Charge carrier transport through 3D assemblies of zincblende CdSe and ZnSe quantum dots in weak size-quantization regime

Abstract

The mechanism of charge carrier transport through 3D assemblies of ZnSe and CdSe quantum dots with zincblende structure in weak size-quantization regime was investigated. The Debye length in the case of ZnSe QDs was found to be 11.5 nm, i.e. almost three times larger than the average diameter of the nanocrystals constituting the films annealed at 250 °C. In CdSe QDs, on the other hand, the Debye’s length of 11.8 nm was almost twice smaller than the average crystal diameter in the films annealed at 300 °C. In the case of ZnSe QD assemblies, it was found that the predominant mechanism governing the charge carrier transport in temperature range from 380 to 650 K is the thermionic emission, with the trap levels taking part in the formation of crystal boundary barrier being located above the Fermi level. Combining temperature-dependent conductivity data with the data from optical absorption studies, the actual position of the trap level was estimated to be at about 0.37 eV (referred to the intrinsic Fermi level at the interface). In contrast to the case of ZnSe, the temperature dependence of conductivity in the case of thin films composed by 3D assemblies of CdSe QDs appeared to be much more complex. In the highest temperature region in which the temperature-dependent conductivity measurements were performed for this system (from 480 to 540 K), it was found that the thermally activated band-to-band electronic transitions govern the conductivity changes, the corresponding thermal band gap energy being 1.85 eV. In the lower-temperature region, down to 300 K, the thermionic emission was found to be predominant charge carrier transport mechanism, with trap levels being positioned above the Fermi level. The two detected trap levels were found to be located at 0.46 and 0.79 eV, corresponding to the measured conductivity activation energies of 0.84 and 0.51 eV.