In-situ STM and XRD studies on Nb–H films: Coherent and incoherent phase transitions

Abstract

Hydride precipitation in 25nm and 40nm epitaxial Nb-films was studied by Scanning Tunnelling Microscopy (STM) supported by X-ray diffraction (XRD) measurements. In combination, these methods yield information about the phase transition, the coherency state, the hydride precipitates’ density and size as well as their lateral distribution, at 293K. For both film thicknesses, hydride formation was detected with STM; it can be easily missed by XRD. While the 25nm film showed a coherent phase transition, the phase transition was incoherent for the 40nm film. This is in good accordance with theory. The phase transition features are found to strongly depend on the coherency state: a large number of small hydrides appear in the coherent regime while a small number of large hydrides evolve in the incoherent regime.