Abstract
Silver interdiffusion is often detrimental to the performance of thin-film coatings and devices that contain Ag/ZnO interfaces. Using a first principles computational analysis, a strategy is proposed which limits this diffusion by incorporating substitutional donor dopants into ZnO. First principles nudged elastic band calculations indicate that interstitial silver is likely to diffuse towards its nearby interstitial positions along [0001] in ZnO, with a relatively small energy barrier of 0.75 eV. Doping ZnO with Al slightly increases this energy barrier to 0.89 eV. In Sn-doped ZnO and Sc-doped ZnO, interstitial silver may bind to Sn or Sc and a neighbouring oxygen atom to form a Ag-O-Sn/Sc trimer in the ground state which increases the Ag migration barrier to 1.01 eV and 1.34 eV, respectively. An analysis of the Bader charges and bond strengths shows that electrostatic interactions and ionic radii have a more significant impact on Ag migration than charge transfer between the interstitial and its neighbours. In particular, the binding between Ag, O and Sn/Sc is found to be the main effect responsible for inhibiting Ag diffusion in doped ZnO.
Highlights
The excellent conductivity and transmittance of Ag/ZnO based thin-films has led to their wide use in a variety of industrial applications including low-emissivity multilayer coatings in the glass industry [1] and transparent electrodes for flat-panel displays, thin-film transistors 5 and solar cells [2, 3]
The ionic geometry was optimised by the conjugate gradients (CG) algorithm with a convergence tolerance of 0.01 eV/ ̊A for the maximum force
In this study the defect was assumed to be in a charge neutral state (q = 0). This assumption is based on the observation that nominally undoped ZnO is an
Summary
The excellent conductivity and transmittance of Ag/ZnO based thin-films has led to their wide use in a variety of industrial applications including low-emissivity multilayer coatings in the glass industry [1] and transparent electrodes for flat-panel displays, thin-film transistors 5 and solar cells [2, 3]. A different AZO/Ag/AZO multilayer 20 stack deposited by radio frequency magnetron sputtering was studied by Crupi et al [8], in which a similar resistivity of 1.8 × 10−5 Ω·cm was measured It was noted in [8] that the AZO (compared to pure ZnO) film was a good barrier to Ag diffusion and contributed to the thermal stability of the multilayer up to 400 ◦C. In both studies the thermal stability was investigated by comparing the optical and electrical properties of the thin-film 25 systems before and after annealing, while no direct data on silver diffusion (concentration profile, diffusivity, etc.) were provided. The detailed diffusion mechanism in ZnO or AZO as well as across the Ag/ZnO interface remains unclear
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