IMPACT PROCESSES IN ELECTROLUMINESCENCE

In gas discharge lamps, atoms are excited or ionized by impact by energetic electrons, which gain their energy from the applied voltage. It has long been thought that one could have a solid state analogue giving bright efficient lamps but progress has been disappointing. Even in commercially-available ZnS electroluminescent panels the efficiency is only of the order of 1%. Ferd Williams was interested in finding what physical principles imposed a limit on the efficiency. A brief survey is given of impact processes and of the theoretical approaches to the calculation of impact cross-sections of centres in crystals. At present, only simple models are tractable. In some circumstances experimental data from optical measurements can be used to estimate the magnitude of impact cross-sections. One can draw some general conclusions about the parameters necessary for useful impact excitation or ionization of luminescent centres. It is suggested that two centres, one for the impact process and one for luminescence, may be necessary for further improvement.