Electronic Transport in Semiconductor Nanocrystal Thin Films

University of Minnesota Ph.D. dissertation. June 2018. Major: Electrical/Computer Engineering. Advisor: Stephen Campbell. 1 computer file (PDF); viii, 103 pages.Semiconductor nanocrystal (NC) thin films have emerged as intriguing materials for low cost synthesis of electronic devices with size-tunable optical and electronic properties that enable unique control over operating characteristics. However, in order to fully realize the potential of these materials so that they can be effectively integrated into useful devices, greater understanding of the electronic transport properties is needed. In particular, the relationship between film morphology, surface chemistry, and disorder leads to unique challenges in engineering the performance of NC-based devices. The standard measurement techniques and modeling schemes developed for bulk semiconductors are not necessarily well suited for these challenges, so a deeper understanding of how they can be applied to semiconductor NC films and how to properly interpret the results is needed. In this thesis, the electronic conduction in two semiconductor NC material systems was explored. First, ZnO was used as a wide bandgap material that was known to have high native doping levels and electronic conduction that can approach metallic behavior. Atomic layer deposition (ALD) Al2O3 was used to passivate thin films of porous ZnO NCs, which have electronic properties that are extremely sensitive to surface oxidation reactions with ambient water vapor. This property was utilized to systematically control the conductivity of ZnO films by photochemically desorbing surface hydroxyl groups in vacuum and performing subsequent electrical measurements in situ. With this technique, we observed conductance increases of up to 105 and associated changes in transport mechanism between Mott and Efros-Shklovskii variable range hopping regimes. Through this analysis, we were able to determine the role of defect states and NC surface depletion in determining the coupling between NCs. Second, Ge NCs were studied as a narrow bandgap material with large quantum confinement effects leading to bandgap increases of up to 50%. Thermal admittance spectroscopy (TAS) and field-effect transistor (FET) measurements were used together to study charge injection in these films. We observed a change from electron conduction to hole conduction in Ge NC FETs after infilling with ALD Al2O3. The dominant barrier for transport in these FETs was determined to be minority carrier injection to the channel due to NC charging. Contact material was not observed to have any effect on the FET polarity, which, along with large hysteresis observed in I-V and C-V measurements, indicates that the transport properties are largely dominated by trap states