(342g) Thin Films from Nanocrystals Synthesized in the Gas Phase and Coated By Atomic Layer Deposition: A Case Study of Transparent Conducting ZnO

Thin films comprised of nanocrystals find applications in a wide variety of optoelectronic devices such as thin film transistors, solar cells and light-emitting diodes. Gas phase aerosol deposition of nanocrystal films is particularly attractive because deposition rates as high as 100 nm/s are possible even at low temperatures. The surfaces of the nanocrystals play a significant role in performance and functionality of these films.  Understanding the origin of surface defects and developing strategies to mitigate them is crucial for successful applications.  In this talk, we describe a new strategy for depositing films comprised of nanocrystals and for eliminating surface traps that degrade their performance. Specifically, the nanocrystals are synthesized in the gas-phase and deposited on suitable substrates through supersonic expansion and impaction. Following, the surfaces of the nanocrystals are coated by atomic layer deposition (ALD).  This is a versatile approach since there are many material options for forming both the nanocrystal network and the ALD coating.  In the particular example that will be discussed in this talk, ZnO nanocrystals are synthesized in a nonthermal plasma containing Ar, O₂ and diethylzinc. Using this approach, we demonstrate fast deposition of nanocrystalline films of ZnO, an earth-abundant, nontoxic, low cost material that can be used in thin film transistors or as a transparent conducting oxide (TCO).  Without intentional doping or gating to fill traps, ZnO nanocrystal films have historically been too resistive because of low electron mobility and low electron concentration.  We found that surface hydroxyl groups and adsorbed water molecules act as surface traps, and are responsible for the high electrical resistivity observed in ZnO nanocrystal films.  We eliminate these surface traps by infilling the ZnO nanocrystal matrix with amorphous Al₂O₃ deposited using ALD.  The ALD film serves two purposes. First, trimethylaluminum reacts with adsorbed OH and removes it from the surface. Second, the Al₂O₃ coating act as a barrier against water adsorption. After coating with Al₂O₃, the ZnO nanocrystal network becomes conductive and exhibits a relatively high Hall effect electron mobility, 0.1 cm² V^-1 s^-1 for as-deposited particle networks and as high as 3.0 cm² V^-1 s^-1 for annealed particle networks.  We will discuss the effects of nanocrystal size and film porosity on electrical conductivity.  Larger particles and less porous films result in higher conductivity.