Pulsed laser deposition and structural evolution of BaF2 nanolayers in Eu-doped BaF2/Al2O3 layered optical nanocomposite thin films

Abstract

We have developed a pulsed laser deposition (PLD) geometry for the growth of uniform BaF2 nanoscale thin films, through control of the deposition conditions. Our goal is to use the BaF2 layers with controllable structure as component layers in layered optical nanocomposites. The structure of the BaF2 nanolayer evolves as a function of the layer thickness: BaF2 grows via a layer-by-layer growth mode on Al2O3; the layers are amorphous for a thickness < 3 nm, and then become nanocrystalline as the layer thickness increases. The BaF2 nanocrystals have an FCC crystal structure with a weak <111> texture that becomes stronger for thicker films. We then demonstrate that our BaF2 films can be introduced into layered Al2O3/BaF2/EuOx nanocomposite films, which allows for control of the relative position of the Eu ions and the BaF2 layer. Cross-section samples of the multilayered films show interfacial intermixing between the layers, which is related to implantation during the PLD process. This intermixing enables the incorporation of Eu ions into BaF2 layers and form a thin Eu-doped BaF2 nanolayer at the interface. The layered nanocomposite films show photoluminescence (PL) emission from Eu3+ ions, and the PL intensity changes can be correlated with the crystallinity and crystal size changes in the BaF2 layer. Our results provide guidance for achieving thin film nanocomposite materials with controllable structure and photoluminescence behavior for light emitting diodes, photovoltaics and other optical applications.