Nucleation and growth of bubbles in He ion-implanted V/Ag multilayers

Abstract

Microstructures of He ion-implanted pure Ag, pure V and polycrystalline V/Ag multilayers with individual layer thickness ranging from 1 nm to 50 nm were investigated by transmission electron microscopy (TEM). The bubbles in the Ag layer were faceted and larger than the non-faceted bubbles in the V layer under the same implantation conditions for both pure metals and multilayers. The substantially higher single defects surviving the spike phase and lower mobility of trapped He in bcc than those in fcc could account for this difference. For multilayers, the bubbles nucleate at interfaces but grow preferentially in Ag layers due to high mobility of trapped He in fcc Ag. In addition, the He concentration above which bubbles can be detected in defocused TEM images increases with decreasing layer thickness, from 0 for pure Ag to 4–5 at. % for 1 nm V/1 nm Ag multilayers. In contrast, the bubble size decreases with decreasing layer thickness, from approximately 4 nm in diameter in pure Ag to 1 nm in the 1 nm V/1 nm Ag multilayers. Elongated bubbles confined in the Ag layer by the V–Ag interfaces were observed in 1 nm multilayers. These observations show that bubble nucleation and growth can be suppressed to high He concentrations in nanoscale composites with interfaces that have high He solubility.