On the synthesis and thermal stability of RuN, an uncommon nitride

Ruthenium nitride (RuN) is a compound that under ambient conditions presents positive enthalpy of formation. Recently, we synthesized RuN thin films in the zinc-blende structure by radiofrequency magnetron sputtering. The effect of changing the synthesis parameters was investigated in detail, but many questions remained open. Now we are able to disclose relevant aspects of the effect of the direct energy deposition by particles striking the substrates during the film growth. This has been accomplished by modifying the configuration of the source magnets from the unbalanced to the balanced one. The thermal stability of the synthesized compound has been investigated by annealing either in ultra-high-vacuum (UHV) or in air. In UHV the nitride was stable up to about 230 °C; at higher temperatures there was a marked loss of nitrogen and formation of metallic hexagonal close-packed ruthenium was observed. In air the stability limit was close to 200 °C; at higher temperatures we observed a slow nitrogen loss and an increase of oxygen incorporation. This behavior is due to the occurrence of chemical reactions leading to the formation of a surface layer containing oxynitride, hydroxide and oxide species. At 350 °C ruthenium oxide and metallic hexagonal close-packed ruthenium are present below the surface.