Opptical and electrical properties of CdTe nanocrystal quantum dots passivated in amorphous TiO_2 thin film matrix

摘要

CdTe nanocrystal quantum dots sequestered in TiO_2 thin film matrix have been synthesized by r.f. sputtering from a composite CdTe/TiO_2 target. CdTe nanocrystal formation is nucleation controlled as their size (11-25 nm), dispersion and volume fraction (0.065-0.2) increases with film thickness, substrate temperature (100 deg C) and thermal treatment. The optical band gap derived from the onset of absorption coefficient showed blue shifts concurrent with the CdTe nanocrystal size reduction due to quantum size effects. These shifts, not consistent with theoretical models based on strong or weak confinement regimes, are explained on the basis of anisotropic growth and formation of CdTe nanocrystal clusters. TiO_2, in addition to being an ideal passivator and providing a barrier for carrier confinement to observe quantum effects, shows O_2 vacancy dependent conductivity modulation. Electrical conductivity variation with CdTe nanocrystal size and density is attributed to electrical coupling and tunneling behavior of carriers between CdTe nanocrystallites.