Study of hydrogen-defect interaction in thin Nb film on Si substrate using positron annihilation

Nanocrystalline thin Nb films loaded with hydrogen were studied in the present work. Thin Nb films were prepared on (100) Si substrates at room temperature by cathode beam sputtering. Microstructure observations by transmission elec tron mi cros copy (TEM) revealed that the films exhibit elon gated col umnlike grains. The width of the columns is smaller than 100 nm. Two “generations” of grains can be distinguished in the columns: (i) “first generation” grains at tached directly to the Si substrate, and (ii) “second generation” grains which grow on top of the “first generation” grains. X-ray diffraction (XRD) studies revealed that the Nb films are characterized by a strong (110) texture. However, the lateral orientation of grains (i.e. in the plane of the substrate) is random. Defect studies were performed by variable energy positron annihilation spectroscopy (VEPAS) with measurement of Doppler broadening (DB) of the an ni hi la tion line. The shape of the annihilation line was characterized by the S parameter which represents a frac tion of positrons annihilating with low-momentum electrons. It was found that the virgin Nb films (i.e. free of hy dro gen) con tain a high den sity of defects. Nanocrystalline grain size leads to a sig nif i cant volume frac tion of grain bound aries con tain ing open volume vacancy-like defects. Thus, most of positrons annihilate from a trapped state in the open volume defects at grain boundaries. Subsequently, the films were step-by-step electrochemically charged with hydrogen and the evolution of microstructure with increasing hydrogen concentration was monitored. Hydrogen loading leads to a significant lattice expansion which was measured by XRD. Contrary to free standing bulk metals, the lattice expansion is highly anisotropic in thin films. The inplane expansion is prevented because the films are clamped to an elastically hard substrate. On the other hand, the out-of-plane expansion is substantially larger than in the bulk samples. Moreover, an enhanced hydrogen solubility in the a-phase in the nanocrystalline Nb films is found. Formation of the b-phase (NbH) starts at a hydrogen concentration of xH = 0.25 [H/Nb atomic ra tio], i.e. it is » 4 times higher than in bulk Nb. Using VEPAS it was found that hydrogen is trapped in vacancy-like de fects at grain bound aries. Hydrogen trapping leads to a local increase of the electron density in these defects and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. Subsequently, when the hydrogen concentration exceeds xH = 0.02 [H/Nb], all available traps at grain bound aries are already filled with hydrogen and the S parameter does not change any more. Formation of the b-phase particles leads to an introduction of new de fects, which is reflected by an increase of the S parameter at xH > 0.25 [H/Nb].