Interactions of oxygen with surfaces of cadmium sulfide single crystals

Abstract The surface photo-voltage of cadmium sulfide, after bake-out in high vacuum, increases irreversibly with increasing oxygen pressure. At a given pressure, the surface photo-voltage is proportional to the logarithm of the light intensity, which is also true for unbaked crystals. Saturation of the photo-voltage was not obtained, even for light intensities as high as 5 × 10 18 quanta cm 2 sec . Parallel experiments on the photo-enhanced reaction of oxygen with CdS surfaces permit the elucidation of some of the possible effects. Thermally induced defects have helped to distinguish several processes which occur at room temperature : (a) a very slow uptake of oxygen in the dark over many days ; (b) a photo-promoted desorption of oxygen from incompletely outgassed crystals ; (c) a photo-promoted, irreversible uptake of oxygen, after rigorous outgassing of the crystal at 400°C; (d) a photo-desorption of oxygen from crystals in which thermally created defects have combined with oxygen.