Size-dependent optical edge shifts and electrical conduction behaviour of RF magnetron sputtered CdTe nanocrystals : TiO_2 composite thin films

摘要

CdTe nanocrystals sequestered and passivated in an amorphous TiO_2 thin film matrix have been prepared by RF sputtering from a composite TiO_2 :CdTe target. The CdTe nanocrystal size and volume fraction increases from 15 to 40 nm and 2 to 20/100 respectively as the film thickness increases, typically from 0.05 to 0.25 μm. A systematic dependence of the optical band edge on the CdTe nanocrystal size shows a strong quantum confinement effect. The optical edge shifts are significantly higher than the theoretical prediction based on single--particle confinement of decoupled electrons and holes. This is understood on the basis of nucleation-controlled growth of CdTe nanocrystals by direct vapour phase condensation, in which small nuclei are rapidly passivated by TiO_2 depositing at much higher rates. The nano-sized CdTe growth island thus formed comprises of several TiO_2 passivated nanocrystals. Electrical conduction behaviour of these films show that tunnelling between the CdTe nanocrystals is not a dominant mechanism, as a three-dimensional network is not realized due to small thickness and lower coverage. The current transport is essentially space-charge--limited. The injection of electrons from nano-sized CdTe crystals follows spherical radial space charge flow which modifies the usual power law dependence from quadratic to 3/2. The analytical description of the current conduction process in composite CdTe:TiO_2 is discussed.