Ultrafast and low-power crystallization in Ge1Sb2Te4 and Ge1Sb4Te7 thin films using femtosecond laser pulses

Rapid and reversible switching between amorphous and crystalline phases of phase-change material promises to revolutionize the field of information processing with a wide range of applications including electronic, opto-electronics, and photonic memory devices. However, achieving faster crystallization is a key challenge. Here, we demonstrate femtosecond-driven transient inspection of ultrafast crystallization of as-deposited amorphous Ge1Sb2Te4 and Ge1Sb4Te7 thin films induced by a series of 120 fs laser pulses. The snapshots of phase transitions are correlated with the time-resolved measurements of change in the absorption of the samples. The crystallization is attributed to the reiterative excitation of an intermediate state with subcritical nuclei at a strikingly low fluence of 3.19 mJ/cm(2) for Ge1Sb2Te4 and 1.59 mJ/cm(2) for Ge1Sb4Te7. Furthermore, 100% volumetric crystallization of Ge1Sb4Te7 was achieved with the fluence of 4.78 mJ/cm(2), and also reamorphization is seen for a continuous stimulation at the same repetition rate and fluence. A systematic confirmation of structural transformations of all samples is validated by Raman spectroscopic measurements on the spots produced by the various excitation fluences. (C) 2018 Optical Society of America