The evaluation of electron trapping parameters from conductivity glow curves in cadmium sulphide

An extensive experimental examination of conductivity glow curves and their analysis has been made on a variety of CdS crystals. A large number of analytical methods have been used to evaluate trap depths and capture cross sections. It is shown that it is possible to obtain consistent results and determine whether retrapping occurs provided an experimental procedure is adopted to produce isolated, non-overlapping, glow curves. Good agreement has been obtained between trap parameters calculated from (1) glow data and (2) photoconductive decay. The crystals examined contained up to six discrete sets of traps at depths of 0.05, 0.14, 0.25, 0.41, 0.63 and 0.83 ev. The cross sections of the first four are all smaller than 10 -20 cm 2 and consequently all four traps empty with negligible retrapping. The trap at 0.63 ev has a large cross section with a lower limit of 10 -16 cm 2 , and empties under fast retrapping conditions. If a single set of recombination centres is assumed their cross section for electron capture is 10 -18 cm 2 . The centre with a level at 0.83 ev is not a genuine trap because it dissociates when an electron is released to the conduction band. Thus although it has a large effective cross section (similar 10 -15 cm 2 ) it empties under monomolecular conditions.