HRTEM and SAED image (inset) of as-deposited HEO film, indicating that (a) film that has typical face-centered-cubic (FCC) structure, (b) low-magnification image that shows a large number of He bubbles (marked by white arrows) relatively evenly distributed in the HEO film with 0.3-Pa He implantation, (c) HRTEM image and (d)–(e) fast Fourier transform pattern of corresponding regions (marked by black rectangle in panel 4(c)). It is indicated that a large number of He bubbles (marked by red circles in panel 4(c)) with a diameter of about 1.5 nm–2 nm mainly distributed at the grain boundaries while few He bubbles existing inside grains (traced by white dash lines).