Laser-induced crystallization of amorphous GeTe: A time-resolved study

Amorphous thin films of GeTe were exposed to excimer-laser pulses of fluences between 3 and 27 mJ/cm². Transient reflectivity and conductivity were simultaneously recorded during the laser interaction with a time resolution of a few nanoseconds. A rapid increase in R and {σ} due to laser-induced heating is observed. This is followed by a decrease of these parameters at a slower rate due to cooldown. During the cooldown process and within a certain temperature interval nucleation and growth are possible and experimentally observed. The cooling rate in this interval is a function of incident fluence and determines the degree of crystallization induced by laser irradiation. Above a fluence of 22 mJ/cm² the films crystallize to a large degree within 200 ns (25% and greater than 90% at 23 and 27 mJ/cm², respectively). Between 15 and 22 mJ/cm² the crystallization is less than a few percent (frustrated crystallization), and application of a subsequent pulse leads to an extremely fast crystallization (50 ns). Below 15 mJ/cm² the irradiated films remain amorphous. A model describing transient R and {σ} in terms of cooling, nucleation, and growth is presented and compared with the measured transient profiles.