Experimental studies on the defect states at the interface between nanocrystalline CdSe and amorphous SiOx

Superlattices of a-SiOx/nc-CdSe and thin composite films of SiOx, doped wit h CdSe nanocrystals have been investigated. The CdSe nanocrystals size in b oth kinds of samples was determined by x-ray diffraction and HREM measureme nts. A significant difference has been found in the size values determined by both methods, which has been ascribed to appreciable nanocrystal lattice deformations. Subband absorption, room-temperature photoluminescence and t hermally stimulated currents have been measured. It has been observed that in the superlattices the absorption in the tail region increases as sublaye r thickness decreases. A new photoluminescence band has also appeared in th e superlattices having thinnest (2.5 nm) CdSe sublayers. Two new maximums a t about 220 K and 240 K. not existing in the CdSe single layers studied, ha ve been found in thermally stimulated current spectra of the composite film s. Both maximums are less expressed in the superlattices. The described res ults have been connected with a size-induced increase in the concentration of interface defect states in CdSe nanocrystals. It has been estimated that these defects are disposed at about 0.35 eV above the highest occupied mol ecular orbit in CdSe.