Electronic defects in CdSe nanocrystals embedded in GeS2 amorphous matrix

Electronic defects in CdSe nanocrystals of a-GeS2/nc-CdSe superlattices and composite films are investigated and compared with results obtained for similar SiOx/CdSe films. A wide band of localized states centred at 0.55 eV below the conduction band edge is seen in both groups of samples and identified with defects in the nanocrystal bulk. A band at similar to 0.7 eV below the conduction band is well resolved in SiOx/CdSe samples but not seen in GeS2/CdSe films. As this feature is ascribed to defects at the CdSe-CdSe interface, a lower density of such defects is assumed in the latter case. In GeS2/CdSe samples a new band located at 0.50 eV below the conduction band appears. It is attributed to defects at the GeS2-CdSe interface. Optical absorption measurements reveal that defect concentration above the valence band of CdSe nanocrystals in GeS2/CdSe samples is lower than in SiOx/CdSe ones. Steady-state photoconductivity of GeS2/CdSe samples shows that at low temperatures the mobility-lifetime product in CdSe nanocrystals decreases with decreasing nanocrystal size. This observation is related to deep defects at the interface of CdSe nanocrystals and reflects the increasing surface to volume ratio.