A novel liquid-mediated nucleation mechanism for explosive crystallization in amorphous germanium

Abstract

We report a novel mechanism for explosive crystallization in amorphous germanium (a-Ge), which operates through liquid-mediated nucleation occurring under extreme thermal gradient conditions. The crystallization kinetics of sputter-deposited films with thicknesses ranging from 30 to 150 nm were characterized using in situ movie-mode dynamic transmission electron microscopy (MM-DTEM). After localized heating from a short laser pulse, explosive liquid phase nucleation (LPN) was observed to occur during the early stage (<2 μs) of crystallization in the thicker (>50 nm) films deposited on silicon nitride substrates. The crystallization front propagated at ∼12–15 m/s and produced nanocrystalline microstructure with ∼50 nm grains. A mechanism involving the existence of a relatively thick (>100 nm) transient liquid layer and a high nucleation rate is proposed to explain the behavior. The key thermodynamic and kinetic features as well as the feasibility of the mechanism are further explored by employing parametric and systematic phase-field modeling and simulations.