Charge transport and photoconductivity in the lead di-iodide layer structure.

This thesis presents the results of an investigation into the hole mobility and its temperature dependence in single crystal specimens of the layer compound Pbl2 in a direction parallel to the c-axis. Thin specimens, between 20 and 70ms thick, were cleaved from, a larger boule crystal and fitted with evaporated metal electrodes. The temperature dependence of the electrical conductivity along this axis has also been studied, as well as the photoconductive response at 290K. The hole drift mobility was investigated by the use of fast electron beam techniques. A 301ceV excitation pulse of l0nsea duration was focussed on to the specimen and generated free carriers near the top electrode. A measurement of the transit time in a pulsed applied field led to a value for the hole drift mobility At room temperature was found to lie between 1.8 and 5cm2volt-1 sec-1. For most specimens the temperature dependence of mobility above about 250K can be represented by an equation of the form /.;cc exji t/feT,where 6 represents an energy that varies between 0.025 to 0.150eV for the crystals investigated. At a particular temperature, it is found that the low mobility specimens correspond to the highest values of C ..On the basis of the results it is concluded that the hole transport takes place in a narrow band and that the mobility is determined by optical mode scattering. At temperatures below 250K, however, specimens with high show the presence of another transport mechanism in which becomes essentially temperature independent. Attempts have been made to fit the mobility results to two transport theories: Holstein's small polaron theory and the model developed by Fivaz and Mooser for transport in layer structures. Possible reasons for the range of observed optical phonon energies are discussed. Measurements of the temperature dependence of the dark conductivity revealed a trapping level 0.55eV below the bottom of the conduction band, which hitherto had not been detected. In addition a defect trapping level 0.26eV above the valence band was found in agreement with previous workers. At temperatures greater than 400K to l4-50K,.the electrical properties are determined by an ionic conduction mechanism with an activation energy of 1.3eV. This appears to be connected with the creation and movement of Pb2+interstitials. The room temperature photoconductive response shows the absorption edge to be located at about 2.4eV. On the basis of the present results and those of other workers, an energy level scheme for PbI2 is proposed which can account for the majority of the observed features.