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 an d composite SiOx-CdSe thin films having varying CdSe sublayer thicknesses a nd average nanocrystal sizes and, for comparison, on SiOx and CdSe single l ayers. Size-induced changes in room temperature conductivity and dark-curre nt activation energy at temperatures T > 320 K have been observed in both m ultilayers and composite films. The high-resolution electron microscopy stu dies performed have shown that: (i) the CdSe sublayers in the multilayers a re nanocrystalline with nanocrystallite size equal to the sublayer thicknes s; and (ii) the CdSe nanocrystals in the composite films are disposed in Si Ox-CdSe 'sublayers' having high CdSe volume fractions. The conclusion has b een 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 nan ocrystal 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 inter face do not appreciably affect the charge transport, due to the great condu ctivity increase, induced by the SiOx matrix. The observed size-induced cha nges in the dark conductivity and dark-current activation energy in these f ilms have been attributed to an upward quantum-size shift of the conduction band bottom in CdSe nanocrystals.