Atomic layer deposition of GdF₃thin films

Gadolinium fluoride is an attractive optical material with applications in, e.g., deep-UV lithography, solar cells, and medical imaging. Despite the interest toward this material, no atomic layer deposition (ALD) process has been published. In this article, an ALD process for GdF3 using Gd(thd)3 and NH4F as precursors is presented. The deposition was studied at temperatures 275-375 °C, but 285-375 °C produce the purest films. The saturation of the growth per cycle (GPC) with respect to precursor pulses and purges was proved at 300 °C. The GPC value at this temperature is ∼0.26 Å, and the deposition temperature has very little effect on the GPC. According to x-ray diffraction, all the films consist of orthorhombic GdF3. The impurity contents, evaluated by time-of-flight elastic recoil detection analysis, is low, and the films are close to stoichiometric. The nitrogen content is less than <0.04 at. %. The antireflection properties were qualitatively evaluated by UV-vis spectrometry in a transmission mode at a 190-1100 nm range: on sapphire substrates, GdF3 serves as an antireflective coating. Dielectric properties of the films were studied, and for example, a permittivity value of 9.3 was measured for a ∼64 nm film deposited at 300 °C.