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Atomistic Simulation @ Cambridge

Department of Materials Science & Metallurgy

Studying at Cambridge

 

Glass Coatings

P. D. Bristowe, T. Wang and P. Warren

Sponsors: EPSRC in collaboration with NSG (Pilkington)

  • Classical and first principles calculations of atomic diffusion across thin-film optical coatings
  • Na diffusion into the coating and Ag diffusion into the glass
  • Development of thin-film diffusion barriers

Research Highlight

A first principles study of the properties of Al:ZnO and its adhesion to Ag in an optical coating

Z. Lin and P. D. Bristowe, J. Appl. Phys. 106, 013520 (2009)

A first principles density functional study of the atomistic properties of Al:ZnO nd its adhesion to Ag is presented. Optical coatings often contain interfaces between ZnO (0001) and Ag (111) layers whose bonding can be improved by incorporating small amounts of Al into the ZnO but the underlying strengthening mechanism remains unclear. It is assumed that Al relaxes the internal compressive stress in the film but the situation is complicated by the presence of hydrogen and/or water which can adsorb on the ZnO surface during fabrication of the coating. Hydrogen and/or water are known to weaken the Ag/ZnO interface particularly when it is O-terminated. In this paper it is shown that aluminum substitutes on Zn sites in ZnO and this does indeed reduce the internal stress in the layer under compression. However, it is also shown that Al segregates to the ZnO surface when it is O-terminated (but not Zn-terminated) and this reduces the propensity for hydrogen adsorption. Thus by eliminating some of the hydrogen from the ZnO surface which is more likely to be O-terminated than Zn-terminated under ambient conditions, the strength of the Ag/ZnO interface can be increased. The effect of aluminium incorporation into the ZnO layer is therefore two-fold: it relaxes the residual stresses in the coating and also improves the chemical bonding at the metal/oxide interface by removing the weakening effects of gaseous adsorption. The changes in interfacial bonding are explained in terms of an electron redistribution and compensation model. 

glass_coatings.jpg
The calculated work of separation (-Wsep), shown as scatter points, as a function of Ag-ZnO(AZO) inter-slab spacing for (a) the O-terminated and (b) the Zn-terminated interfaces with and without Al incorporation. The curves show the data fitted to a binding-energy equation. The data labeled adj have been adjusted to remove the strain energy associated with the applied compressive stress. The data labeled AZO-1 and AZO-2 refer to the position of the Al substituent adjacent to the Zn-terminated interface.