Charge transport in CdSe nanocrystalline sublayers of SiOx/CdSe multilayers and composite SiOx-CdSe thin films

Dark-current measurements have been carried out on SiOx/CdSe multilayers and composite SiOx-CdSe thin films having varying CdSe sublayer thicknesses and average nanocrystal sizes and, for comparison, on SiOx and CdSe single layers. Size-induced changes in room temperature conductivity and dark-current activation energy at temperatures T > 320 K have been observed in both multilayers and composite films. The high-resolution electron microscopy studies performed have shown that: (i) the CdSe sublayers in the multilayers are nanocrystalline with nanocrystallite size equal to the sublayer thickness; and (ii) the CdSe nanocrystals in the composite films are disposed in SiOx-CdSe 'sublayers' having high CdSe volume fractions. The conclusion has been reached that in both multilayers and composite films charge transport, in the layer plane, involves networks of CdSe nanocrystals contacting each other. It has been found that in the SiOx/CdSe multilayers charge transport is controlled by potential barriers for electrons existing at the CdSe nanocrystal interface and that the barrier height does not exceed 0.25 eV. In the SiOx-CdSe composite films the potential barriers at the CdSe-CdSe interface do not appreciably affect the charge transport, due to the great conductivity increase, induced by the SiOx matrix. The observed size-induced changes in the dark conductivity and dark-current activation energy in these films have been attributed to an upward quantum-size shift of the conduction band bottom in CdSe nanocrystals.