Atomic Layer Deposition of ScF₃ and ScAlF Thin Films

In this paper, we present an ALD process for ScF₃ using Sc(thd)₃ and NH₄F as precursors. This is the first material made by ALD that has a negative thermal expansion over a wide-temperature range. Crystalline films were obtained at the deposition temperatures of 250-375 °C, with a growth per cycle (GPC) increasing along the deposition temperature from 0.16 to 0.23 Å. Saturation of the GPC with respect to precursor pulses and purges was studied at 300 °C. Saturation was achieved with Sc(thd)₃, whereas soft saturation was achieved with NH₄F. The thickness of the films grows linearly with the number of applied ALD cycles. The F/Sc ratio is 2.9:3.1 as measured by ToF-ERDA. The main impurity is hydrogen with a maximum content of 3.0 at %. Also carbon and oxygen impurities were found in the films with maximum contents of 0.5 and 1.6 at %. The ScF₃ process was also combined with an ALD AlF₃ process to deposit ScAlF films. In the AlF₃ process, AlCl₃ and NH₄F were used as precursors. It was possible to modify the thermal expansion properties of ScF₃ by Al³⁺ addition. The ScF₃ films shrink upon annealing, whereas the ScAlF films show thermal expansion, as measured with HTXRD. The thermal expansion becomes more pronounced as the Al content in the film is increased.