Chargetransport in CdSe nanocrystalline sublayers of SiOx/CdSemultilayers and composite SiOx-CdSe thin films

Abstract

Dark-current measurements have been carried out on SiOx/CdSe multilayers and composite SiOx-CdSethin films having varying CdSe sublayer thicknesses and averagenanocrystal sizes and, for comparison, on SiOx and CdSesingle layers. Size-induced changes in room temperatureconductivity and dark-current activation energy at temperaturesT > 320 K have been observed in both multilayers and compositefilms. The high-resolution electron microscopy studies performedhave shown that: (i) the CdSe sublayers in the multilayers arenanocrystalline with nanocrystallite size equal to the sublayerthickness; and (ii) the CdSe nanocrystals in the composite films aredisposed in SiOx-CdSe `sublayers' having high CdSevolume fractions. The conclusion has been reached that in bothmultilayers and composite films charge transport, in the layerplane, involves networks of CdSe nanocrystals contacting each other.It has been found that in the SiOx/CdSe multilayerscharge transport is controlled by potential barriers for electronsexisting at the CdSe nanocrystal interface and that the barrierheight does not exceed 0.25 eV. In the SiOx-CdSecomposite films the potential barriers at the CdSe-CdSe interfacedo not appreciably affect the charge transport, due to the greatconductivity increase, induced by the SiOx matrix. Theobserved size-induced changes in the dark conductivity anddark-current activation energy in these films have been attributedto an upward quantum-size shift of the conduction band bottom inCdSe nanocrystals.