In situ studies of amorphization of the GeAl and SiAl systems induced by 1 MeV electron irradiation

Abstract

GeAl and SiAl bilayer specimens, and also A1–2.3at.%Ge two-phase alloy specimens, were irradiated in situ with 1 MeV electrons at temperatures in the range 10–190 K in a high voltage electron microscope. The Ge precipitates in the Al2.3at.%Ge alloy disappeared completely at a critical fluence ( Φ c) of approximately 2.4 × 10 23 cm −2 (24 displacements per atom (dpa) in Ge and 14 dpa in Al) for specimens irradiated at 10 or 50 K; this is the same value of Φ c at which GeAl bilayers are found to contain an amorphous phase. At 10 K an irradiated GeAl bilayer specimen is found to have an amorphous phase at the interface only when a Ge layer faces the incident electron beam, while for a SiAl bilayer specimen amorphization occurs at the interface independent of the direction of the incident beam with respect to a bilayer. For GeAl bilayer specimens Φ c is approximately 2.5 × 10 23 cm −2 (25 dpa in Ge and 15 dpa in Al). For SiAl bilayers Φ c is approximately 3 × 10 23 cm −2 (19 dpa in Si and 18 dpa in Al). The temperature dependence of Φ c is also studied for a GeAl bilayer specimen. The value of Φ c is a constant for T < ≈160 K, and then it increases rapidly with increasing T; Φ c becomes immeasurably large (more than 70 dpa in Ge and more than 43 dpa in Al) at a critical temperature of about 190 K. The temperature dependence of the crystalline to amorphous transition is explained in terms of a recoil-implantation mechanism, coupled with a radiation-enhanced monovacancy diffusion mechanism for Ge atoms in Al, and an athermal contribution to Ge diffusion in Al via an electron-beam stimulated monovacancy migration mechanism.