Impact Ionization of Free and Bound Excitons by Free Carriers in an Electric Field in CdS. Investigation by Optical Methods

Quenching of the exciton luminescence of cadmium sulfide at 1.8 K in a static electric field is observed and attributed to impact ionization of free excitons and delocalization of bound excitons by hot electrons. A continuous transition from the T ownsend‐Shockley to the Davydov‐Wolf law is observed when the field is increased to 1 kV/cm. A theoretical analysis of the results on the quenching of luminescence lines of bound excitons makes it possible to identify the mechanisms of relaxation of high‐energy electrons dominating in CdS at low temperatures. It turned out that these electrons lose their quasi‐momentum by spontaneous emission of piezo‐phonons in “pure” samples and are scattered by impurities in “less pure” samples. The mean free path of 4 meV electrons in moderate fields is 10⁻⁴ cm in the “pure” samples and 5 × 10⁻⁶ cm in the remaining samples.