Defect studies of Mg films deposited on various substrates

In the present work the structure of Mg films deposited by RF magnetron sputtering was characterized using variable energy positron annihilation spectroscopy combined with scanning electron microscopy and X-ray diffraction. The effect of deposition parameters, namely temperature, type of substrate and deposition rate, on the microstructure was examined. All Mg films studied grow with the basal (0001) plane parallel with the substrate and exhibit only negligible in-plane stress. Films deposited at room temperature are characterized by nanocrystalline structure with high volume fraction of grain boundaries. and positrons are preferentially trapped in open volume defects present at grain boundaries. In these films positrons are trapped predominantly in open-volume defects present at grain boundaries. With increasing deposition temperature the mean grain size increases and the volume fraction of grain boundaries decreases. Hence, in Mg films prepared at elevated temperatures positrons are trapped mainly at misfit dislocations compensating different atomic spacing in the films and the substrate. Moreover, it was found that slow deposition rate leads to higher density of defects compared to fast deposition rate. By annealing of Mg film with thin 20 nm Pd over-layer at 300°C for 1 hour Pd layer is mixed with Mg film forming a Mg-Pd compound. The Mg-Pd phase likely contains structural open-volume defects which trap positrons.