavelength near the fundamental absorption edge forms electron‐hole pairs of which the hole is captured by the oxygen, converting the oxygen to physically adsorbed. The physically adsorbed oxygen easily desorbs and the electron remains to increase the conductivity. The kinetics of the adsorption process when the light is removed are found to obey the Elovich equation, dq/dt=a exp(—bq), where q is the density of adsorbate, and a and b are constants. It is believed that the limiting process is electron transfer through the Schottky barrier, since it results in an equation equivalent to the empirical one above. Experimentally a continuous spectrum of time constants is found. A technique has been developed for analyzing experimental curves into component time constant processes. The Elovich equation results in time constants depending upon the intensity of illumination. The continuous spectrum of time constants can therefore be attributed to this variation of intensity with depth in the sample.

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