In situ dynamic transmission electron microscopy characterization of liquid-mediated crystallization of amorphous Ge

Abstract

Crystal growth rates during laser-induced, liquid-mediated crystallization of amorphous Ge were measured with movie-mode dynamic transmission electron microscopy (MM-DTEM), a photo-emission microscopy technique with nanosecond-scale time resolution. Films of 50-nm thick amorphous Ge were crystallized using a 12-ns laser pulse with a Gaussian spatial profile, which established high local temperature gradients in the specimen. Crystallization proceeded by formation of a central zone with a high nucleation rate (Zone I), followed by liquid-mediated outward growth of columnar grains (Zone II), followed by spiraling growth (Zone III) until the crystallization halted in cooler parts of the specimen. Zone II growth was imaged for several laser pulse energies with 20-ns electron pulses with 95 ns between frames. A thin liquid layer between the solid amorphous phase and the advancing crystallization front during Zone II growth was imaged. The Zone II growth rate for each experiment remained nearly constant although the crystallization front passes through a large temperature gradient. Measured growth rates ranged from 5.7 to 13.6 m/s, consistent with transient-liquid-layer mediated growth rather than solid-state growth. In contrast with a previous report, the growth rate did not increase systematically with laser energy or absorbed energy. The new results, together with previously reported data, suggest both sets of experiments were conducted under conditions where the growth rate saturates near its maximum value. A phenomenological model based on the concept of an upper and lower threshold temperatures for the Zone II growth was fitted to the data from these experiments and previous MM-DTEM crystallization experiments.

Impact of supercooled liquid structures on the crystallization processes of amorphous Ge

“…Previous studies have suggested that nanocrystals form by solid phase crystallization at the highest temperatures in the laser irradiation region. 45) On the contrary, our present study revealed that nanocrystallites are initially formed by liquid phase crystallization at the low-temperature region, suggesting that Zone I is formed during the temperature rise. As the temperature increases further, the low-density liquid phase changes to a high-density liquid one.…”

Расчет Динамики Границы Аморфная Фаза-Кристалл При Твердофазной Взрывной Кристаллизации

“…В последние годы произошел значительный прогресс в экспериментальных возможностях наблюдения in situ процессов, сопровождающих ВК. Он связан в первую очередь с разработкой новой методики (dynamic transmission electron microscopy), с помощью которой удалось добиться гораздо более высокого пространственного и временного разрешения структуры кристаллизующегося материала [28][29][30]. В частности, проведенные в [28][29][30] замечательные эксперименты показали, что переход аморфного состояния в кристалл Ge происходит по механизму бокового, а не рассмотренного в теоретических работах [5,6,[17][18][19][20][21] нормального роста.…”

“…Он связан в первую очередь с разработкой новой методики (dynamic transmission electron microscopy), с помощью которой удалось добиться гораздо более высокого пространственного и временного разрешения структуры кристаллизующегося материала [28][29][30]. В частности, проведенные в [28][29][30] замечательные эксперименты показали, что переход аморфного состояния в кристалл Ge происходит по механизму бокового, а не рассмотренного в теоретических работах [5,6,[17][18][19][20][21] нормального роста. Т. е. сначала на фронте возникает локальный закристаллизовавшийся выступ, который затем расширяется и охватывает всю поверхность фронта.…”

Nanosecond Time-Resolved Transmission Electron Diffraction Measurements on Ion Substitution Dynamics

“…Its fast heating rate and precise local processing capacity make laser annealing a powerful technique to reveal novel phenomena and mechanisms in the crystallization process of amorphous materials. Previous time-resolved studies using ultrafast transmission electron microscopy (UTEM) and dynamic transmission electron microscopy (DTEM) on amorphous films − have provided an in-depth understanding of crystallization mechanisms. Thermally driven liquid-to-crystal transformation would occur in various regimes after laser irradiation involving fast nucleation and consequent complex growth dynamics.…”