AVS 61st International Symposium & Exhibition
    Thin Film Wednesday Sessions
       Session TF+MS+PS-WeM

Paper TF+MS+PS-WeM4
Combining Gas Phase Aerosol Deposition with Atomic Layer Deposition for Fast Thin Film Deposition: A Case Study of Transparent Conducting ZnO

Wednesday, November 12, 2014, 9:00 am, Room 307

Session: Applied ALD: Nanoelectronics and Emerging Applications
Presenter: Elijah Thimsen, Washington University, St. Louis
Authors: E. Thimsen, Washington University, St. Louis
M. Johnson, University of Minnesota
A. Wagner, University of Minnesota
A. Mkhoyan, University of Minnesota
U.R. Kortshagen, University of Minnesota
E.S. Aydil, University of Minnesota
Correspondent: Click to Email
Atomic layer deposition (ALD) has emerged as a powerful and scalable technique for a variety of applications where layer-by-layer control over film properties and conformal deposition in tight geometries are needed. One common criticism of ALD is that it is slow and may become uneconomical when thick films and high deposition rates are needed. In fact, deposition rate is often an issue even with physical vapor deposition methods such as sputtering and also chemical vapor deposition. One way to deliver material onto a substrate at high rates is through deposition of nanoparticles. Gas phase aerosol deposition is particularly attractive because rates as high as 100 nm/s are possible even at low temperatures. However, aerosol deposition often yields porous films unsuitable for optoelectronic applications. In this talk, we describe a new two-step strategy for depositing dense thin films at high rates. Our strategy combines the high rates of aerosol deposition with advantages of ALD. In the first step nanoparticles are synthesized in the gas-phase and deposited onto suitable substrates by aerosol deposition. In the second step, the space between the nanoparticles is infilled by ALD. This is a versatile approach since there are many material options for forming both the nanoparticle network and the ALD coating. In the specific example that will be discussed in this talk, the crystalline nanoparticles are synthesized in a nonthermal plasma containing the precursors that lead to nucleation and growth of the desired material. These nanocrystals are deposited on suitable substrates through supersonic expansion and inertial impaction. Using this approach, we demonstrate fast deposition of nanocrystalline ZnO films, an earth-abundant, nontoxic, low cost material that can be used as a transparent conducting oxide (TCO), from a plasma containing Ar, O2 and diethylzinc. The space between the particles is filled either by Al2O3 or Al-doped ZnO (AZO) to give continuous TCO films. After annealing in H2 and coating with Al2O3, the ZnO nanocrystal network becomes conductive with Hall effect electron mobilities as high as 3.0 cm2 V-1 s-1. Depending on the combination of the nanocrystals, ALD coating, and post processing, we have obtained transparent films with resistivity values as low as 3.8 x10-3 Ohm cm. The lowest resistivity films were obtained with undoped ZnO nanoparticles coated with AZO. The resistivity can be improved by doping the nanocrystals, which has proven to be challenging. We will discuss the effects of nanocrystal size, doping of nanocrystals in the gas phase, and film porosity on electrical conductivity.