The Basic Significance of Oxygen Chemisorption on the Photoelectronic Properties of CdS and CdSe

The long‐known phenomenon of oxygen chemisorption on CdS and CdSe , more intensively studied recently, is shown to have a basic significance for the photoelectronic properties of thin crystals or layers. It is proposed that every “pure” crystal of CdS or CdSe is of relatively high conductivity [ ≥ 10 − 6 ( ohm cm ) − 1 ] and high photosensitivity (electron lifetime of 10 −3 sec) as prepared by standard techniques. If the crystal or layer is thin enough, oxygen adsorption obscures these properties and gives the appearance of a highly insulating insensitive material. Smaller effects are found in crystals containing deliberately incorporated impurities as long as the Fermi level at room temperature lies above the level associated with the surface state for chemisorbed oxygen. Because of the universal presence of oxygen chemisorption effects, every experiment in which the effect of heat, vacuum, or photoexcitation are investigated toward understanding the nature of photochemical changes, must be interpreted in the light of the significant effects associated with the simple desorption of oxygen.