AFM allows high resolution visualization of laser-crystallization experiments of phase-change memory materials. Crystallization speed limits the data rate in phase-change solid-state memory devices. AFM has been used to reveal the physics of crystal nucleation and growth, since the density change accompanying crystallization produces observable topography in AFM.

Crystallization speed of amorphous GeSb films: Sputter-deposited amorphous films of GeSb are slow to crystallize when heated with a laser pulse, because formation of a critical-size crystal nucleus is much slower than the enlargement of crystalline regions by propagation of the crystal-amorphous phase boundary. However, nucleation and growth rates are both temperature dependent, and a sequence of two laser pulses of different powers can produce conditions that are optimal for both the production of nucleation centers (first laser pulse) and crystal growth (second laser pulse). This plot shows the much faster crystallization of GeSb by a sequence of two pulses (dot-dash lines) compared to a single pulse (solid lines); in the two-pulse experiment, the first pulse is always 150 nsec, 100 mW, while the second pulse is longer and one of 60, 80 or 100 mW.

This AFM image shows crystalline areas (dark color) growing from multiple nucleation sites produced in the center of the focussed laser beam by the high power first pulse. The crystal growth was induced by a second pulse of lower power, such that the temperature for optimal growth was produced at the center of the beam as well.

When the second pulse is of high power, comparable to that of the first pulse, nucleation still occurs at a single place near the edge of the beam, since for this power, the temperature at the center is higher than optimal for rapid crystal growth.