Mechanism of Nanovoid Formation at the ZnO-Glass Interface in Planar Multilayered Structures of AlOx-ZnO-Glass

Abstract

Functional porous materials require easy fabrication methods with controllability of a wide range of pore size and its density for practical applications including optical devices. The Kirkendall effect based on unbalanced material diffusion provides such a possibility in conjunction with material configurations of multilayers. This study reports a formation of nanoscale pores within ZnO films in planar multilayered structures of Al2O3-ZnO-aluminosilicate glass and demonstrates the mechanism of forming relatively large nanopores in ZnO near the ZnO-glass interface via stress-promoted Kirkendall diffusion. Experimental characterizations supported by atomic simulation reveal that an enhanced in-plane tensile stress in the ZnO films with increasing the thickness of the neighboring Al2O3 films can promote the diffusivity of the Zn atoms and the pore growth in the ZnO films. The pore size and location in the intermediate ZnO layer of the Al2O3-ZnO-glass is alterable by simply selecting the thickness of the Al2O3 layer. Promoted diffusion of the Zn atoms enables to fabricate porous planar ZnO films with pore sizes up to a few hundred nm with an enhanced light scattering ability. These findings offer a promising route to produce porous planar films through in-depth understanding of diffusivity enhancement in glass-metal oxide couples.