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, optoelectronics, 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/cm² for Ge₁Sb₂Te₄ and 1.59  mJ/cm² for Ge₁Sb₄Te₇. Furthermore, 100% volumetric crystallization of Ge₁Sb₄Te₇ was achieved with the fluence of 4.78  mJ/cm², 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.

Femtosecond laser-induced ultrafast transient snapshots and crystallization dynamics in phase change material

We report here femtosecond laser-driven transient snapshots of ultrafast crystallization of Ge₂Sb₂Te₅ films from its as-deposited amorphous phase, and the local structural change is validated by micro-Raman spectroscopy and x-ray diffraction. The decay time constant of ∼5  ps in transient spectra with a precise temporal resolution using 400 nm (pump) reveals about 68 volumetric percentage crystallization at a remarkably low fluence of 4.78  mJ·cm^-2. This is attributed to reiterated excitation after a complete carrier relaxation and formation of a long-lasting transient phase at sub-threshold fluences. Furthermore, Raman spectra of irradiated spots confirm defective-octahedral modes at 110 and 160  cm^-1 validating crystallization.