Optimal nitrogen composition of hafnium nitride thin films for field-emitter array applications

The optimal nitrogen composition of HfNx films was proposed for the emitter material of field-emitter arrays for stable operation. The dependence of the electrical resistivity, oxidation resistance, and work function on the nitrogen composition was evaluated within a nitrogen composition range of 0.0–1.4. As a result, the electrical resistivity decreased with increasing nitrogen composition from N/Hf = 0.3 to 1.0, then increased rapidly for N/Hf > 1.0. The oxidation resistance improved with increasing nitrogen composition at 0.0 < N/Hf < 1.1 but deteriorated at N/Hf > 1.1. The highest oxidation resistance was achieved at N/Hf = 1.1. The work function was evaluated in an ultrahigh vacuum, then also evaluated during thermal treatment with increasing sample temperature up to 300 °C, and after cooling to room temperature. Thermal treatment lowered the work function. Most of the samples showed the lowest value at 300 °C, but some of them showed the lowest at 250 °C. The lowest work function increased with increasing nitrogen composition from N/Hf = 0.3 to 1.0 and reached a plateau at N/Hf > 1.0. The smallest difference between the work function before thermal treatment and the lowest work function was obtained at N/Hf = 1.3. Considering the dependence of these film properties on the nitrogen composition, over-stoichiometric HfN with slightly excess nitrogen, such as N/Hf = 1.1, presumably works better than the other compositions.