Atomic Layer Deposition of ScF and ScAlF Thin Films.

In this paper, we present an ALD process for ScF using Sc(thd) and NHF 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), whereas soft saturation was achieved with NHF. 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 ScF process was also combined with an ALD AlF process to deposit ScAlF films. In the AlF process, AlCl and NHF were used as precursors. It was possible to modify the thermal expansion properties of ScF by Al addition. The ScF 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.