Kinetics of liquid-mediated crystallization of amorphous Ge from multi-frame dynamic transmission electron microscopy

Abstract

The kinetics of laser-induced, liquid-mediated crystallization of amorphous Ge thin films were studied using multi-frame dynamic transmission electron microscopy (DTEM), a nanosecond-scale photo-emission transmission electron microscopy technique. In these experiments, high temperature gradients are established in thin amorphous Ge films with a 12-ns laser pulse with a Gaussian spatial profile. The hottest region at the center of the laser spot crystallizes in ∼100 ns and becomes nano-crystalline. Over the next several hundred nanoseconds crystallization continues radially outward from the nano-crystalline region forming elongated grains, some many microns long. The growth rate during the formation of these radial grains is measured with time-resolved imaging experiments. Crystal growth rates exceed 10 m/s, which are consistent with crystallization mediated by a very thin, undercooled transient liquid layer, rather than a purely solid-state transformation mechanism. The kinetics of this growth mode have been studied in detail under steady-state conditions, but here we provide a detailed study of liquid-mediated growth in high temperature gradients. Unexpectedly, the propagation rate of the crystallization front was observed to remain constant during this growth mode even when passing through large local temperature gradients, in stark contrast to other similar studies that suggested the growth rate changed dramatically. The high throughput of multi-frame DTEM provides gives a more complete picture of the role of temperature and temperature gradient on laser crystallization than previous DTEM experiments.

Effect of medium range order on pulsed laser crystallization of amorphous germanium thin films

“…Laser processing with its short duration has the additional advantage of putting the material in a far-from-equilibrium state, creating phases not accessible via other methods. 4,[7][8][9] It has been shown that the crystallization behavior of amorphous semiconductors can vary significantly based on their preparation methods 3,10 and thermal treatment history. 11 In particular, the differences have been linked to the changes in the medium range order (MRO) of the materials.…”

“…11 While the classical theory is successful at describing solid-state transformation at slow rates, it has been reported that the laser-induced rapid crystallization of a-Ge is facilitated by the formation and propagation of a thin, undercooled liquid layer. 9,[15][16][17][18] The effect of MRO on liquid-mediated crystallization has not been well understood so far. Figure 1(a) shows a typical bright field transmission electron microscope (TEM) image of a laser-crystallized region of 33 nm a-Ge thin film supported on a 20 nm amorphous silicon nitride (SiN x ) membrane.…”

“…Figure 1(a) shows a typical bright field transmission electron microscope (TEM) image of a laser-crystallized region of 33 nm a-Ge thin film supported on a 20 nm amorphous silicon nitride (SiN x ) membrane. Following the nomenclature of previous studies, [7][8][9] we divide the different microstructures of the crystallized region into three zones. The nanocrystalline central region, zone I, is formed within the hottest part of a Gaussianprofile laser that results in extremely high nucleation rates.…”

“…In this study, we focus on the effect of MRO on the kinetics of zone II growth, where nucleation rate is low and extremely rapid, liquid-mediated growth occurs. [7][8][9] Previous time-resolved in-situ TEM measurements have shown that during the pulsed-laser crystallization of a-Ge, zone II growth proceeds with a radial velocity of $8-12 m/s, much faster than typical solid-state diffusion controlled transformations. 4,8,9 Such speed also requires a temporal resolution well beyond the capability of a traditional CCD camera inside a TEM.…”

“…[7][8][9] Previous time-resolved in-situ TEM measurements have shown that during the pulsed-laser crystallization of a-Ge, zone II growth proceeds with a radial velocity of $8-12 m/s, much faster than typical solid-state diffusion controlled transformations. 4,8,9 Such speed also requires a temporal resolution well beyond the capability of a traditional CCD camera inside a TEM. We use the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory (LLNL) to image the crystallization process in real time.…”