Plasma Enhanced Atomic Layer Deposition Of Cobalt-based Thin Films And Their Applications In Hydrogen Evolution Reaction

Transition metal nitrides（TMNs）and transition carbides（TMCs）have attracted attention as promising electrocatalysts which are identified as alternates to expensive noble metals.Very recently,they were reported as highly active and stable electrocatalysts for hydrogen evolution reaction.ALD is renowned for its atomic-precise control of film thickness,remarkable large-scale film uniformity,and extraordinary process reproducibility.ALD utilizes alternate self-limiting surface chemistry reactions to grow the target material one layer at a time,and therefore it can enable uniform conformal film coatings on virtually any complex 3D structures.Nevertheless,there is no report on the deposition of cobalt carbide and cobalt nitride thin films by low temperature ALD to the best our knowledge.In this work,we report a new plasma-assisted ALD process for depositing cobalt carbide and cobalt nirtide at low temperatures down to 100°C,using an amidinate-type cobalt precursor,i.e.,bis（N,N′-diisopropylacetamidinato）cobalt（II）（Co（amd）₂）,along with H₂ plasma or NH₃ plasma as the coreactant.The main results are summarized as follows:（1）For the first time,we reported a new process for depositing cobalt carbide thin films,The influence of Co（amd）₂ pulse,purge after the Co（amd）₂ pulse,H₂ plasma pulse,RF power were carefully investigated at various deposition temperatures.By using Co（amd）2 as the precursor and H2 plasma as the coreactant,highly conformal and pure cobalt carbide films were successfuly deposited into 20:1 high-aspect-ratio trenches which showed a good self-limiting ALD growth behavior,with a saturated film growth rate of 0.066 nm/cycle at 80°C.High-resolution XPS and Raman measurements show that all the carbon in the as-deposited films exixts in Co-C bonds.The deposited films were nanopolycrystalline,and the crystal structure was determined by TEM to be the hexagonal Co₃C structure.The in-situ QCM was used to evaluated the deposition mechnism.In the last,the PE-ALD Co₃C_x/CC is an excellent candidate electrocatalyst for HER,which shows superior catalytic activity and stability.（2）Cobalt nitride（Co₃N_x）thin films were successfully deposited via the technique of plasma-enhanced atomic layer deposition at low temperatures down to 100°C by using Co（amd）₂ and NH₃ plasma.The deposited Co₃N_x films were fairly smooth and pure,and the x in the nominal formula of Co3Nx was determined to be approximately0.78 by XPS.Both XRD and TEM showed the Co₃N_x films formed a hexagonal phase,with a low resistivity of 182μ?cm.The PE-ALD Co₃N_x/Co₃C_0.69/CC catalyst（8:1cycle ratio）exhibited a small overpotential of-159 mV corresponding to the current density of 10 mA cm^-2 with a good long-term stability.