Plasma Enhanced Atomic Layer Deposition of Thin Film YSZ Electrolytes

• Author / Creator

  Zhang, Wenfei

• In order to keep the performance of solid oxide fuel cells (SOFCs) at intermediate operating temperature range (500℃ - 700℃), thin and fully dense yttria-stabilized zirconia (YSZ) electrolytes are desired to provide shorter oxygen ion transport distance and prevent fuel crossover and internal currents losses. Atomic layer deposition (ALD) is one of the promising fabrication candidates, yielding conformal and pin-hole free thin films due to its self-saturation reaction. For YSZ thin films, Y2O3 concentration, grain size, thickness, and ALD recipe setup are critical to their conductivity. All reported studies in the literature used the ALD recipe with various Y2O3 and ZrO2 ALD cycles in one ALD super-cycle to manipulate the Y2O3 concentrations. With this method, annealing is required to homogenize the distribution of Y2O3, during which the properties of thin films are possibly compromised. A recipe with both yttrium and zirconium precursor pulses in one ALD cycle was developed in this study. Plasma enhanced ALD (PEALD) YSZ thin films using tetrakis-dimethylamido zirconium (Zr(NMe2)4), Tris(methylcyclopentadienyl)yttrium (Y(MeCp)3) and oxygen gas as precursors were grown on p-type Si(100) at 150℃. Several characterization techniques, including in-situ spectroscopic ellipsometry (i-SE), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), and four-point conductivity measurements were utilized to characterize the optical, chemical, morphological and electrical performance of the YSZ electrolyte thin films. As-deposited YSZ thin films were dense and with high refractive indices (2.15 to 2.3 at 550nm), showing an outstanding oxygen ionic conductivity (0.2 S/cm at 600℃) with low activation energy (0.98 eV obtained from 400℃ to 600℃). The true size effect was considered to be responsible, introducing the intrinsic modification of the local physical properties in the vicinity of the grain boundaries. The true size effect was compromised due to the coarse grains of thick YSZ thin films, and consequently they exhibited poorer conductivity. From the results it has been found that the ALD recipe setup affects the structural and electrical properties of YSZ thin films. Annealing improved the conductivity of samples using YOZO and ZOYO recipes due to the homogenization of Y2O3 distribution. However, the conductivity of samples using YZO and ZYO recipes decreased after annealing because of the weakened true size effect.

• Subjects / Keywords

  • YSZ
  • Thin film electrolyte
  • ALD

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3Z02ZP4S

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.