Use of paramagnetic resonance to detect optical processes in photoconductors

Abstract

In photoconductivity the light induced transfer of charge from one impurity or defect site in the lattice to another is a common process. It is in fact while the charge is free that it contributes to the photocurrent. As a result the valence of the sites involved in the process will change by one. In principle this valence change should be observable using paramagnetic resonance absorption techniques because at each site we are going from a state with an even number of electrons to an odd number or vice versa. The use of paramagnetic resonance to detect light induced charge transfer will be illustrated by several experiments carried out on II–VI compounds doped with various impurities. In addition to the observed valence change, interaction between defects in the lattice has been detected. The resonance technique is useful in that it isolates a single microscopic process occurring during photoconductivity and hence is a definite aid in setting up a model for the photoconductive process. It will also be shown how the apparatus used to detect paramagnetic resonance can be used to measure microwave photoconductivity. The measurement does not require any electrodes on the sample.