Double-locked nucleation and growth kinetics in Nb-H thin films

Abstract

The kinetics of hydride precipitation in epitaxial Nb films are studied by means of scanning tunneling microscopy (STM) using hydrogen gas loading. Due to the clamped state of thin films, hydride formation results in strong unidirectional out-of-plane film expansion that can be easily detected with STM. Hydrides are found to initially form with cylindrical morphology, leading to typical surface topographies. Their localized expansion allows the analysis of the hydride lattice matching, which is coherent (H1) at the initial stages and semicoherent (H2) at later stages. The volume fraction of H1 and H2 precipitates changes with time. At initial stages, the coherent precipitates dominate, while at later stages semicoherent precipitates become the dominant ones. The relative occurrence of H1 and H2 is bimodal. A maximum occurrence of 30-40 nm sized H1 hydrides is found, which is related to coherency stress between the hydride and the Nb matrix hindering a further hydride growth. It is further demonstrated that for Nb-H films adhered to substrates, the system can be locked in the two-phase region of the phase diagram (here at 10(-4) Pa at about 50% of hydride). This is different from bulk Nb-H, where the complete sample transforms into a hydride when the hydride formation equilibrium pressure is exceeded. Impact parameters on the lateral hydride arrangement are studied. The impact of the Pd-island surface coating and the intrinsic dislocation network on the precipitation density and arrangement appear to be negligible. However, the substrate miscut and, thus, the surface roughness exhibit a strong influence on hydride nucleation. The H1 hydride arrangement along (111) and the directed H2 hydride growth along ( 111) are governed by the elastically soft matrix lattice orientations.